To go further in the design of amplifiers, I ran into the problem of a quality source. I really needed a good DAC. I was not fully satisfied with the quality of those that I had at home and which I had to listen to before. If this is a classic DAC with operational amplifiers at the output, then this usually leads to the problem of reproducing the upper mids and highs. The middle becomes slightly cutting the ear, harsh, as if with sand or metal in the voice, especially at high volume. With tube DACs, too, not everything is in order - often there is no good bass or a flat, inexpressive sound, and besides, for some reason, developers are very fond of putting a cathode follower at the output, which, although it lowers the output impedance, but in my humble opinion of sound does not decorate, to put it mildly. In general, I came to the conclusion that I have to do it myself.
Why did I choose Ad1955? Its output is designed for an I - U converter with a current of 3 - 5 mA of positive polarity. And here - a wide field of options for connecting to a high anode voltage in such a way that the output current of the DAC microcircuit passes through the lamp.
Yes, of course, I wanted a DAC with a tube output. And given my weakness for cascades with a common grid and transformers, the output was planned for my favorite 6E6P lamp with a transformer output. The choice of this lamp is also due to its low internal resistance in the triode, as well as its high slope (30 mA per volt), and in the case of a cascade with a common grid, this gives a lower input impedance - and this is very good for I - U DAC converters, for which the input impedance should tend to zero. It is logical to make the input of the I - U converter on a germanium transistor connected according to a circuit with a common base. Hence the scheme was born. According to my rough estimates, the input impedance of my hybrid cascode is somewhere on the order of 1 ohm. How did you count? We take the formula for calculating the input impedance of a stage with a common grid Rin = (Ra + Ri) / (u +1). In a 3.3 KOhm lamp load, the 6E6P itself in a triode has about 1500 ohms. Add and divide by 30 - this is the lamp gain. It turns out 160 ohms. This is the input impedance of a lamp connected to a common grid. Now for the transistor - the lamp is a load of Ra. I don't know the internal resistance of the germanium transistor, but we take roughly 50 Ohms, then if its Cus is about 250, then (160 + 50) / 250 = 0.84 Ohms.
If 6E6P seems to someone too emphasizing the middle, then it can be replaced with 6Zh9P, 6Zh11P or 6Zh49P. Only in this case, you should pay attention to the fact that the collector of the transistor is connected to pins 1 or 3 of the lamp socket (and not to pin 6) - then you can simply select the light bulb that seems more melodious to you with a simple crossover.
I give the first version of the scheme, although I am sure it will have to be finalized, because there is no limit to perfection….
In order not to do the digital part myself, I took the DAC board on the AD1955 on e-Vay and removed the operational amplifiers from it, also soldered the 2K resistors from the power supply put on the datasheet from the AD1955 outputs, and left 100 pF (capacitors C1 and C2 in the diagram) those that were on the board. I will give more detailed details a little later.
I tried a transistor stabilizer as a power supply, but still it turned out to be the best sounding tube doubler on 6N1P, which was nevertheless later replaced by ECC99. The reason for using this rare lamp is simple - to pack my DAC, I used a case from the Chinese Lite DAC, which ordered to live a long time, thank God, I did not throw out the case. Both network transformers, a network button and input / output connectors came in handy. Here is the power supply diagram:
As you can see, the 6E6P filament is powered by direct current, but unstabilized.
Now a little about listening. Source - CD-player Denon 1500 and compared with my DAC, signal delivery via optical digital cable. The amplifier is my cascode for 6E5P - 2A3. Columns - broadband in OY from 3AC505. The first impression was very bad, I was very upset and was about to take my creation to the closet in the company to other unsuccessful projects. It seemed to me that my DAC gives too harsh female vocals and trumpet. But then - lo and behold! - it turned out that I confused the inputs on the switch in front of the amplifier - what I was disappointed in - it was just Denon's DAC, but my DAC gives an excellent presentation of the material! And the timbre balance, stage width, and emotional saturation will be higher than Denon's. In general, he sings cleanly, in detail, transparently, and what is especially different from my trademark Denon is a very soft delivery of vocals and, in general, the upper mids and highs - no ringing, excessive harshness at almost any volume, in general - much more natural. Here it is appropriate to say about the "coloring" of the sound. As in colorimetry, speaking about color, it is important to answer the question - what is accepted as the standard of white? If we take transistor sound as this standard, then yes, lamps give “color”. But in my understanding, tube sound is the standard of white. And the operational amplifiers at the output (by the way, always used with deep OOS) give a slightly metallic color and a slightly unnatural upper register, which is not inherent in live performance. In general, I was very, very pleased with my creation.
Here are its characteristics
- output voltage at the level of 0 dB - 2 Volts;
- the noise level is less than -80 dB, there is simply nothing less to measure;
- the total harmonic distortion at the maximum level - less than 0.15% - again, I still cannot measure more accurately.
- inputs - optical and SPDIF;
- outputs - unbalanced 2 Volts and balanced 10 Volts;
- output impedance - at unbalanced output - less than 100 Ohm, balanced output - about 2 KOhm;
- the circuit does not contain OOS circuits.
This is how the device, packed in a case, looks like and a photo of the entire set of listening equipment.
Output transformers were wound to order in the Audioinstrument firm, for which I bow to Sergei Glazunov. And also - read on the forum http://www.diyaudio.ru/forum/index.php?topic=4180.0. My first attempts (not entirely successful) to make a DAC only on lamps are in another branch on the same forum http://www.diyaudio.ru/forum/index.php?topic=1267.570.
