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Unequal sound levels on channels of BZLS
Unequal sound levels on channels of BZLS
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Old 22nd February 2019, 10:16 PM   #1
jfuquay is offline jfuquay  United States
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Unequal sound levels on channels of BZLS
Default Unequal sound levels on channels of BZLS

I recently completed a Pass Balanced Zen Line Stage to drive two Pass ACA monoblocks I built last year. I thought it sounded good on startup, but as I was checking pots and switches I noticed I had miswired the output potentiometers (that's what happens when a noob wires pots that face opposite directions) and left off the resistors leading from the input pots to the main board. I fixed and restarted.

I now have a left channel that is noticeably softer than the right at all volumes, and I think gain is down on both channels. I have swapped inputs (XLR and RCA both) and interconnects and speakers with no change, but swapping the outputs (XLR) moves the lesser channel, so I assume the left channel board is the problem. I have checked wiring and reflowed some wiring connections without improvement. I have the on-board trimpots installed, one on each channel, and both are set evenly. (Let me say here that I can't hear any change when adjusting either trimpot.) I thought I'd post the voltage numbers and see if anyone might recognize an issue.

The recommended voltages from the BZLS article are as follows:
Power Supply: about 80 volts unregulated; 60 volts regulated
Main Board: -3.5 v on Source; +30v on Drain, ground on G

My measured voltages are:
Power Supply: +- 89v unregulated; +-60.5v regulated
Main Board: Right: -3.75v on Source pin; 31.6v on Drain; -150mv on G
Left: -4.11v on Souce pin; +32v on Drain; -324 mv on G

One other thing. During all the checking, I realized that even after turning off the BZLS (but not touching the volume), the source input is audible at very low levels both from the DAC and the tuner when each is selected. I was surprised, but then I've never had a separate preamp.

If anyone can suggest a fix I'd appreciate it. Thanks.
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Old 22nd February 2019, 11:18 PM   #2
PRR is online now PRR  United States
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> after turning off ...the source input is audible at very low levels

This is not a fancy-pants amplifier. When powered-down, there is a path through the large MOSFET's gate-drain capacitance from input to output. "Very-very low" seems reasonable. "All highs" is also likely, though hard to notice because the ear loses sensitivity to bass at low level.

More complex preamps mostly don't do this. It is not significant; we normally don't expect signal to go through a "dead box", we turn everything on.

Your DC voltages look fine. Drain at half supply or a pinch more, and nearly equal; with all the source degeneration the gains will be =equal= for any practical purpose.

I would look again at that pot wiring, which you report was wrong at one point. Maybe it is still wrong a different way? A particular note: there are two inputs and two outputs. If one is not connected, signal passes but may be weaker.
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Old 23rd February 2019, 01:23 AM   #3
jfuquay is offline jfuquay  United States
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Unequal sound levels on channels of BZLS
Good information, thanks. I’ll revisit the pot wiring, you raise a good point. “Still wrong a different way” is a distinct possibility.
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