Hi !

I have a Marantz PM-66SE audio amplifier.
The problem is a nonlinear imbalance between the channels.
What I mean is that under an identical stereo test signal one channel measures from under 10% louder ( at lower volume levels) to over 40% louder (at higher volume levels).
Any ideas about possible causes ?
Thank you.

 

One channel has an increase in gain or lower in gain compared to the other channel, do you have the service manual?

Is the balance set correct?

 

Equally (pun intended) are you certain that the same signal was supplied to both channels?

 

There could be a dozen reasons, from cheap volume controls to dirty connections in the speaker wire.

A sound level meter is a tool. It enables you to find the difference, then find the cause, then find out if the amplifier is fixed. If only the sound level meter was here so we could see it as we ran all the adjustments through their range and wiggled the wires.

 

One channel has an increase in gain or lower in gain compared to the other channel, do you have the service manual?

Is the balance set correct?

Sorry. I don't have the service manual.
All I could find was the attached schematic.
But wouldn’t a difference in gain be constant - independent of volume ?

 

Equally (pun intended) are you certain that the same signal was supplied to both channels?

I used a 4000Hz square wave. The other channel I obtained by doing a copy of the first one.
I checked the stereo wave before writing it on an audio CD.
I used a a CD player with that CD as a source.
I checked both the CD output and the amplifier output with an oscilloscope.
So I’m pretty sure that the same signal was supplied to both channels.

 

Ok can you input the same signal into both channels at the same time by shorting both the left and right inputs together, and see what the signal is like at the wipers of the volume control with the scope?

 

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Ok can you input the same signal into both channels at the same time by shorting both the left and right inputs together, and see what the signal is like at the wipers of the volume control with the scope?

That would be the conventional way to check or adjust channel balance in stereo amp. You should check at various stages through the amp from input to output to localise where any discrepancies occur, not just at the volume control.

 

I used a 4000Hz square wave.

Did you have a 8 Ohm load on both channels when you measured the output signal?

 

But wouldn’t a difference in gain be constant - independent of volume ?

If there is a load on the output, that apparrent gain (measured) depends on the output impedance of the amp which can vary for a number of reasons.

 

Did you have a 8 Ohm load on both channels when you measured the output signal?

Good point. Is the amp rated for 8 ohms or 4 ohms?

The loads need to be proper resistors not speakers.

 

Good point. Is the amp rated for 8 ohms or 4 ohms?

The loads need to be proper resistors not speakers.

It is rated 50W at 8 ohms.
I don't have resistors with this amount of dissipation power.
I will try to get a resistive dummy load of this amount of power before testing again.
Do you think it is possible that the trouble is with the loudspeakers ?

 

I once made some (speaker and power supply) testing non inductive loads by from old electric fire wire by determining the length for 1 ohm and hammering two rows of copper nails, set this distance (about 120mm) apart into a base board.

The nichrome wire was then would round back and fore between the nails in a zig-zag fashion, above the baseboard in free air for cooling.

This created resistors with taps at 1 ohm intervals. I could get a pair onto the baseboard.

It was very cheap and very successful, though not very pretty.

 

...
Do you think it is possible that the trouble is with the loudspeakers ?

Number 1 trick of fixing weird audio amp faults... Swap stuff!!

Swap the speakers from right to left, if the problem swaps, then the fault is the speaker.

If the amp has 2 power amp modules, then as another test you can swap them. If that swaps the weird fault then you proved the fault is inside the module.

 

Number 1 trick of fixing weird audio amp faults... Swap stuff!!

This is a really good service technique that has far wider applications than just audio amps.

It's official name is test by substitution.

 

Where is the volume control?

Is the volume control being used to adjust the signal level?

Most potentiometers have pretty bad tracking channel to channel.