Updated June 6, 2015. I had to slightly correct the schematic. Firstly, excitation (resonances) was observed at loudness peaks and therefore it was necessary to add capacitors C3 and C5 to the lamp grids, as well as C1 and C6 to the anodes. Also, due to the voltage drift at the AD1955 output, it was necessary to stabilize the bases of the transistors with the help of a 3.0 volt D1 Zener diode. Well, and nevertheless I replaced 6E6P with 6Zh49P - to me it seemed to me the most balanced timbre of all those listed earlier.
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Is a multi-bit digital-to-analog converter based on four industrial 18-bit AD5871 DAC chips.
- tube headphone amplifier with an impressive power of 8 watts and the ability to replace tubes with "solid state" amplifier modules, which are purchased separately.
The devices are designed with a fully balanced gain topology.
Appearance
All Schiit devices are made in the same style, and models from the upper price category are no exception. No sapphire glasses and diamonds in the pens, unnecessary overpayments for the gilding of the case and screens. However, the enclosures are now full-sized and look harmonious in any audiophile equipment rack.
The controls are still minimalistic: the only button on the DAC that selects the desired input.
On the headphone amplifier, in addition to the volume control, there are switches for gain and selection of balanced or unbalanced input.
The rear panels of the devices are also laconic.
The Gungnir DAC has USB, optical and two coaxial inputs, one of which is BNC. It should be noted that BNC is a connector specially designed for high frequency transmission (as opposed to low frequency RCA). BNC is also optimal for high quality digital signal transmission.
There are two pairs of unbalanced RCA outputs and balanced XLR outputs that operate simultaneously.
The Mjolnir 2 amplifier has balanced and unbalanced inputs on the rear panel, as well as outputs for connecting other equipment, such as a power amplifier for speaker systems.
The toggle switches for the power switches on both units are also located on the rear panel. And if in the case of a DAC consuming a relatively small power of 20 watts, you can close your eyes to this and leave it constantly on, then in the case of a headphone amplifier that consumes 45 watts at idle and has a limited lamp resource, this is rather inconvenient. At least in the rack, you can't just turn off the power. This is exactly the case when convenience comes at a price for the beauty and design of the front panels.
Passport technical characteristics
Gungnir Multibit
- D / A IC: Analog Devices AD5781BRUZ × 4 (two per channel, balanced)
- Digital Filter: Proprietary closed loop with bit-precision function, implemented on Analog Devices SHARC DSP
- Analog path: fully discrete JFET buffer stages for balanced output and JFET summing stages for single-ended output, direct coupled
- Operating frequency range: 20 Hz - 20 kHz, ± 0.1 dB; 1 Hz - 200 kHz, -1 dB
- Maximum Output Amplitude: 4.0 V RMS (balanced output), 2.0 V RMS (unbalanced output)
- Total Harmonic Distortion (THD):<0,005% (20 Гц — 20 кГц, при полной выходной мощности)
- Intermodulation Distortion (IMD):<0,004% (измерены по стандарту CCIR)
- Signal to Noise Ratio (S / N):> 115dB (Relative to 2V RMS)
- Inputs: coaxial S / PDIF (RCA and BNC), optical S / PDIF (Toslink), USB
- Supported formats: up to 24 bit / 192
- Outputs: One pair of balanced XLR connectors and two pairs of unbalanced RCA connectors
- Output impedance: 75 ohms
- Clock recovery: bit-accurate at all original sample rates via Adapticlock analysis and VCXO / VCO regeneration
- Power supply: two transformers (one for digital, the other for analog) with 8 regulation steps, including separate power rails for critical sections of the digital and analogue path
- Upgradeable: Separate, replaceable USB I / O and DAC / AO cards
- Power consumption: 20W
- Dimensions: 406 × 223 × 60 mm
- Weight: 4 kg
Mjolnir 2
- Operating frequency range: 20 Hz - 20 kHz (−0.1 dB), 2 Hz - 400 kHz (−3 dB)
- Maximum power at load impedance:
- 32 Ohm: 8.0 W RMS / ch
- 50 Ohms: 5.0W RMS / channel
- 300 Ohms: 850 mW RMS / channel
- 600 Ohms: 425 mW RMS / channel
- THD: Less than 0.005% (20Hz - 20kHz, 1V RMS)
- Intermodulation distortion: less than 0.006% (CCIF test, 1V RMS)
- Signal-to-Noise Ratio:> 104dB (unweighted, referenced to 1V RMS, in low gain mode)
- Crosstalk: less than −75 dB (20 Hz - 20 kHz)
- Output impedance: 1.0 ohm (high gain), 0.3 ohm (low gain)
- Gain: × 8 (18dB) or × 1 (0dB), front switch
- Topology: tube voltage amplifier or solid state LISST voltage amplifier, push-pull parallel output stage Crossfet, non-inverting single voltage gain stage
- Power supply: a special transformer for the Cyclotron 4 output stage, operating on filter capacitors with a capacity of more than 65,000 μF, plus a separate transformer with a voltage of 200 V and storage capacitors with a capacity of more than 4000 μF - for a high-voltage discrete-controlled input stage
- Inputs: One pair of balanced XLR and unbalanced RCA sockets, switchable via front toggle switch
- Outputs: 4-pin balanced XLR, 6.3mm minijack, a pair of 3-pin XLR pre-outs, one pair of unbalanced RCA connectors
- Power consumption: 45W
- Dimensions: 406 × 223 × 60 mm
- Weight: 5.4 kg
- Approximate price: 76,500 rubles (only with 6BZ7 lamps) at the time of preparing the review
Internal structure and measurements
The internals of the Gungnir Multibit DAC will evoke a positive reaction from any engineer. To a burnt out audiophile, it may seem that gold parts and film capacitors were not supplied for such a price. But wait, Schiit engineers have another surprise for you!
