A New Take on the Classic Pass Labs D1 with an ESS Dac

[regal]

Assume if one wanted to use these boards with the original PCM63k's, that their current output +-2.1 is not enough to use the new mofsets to get these lows THD results?

Would it be worth simulating with higher supply voltages ?

Also I believe I am down for a board, want to make sure that is good for stereo balanced, if not I need 2.

Last thanks in advance for putting up with a dac neanderthal's questions

[lil' knight]

Just wondering if this board would work with the AD1865? I have some spare DAC END 2 boards that I have no use. DAC End 2 - the AD1865N-K with single ended vacuum output stage

[opc]

Hi Guys,

Generally speaking, I would say that it's probably not worthwhile to mate this with the AD1865 or the PCM63K. It can certainly be done, but with such low output current swing, the gain has to be adjusted way up which requires larger resistor values. This in turn drops the current, which drops the transconductance and the whole thing quickly spirals downhill. The output impedance also climbs up as those resistors are increased, and you'll soon find that you need a buffer or a second stage to lower the output impedance. This basically lands you right back at the original D1 circuit, which is really what you would want to build if you had the PCM63K.

The only viable option I see would be to just drop one of those dacs in as it is, and live with the very low output. You could always add your favorite voltage gain stage after this, and that would get you what you need.

Regal: indeed, one board does provide differential stereo output and all necessary power supplies. Just add transformers and a heatsink!

Cheers,
Owen

[Algar_emi]

Almost ready to test the FET into the actual circuit after a weekend spent in drilling and mounting into the recycled case. Everything is mounted on a sub plate of 1/8 aluminum for better heat dissipation and easier testing. All supplies tested ok. I'll mount the FET on the bottom plate near the heatsink for the test.

[Algar_emi]

To what value do I adjust the PCB pots? Is it for 3V at the pot as in the transconductance procedure? Then I measure the 200R Resistor Voltage? Thanks.

[qusp]

no, the pots on the PCB are a completely different purpose, simply to adjust gate bias to match AVCC/2 on your particular dac. so power up your buff and measure what the DC voltage from AVCC L/R reg to ground is and divide that by 2 (normally around 1.65-1.7vdc), note that down, power up the board with the dac not connected and adjust all the pots to match that voltage on each mosfet source ->G. power down and install the dac, power the lot up, measure each source to ground and readjust to match the voltage again as it will change a touch under load.


hope that makes sense

[qusp]

the other operating points as set in the matching procedure, are already set by the fixed resistors given +/-45vdc input

[Algar_emi]

What is the AVCC L/R reg? Is it the analog Vcc regulators of the dac?
I have a Buffalo ESS ver 1 DAC. See the included schematic from the Buffalo. Is this the supply I need to measure to find the AVCC L/R? Thanks...

[opc]

Hi Algar,

It is indeed this supply. It should be 3.3VDC so the pot should be adjusted for 1.65VDC on the source of the mosfet (where the DAC output connects). The DAC is happiest when its output is at exactly 1/2 AVCC.

Regards,
Owen

[qusp]

oops...., yes source, now that was a brain fart; i started thinking of matching too even after mentioning these were already set. its best to measure the reg rather than assuming 3.3/2 as the tp boys at least with the buf 2 onwards increased this voltage to 3.4 and even a touch higher on some modules. I have gone as far as 3.6v in my experiments; Dustin gives this tip to increase DNR; but i'm not sure if they were doing it back then; sorry i forgot you were using the V1 dac.