NAD 7100 / 3100 - one channel louder than the other

[mashley_nad]

I have a NAD 7100 receiver, which works just fine except the left channel is louder than right.

To get channels balanced, the balance control needs to be at almost 3 o'clock!

It does not matter what input is used or if the mono button is pressed. The mismatch is the same.

If I put same signal into left and right power amps (e.g. left pre-amp out into both left and right power amps) - then the channels are balanced.

So clearly something is wrong in the pre-amp section.


I'm hoping someone might have experienced the same with either the NAD 7100 receiver or the 3100 amp on which it is based, or a simlar NAD amp, and can give me a clue about to fix this issue ...

[Artie]

NAD is more-or-less, the same design as Proton. I had this same problem in my Proton 1100 preamp. It was a bad cap in the final output stage. (Remember, this is a preamp.) Its been a couple years since I fixed it, but I seem to recall that the cap was easy to spot. Bulged out if I recall. It was an electrolytic. Around 1/4" diameter.

Look at the caps in the preamp section. Perhaps, replace them all, if you aren't sure. (Also look for leakage.)

Hope this helps.

[mashley_nad]

Originally posted by Artie
NAD is more-or-less, the same design as Proton. I had this same problem in my Proton 1100 preamp. It was a bad cap in the final output stage. (Remember, this is a preamp.) Its been a couple years since I fixed it, but I seem to recall that the cap was easy to spot. Bulged out if I recall. It was an electrolytic. Around 1/4" diameter.

Look at the caps in the preamp section. Perhaps, replace them all, if you aren't sure. (Also look for leakage.)

Hope this helps.

caps seem to look OK --- but that doesn't mean they can't be sick!

thanks for the tip - I will investigate those output stage caps

[mashley_nad]

Some progress: All electrolytics in pre-amp signal path replaced, but to no effect.

So got the 7100 service manual (BTW a very professional copy supplied by vintageaudiomanuals.com) which has a preamp channel balance adjustment, performed at full volume with 100mV 1kHz CD input. This is via a trimpot (R 315) in the feedback volume control circuit.

OK. So I adjusted for equal output. However, at normal listening levels the channels are still out of whack requiring a 3 o'clock balance adjustment.

OK, so tried adjusting at normal listening position, and with the trimpot at R=0, still out of whack by about 2 o'clock. If I use the low level function (-20dB mute), then the channels are almost balanced.

Could it just be that the volume pot is out of whack???

Attached is relevant section of preamp circuit. Output is at P301 and P302 which goes to tone conrol section.

Next post has the tone control section which is on the power amp board.

[mashley_nad]

Here is tone control section

[mightydub]

Some NAD products have jumpers on the back between pre-amp out and power amp in. If yours has this, you can figure out if it is the pre-amp or power amp using a known good source plugged directly into the power amp.

It could be the power amp - off the top of my head, things to check are the input AC coupling cap (if present) , the input termination resistor, and the resistors for the feedback voltage divider.

[wwenze]

Originally posted by mashley_nad
If I use the low level function (-20dB mute), then the channels are almost balanced.

Dead resistor? The low-level function just enables a voltage divider, since that works it might be a good clue.

[mashley_nad]

Originally posted by mightydub
Some NAD products have jumpers on the back between pre-amp out and power amp in. If yours has this, you can figure out if it is the pre-amp or power amp using a known good source plugged directly into the power amp.

Thanks for the suggestion, but I have already confirmed the power amps are not the issue. The channel difference is seen at the pre-amp outputs (I'm using an oscilloscope).

[mashley_nad]

Originally posted by wwenze


Dead resistor? The low-level function just enables a voltage divider, since that works it might be a good clue.

Thanks for the suggestion.

Using the low level function means you have to turn up the volume quite a bit for the same output.

I begin to think the left and right volume pots are just out of whack.

As per the service manual, the outputs can be set exactly equal at full volume. The discrepancy only arises as the volume approaches normal listening levels.

[mightydub]

Originally posted by mashley_nad


Thanks for the suggestion, but I have already confirmed the power amps are not the issue. The channel difference is seen at the pre-amp outputs (I'm using an oscilloscope).

Good - if you have a scope that makes it easy - set balance to the midpoint, volume to normal listening level, and follow the signal path looking for differences between L and R. Chances are there's a cracked resistor, cracked etch in the pcb, bad solder joint, etc.

On 3 different pieces of equipment (TV, car stereo, NAD receiver) I have fixed various problems by reflowing the solder joints. The trick is finding the one that needs it.