On the motherboard, according to the accepted Schiit concept, there are separate multi-bit DAC and USB-input modules. Note that this does not reduce the cost of the device, but it allows you to achieve a higher quality of the wiring and operation of individual nodes compared to how everything would be mounted on one board.
The personal thanks to the engineers written on the boards evoke special affection, this once again confirms that the device was designed by people for people, and not by obscure OEMs solely for earning money. The DAC has many interesting solutions to improve sound quality.
The USB receiver is made on the familiar CM6631A controller, but it has galvanic isolation, and done correctly: the master oscillators are located on the "clean", galvanically isolated side. Yes, it is more expensive, yes, it is more difficult in terms of implementation, but this is the only way to get a good result. And here it is done. So you can safely connect the DAC via USB to your computer and not worry about noise and ground loops. Note that in our case, Windows 10 independently found and installed the required driver. The USB drivers from the official website could not be installed.
The S / PDIF receiver is made on the old familiar, but not worse, CS8416 microcircuit.
Also on the motherboard, in addition to transformers, rectifiers and primary stabilizers, there is a rather interesting phase-locked loop, with its own generators for frequencies of 22.579 and 24.576 MHz. This proprietary technology is called Adapticlock and serves to additionally suppress digital signal jitter.
On the board of multi-bit DACs, in addition to the AD5871 itself, there is an Analog Devices ADSP-21478 digital processor, which serves for digital signal filtering. After him and before the AD5871, according to all the canons of building high-quality DACs, there is a reclocking made on separate digital D-flip-flop microcircuits.
The post-DAC filter amplifier is a separate, audiophile topic. Surprise! It is implemented on JFET field-effect transistors using non-feedback circuitry. Yes, there are microcircuits, but the signal does not pass through them, they are only needed to maintain zero at the DC output. This is a gift to those who believe that negative feedback in the sound path is evil. Yes, it affected the measurements, but not the sound.
Objective measurements were carried out using USB under Windows 10.
In this case, the measurements can be characterized simply: the manufacturer wanted to sneeze at them, the safe concept and sound were put at the forefront. A specifications with many zeros after the decimal point, posted on the official website, serve rather to avoid unnecessary excitement among people who listen not to sound, but to graphics. We do both.
The lamp distortion spectrum gives field exhaust, and, apparently, this was done on purpose.
To verify this, the measurement card was connected directly to the DAC output, before the JFET filter amplifier.
In this case, we see very low distortion of the AD5781 DACs proper with a typical multi-bit signal spectrum. For the sake of interest, an audition was made in this version. Let's just say: without a harmonizing filter, the sound is not very good. Despite their low distortion, DACs subjectively sound very harsh.
We also ran the J-test test file, which allows us to show flaws in the construction of the digital part, recalculation or increased jitter of a digital signal. The result is perfect: no collateral interference other than the main comb. This confirms the very high design quality of the digital, “pre-audio” part of the device.
A few words about the applied multi-bit converters. The AD5781 is one of the best multi-bit converters available today, but they are very expensive at around $ 40 apiece. There is also the AD5791, 20-bit precision, they cost $ 100 apiece and are used in the top-end Schiit Yggdrasil DAC.
Despite the costs, we support the manufacturer in using parts that are produced here and now, and not obscure old warehouse leftovers or even Chinese fakes. This guarantees the quality and repeatability of the product characteristics.
The post-DAC filter and output are made on JFET transistors in the SOT-23-5 package marked XL, which could not be identified. Wima foil capacitors and Nichicon KW electrolytic capacitors are also used.
The filter circuitry is fully balanced, so the unbalanced output receives only half of the output signal, in fact only two DACs work instead of four. This affects measurement and subjective sound quality, so we do not recommend using an unbalanced connection to an amplifier, although it is possible.
The Schiit Mjolnir 2 headphone amplifier is designed with a similar balanced ideology. But here the emphasis is on the power output and the stability of the power supply, for the buildup of the most difficult load.
Powerful 30-watt transformers, a capacitor bank with a total capacity of 65,000 μF, IRF610 output transistors capable of dissipating 54 watts of power, included in the proprietary Crossfet topology - all this allows the amplifier to work even with an 8-ohm load connected to a balanced output.
The manufacturer uses transistors of the same structure in both amplification arms, which ensures their excellent identity and lower distortion.
The heart of the amplifier, which provides the main signal amplification, is a vacuum tube with JFETs at the input. The microcircuits present are used only to maintain a zero at the DC output, the signal does not pass through them. The amplifier circuitry is unique and does not resemble standard solutions.
The unbalanced output is implemented in a completely different way, a separate bipolar transistor amplifier is provided for it, although the main amplification is also provided by the lamp. The maximum unbalanced output power is limited to 2 watts. The balanced and unbalanced outputs operate simultaneously, but at the same time independently of each other.
Any music lover is warmed by the soft glow of a vacuum tube. Lamps give out a somewhat special sound, emphasize voices, timbres of instruments, muffle and soften the sound, veil the garbage in the recording ... But what if you want to cheer up and listen, for example, to several Death Metal albums? Schiit took care of such desires with a solid-state, lamp replacement, LISST. Basically, it is a two-stage amplifier mounted in a box. And with him there will definitely not be any veil - checked!
Technically, the Mjolnir 2 amplifier makes a very good impression, and in terms of circuitry it is not inferior to the Gungnir DAC, but what about the measurements?
For testing we used a professional Lynx L22 balanced card, and in most cases the measurement results were limited by its quality, and not by the amplifier.
Whether a 6BZ7 vacuum tube or LISST solid-state circuit is used, the balanced amplifier performs excellently at 300 ohms. When the load is reduced to 32 ohms, only the second harmonic grows, which will not affect the sound in any way.
An unbalanced amplifier is more demanding on the load, and at a power of more than 100 mW at a load of 32 ohms, distortions grow catastrophically. At a 300-ohm load, nothing like that happens. Therefore, for maximum quality, we still recommend using a balanced output.
The output impedance of the balanced output is about 0.8 Ohm, it perfectly dampens any headphones, prevents uncontrolled resonance phenomena, which ultimately provides a natural and dynamic sound.
Listening
Let's not bore you by listing the musical material used for listening, it all depends on personal preference. Note that we were not limited to any specific styles of music, listened to the Gungnir Multibit DAC on different systems, and used different headphones with the Mjolnir 2 amplifier, from Oppo PM-2 to Audio-Technica M50x. A person who has "grown" to this class of audio equipment knows perfectly well what he wants to get in sound and what he will use to listen to his favorite music.
The Gungnir Multibit DAC can be described as a source that is ready to play almost any music with high quality. It cannot be called too harsh or, on the contrary, too delicate. But the multi-beat heart is undoubtedly more predisposed to dynamic and fast music such as rock and metal, and pop music performed by Gungnir will sound much more interesting. Of particular note is the way the voice is reproduced: it seems much more alive. And in general, the mid-frequency range is reproduced perfectly. Instruments are played separately, there is no mess of sounds at all. In many old familiar compositions, many new sounds were heard, which were previously masked in the general rhythm. The bass is firm and biting, but at the same time there is not a hint of buzz. In some systems, the bass may not be enough, but where it is - it will be excellent. High frequencies have never been the strong point of multi-bit DACs, in this case the manufacturer tried to make them as neat and comfortable as possible. Yes, there is some roughness, but this is by no means dirt! On the contrary, in high-quality headphones you can hear much more detail in the high-frequency range than when using modern "slick" boring DACs like AK4490. The limiting factor here is the quality of recording and mastering of the phonogram itself. The very low level of jitter also contributes to the correct sound reproduction of the high-frequency range, when connected via the USB input the DAC plays great! This is what we recommend listening to, but if you have a high quality audiophile digital source, you won't be disappointed with the coaxial connection.
To summarize, Gungnir Multibit can be described as the most neutral and even, perhaps, a little distant source. He does not dump the musical picture directly on you (and deal with it as you like), but delivers, delicately and accurately revealing the intention of the composer or sound engineer, without hiding or embellishing details. The listener is more in the role of an observer, contemplating a riot of sounds from the side. You will not find yourself in the center of an orchestra or on stage next to the musicians. But hear everything. This is how music is listened to. Dynamic, fast, open. For this they love the old multibit players, and this trait has been preserved in the new Schiit Gungnir Multibit.
We must admit that, having received this DAC for a test and carried out preliminary measurements, we were somewhat disappointed with the low measurement results, and there was no wah effect during the initial listening. However, the longer this DAC was in our possession, the more we liked it, revealing more and more secrets of old familiar compositions and without causing absolutely any irritation even with very long listening. The sound is very comfortable, but at the same time clear - a rare combination. That is why it is recommended, when purchasing equipment, to arrange for a sufficiently long test, at least for several days, and only after that draw conclusions. Perhaps the option that "hooked" you in the store with a dynamic and bright sound, at home will take out the brain in three days. With Gungnir Multibit, it turned out the other way around.
The sound of the Mjolnir 2 headphone amplifier also deserves the most flattering epithets, it's not for nothing that it is the most expensive in the manufacturer's lineup.
The amp sound is absolutely neutral and clear, and the bass control is amazing. Listening to headphones through this amplifier allowed discovering new subtleties of bass parts: for example, it turned out that in some compositions the sound engineer intentionally introduced distortions in the bass, which was never noticed when listening to the speakers. In general, the bass stopped simply setting the rhythm, it became an interesting object of observation. The amplifier is so masterful at controlling any headphones that it seems that you can connect external speakers to it and listen to music calmly.
The use of 6BZ7 vacuum tubes or LISST solid state modules is purely a matter of taste. In both cases, the sound is very good. Tubes tint and soften the mids, but give some veil at the highs. Solid-state modules give the cleanest high frequencies, wide stage and neutral sound, they are more preferable for modern electronic and rock music.
To understand the subjective difference, we tested the balanced and unbalanced outputs of the amplifier using Oppo PM-2 headphones and interchangeable cables with matching connectors. The outputs have only minimal differences in color and sound quality, but balanced overall gives a higher grade sound, which translates into better bass control and cleaner sound at the same volume. If you do not go beyond a reasonable volume, then both outputs have excellent sounding, the difference is purely tasteful. If you want to "give in", remember that the balanced output in high gain mode has a maximum amplitude of 20 volts! Probably, after all, it was necessary to connect the speakers.
From the characteristics of the amplifier, it should be noted that due to good control of the bass, it does not "inflate the low frequencies", so if the headphones have a roll-off in the low-frequency region, then the bass will be low in the end.
conclusions
And made for each other! You understand this when you turn them on together and start just listening to music. You can spend more than one hour contemplating music, only time makes you interrupt listening. Isn't this the main criterion for the quality of technology? We think yes. Schiit engineers think the same way, creating products that are both technically and sensationally admirable.
For DAC or CD player. At first glance, the scheme may seem somewhat complicated, and some of the functions incorporated in it (such as a phase shifter) are useless for a simple music lover.
Today we offer you a version of such an amplifier on just one tube (in each channel), without unnecessary marketing tricks, but, as before, with good characteristics and high sound quality.
When tested on various focus groups for different types of DACs revealed one common result - the sound quality of a CD-player is higher, the better the signal at its output from the high-frequency components is filtered. That is, the output filter should have a rather steep slope in the frequency response at the edge of the audible range.
