A couple of weeks ago I played with the different impedance settings, comparing 1 @ 20, 2 @ 40, 3 @ 60, 4 @ 80 and 5 @ 99. 2 @ 40 was the obvious winner so I set it there and left it. Also a couple of weeks ago something happened to my balance and it actually switched to exact center, which was odd because it has been off to the right forever. I was pleased that I could stop turning down the right input volume knob on my amp to correct this. Today I started having a channel balance problem - as I turned up the volume the image moved to the right. Running auto-calibrate stuck while running LDR #2 so I switched it out with some of the spares I have from when I fried an earlier board. I switched the #2 LDR based upon your explanation in an earlier email of which was which and then auto calibrate ran all the way through, and now the balance was a little to the right (this is where it had been before the change a couple of weeks ago- so I again corrected this at the amp which has a volume control for each channel). I played with switching various LDRs because I am a masochist and finally just switched out all 4 LDRs and ran auto-calibrate, but there was no effect on the balance - it is always just a bit to the right which is OK. But now the impedance changes have no effect. 1,2,3 and 5 sound the same and have the same level of output. And now setting #4 is much louder and switches balance way to the left. Any ideas? And I may hold some kind of record for running auto-calibrate the most times in a 4 hour period.
I see two different possible issues reflected in this information.
1) Bad LDR - If channel balance shifts enough to be noticeable it's a fair bet that the performance curve of at least one LDR has drifted substantially. If running autocal hangs up on a given LDR it's also a good bet that the given LDR (1, 2, 3 or 4) is the problem and needs to be replaced after which rerunning autocal should go smoothly and balance will be centered again.
2) Channel Balance Offset - Boards should only need to be precalibrated with precison resistors (in place of the LDRs) one time at board commissioning to capture the nuances of the non-LDR hardware on a given board. This hardware includes DACs, op amps, ADCs, voltage regulator, and a few resistors etc. It's been my experience that once this is done, and you install the LDRs and run autocal, you get a consistent solidly centered channel balance. When running autocal for the first time at any given setting number and impedance level I've also discovered that it's helpful to run autocal twice. The first time through it has to work harder to hit the target points. Subsequent times through it starts with the results of the prior autocal so the results seem to be tighter. If you are experiencing consistent channel balance offset after successful autocal, especially with a new preamp/board, I would first double check everything outside of the preamp that could account for this. If the balance offset is something that crept into the picture over time but the unit otherwise runs through autocal with no problems, I would run the board through pre-calibration again and see if this clears things up. To date, I've not had to do this.
And to clarify Craig's comment, replacing LDRs does not require redoing pre-cal, only auto-cal.
