ProtoDAC TDA1387 X8 project

[thomaschan]

Finally got a chance to put everything together. I have tried to solder 680 ohm resistor and 470 ohm resistor in parallel to the 90 ohm cathode resistor. 680 showed improvement while 470 gave me best sonic results.

Typo. I actually mean input grid resistor not cathode resistor.

So that is about 75 Ohms. Have you tried any lower values? How is the gain with 75 Ohms?

I haven't tried any lower value yet. At 75 ohm, my volume control is at 8:30 to my DIY Goldmund 27 preamp. This preamp is about 12X gain. I plan to connect the streamer to my 6SN7+ 300B tube amp and hear how it sounds first before making any more changes to the resistor.

With the passive I/V resistor circuit of ProtoDAC, I have noted that there is an inverse relationship between the I/V resistor value and the power supply decoupling capacitor value (C3 + C4). Too low a capacitance and dynamics are lost. Too high and the soundstage depth shrinks. Does this also apply to the Lampizator circuit? The decoupling capacitor might need to be fairly large with the 75 ohms I/V resistance, at least 4000uF (just a guess).

Also, your preamp has 21.5dB of gain, I wonder how it sounds going directly to the preamp with the 75 or 90 Ohm I/V resistor (i.e., eliminate the Lampizator circuit). Or use the Lampizator circuit with a high quality pot or switched attenuator and eliminate the preamp.

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.