HiFi USB DAC PCM2707 – Preamplificatore a valvole – Mini DAC valvolare

In the modern hi-fi world, it seems that without a 32-bit / 384 kHz DAC, femto oscillators, and zero quantum-noise regulators, you cannot even listen to “Il ballo del qua qua”. Everything has to have a pompous name, a spec sheet that screams miracle, and maybe a power supply bigger than the one in your power amp.

Yet the reality of listening is a bit different. This article was written to give dignity back to the essentials, with a project that, at first glance, may look minimal, cheap, or even toy-like, but proves that sound quality is not tied to the number of bits or the kilohertz printed on the box. It is mostly about how the signal is treated once it leaves the converter chip. The analog section makes the sound. And when it is done properly, even with a simple foundation, it can outperform products that are far more expensive and pretentious.

After years of testing, listening sessions, and even a bit of mocking of certain audiophile rhetoric, this project became a small classic. And that is exactly where its evolution comes from, a refined, rugged, CE-certified unit, ready to use, yet still faithful to the original spirit.

USB MiniDAC PR-Zero ECC86

The MiniDAC PR-Zero ECC86 is the definitive, professionally built version of the project below, originally born out of curiosity and experimentation. It is still based on the well-proven PCM2707, a synchronous USB converter with a maximum resolution of 16-bit / 48 kHz, more than enough to reproduce any CD accurately, as well as all 44.1 kHz / 16-bit recordings available online.

But the analog stage is what makes the difference. The signal passes through a real ECC86 tube, specifically designed to operate at low voltages, and driven correctly. Downstream, a JFET buffer with BF256 lowers the output impedance, making the DAC compatible even with amplifiers whose input stage is less tolerant, without ruining the musicality of the tube section.

The build uses a printed circuit board, in a compact enclosure, with a well-filtered power supply, switch, LED indicator, robust connectors, and an optimized layout. No flimsy floating “Chinese stuff”. It is a complete, tested, ready-to-use product, made for anyone who wants an entry-level DAC that is not the usual sterile box. It has a real analog soul, warm but genuinely good-sounding, not just good on paper.

It does not pretend to “destroy” multi-thousand-euro DACs, but in its price range, and even above it, very few units can offer the same sonic personality, technical coherence, and build care. It is perfect for anyone taking their first steps into tube hi-fi, for anyone who wants a enjoyable second system, or simply for anyone tired of digital smoke-and-mirrors and who wants to listen to music, not firmware.

Kit PCB + Transformer + PCM2707 Carrier Board

For those who prefer DIY but want to start from a solid and proven base, the PR-ZERO PCB is also available, ready to be assembled and paired with a dedicated power transformer, custom designed specifically for this circuit. An ideal solution for anyone who wants to build their own tube DAC starting from reliable parts, avoiding compromises on power supply and layout. Contact me if you are interested in purchasing the PCB + transformer kit.

Below, you will find the original article that started it all.

Low-voltage version with ECC86

To test my ideas, I bought on eBay a small synchronous USB DAC board based on the PCM2707 chip, with a maximum resolution of 16-bit at 48 kHz, spending under 10 euros, specifically €5.69. At first glance it might look like a cheap little board, but the goal was not to build yet another desktop toy. The point was to see what could be achieved by pairing this inexpensive converter with a real tube preamplification stage, not a simple passive buffer or some random op-amp.

As the first modification, I removed the jack connector and the two electrolytic capacitors on the output (marked with a red star in the photo), so I could interface the signal directly to the new analog stage, eliminating anything that could degrade it unnecessarily.

I also removed the yellow RCA connector, which many might mistake for an input, but is actually a S/PDIF output that is basically useless in the context of this project. At that point I mounted everything on a perfboard, and given the weak output level of the PCM2707, too low to properly drive most amplifiers, I added a real tube preamplifier stage based on the ECC86, a tube specifically designed to operate at low anode voltage.