If in the 90s analog filters were most often used, then recently digital filters have become more and more popular. It is not surprising, because with the relative simplicity of implementation, they show much higher characteristics compared to analog filters. Meanwhile, the test results showed that listeners prefer CD-players with analog filters, since digital ones, although they have higher characteristics, use clock signals in their work, which leads to an increase in the RF noise level.
Well, if the filter is not only analog, but also made on tubes, then this, in addition to cleaning from high-frequency noise, makes the sound of CDs more pleasant, "warm", eliminates digital sound harshness.
The filter circuit is shown in the figure:
Click-to-zoom
The amplifier has an absolutely flat frequency response throughout the entire audio range. The rolloff starts at 20 kHz (-0.5 dB) and is -24 dB / oct. This allows you to very well filter out all the high-frequency noise of digital audio reproduction and make a CD sound like vinyl.
In addition, the circuit has a low output impedance, which significantly reduces the requirements for interconnect cables.
The power supply circuit is shown in the figure:
Click-to-zoom
Here we see the traditional incandescence of lamps with direct current to reduce the background level of the network.
The high-voltage stabilizer is made according to the circuit of an electronic choke with current protection.
To improve the separation of stereo channels, each channel is assembled on a separate printed circuit board. The power supply is designed to work with two channels.
This design can be designed as a separate unit or, if space permits, integrated into an existing CD-player.
To achieve high sounding results, the capacitors in the circuit must be of the highest quality. Capacitors C4 and C7 not shown in the diagram are places on the printed circuit board in case you cannot find capacitors C3 and C6 of the required capacity and you have to connect several in parallel, or to change the cutoff frequency of the filter.
Listening has shown a significant improvement in the sound quality of a CD-player with this filter. This is a fairly cheap and easy way to upgrade your device to a higher price point.
PCB drawings and layouts can be downloaded
Printed circuit boards in SLayout format (rar-archive, 47 kb).
The article was prepared based on the materials of the journal "Elector"(Germany)
Happy creativity!
Editor-in-chief of Radio Gazeta.
28 responses to “A simple tube filter for a DAC or CD player”
- Kirill:
February 27, 2017Does it make sense, in your opinion, to "raise" the heating potential?
- Chief Editor:
February 27, 2017Here the glow is supplied with constant voltage. So this is superfluous.
Note that the filament circuits of the lamps are connected in series! Consider this when repeating the design.
If there is no desire to assemble a stabilizer for heating circuits, then yes - "it will not hurt to raise the heat.
- Kirill:
February 28, 2017Constant is understandable. However, the potential difference still remains, since the stabilizer of the heating circuits sits on the ground. I agree, in theory, there should not be a background. However, isn't this harmful to the lamp itself? Unclear...
- Chief Editor:
March 1, 2017It's called "I heard the ringing, but I don't know where he is" 🙂
The filament potential is raised when it is supplied with alternating current, just so that there is no background. Since through the heater-cathode section (which is essentially a diode), the background of the filament voltage (50 Hz) successfully crawls through. By raising the potential, we block this diode and block the way to the background.
If the glow is powered by a constant (and even stabilized), then the background should not be there, so the diode does not need to be locked. We save two resistors. This does not affect the lamp itself.
It is important for the lamp that the MAXIMUM PERMISSIBLE heater-cathode voltage is not exceeded. Indicated in reference books. Usually occurs in cathode followers and high-power output stages.
- Kirill:
March 1, 2017Why, I just know where this ringing comes from - from datasheets. For example, for a 6F5P lamp, the maximum permissible cathode-filament voltage is only 100 volts. Structurally, this lamp is close to ECL86 / PCL86, therefore, I believe that this is also true for it. In the presented scheme, this condition does not seem to be met.
- Chief Editor:
March 2, 2017For me, “seemingly not being complied with” and “not being complied with” are very different things.
What voltage do you think is present in this circuit at the cathodes of the lamp?
- yuriyruss:
March 9, 2017PCL86 and 6f5p datasheets are completely different lamps. Put them on the forehead. It is necessary to recalculate the entire voltage bias circuit. Later, when I check this filter for 6f5p, I will post the resistor values and the voltage on the lamp here.
- Chief Editor:
March 10, 2017 - Chief Editor:
March 10, 2017By the way, we did not write in the article that 6F5P is an analogue of PCL86.
This is assured on the mass of other sites.
On the heating circuits, they are exactly different.
- Sergey Khraban:
July 18, 2017Hello! Please tell me what type of zener diodes D1-D3 and D4 are in the power supply?
- Chief Editor:
July 19, 2017D4 - BZX55C18 (or analog), KS218Zh, KS508G, 1N4746A
D1-D3 - NTE5157A, 1N3045 and similar.
- Sergey Khraban:
July 19, 2017Thanks a lot! All the best!
- kagantsov:
October 5, 2017There are errors in the 12V power supply unit: 2200 electrolyte must be turned over, otherwise it has + on GND (it bangs like that). + and the AC needs to be redistributed, it turns out the AC goes to the microcircuit and + goes to the input of the change. Nonsense is obtained on the signet, it bangs 100%. Correct or warn that there are errors. I will also look at other PPs later. I want to build this device. If I collect and everything is ok, I will share my PP. Thank you.
- kagantsov:
October 5, 2017In PP BP 330V with a diode bridge, the same situation.
- Chief Editor:
October 5, 2017Printed circuit boards in pdf format from the original source.
Boards in SLayout format from Mars corporation.
Neither one nor the other was checked by the editors.
Thanks for the information!
In any case, care and caution when repeating any design will not hurt.
- Mars_Atlant:
October 5, 2017Good evening.
Thank you for your notes, I corrected the silk-screen printing and sent it to the archive updates.
- Chief Editor:
October 5, 2017The archive with boards in the SLayout format has been updated!
- kagantsov:
October 5, 2017So it all converges, but the diode bridges have AC in the middle. Sorry for the obstinacy, but if you really do that - good.
- Chief Editor:
October 6, 2017Obstinacy is encouraged! We are “for” working completed constructions with both hands.
By the way, I would like to hear the opinion, impression, etc. later. about the scheme ...
- kagantsov:
October 6, 2017OK. But it won't be soon. Time is short and projects are moving slowly. I finished the acoustics 2 weeks ago, I did it for 2.5 years. Well, it might be more fun with a filter.)
- Mars_Atlant:
October 6, 2017Good morning.
In part, you are all right about the diode bridge, but this type of diode bridge also exists with a different pinout, leg order. You can check it yourself on the net.
I "did", or rather sketched, the PP according to the PP photo presented on this resource.
Everything corresponds to the original materials, so as not to cause confusion if I have questions about this design.
You can also post your versions of the PP on the forum. Perhaps it will make it easier for someone to assemble this structure.
Good sound everyone.
- kemper:
October 11, 2017I used 6n2 and 6p43 lamps, the sound pleased me, though I don’t know how the 86 lamp sounds, I didn’t find it, (maybe the 6n1 sound seemed a little harder) reduced the supply voltage to 250v
- Chief Editor:
October 11, 2017PCL86 is very similar to our 6F3P (and this stuff is like mud) and ecl82.
Only with the glow it is necessary to take a closer look - for the listed lamps it is 6.3V!
- kagantsov:
October 13, 2017Good afternoon. I just have 2 PCL86 lamps, and the difference between 6F3P is only in the heat? Is the power supply the same - 330V?
- Chief Editor:
October 13, 2017Does religion not allow you to look into the reference book?
It clearly says: the limit voltage at the anode for the triode is 250V, for the pentode is 275V.
We draw conclusions based on the information received.
- Gray:
August 14, 2018Hello! I want to use the circuit of this high-voltage regulator to power the 220 V Morgan Jones ULF. There are many similar schemes on the sites, I almost figured out the method of calculating the elements. But they are missing R2. A similar circuit in the "Modern Hybrid Amplifier" dated 08/02/2014, but there the R2 rating is completely different. Please tell me the purpose of R2 and how to calculate it for a 220 volt circuit.
- Chief Editor:
August 14, 2018R2 here is a type of small decoupling (filter).
The denomination is not very principled.
With significant consumption currents, it is better to completely remove it so as not to reduce the efficiency.
Otherwise, you can leave it at 100Ω.
- Gray:
August 15, 2018Thanks a lot! All the best!
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The Aune T1 is a USB tube DAC with built-in semiconductor headphone amplifier, sold over 50,000 units. around the world.
Main characteristics
1. External linear high-quality power supply unit with additional filtering. This solution helps to eliminate noise from the power supply.
2. The DAC is implemented on the SA9027 asynchronous USB controller and the PCM1793 chip.
3. Aune T1 Mk2 USB DAC is an external sound card, DAC and high quality headphone amplifier in one package. The Aune T1 can also be used with powered speakers in your home hi-fi system.
4. Aune T1 works under Windows 7, 8, Vista, XP, Mac OS operating systems. Connection to iPad is possible. No additional driver installation is required.
5. The headphone amplifier module is made separately and can be replaced later when the corresponding upgrade is released. The lamp must be fully warmed up before playback starts. When the device is turned on, the lamp heats up for 30 seconds, after which the white indicator underneath it lights up, and only then the device begins to function. The Aune T1 Mk2 USB DAC also has a gain mode switching function.
6. New modular design. Multiple boards inside the device are powered independently, which eliminates crosstalk. The DAC also has a Safe Shutdown feature that prevents damage to your headphones or speakers when you turn off the device.
7. Aune T1 Mk2 USB DAC is made on high quality audio components: ALPS potentiometer (Japan), WIMA capacitors (Germany), ENLA electrolyte professional sound capacitor and so on.
8. The amplifier will pump headphones with an impedance of 30-600 ohms. Amplification circuit - OP + BUF.
9. The Aune T1 Tube USB DAC has one line input and one line output.
Video (promo, English)
Specifications
Lamp: 6922EH Electro-Harmonix (Made in Russia)
Frequency response: 20 Hz - 20 kHz
SNR:> = 120 dB
Output power: 1000 mW / 32 Ohm, 400 mW / 120 Ohm, 150 mW / 300 Ohm (maximum 20 V)
Output impedance: 100 ohm, 10 ohm (headphones)
USB interface:
Data up to 24 bit / 96 kHz
Operating systems: Windows XP / Vista / 7/8, Mac OS
Power supply: AC 220/110 V
Size: 155 * 97 * 40mm (L * W * H)
Package contents: power supply, USB cable, 6.35 - 3.5 mm adapter
The main thing in our business is to take the right start! I don't have to worry about building a product line from cheap consumer goods to the very high-end "a. Therefore, I can afford to immediately choose the D / A converter chip I like and build a design around it. So, the mystical DAC was taken as a basis. "as it is called on the Web. I will not make a big secret out of a small microcircuit, but let's keep the intrigue for a start.
Build a good DAC for my beloved, I had been planning since the last century, but somehow all my hands did not reach and higher priority tasks took over. And here, to my delight, a customer appeared, on the one hand able to appreciate a good sound, on the other hand - willing to put up with a certain level of "homemade" in the finished device. Naturally, I will do my best to make my clients happy with their choice. What my "pre-production" products lose in comparison with serial devices of well-promoted brands is:
- part of the editing is done with a cobweb on the "mole rats", and not on the print, which has a positive effect on the sound quality, but, alas, will not be available in serial samples;
- I do not skimp on trifles such as a surge protector or shunt capacities, which, by the way, have been caught by all recognized authorities more than once;
- my "brand" is not yet widely known in narrow circles 🙂
Ready, set ...
Where to begin? That's right, best of all from a ready-made device, even if it is simple, but containing key components. In China for US $ 50 a pretty good kit was purchased for self-assembly of a DAC. As I have already, the Chinese economic genius is not distinguished by special technical talents, so everything in that set was at a minimum, exactly according to the datasheet "am. Except that the creators of the set built food, as it seemed to them, downright very high quality: they poked it" KRENok "garlands. But the sets were supplied with very consistent R-core transformers.
At this stage, the task was not to somehow control the digital receiver or DAC "ohm", so the hard-wired minimalist S / PDIF-> I2S-> DAC chain suited me perfectly.
I deliberately did not seek to find a DAC with USB input. The reason is simple: the computer is very strong and there is no desire to let all this rubbish into the audio device. Of course, there are methods, but I still have not come across a single DAC with a competent decoupling of the USB input (devices for 1K green and higher, as well as products of Russian audio "lefties" do not count).
I consider it necessary to note that despite all my nitpicking about circuitry, etc., the quality of the printed circuit board is simply excellent!
Taking control of the situation into our own hands
In the documentation for the DAC, in one place it is written that the analog power leg must be shunted with an electrolyte of 10 mkF and ceramics of 0.1 mkF. In the diagram, leg 18 is shunted just like this.
A little further in the same document it is said that it is advisable to shunt the input on leg 17 with an electrolyte of 10 μF and ceramics of 0.1 μF. The developer acted in full compliance, the executive comrade, just great!
Another place in the documentation says that 17 leg can start directly on analog power. What we see in the diagram 🙂
What's funny is that not only in the circuit, but also on the printed circuit board, everything is wired like this: with two electrolytes and two 0.1uF capacitors, with a shorty right between the 17th and 18th legs of the chip (the path to the capacitors from 17th leg goes under the microcircuit case) :
Everything came just so dirty from the factory. How I laundered it is another story 🙂
For those who are especially curious: the step of the legs of the microcircuit case is 0.65mm.
My friend Vadich-Borisych got a gorgeous picture on VKontakte: " resistance is futile". Here, inspired, it is just as useless here as the duplicated bypass capacitors in the diagram above, redrawn the" circuit "especially for you:
I needed to control what happens on the 17th leg. I had to cut it alive. It's good that a jumper hasn't been installed yet under the chip - the prospect of soldering one leg of the SSOP case is somehow not encouraging.
Mediocrity - overboard
What digital-to-analog converter does without operational amplifiers?
That's right, only quality DAC... So I just didn't solder the modest filter on the NE5532. Maybe it was worth it to have something to listen to for comparison and make sure how unconvincing the deep loop feedback plays ... But I already have a CD-player from a venerable manufacturer, which very diligently plays out the very mediocre sound of the op-amp, albeit hidden behind the sonorous name HDAM and buried in small screens. And there are enough other similar "samples".
Learn, learn, and ... think!
Perhaps on all, without exception, DACs from manufacturers from "China" I observe the same locomotives from "KRENok" (the photo on the right is not mine, it was caught on the Web). By fanning out series voltage regulators, developers are obviously trying to achieve better power isolation and reduce digital-to-analog noise penetration. Unfortunately, the masses lack what I call "current thinking" in circuitry. In fact, everything is simple and ... a little sad.
Look at some LM317 from the exit side. Surely you will find a 10μF electrolyte and a few more small containers. Now let's estimate the time constant in this circuit: just look at the datasheet and make sure that the output resistance of the "rolls" is very small, which is what the developers of the integrated stabilizer were trying to achieve. To be precise, I honestly admit that it’s too lazy now, but interference with frequencies from, say, 100 KHz and below the roll “sees” right at its output, that is, the control electrode and, as it was designed, transfers these pulsations “upward on command”, diligently trying to keep the voltage on your way out.
Current fluctuations go to the output of the higher voltage regulator. Following the same logic, still fairly high-frequency current changes walk almost unhindered along the entire chain of stabilizers. And they whistle and make noise at all the surroundings.
The only rational grain in the use of two linear stabilizers in a row, I see only in the fact that small accurate stabilizers usually do not tolerate high input voltages, and kits for self-assembly of DACs often fall into the hands of soldering riggers who often do not even bother to look into docks for the components used. And those sets should still work ...
The propagation of sufficiently high-frequency interference can be easily prevented by adding ... ordinary resistors to the circuit. Simple RC filters at the entrance linear stabilizers will provide excellent isolation of RF ripple in both directions, drastically reducing the "distance" along the circuit, until the current surges reach (including the "earth" wire!)
So the power supply has undergone major changes on the board. Alas, it was not without a couple of cut paths and hinged mounting.
Sometimes a small resistor is much more efficient than a large capacitor:
We respect the heritage of our ancestors
Instead of a blunt bridge, we put super-fast diodes in the rectifier, which significantly reduces the "shocks" of the current at the moments of blocking of the diodes. This technique is quite popular and quite meaningful, so we will use it too:
By the way, it is precisely the lack of understanding of how to decouple linear stabilizers in RF that leads meticulous developers to the fact that they begin to put a separate transformer on each block of the circuit. Another very popular, but also costly solution to the problem of series stabilizers: the use of bundles of a current source - a parallel stabilizer. In this case, everything is in order with the decoupling, only that the power has to be dissipated with a considerable margin.
