Question about adding volume control to DAC and reducing output voltage


2016-04-28 3:52 am
I have a Zhaolu D2.5C DAC without volume control, i.e. it only has fixed line-level output. (How? Long story short, the headphone amp PCB burned out, so I ripped it out; that PCB also does volume control.)

Recently I got an integrated tube amp. When I connected the fixed line out from the DAC to the amp, there is a problem. There is only a very small usable range in the volume control on the amp: it'll go from dead silence to "way too loud" in less than 1/4 turn. My understanding is that the Zhaolu DAC line-level output is at ~2.5V, while the amp has a sensitivity of 200mV, i.e. a 200mV input signal is enough to drive it to full power. This mismatch is the reason why I have to keep the volume knob on the amp all the way down.

Unfortunately the volume pot on the amp sucks big time; at the beginning of its travel, there is a very audible imbalance. So I can either have balanced sound that's too loud, or reasonable sound level with a dominant right speaker. :mad:

Another problem with the current setup is that I can't use my subwoofers.

I cooked up a temporary solution by adding a standalone headphone amp to the signal path. Now the system is connected like this:
DAC (fixed lineout) ---> headphone amp (variable lineout) ---> RCA splitters --> tube amp, and subwoofers.

This works fine. Now I can keep the volume knob on the integrated amp at 50%, and then use the headphone amp's volume knob to control the overall sound level. The sound is balanced and at reasonable level.

But I don't like the setup. I want to fix the fundamental problems. I also want to use the headphone amp in another room.

On the tube amp side, I plan to bypass the crappy volume pot using the "shunt" mod.

On the DAC side, I want to reduce the maximal output level, and add a high quality volume control. I'm thinking this wiring diagram:

Left fixed line out ---> 20k resistor ---> 10k volume pot ---> pair of RCA jacks --> left input on tube amp, and left sub.
Right fixed line out ---> 20k resistor ---> 10k volume pot ---> pair of RCA jacks --> right input on tube amp, and right sub.

The reasoning is that, when the volume pot is turned all the way up, the output voltage is just a hair below 1/3 of the original 2.5V output from line level. This should in theory give me more usable range.

Would this work? Will there be any negative impact on the sound quality? The IC between DAC and tube amp is really short (1.5ft).

I guess a even better solution is to mod the lineout PCB in the DAC to not output so damn high. But I have neither the knowledge nor the soldering skill to do that.

Advices are really appreciated!
wshtb said:
Re to DF96: I assume you mean adding this resistive attenuator to the volume pot inside the amp?
No. Put it between the source and the amp. Where you were intending to put your extra volume control?

A somewhat related question: how feasible it is (in a DIY sense) to reduce the sensitivity of my amplifier?
Easy to add an attenuator at the amp input. Harder to do it any other way, such as modifying feedback or otherwise changing the circuit; too many things to go wrong.