Stereo 1176 channels have different levels despite some careful matching

Help Support GroupDIY:

midwayfair

Well-known member
Joined
Apr 7, 2015
Messages
526
Location
Baltimore
I'm a little loath to open this up, but I think the device is actually behaving a little strangely.

What I'm looking at:
-Two Hairball Rev D boards, plus their transformers, pretty certain it's this version: https://www.hairballaudio.com/blog/resources/build-guides/building-the-power-supply
-Some modifications from me -- a bunch of extra ratios, using matched 1% resistors, and extended range on the attack and release knobs. However, the imbalance persists even with the compression bypassed (pull pots).
-I matched all the transistors at the time of building
-I matched the pots as best I could, measuring the resistances at four spots, and they're with some reasonable percentage (probably like 5%).
-Right now I've got the volume balanced, and the compression balanced when I turn it back on, with the outputs at identical spots, and the left channel at 5:00 and the right channel at 2:00. This feels like a pretty big discrepancy.
-My metering on my interface tells me it's an 8dB difference between the two channels at max. This feels like a LOT.
-I can't detect a sound difference between the two channels when I run a mono signal through them and I don't see anything funky on the metering on coming back in, so I'm pretty confident. I don't see any damage and my voltages appear to be correct.

I can live with this (hell, I have for years), but I'm actually a little mystified considering the effort I went through to match the two channels. And while I can't be absolutely certain, I actually think that the left channel must have lost some gain at some point. They've always been a little mismatched, but I don't remember them being this far off, I I mostly remember just needing a small adjustment. Unfortunately I've lost my notes from the last time I used it for its stereo purpose so I don't have the settings I was using before.

Is there a part that could have gone wrong that isn't obvious?
 

radardoug

Well-known member
Joined
Oct 10, 2009
Messages
1,029
Location
North New Zealand
Well, you need to get out the millivoltmeter and scope and start measuring from the front. Measure after the transformer, after the input pot, after the first gain stage, after the line amp. You dont have a scope and a millivoltmeter? Then you need to get those tools, or take the units to someone who has those tools.
As to us guessing which component is faulty, contrary to popular opinion, no-one on GroupDIY is psychic, and so we just dont know.
Also, you need to do this with the two channels separate. On the original 1176, stereo connection was via an adapter box with a battery in it to match the different fet thresholds. I dont know if your boards have this. But you will need it.
 
Top