Hey guys,
Just wanted to share the results of some experiments with you before I go off to work.
I've had a Valab since around January or February of this year, and I've enjoyed it, but I was getting a little bothered by some clipping on very loud tracks that I have. (I listen mainly to metal.) I had already tried lowering the I/V resistor down to about 300 ohms, but I couldn't get rid of all of the distortion. On top of that, I was getting a bit tired of the emphasized bass and harmonics on the lower end. It can really bloat the sound of some guitars.
Enter lampizator.eu and the FETishizator. I'm not a big fan of vacuum tubes, but his simple output stages seemed interesting, so I decided to give the FETishizator a try.
Check it out here.
My theory is that the DAC chips are somewhat fighting each other when outputting the current in parallel. They need to make a voltage adjustment in order to flow current, and they're probably all competing and continually adjusting their output voltage based on what the other chips are doing. I'm guessing that's why the stock Valab gets so warm, and it's responsible for the harmonics too.
So, here's the short version of what I did:
1. Open up the case and remove all but 2 of the DAC chips. This is reversible; the chips are in sockets.
2. Lift the I/V resistors to steal the signal from the side closest to the DAC chips. There might be some DC bias there; I haven't measured it, but I put a cap inline just in case. The 300~400 ohm resistor in there right now is too small for proper operation; the TDA1543 datasheet shows a 1.2K I/V resistor, so you need to use 1K to 1.2K on the FETishizator board.
3. Now we need to steal power. On the bottom of the DAC, you can tap into the legs of the power supply filter caps to get the roughly 13.20 volts found there.
4. Build the circuit that either Lukasz drew on paper, or build the one that is in the PDF by Dietmar. Lukasz uses just the single JFET; Dietmar uses a source-follower MOSFET in addition. Lukasz's is just a simplified version of Dietmar's. Note: You don't need the resistor in series with the JFET gate after the I/V resistor; the JFET gate is in the megaohms, and the tiny resistor before it is like a drop in a lake in comparison.
If you follow Dietmar's JFET_CDOut schematic, skip his little power supply portion in the top right corner. Just use a single voltage regulator to provide for both channels; it's much simpler this way.
5. Connect back up to your output caps on the main Valab board, or use your own caps and wire straight to the RCA's on the case.
I can go into more detail when I get home, but the main advantage of this is to get rid of any remaining distortion and restore balance to the sound spectrum. There is some hiss from the TDA1543's, but I greatly prefer the Valab this way. Voices are amazingly clear in this configuration, and highs aren't drowned out anymore.