Salas DCG3 preamp (line & headphone)

[Salas]

Those voltage drops indicate 125mA & 129mA output stage bias if with 10R R10s or 104 & 107.5mA if with 12R R10s. Very close bias matching considering Mosfets and resistors tolerances. Offset behavior looks good.
Not with output shorted, with input shorted I believe you meant for offset adj. Check offset again when the volume pot will be installed. At 12 o' clock position on open input for a final pass/adj with the op-amp in place.

 

[Salas]

I may try another power amp in the near future. I am thinking of giving a try to purifi. Any suggestions are welcome.

Such champion measuring amps achieve fantastic SNR figures by exploiting low own voltage gain as well. This can be a subdued dynamics trap when on medium sensitivity speakers if not with high output XLR connected (doubling the signal voltage) digital sources. I suggest around 20dB power amp voltage gain for single ended signal if digital source only or around 25dB for phonographic source in your case with the Totem Hawk loudspeakers. That's on top of your DCG3 9.54dB line gain (x3). A well proportioned system voltage gain structure is key. Not too sleepy not too noisy.

 

[voxxonline]

Sorry yes ,inputs

 

[Salas]

You put 10R or 12R R10s?

 

[voxxonline]

10

 

[Salas]

Thus 125mA & 129mA bias. How's alu mounting plate's temperature for now, logical?

 

[voxxonline]

Barely feel anything. It is coupled to a chassis as well

 

[Salas]

Great. Put some thermal paste between plate and chassis when you will finalize it though. For good thermal travel to the main metal mass. Helps when the top lid will be closed. Especially in summertime when class A living isn't easy.

 

[voxxonline]

Thank you. Still need to make a potentiometer support bracket, but looks like I am on the final stretch got a thermal paste too

 

[voxxonline]

So this is how I wired it,
Yet to power up with a potentiometer installed.
How do I know it is working- what to check, except bias?

 

[Salas]

Looks good except the wrong red and black colors on the input RCAs vs your L,R designations (trivial). They are basically channels 1,2 and you call them anything you like as long as they do not get mixed up.

 

[voxxonline]

Thanks. Can I check output resistance as an indicator of attenuation? Or something else to know I am not putting out max volume and see if pot wired and working as intended?

 

[Salas]

You can put say 400Hz 1V RMS sinewave in the pot from a computer gen app if you don't have a bench generator and see it attenuating at the pot's output with a DMM on ACV mode or on a scope if you got one.

 

[voxxonline]

There was a humming noise in both channels, increasing as I turn volume up. Will check on thursday evening all wiring again.

 

[Salas]

Try also connecting the two input RCA barrel pins together to see if that helps.