Channel imbalance issues

[arkrazor]

Unfortunately, yes.

[Paul Birkeland]

OK, start by playing a 60Hz tone into the RCA jacks.  Measure the AC voltage from ground to the center pin of each RCA jack and post those here.

-PB

[arkrazor]

AC Voltages from Ground to the Center pin of each RCA jack measure at around 1.538V.

[Paul Birkeland]

Next, turn the volume control all the way up and perform the same measurement at each of the center lugs of the volume pot.  (Amp still off)

[arkrazor]

I'm getting .375 for the top lug and .370 for the bottom lug

[Paul Birkeland]

Just to be thorough, with the amp off, measure the AC voltage again at A2 and A7.  It should match what you measured previously.


If that checks out, then with the amp on, carefully measure the AC voltage at terminal 6 and terminal 10.  If you have the shorting modification at the headphone jack, let me know and I can give you some options for bypassing it.


-PB

[arkrazor]

A2 and A7 voltages check out. Maybe + or - .001V

I do have the shorting mod for the headphone jack installed.

[Paul Birkeland]

The next thing to do is to try measuring the AC voltage at terminals 1 and 5 with the amplifier on and the volume control all the way up.  Your meter may not be able to measure the AC voltage there with the DC voltage present, but it's worth a shot. 

-PB

[arkrazor]

If I measured correctly, terminal 5 is at 7.68 and terminal 1 is at 7.73

[Paul Birkeland]

Excellent, that's 0.05dB of difference between channels, which is difficult to measure, and impossibly hard to hear.

The next set of measurements should be done at terminals 6 and 10.

Since you have the shorting jack, these will both be 0V unless we take some action.  If you can find a pair of toothpicks, you can pop them in under the spring contacts for the tip and ring on the jack, just to lift the short.  Alternatively, if you have a 1/4" TRS to 1/8 TRS adapter plug, go ahead and plug that in to release the spring contacts. 

I would not use a pair of headphones to break this connection, as we want the signal level high and we want the headphones themselves out of the equation.

-PB

[Strikkflypilot]

Or attach headphone cables but detach at the cups if lacking an adapter?

[Paul Birkeland]

Yeah, if the cables can be unplugged from the drivers (HD580/600/650), that may work.  There still may be a cabling issue, but it's a place to start.

-PB

[arkrazor]

Hey guys, I managed to find a right angle 1/4" TRS to 1/4" TRS and it looked like it did the trick. Terminal 6 measures at 4.77 and Terminal 10 measures at 4.88

[Paul Birkeland]

That's 0.2dB of difference, still not much.

With the adapter still plugged in, measure that voltage at the headphone jack (red and white wire terminals on the jack).

-PB

[arkrazor]

I'm getting similar measurements. White terminal is measuring at 4.87; red terminal is measuring at 4.77. Nothing out of the ordinary, it seems.