I have a 1999 Headroom Maxed out Home headphone amp I've owned since new and love it. Internally it uses OPA627 opamps. I'm driving Audeze LCD-2 headphones and it works well. However, it's too loud. It has so much gain, the volume knob is around 7:00 to 9:00 for my listening. This volume knob position is so low, it's very sensitive and sometimes it gets into the volume knob's non-linear area where the channels aren't balanced. Unity gain position on the knob is around 12:00 to 1:00.
I'd like to reduce the gain. I can think of 3 ways to do it, what I think of as "best" first:
1. Change (reduce) the gain across the OPA627 opamps internally. I know they're stable at unity gain, but they might already be at unity gain.
2. Install a different volume pot to reduce the gain at any given volume position (Unity gain would move from 1:00 to 3:00 or maybe 4:00).
3. Put a ladder voltage divider on the input to reduce the input signal voltage.
The short question: can anyone help me with options (1) or (2) above? I have basic EE skills - designed and built my ladder stepped attenuator - and decent soldering skills but I haven't worked with opamps or power amps. For example to display my ignorance, for solution (2) for less gain do I want a potentiometer with a lower, or higher, value?
I've already done (3), wired a cable with 8 kOhm Dale metal film resistors so it's 16 kOhm input impedance and -6 dB of attenuation. This puts the volume knob pretty much exactly where I want it - that -6 dB made about 1/4 turn difference in knob position. And there is no audible deterioration in the sound quality.
However, I know (3) reduces the S/N ratio. And it's reducing the input impedance of my amp from 100 kOhm (its actual input impedance) to only 16 kOhm. This may be an issue since my preamp is a 10 kOhm ladder stepped attenuator, and source devices see the headphone amp in parallel with my 10 kOhm attenuator.
Normally I'd be fine with step (3) above but reducing the MOH input impedance from 100k to 16k brings the overall load impedance too low. That's 16 kOhm in parallel with 10 kOhm which is only 6.1 kOhm. My CD player (Onkyo DX-7555) has a 470 ohm output impedance, and I'd like a 20:1 ratio, which would be about 10 kOhm total. The MOH's native 100 kOhm input impedance is fine but I don't want to lower it.
Edited by MRC001 - 4/11/14 at 1:15pm