Cutting Voltage / Gain from DAC directly? volume

I am looking at a friends custom DAC kit and its got about 2.0 volt output.

This is some pretty high gain once run thru a preamp. Does not want to have to mod the preamp gain as this would effect the gain on analog sources feeding it too universally.

I am looking for a way to reduce this things output by probably close to 30% or so without of course going to a passive volume or something hooked to the output jacks, and of course do not want to effect the sonic integrity of the unit.

It sounds perfect, just don't want the gain so high as it gives very little volume control control thru the main system, meaning you can not turn the knob to accurately especially by the remote in the preamp without rather large changes in gain.

This is only thru the DAC, all other sources are great.

This unit from what I can see has a dual amp opamp per channel output, we threw a couple different types just because we had them from some other gear which worked perfectly but very little changes in gain, but did change the sound. We only tried just to see what would happen and if it would directly soften up the output a little.

Anyway its a Cirrus like CS4398 dac chip I believe? I know we could to in and solder up a digital volume board directly to the dac and have it control it, but not much space to mount it etc... And its unnecessary as in this project not looking to have a variable volume, just a single clean way to cut the gain on the output or input without effecting the frequency response and sound?

I guess there is some ways to do some parallel / series style resistor stacks, but asking for some other options as doing that seems we might as well just put a variable volume control on the outputs doing the same thing vs. fixing the volume with a couple standard resistors.

Anyway any ideas I am sure it can be done, some units do come with switches internally that can change gains on some higher end units, I have seen a wadia and a few others that do this with like a 1 volt, 2 volt, 4 volt type output. I don't know where to get a part or where to hook this thing up so any help appreciated!

Oh and there is the Cirrus receiver chip in this thing too, its the popular model used in almost everything I have seen, don't remember the number like a CS8486 or something, not sure if it can be done there on the input or somewhere at the output, all chips in this dac are on removable pin boards, including the DAC and receiver so access is pretty much unlimited.

Nevermind, I think I found the answer to this… Seems I need to figure out if there is a feedback resistor used on the opamps and if in fact it is a feedback resistor or what depending on what pins of the opamps it is connected, and those would have to be replaced with a different resistance directly proportional to the one that’s in there…

I am told for example if it’s a 10,000 ohm resistor then for a 30% difference you need to add or subtract about 3000 or 3500 ohms? I don't remember if going up in resistance would bring down the output or going down in resistance would bring it down in the example I was told? I am assuming going up in resistance would bring it down, but the guy maybe told me not thinking that going to a 5000 ohm would be cutting it in half.

If it is a simple inverting op amp configuration, with a single-ended input and a single-ended output, then yes changing a 10K feedback resistor to 5K would cut the gain of the stage in half. However, there are several complicating factors that I think may be present.

And before I get into those, consider that cutting the gain in half will only result in raising the preamp's volume control setting by 6db from the positions it is now used at, which while significant is not a huge difference.

Here is the datasheet for the CS4398:

Note on page 20 the circuit configuration they recommend be applied at the output. It performs differential-to-single ended conversion, as well as 2-pole filtering. If the configuration you are dealing with is similar (and it probably is, since the dac chip itself outputs differentially, and presumably requires similar filtering), simply changing the feedback resistor value will not result in a proportional re-scaling of the output. And more significantly, it will upset the filter parameters, with the likely result of significant sonic side-effects.

Within the audio passband, where the filtering presumably does not do anything, I believe that the relation of output voltage to the two input voltages of the stage shown on page 20 would be:

Vout = -(Out-)*(1.27/1.58) + (Out+)*(.487/1.58)*((1.27+1.58)/(.487+.604)).

Keep in mind also that as the closed loop gain is reduced, the bandwidth is increased, with possible adverse effects on ultrasonic spurii or even stability.

Sorry to muddy the waters!

-- Al