ProtoDAC TDA1387 X8 project

[hifinet]

@thomaschan are you using the Lampizator with the 5670 and the AN DAC3 tube output with 6922/6DJ8? Both circuits are SRPP. Transconductance of the 6DJ8 is much higher. Also, the cathode resistor is bypassed with an electrolytic cap (220uF) in the AN SRPP. This will add gain. The Lampizator circuit has some local degenerative feedback without the bypass cap. I am not a fan of the 6DJ8. Too cold and analytical sounding.

[thomaschan]

@thomaschan are you using the Lampizator with the 5670 and the AN DAC3 tube output with 6922/6DJ8? Both circuits are SRPP. Transconductance of the 6DJ8 is much higher. Also, the cathode resistor is bypassed with an electrolytic cap (220uF) in the AN SRPP. This will add gain. The Lampizator circuit has some local degenerative feedback without the bypass cap. I am not a fan of the 6DJ8. Too cold and analytical sounding.

Yes, I am using 5670 in the Lampizator circuit and 6DJ8 in the AN circuit. You are spot on about the characteristic of the tubes. 5670 is definitely more musical. I had previously built a McIntosh C2200 preamp with 4 pieces of 5670 and it was very very musical.

For the AN circuit, I started with 75 ohm resistor in the I/V conversion but the gain was just way too high. I eventually tuned it down with a 180 ohm parallel to 75 ohm and this is the value I use in recording the Youtube video.

As for 6DJ8, it has low output impedance and quite many brands use it in preamp. I have a vintage Sonic Frontier Line 3 which use 10 pieces of 6DJ8. Convergent Audio Technology CAT SL1 use one piece of 6DJ8 in the output stage and this preamp is extremely good.

[thomaschan]

Before I add the Lampizator tube output, I had been using the ProtoDAC with 90 ohm resistor. Sound quality was OK. However the SRPP tube output makes a huge improvement. Especially when I use separate power supply for the DAC and the Pi.

As for the Pi +5V decoupling capacitor, I am using 220uF Nichicon MUSE. I haven't got a chance to try different value yet but I will. At the moment, I am planning on a better power supply for the B+. Looks like a lot of tuning is needed for this streamer.

Will you be building one as well? Love to hear from others.

Yes, I would like to try the SRPP with a 6021 medium mu dual triode subminiature tube. This cascode JFET buffer should also be tested.

That buffer, is that in conjunction with the tube output device or in place of ? Where in the chain ProtoDAC-xxx-xxx would you place it ?

The I/V resistor is connected to the grid of the SRPP tube.

[hifinet]

Yes, the CAT SL1 seems to be very highly regarded. I had to look up the CAT SL1 preamp, and there is a diyA thread with apparently the correct schematic here. It's a mu follower. Where the 6DJ8 is used is more as a cathode follower, so I wouldn't think it contributes that much to the overall sound. Most of the sound being from the 12AX7 (and 12AU7), but I need to read the Morgan Jones circuit analysis.

[thomaschan]

Yes, the CAT SL1 seems to be very highly regarded. I had to look up the CAT SL1 preamp, and there is a diyA thread with apparently the correct schematic here. It's a mu follower. Where the 6DJ8 is used is more as a cathode follower, so I wouldn't think it contributes that much to the overall sound. Most of the sound being from the 12AX7 (and 12AU7), but I need to read the Morgan Jones circuit analysis.

I had built the SL1 before with modified tube power supply for the B+. Very musical with punchy bass. Original design is solid state power supply which I haven't tried yet. Maybe one day I will. There is PCB available on Taobao for both versions.

https://www.youtube.com/watch?v=SVlRRSrfzM4&t=75s

[hifinet]

Yes, the CAT SL1 seems to be very highly regarded. I had to look up the CAT SL1 preamp, and there is a diyA thread with apparently the correct schematic here. It's a mu follower. Where the 6DJ8 is used is more as a cathode follower, so I wouldn't think it contributes that much to the overall sound. Most of the sound being from the 12AX7 (and 12AU7), but I need to read the Morgan Jones circuit analysis.

I had built the SL1 before with modified tube power supply for the B+. Very musical with punchy bass. Original design is solid state power supply which I haven't tried yet. Maybe one day I will. There is PCB available on Taobao for both versions.

https://www.youtube.com/watch?v=SVlRRSrfzM4&t=75s

A discrete MOSFET regulated high voltage supply. I used Maida LM317 high voltage regulators on all my tube stuff. It is much simpler than the CAT discrete regulator, and there is a significant benefit to regulating the tube supplies. Highly recommended. Easy to put one together on a perf board. Since you don't want a widely variable supply (as in the original Maida circuit), just use fixed resistors for the voltage divider with a parallel trimmer. Online calculators available. Wire wound for the large value resistor makes the voltage more stable. I have used old TV horizontal transistors directly mounted to the metal chassis for high dissipation applications. Tubes can also be used as the pass devices. The snubber network (2R7 + 1uF mylar cap) on the output and the cap on the adjust pin are very important for proper transient response. Test your LM317s at low voltage to be sure they are stable before using them at high voltage. Maida PCB sold on eBay.

[hifinet]

Continuing off topic. The following is my two cent opinion. Looking at a clearer CAT SL1 schematic, it is using LM317 voltage regulation and constant current sources. You can first try a Maida LM317 regulator on the mu follower, and see what you think. I think you could probably run the mu follower stage and 6DJ8 cathode follower stage for both channels with one Maida regulator. CCS could be added later.

If the source of the line stage is a DAC and Moode, I think you can omit the 12AU7, because I think it is only there so that the line stage is noninverting. With Moode it is easy to digitally invert polarity if needed (thank you, Tim). The 12AU7 is not a very good sounding tube. Some of the mods substitute and redesign for a 6CG7, which is much better sounding. I think just get rid of the 12AU7 stage.

[rh2018]

Another tube buffer well worth considering is a kit (K295) from Oatley Electronics here in Australia. 
https://oatleyelectronics.com/files/k295notes.pdf

The K295 uses two Raytheon JAN6418 miniature pentodes wired as triodes.

I don't know about the buffer introducing "tube sound" but used to buffer the ProtoDAC (420hm I/V resistors) it works as intended to isolate the DAC from varying loads.

The 6418 is said to be microphonic but using the rubber grommets shown in the notes I could not detect any in practical use.

At US$24 the K295 is well worth an hour or so with the soldering iron

[hifinet]

The K295 circuit doesn't look like a buffer preamp. It is a common cathode configuration, which is a triode gain stage. This preamp is inverting (no need to digitally perform polarity inversion with ProtoDAC). The interesting thing about the 6418 is that it can run on batteries. It is also a directly heated cathode. Apparently it was used in hearing aids and other portable electronics. Pentodes can make very good sounding triodes. I was not familiar with this tube. If it doesn't sound like it is adding a "tube sound", that seems like a good thing to me. Thank you for sharing.

[hifinet]

An inexpensive reclocker for ProtoDAC:

Forum member @ECELO confirmed recently that the Inno-Maker HiFi DAC HAT ( InnoMaker ) functions well as an inexpensive reclocker at $32.99 USD. It does not have the best quality clock crystals, but sound quality is reported to be very good. It will certainly be an improvement from the I2S signals of the RPi, especially for 44.1 files. Based on the photo, the clock crystal is an SCTF clock oscillator (located near the midpoint at the top, marked X1 and X2 on the PCB). Disclosure: I have no affiliation with this product, and no personal experience.