Let's not ask too much of the "whale"
A separate article is needed to describe a series of experiments with various stabilizers. Here I will just note that, to the credit of the developers from the Middle Kingdom, the lm1117 stabilizer they have chosen for the LDO may be the best option from commercially available and relatively affordable integrated stabilizers. Any 78XU, LM317 and others like them simply rest because of the incongruously large output impedance (measured at 100KHz). Alas, the precision LP2951 went into the same basket. The TL431 behaves a little better in the shunt stabilizer circuit, but there is a story there: TL431s are very different, depending on who made them. 1117 wins by a large margin. Alas, it also turns out to be the noisiest stabilizer. Grumbles, squeaks with and without load.
I had to assemble the stabilizer myself, using discrete components. From just two modest transistors, following the HotFET ideology, it was possible to "squeeze out" everything that requires dozens of transistors in an integral design and all one does not hold out. Of course, a few more active components were required to ensure the work of the "sweet couple" ... but this is again a completely different story.
An interesting result of macro photography: with the naked eye, I did not notice that the board had not completely washed off the flux.
Polymers run the show
The last refinement aimed at achieving the most accurate sound transmission was the "smoothing" of the power supply.
In critical places, the usual (albeit not bad ChemiCon) aluminum electrolytes from the kit were replaced with solid-state aluminum Sanyo OS-CON. Since I collected two identical sets in parallel, it was possible to arrange "A / B" testing. The difference is on the verge of audibility, but it is! Without a signal with ordinary electrolytes, at a (very) high gain, there was a kind of "noise space" in the headphones. Polymeric electrolytes transport us to the absolute.
Sanyo OS-CON - purple barrels without a notch on the lid.
If you don't want to think with your head, work with your hands
On almost all DAC boards and sets using the CS8416 digital receiver, the Chinese put a toggle switch so that the user can choose between the optical and copper S / PDIF input (the photo on the right is a typical example found on the Web). So: there is no need for a switch, the receiver microcircuit may well listen to two inputs without any outside help, be it a crude toggle switch or a wise microcontroller.
I am sharing with you a trick I saw on a demo board from Cristal Semiconductor themselves. It is enough to connect, for example, a copper S / PDIF to RXN, and the output of an optical TOSLINK receiver to RXP0.
I hope you don’t need to explain how this works? 😉
Even in the reference design, the firms plowed, they forgot the bypass capacitor in the TORX power supply 🙁
Saving or illiteracy?
It is very useful to read the documentation of manufacturers, especially those that make the very microcircuits, which audiophiles later pray for. Here's the top secret: reference design boards, evaluation boards and similar "probes" from manufacturers usually contain examples literate the use of those very microcircuits. Moreover, it is not at all necessary to buy all these boards, and the price tags for such "samples" are very different: 50, and 400, and for a thousand green ones can pass. But, my dear developers, the documentation for all these boards is openly available! Okay, good to teach.
So, what the Chinese missed, or what they saved on: modest shunt ceramic capacitors of 1000pF in parallel to 10uF and 0.1uF. It would seem - why, because with such capacities we shunt frequencies from tens of megahertz and above. The audio range is considered to be up to 20 kHz, well, up to hundreds of kHz. But nobody canceled the digital part in the digital-to-analog converter. So it is precisely the interference at tens of megahertz that freely walk on inexpensive self-built DACs, forcing all PLLs to tremble in fear and thereby creating ideal conditions to create the terrifying JITTER.
Another popular way to save money on matches
The vast majority of manufacturers of both digital audio signal sources and digital-to-analog converters save 30 ... 50 cents on each device. We, users, are paying for this. Read the details.
What's high-end without lamps?
I am amused by the hordes of tube-DAC and tube-headphone-amplifier "s in the price range from one and a half hundred to hundreds of dollars, which have flooded the market lately. Looks like people like how the light bulb hisses and distorts at 15 ... 24 volts of the anode. However, analysis of all the sores of such DACs and pseudo-tube amplifiers for headphones is a topic for a separate article, but not just one.
(the photo on the right is for example, I don't have such a lamp dump)
Rich theme. I just ran over the tops here, I did not touch the analog part at all. And how interesting it can be to separate the "ground" correctly or to organize a simple and yet convenient control of the device. And what are some attenuators worth - after all, you can choose them of different resistances, build according to different topologies, and turn them on in different parts of the path. Matching the sources with the load is a very, very interesting question, you know! ... But for today it's time for me to wrap up.
BOM, or Bill of Materials
Of course, the deal is not limited to fifty dollars. The ceramic capacitors from the kit have been replaced with foil. Schottky diodes, high-quality electrolytes, and a lot more had to be added, not to mention the case. And, of course, my HotFET amplifier: only 2 (two) amplification stages from the DAC output to the headphones or the amplifier output. Neither more nor less, but only in the amplifier itself 32 transistors counted in a stereo version. Yes all transistors are JFETs and depletion MOSFETs. No way I don't fit in fifty dollars even for components 🙂 And note, this is without any audiophile esotericism. Well, on this score, I also have my own opinion. After all, there are people who believe that by installing the "correct" components, any circuit can be made to sound. If you, dear reader, are from their ranks - teach, I will listen, argue, listen and tell everyone about my experiences right on this site.
So where is the promised freebie ???
Friends, this article is just a reflection, notes in the margins, was written in hot pursuit of the rework of the Chinese TSAP. I myself will never get involved in such an adventure anymore: although it turned out well, it was too expensive in terms of time and effort. And I do not advise anyone. When I was dealing with that set, the poison just oozed, which was reflected in the article 🙂 I apologize for the slightly arrogant style of presentation, and if I did not meet your expectations and did not offer the distribution of almost free high-end DACs to the population 😉
If you were interested, please let me know. There is still a lot of material in the bins, but the strength, motivation to publish and arrange all this is given mainly by the reviews, comments of my readers.