Matching Output Voltage

Audio Components Amplifiers
1

I was testing the output voltage on my amp today fearing a channel imbalance. Using the XLO test disk (Track 5 test tone) I get 5.30V in one channel and 5.15V in the other. Swapping the tubes in each channel (NAT Magnetic preamp) and BHK 250 didn’t really make much of a difference. Closest I could get them matched near that output level was .13V.

  1. Is this difference significant? I have setups where I could match the channels closer than that. I’m not sure how audible it really is and how much is in my head.

  2. Is there an adjustment for gain in the BHK 250?

I’m using PCC88 tubes in the 250 and just ordered a matched pair to see if that helps.

Thanks.

Ian

2
That is about a 0.25 dB difference. Probably not that significant.
I would suggest swapping the left and right input cables on the BHK and measure again. If the left and right channels voltages stay the same as before I would suspect the BHK.

If the left and right channels voltages are changed, that points to a preamp imbalance.

Of course both units could be off slightly also. Post the numbers if you have any doubts.
3

Did you measure the outputs out of curiosity, are you hearing the balance as amiss?

4

Yes and Yes. I’m always tinkering, (new tubes, new cables) and was letting Red Cloud burn-in. I THOUGHT i heard a difference and measured. I like when I can swap tubes and bring down imbalance to .10db or less. Couldn’t do it here. Will try what st50maint suggested.

  1. The preamp I’m trying out, NAT Magnetic (I have a BHK on loan to a friend) is kind of casually built from what I’ve read. People say no two are built exactly the same.

  2. The other variable is the BHK 250 just came back from Boulder where it needed some parts replaced in the left (higher measuring) channel that got damaged from DC getting into the amp (we suspect). BTW FAST!! turnaround from the PS Audio folk on the amp repair. happy-132_gifIt could be that the new parts need to burn-in or might not be matched as closely to the ones that were in there now.

5

Interesting background. :slight_smile:

My hope is PS will chime in and let you know what to expect for channel balance. This seems to large of a difference to me and, as you note, you can hear it.

However, I would try using a single channel of the NAT to separately measure each channel to remove one variable.

6

It’s the preamp. When I swap channels into the BHK the levels change. The right channel of the preamp is down about .2db from the left. My gut told me more likely that than the amp.

I’ve got new tubes coming for the preamp. I can futz around with them to try and bringing the channels closer. If there is a fine gain adjustment in the preamp I don’t know where it is…

Thanks to all for their help and Merry Christmas to everyone on the board and at PS Audio. smile

7
ihmeyers said

It’s the preamp. When I swap channels into the BHK the levels change. The right channel of the preamp is down about .2db from the left. My gut told me more likely that than the amp.

I’ve got new tubes coming for the preamp. I can futz around with them to try and bringing the channels closer. If there is a fine gain adjustment in the preamp I don’t know where it is…

Thanks to all for their help and Merry Christmas to everyone on the board and at PS Audio.


If you are getting channel gain match of about 0.2 dB, you are ahead of the game! You are talking about matching gain between tubes and there is considerable variation of their gains of the same type. If you bought a batch of a particular type of tube, you might be able to get closer - but why? It truly isn’t worth it in the cosmic scheme of things!

BHK

8

If it’s close in matching but not perfect (or greater than your intended limits) why not use the pre-amp (or if using DSD-DAC) balance control to fine tune.