Since I can't seem to sleep, I thought I'd take a look at the DAC60 schematic and see what I can see. Of course this all changes when you pop the hood and see how much (or how little) room there is to work with and whether the boards make it practical to mod.
Here is what I see....
From the digital side, I see U1, your receiver, U2 is your digital filter, and U3 & U4 are your left and right channel DACs. From my quick glance they don't look to be balanced dacs, somehow I thought the 1704s were....not that that is a bad thing mind you.
X1 is the local clock, but I am clueless as to what clocking scheme is employed. I just see it is fed into the receiver and passed from there to DF & dacs. Is it really a 6.1Mhz clock or is this a typo? I thought clocks were typically 11, 16, 33Mhz.
From the Dacs' current output there is an LCLC filter (lowpass filter) and then an SRPP tube stage, which provides both gain and low output impedance. Below the lower dac is the schematic for the tube's B+ supply (some call it HT (high tension) supply). The B+ has a regulator for each channel. Someone correct me if I am wrong, but I beleive this to be the shunt type of regulator (as opposed to the series type).
The rest of the schematic is the psu and regulators (vanilla lm317 chip regulators) for the various voltages (dacs, cpu, etc). I didn't see on the schematic where the heater were, are they AC heated? Then there is a schematic for a CPU. I assume this CPU is for control of logic & switching, but I am not sure.
Fred, you replaced the B+ CRC filter resistor with a choke correct? That will improve regulation a bit further.
Myself, I'd be inclined to change the analog stage a bit dramatically, loosing the SRPP stage by ditching the top triode and adding a cascoded CCS. For the 6922/6DJ8 at least, it would seem that you have enough B+ to do it. The alternative would be to add in some resistors, a cap, maybe an extra dual triode per channel and turn it into an Aikido output. In these cases, the extra regulation of a choke in the input filter wouldn't be necessary, I'd think.
Having a look at this DAC's schematic has gotten me pretty excited about it's potential. I think I am definitely going to have to pick one up.
The easy mods are most certainly changing the output coupling caps (2.2uf blue caps) or maybe try bypassing with .02uf of your favorite exotic cap (teflon, platinum, et al). Replace the SPDIF RCA with a true 75ohm BNC connector (do your transport side too) and use a true 75ohm cable.
Then I might take a look at what electrolytics are being used in the PSU's, but this only yields just so much IMO. Dampening (physically) the clock. Ponder whether the low voltage regulators for the DACs would benefit from better regulators, maybe even consider bursonaudio's discrete regulators.
What are your guys' thoughts?
