Issues with testing the impact of volume pot removal

[Mike L.]

I use my Stereomour as a power amp, with an upgraded AES AE3 handling volume and switching duties, so I've been considering the removal of the volume pot.  I figure that getting a superfluous pot out of the way would result in better sound, right?  To test how well my system would respond I've turned the Stereomour volume all the way up and adjusted the pre-amp volume accordingly.  To my surprise, the quality of the sound degraded considerably.  Even at low volumes it sounds harsh and brittle, like the amp is distorting.  Is this typical?  Was that not a good way to approximate what it would sound like without the pot?  My amp seems to sound best with the volume set at about 60-70% of maximum, but with only 3.5 watts on tap I can't afford to leave anything unused.

Any insight would be most appreciated.
Thanks!

[Doc B.]

It's a little hard to say what is happening without knowing what is happening to the signal with the different level settings. It's possible that the carbon pot track in the Stereomour is smoothing out an overly bright presentation from the preamp or the interconnects. When you turn the pot all the way up none of that track is in the signal path. When you back off to 60 or 70% volume, part of the track is in the signal path.

Maybe try connecting your source directly to the Stereomour rather than running it through the preamp, and see if the sound changes.

[Mike L.]

Thanks, Doc.

I added the AE3 so I could run a Crown Class D amp for my woofers with a single volume control, and to get a bit more gain when I use my turntable.  Taking it out again is no big deal for testing purposes, but it'll be quite loud at full throttle and I expect driver distortion would confound the test.

I just soldered up a couple sets of interconnects using Duelund 20 gauge tinned copper.  I'll swap them around with some other interconnects to see how the sound changes.

I've used the AE3 in a bunch of systems, and it has always performed very well.  Definitely wasn't bright before, but synergy is everything.  I'll keep playing around.

[Doc B.]

You left out some pretty important info in not mentioning that the woofers are run off the preamp via a class D amp. What exactly does the Stereomour drive? Just tweeters? Tweeters and midrange? Is there a line level crossover? What is the input impedance of the class D amp that is a parallel load for the preamp along with the Stereomour?

[Mike L.]

I'm embarrassed to say that I didn't realize that the Crown could impact the sound of the Stereomour.

The Stereomour drives a single Fostex F120a on each side.  Currently running full-range, but I have tried to high-pass it passively at the speaker in the past with mixed results.  I plan to add a PLLXO as soon as I can figure it out.

The output impedance of the AE3 is not clear, as I don't have the specs.  A factory upgraded version was 560 ohms.

The input impedance of the Crown XLS 1500 is 10k.

Thanks very much for the help, Doc. 

[Doc B.]

OK, the input impedance of the Crown amp is dominating the load seen by the preamp. From what I can find online 500 ohms is about right for the published output impedance of the AE3. It is looking into about 9K with the two amps in parallel as a load. That should not be a problem. In theory, at least.

What I suggest next is to remove the Crown amp from the equation by disconnecting it from the preamp and try the experiment of listening with the Stereomour volume control all the way up again, to see if that changes the tone at all.

[Mike L.]

Thanks, Doc!

I'll give it a try and report back ASAP.

[Mike L.]

Hi Doc

Well, I finally got to try the experiment you suggested, this time using my musician wife and her fabulous ears to remove confirmation bias.  She couldn't hear a difference between max volume on the Stereomour or mid way (total volume corrected by the AE3, of course).  She also couldn't tell when the Crown was plugged in or not.

I guess it was all in my head, eh?

Now to research the proper value and position of the resistor I need to remove the volume pot.

Thanks for your help.

[Doc B.]

When we do these experiments we have to be sure that the levels are matched exactly between the two test setups. It may be that you were able to focus more on getting the levels to match with your wife concentrating on listening for any differences than you were when you tried the experiment by yourself.

I'm of the opinion that a large percentage of A/B listening tests and the subsequent reviews are tainted by level mismatches of 1dB or less, that are audible enough to affect the results. It's very common, even among "professional audiophiles".