On that point, an important note. Online you often see projects that use tubes at low voltage, but not tubes designed to work at low voltage. People use ECC82, ECC83, ECC88 and similar, which were designed to run at 150 to 250 V, and instead feed them with 12, 24, or 30 volts. The result is a signal with very high impedance, heavy distortion, and worse, early degradation due to cathode poisoning (cathode poisoning), a real and well-documented phenomenon that causes progressive loss of performance.

That is why I chose the ECC86, a tube developed for vintage tube car radios, designed to work correctly at low voltage, with no compromises and no long-term damage risk. A technically correct solution, not a gimmick imitation. Here is the premium schematic:

To complete the project I made a 3D-printed plastic enclosure, a choice driven both by cost and by a bit of provocation toward those who build devices with absurd price tags, often with performance that does not match the cost. Everything was finished with a switch and a power LED, to keep it simple and functional, with no useless frills.

In listening tests, in a room acoustically treated by professionals and with top-reference loudspeakers, this little DAC surprised us with its sound quality, airy, detailed, bright, with performance far beyond what you would expect if you only looked at the cost or the spec sheet.

Out of curiosity, and partly as a challenge, we compared it against a commercial DAC costing around €3,000, featuring an AK4495S converter and a built-in tube stage, using the same chain and 24-bit / 192 kHz files. You would expect an abyss between the two. On one side, a cheap DIY project based on a 5-euro board, totaling around €150 including labor. On the other, a branded high-end product sold as the ultimate solution. And yet, no.

You could hear a difference, sure. The €3,000 DAC had a finer grain, and a sense of greater control in the micro-details. But the real sonic gap was much smaller than the price gap. To put it plainly, it was not like going from an intercom to a studio setup, but more like going from 500-grit sandpaper to 1000-grit. Except the jump was not from €150 to €300, it was from €150 to €3,000.

I am convinced that even less experienced builders, as long as they are curious and willing to pick up a soldering iron, can build this project with little expense and great listening satisfaction. The result is enjoyable, musical, and concrete, far from the idea that “cheap project equals mediocre sound”.

Of course, nothing prevents you from using this tube stage with higher-performance DACs, maybe with 24- or 32-bit converters, if you want an upgrade while keeping the analog character. The only important caution is not to load the ECC86 output too heavily. It is still a low-voltage tube, but it runs at about 1 mA of anode current, so it should be interfaced with amplifiers that have an input impedance of at least 47k?. Going below that would force the tube to operate outside its optimal range. As for compatibility, the DAC is plug & play, recognized without additional drivers by all operating systems: Windows (from XP onwards), Linux, and macOS.

Variant with external 24 VDC supply and output buffer

This version of the circuit was developed at the request of “G.M.”, to be paired with a DIY solid-state power amp that already had a 24 VDC power supply. The core project is the same, but in this case an adaptation was necessary. The amplifier had an input impedance that was too low to be driven properly by the ECC86 alone, which performs best with much higher loads.

To solve that, an output JFET buffer stage was added, useful to lower the impedance and allow the DAC to interface easily with the power amp. The result keeps the sonic character of the ECC86, while improving compatibility with a wider range of amplifiers.

Comment from “G.M.” received by email:

I am astonished, stunned, speechless, and moved to tears. It cannot be, it is not real. I finally managed to connect everything and just to test it I hooked up two worthless speakers, but the sound is unimaginable. I finally have an idea of what definition, soundstage, and bass presence are. I cannot wait to connect it to more serious speakers. You have no idea how grateful I am to you.

Thank you, thank you, thank you. With music you made me discover a new world. I believe I should also thank your grandfather who supported you in your passion. We will talk again later, thanking you again, best regards.

ECC82 variant

In this version, instead of the low-voltage ECC86, an ECC82 powered at high voltage was used, for those who want a sound with slightly different characteristics and can handle higher anode voltages. In the photos you can see a 9AU7, which is simply a variant of the 12AU7 with a 9-volt heater instead of 12. Functionally, the difference is limited to the heater. Electrical behavior and performance remain practically identical. Below is the premium schematic: