Channel imbalance: testing/verifying

[htbyron]

A recent post by voodoochile in the thread about headroom manufacturing got me thinking about channel imbalance in amps. I just received a Little Dot II tube amp, and noticed that at the bottom end of the pot range the left channel is present, but not the right. It doesn't take long (when increasing the volume) for the right channel to come in, but it sounds to me as though the left channel remains louder than the right even at normal levels. But, I'm not sure! Is there any way to test this? (I have tried this with different source, different recordings, & different headphones. The problem is present throughout, so I'm pretty sure it's the amp.)

Thanks,
Tom

 

[bobeau]

Do a reverse channels in Foobar then turn the phones around.

 

[Tim D]

You can use the RMAA calibration test (under the presumption that your sound interface itself has matched L/R channels). Get a stereo phono to 2 RCA cable or necessary adapters. Looking at the volume control on those amps I wouldn't be surprised, however best tracking is usually always at 12' o clock area.

I had a Grace 901, that was pretty much perfect in L/R tracking (and I'm sure the 902's are the same in that regard if not better). The Gilmore Lite is surprisingly well matched making me think that Headamp does some good QA on those Pana EVJ's. My Mad Ear+ has stock pots with average tracking but with balance control which I calibrated myself to be even at listening levels.

Actually once you have ascertained that your sound interface in RMAA is L/R matched you can then compare it to your other sources (which I have found in consumer gear even without a pot sources may not be matched to the nth degree). Just go through the test suite and you can get a pretty good visual indicator by zooming in on the Left and Right channels of frequency response as well. My m-audio usb audiophile L/R matching is pretty good, with Grace901 (DACT) as reference.

Remember the accuracy of the above is only as good as the soundcard though. I was able to confirm using the Grace 901 in several tests. Another way to know is to use the same RMAA tests(and your ear) and switch L/R cables to see at what device the mismatch occurs (i.e. if you switch L/R cables going into the headphone amp and the mismatch still occurs on the same channel you know its the amp).

 

[Pappucho]

One additional simple test you can do is to reverse the headphones on your ears, in other words, listen to the right channel with your left ear and vice versa. If the sound is still louder on the same driver then you can be pretty certain it's the amp, after ruling out the headphones and sources (of course).

 

[htbyron]

Quote:

Originally Posted by Pappucho One additional simple test you can do is to reverse the headphones on your ears, in other words, listen to the right channel with your left ear and vice versa. If the sound is still louder on the same driver then you can be pretty certain it's the amp, after ruling out the headphones and sources (of course).

Thanks, Cesar -- I did that & I'm convinced the problem is the amp. I also tested with different sources & different headphones.

 

[htbyron]

Quote:

Originally Posted by Tim D You can use the RMAA calibration test (under the presumption that your sound interface itself has matched L/R channels). Get a stereo phono to 2 RCA cable or necessary adapters. Looking at the volume control on those amps I wouldn't be surprised, however best tracking is usually always at 12' o clock area. I had a Grace 901, that was pretty much perfect in L/R tracking (and I'm sure the 902's are the same in that regard if not better). The Gilmore Lite is surprisingly well matched making me think that Headamp does some good QA on those Pana EVJ's. My Mad Ear+ has stock pots with average tracking but with balance control which I calibrated myself to be even at listening levels. Actually once you have ascertained that your sound interface in RMAA is L/R matched you can then compare it to your other sources (which I have found in consumer gear even without a pot sources may not be matched to the nth degree). Just go through the test suite and you can get a pretty good visual indicator by zooming in on the Left and Right channels of frequency response as well. My m-audio usb audiophile L/R matching is pretty good, with Grace901 (DACT) as reference.

Wow -- that went almost entirely over my head! (Sorry, I'm quite a newbie at all this.) I'll do a little reading then come back & look at your post again. Thanks for the info & the learning challenge.

 

[htbyron]

Quote:

Originally Posted by bobeau Do a reverse channels in Foobar then turn the phones around.

Good idea! (same problem)

 

[robzy]

Potentiometers (Read: volume controls) have a tendancy to have worse tracking down in their bottom ranges. Both my cMoy and PIMETA display that problem with Pana pots.

Rob.

 

[htbyron]

Quote:

Originally Posted by robzy Potentiometers (Read: volume controls) have a tendancy to have worse tracking down in their bottom ranges. Both my cMoy and PIMETA display that problem with Pana pots. Rob.

Thanks, Rob. That confirms that I'm gonna have to figure out this RMAA software to test performance throughout the pot range (since I don't entirely trust my own ears on this at louder volumes). OK -- new challenge ahead, I guess.

 

[jefemeister]

make a CD with the exact same close to full scale, 1kHz sinewave on each channel. Hook a multimeter up to each channel's output. Measure the signal at different volume steps for each channel.

 

[lazyfly]

What's with all the software testing..? The suspect is the amp and as robzy pointed out, the pot. Unplug the amp from the power, remove the source, pop the lid off and hopefully you'll have access to the pots in/out pins. Use a multimeter to test the impedance at various pot locations.

eg: Use the diagram below, hookup the meter to the rear Signal pin then the other probe of the meter to the rear Output pin. Jot down the value read from the meter. Move the probes to the Signal and Ouput front pins, jot down that value. Compare the values you wrote down and that's the level matching at that pot position. Move the pot a little and repeat. You should get a fair idea how unmatched the wiper is at low levels.

If possible replace with a better pot or get a stepped attenuator.

It's early so I may have buggered this up entirely

oops, didn't see, jefemeister's post... sorry!

 

[braillediver]

If you can connect the amp to a pre-amp that can be switched to mono then the signals should be equal on both sides.


Mitch