CS4397 dac

How come leo's board did not come with R28 and R29? I have resistors there. Should i remove those since I added in 30k resistors?

Hello, again.

I tried Leo's mod for the r's and c's around the opamp. (R's 30k C's 15-pf). But i am still getting about 25mv on each side (left +27mv and right -23mv).

tried matching the resistors: resistors went down from about 30,2k to about 29,9k dc output went up from about 25mv to 30mv. Fet based opamp also had no result.

Maybe it's the way i am measuring ?!

Ouput caps and r's are still in place and i measure between pin s1 and 7 from the opamp to ground (middle) pin on the output connector. The DAC is not connected to an amp. Opamp is JRC5532.

Is this the correct way to measure the DC on the output or do i have to remove output caps and resistors first to get correct measurement ? Or do i have to connect the dac to an amp ?!

I would be pleased to hear from someone who knows and can help me eliminate the offset.

Thanks in advance !

Naqua

@Naqua
your measurement is correct. Pin 1 and 7 from op-amp to ground. I am using 30 K resistor (matched set) and my offset is 1 mV left side and 2 mV right side.
Use matched set od 4 resistors. You need to buy 10 or 20 resistors & use your multimeter to match 4 of them. If you are using coupling capacitor, capacitor will block DC, no need to worry about some DC offset left. And C's are not 15pF, they are 150pF.
Resistors are responsible for DC offset, no capacitors. Capacitors+resistor are RC filter.

@JDMZ
why to remove R28 and R29? Look again on board shematic I posted, what will you accomplish with this action?

Thanks Firefly! I'm glad somebody else can help answer questions , I think the manufacturers of this dac should be paying us:lol:

I've noticed a few of the dac boards had R28/28 missing, maybe they just forgot to fit them on some batches

Thanks Firefly,

To resume i understand correctly:

I'll try to macht 4 resistors of 30k. My multimeter just has 1 digit after the decimal point. Will that be enough to match ??

Also my Resisitors are matched as follows:

30.0k 29.9k for left channel
29.8k 29.8k for right channel

I assume this is not matched enough then ?!
Is it better to have precision resistors of 0.5% or 0,1% tolerance or do i have to match them also.

Is it ok to raise the resistor value if it helps to eliminate the offset? (ie to 31k or 33k)

Thanks again

@Naqua
30.0K vs. 29.8K, is this matched? For me this is NOT matched. Maybe you can use 27K resistor and 5K multiturn trimmer in series to set offset to 0 mV :eyebrows:
If you change resistors, you will have to change capacitors values too. Stay with 30K resistors and capacitors 120-180pF.

Pic of multiturn is bellow

http://shrani.si/f/t/YV/3wOuhRIJ/multiturn.jpg

@leo
thanks, will try to help, but credits for major work goes to you :)

Thanks Firefly (and of course Leo) :)

Will try 0,1% 30k resistors first.
Then will try your multiturn option.

Thanks again (and again and again etc) :)

My opa hdam opamp has a ground wire. Can I hoook this to AG ground? between the two outputs? or one of those other grounds?

The output caps i used.. does not show polarity. Is this normal/ok?

Thank you

Some new things about op-amp output stage.
Look to shematic I posted on page 28
http://theartofsound.net/forum/showp...&postcount=288

Capacitors C32 and C33 are connected as negative feedback. This is very very bad practice, to put capacitors as negative feedback, they are destroying trebles & fine high details. It is asking for trouble or like giving pacient a medicine, after he is already death :lolsign:
They must go out. I cut them from the my board.

And resistors R8 and R9 are also negative feedback. Too much negative feedback is poison for sound, like hitting a pacient with hammer to sedate him :lolsign:
Values must go higher, do decrease neg. feedback, I will make some tests & calculation and post results.

Capacitors C37 and C39 are part of RC lowpass filter for L and R channel, They have too big values, will make some calculations & tests and post results.

Also capacitors C44 and C51, near wide white power connector, with original values of 100μF. Values should NOT be increased, they should be decreased something like 47μF, to give sound more air. But this depends on your ears :lol:

So you are saying I should just desolder those two capacitors? Anyone know where the ground of my opamp can go? AG or the other ground?(or however many types there are)

Looking forward to your results!