Channel Imbalance Issue (Left Channel Bias) [resolved]

[AyeVeeN]

I've just replaced the stock pot with the blue alps I'm still getting a left channel bias across the entire pot range. The channel imbalance was more prevalent with the stock pot.

I did recently upgrade from the Crack to the Crack + Speedball, but the regular Crack build also had this channel imbalance.

I measured the voltage output of the amp at 12 o'clock, and the left channel sits at 0.772 V, while the right channel sits at 0.662 V with a 60 Hz test tone, correlating to an approximately 1.3 dB difference. My source measures 0.501 V on both outputs, so it has to be the amp (also tried reversing the RCA cables and left channel bias persists). It's not the headphones themselves since the bias occurs only when adding the Crack into the signal path.

All components are stock except for the pot.

Here's the pre-Speedball data; I haven't documented the post-Speedball data since I just measured to check that everything was installed correctly. If needed, I can get the post-Speedball measurements.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FQAKzak0.png&hash=1c4a70ca15186e9d48e378315ff7f4cab19b7b19) (https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FpuElfVG.png&hash=f76c4a5e82bbe13313f681a3ed00106b3dc67d99) (https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FumwfS8U.png&hash=8306d80be91eb9119e70e20bed4d19ac3622441d)

[borism]

Did you try different tubes? Could it be that the triodes are not balanced in either the 12AU7 or the 6080?

[Paul Joppa]

The mil-spec 12AU7WA/6189 has a +/-1.5dB gain variation for any one given triode. The 6080WA has an even larger variation, though as a cathode follower the negative feedback reduces the variation. So this difference is well within the normal range, unless you seek out balanced-section tubes.

A 16K resistor between the RCA jack and the "top" of the high-gain side potentiometer will make a suitable corrective, as long as you continue to use the same tubes.

[AyeVeeN]

I have not tried out new tubes yet. Will soon; just wanted to see if anyone could pinpoint an issue anywhere else before I went to buy one. I'll try this rather than doing the 16k in-line resistor since I don't want to have to mess with the wiring. Thanks!

[AyeVeeN]

I tried new tubes and it does appear to primarily be the 12AU7. Bought a 3100/3100 RCA 12AU7 to test it out and my voltage differential dropped a slight amount, though I still appear to be getting a 0.5dB left channel bias instead of 1.5dB. Could the 12AU7 still be imbalanced despite being rated 3100/3100? Swapping the 6080 showed no difference.

It's at least usable now since the left channel bias before was way too prominent to ignore it. Both 60 Hz and 1 kHz showed about the same voltage differential.

[Paul Birkeland]

Play a 60Hz tone into the Crack with the volume pot turned all the way up, then let us know what the AC voltage is at the tip and ring terminals of the headphone jack (if you have performed the shorting mod, you'll need to do this with headphones plugged in).

[AyeVeeN]

Tip: 5.12V
Ring: 5.11V

I did this with just a 6.35mm to 3.5mm adapter plugged in and measured on the inner terminals; is that fine?

[Paul Birkeland]

That is 0.017dB.  This is way, way below audible.

Other options would be an imbalance in headphone drivers, your ears, or abnormally uneven ambient noise.

[AyeVeeN]

Blue Alps pot tracking? Measuring it through my source at max volume (AQDF v1.2) gives me .471 V for both channels. Measuring it through the Crack with blue alps and AQDF at max gives

10:30       | 0.274 V tip, 0.254 V ring
12 o'clock | 0.700 V tip, 0.660 V ring
3 o'clock   | 3.01 V tip, 2.91 V ring
max          | 5.12 V tip, 5.11 V ring

Measuring it through cables gives approximately the same exact values for the respective channels. Purchased the blue alps through mouser.

[Paul Birkeland]

Yeah, there's some consistent imbalance there until you get up to max volume.  It's always 1/4 to 3/4 of a dB favoring the left channel.