how to check the amp output voltage symetry ? A channel playing louder enough from the other to waste the soundstage


It is not the pre not the dac, I already made the tests by swaping the links between the pre and dac and pre to amp, to see if the louder channel swapped between L and R, but no.

I checked impedance of mid and treble with DVM and sweep with ARTA : seems ok both side. So I assume a channel of my SMPS Chord that has 25 years play a little louder on a channel maybe ?

How to check the symetry of the amp output with a DVM please (have no scope) ? Can I inject via the Arta or the pre a signal à la Pink or else and check if the votage on the binding posts are the same L and R, please ?

Many thanks

PS : the passive filter value of caps are the same to 0.1 uF , film caps. It is not the room , the difference is listenable the ear on the mid drivers for instance.
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Inject a signal about 100Hz into the preamp with the amp output loaded with an appropriate resistor/speaker and DVM on AC.
Raise the volume to a comfortable level and note the reading.
It helps to have two VTM's to compare each channel's output, otherwise test each channel momentarily.
And use a mono adapter to feed both channels the same signal.
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The most basic is to inject a 1 k sine wave, measure and set the input voltage (AC) to a low value, say 200mV and then without changing anything plug into your amp and measure the output voltage. Don’t need a load or speakers to get a rough estimate.
Again, without changing any input or output settings, switch channels and measure-this will tell you the absolute difference.

I will warn you that this will no catch an input or output capacitor issue, where the FR is being cut, unless you run the same test at multiple frequencies in the low and high range (typically would low range if you can hear it clearly on all music) so you can try same tests at 50 or 100 Hz and see if the channel difference increases significantly
If you can connect your PC as source for your DAC then download Audacity and use that to generate a slow sweep ('chirp' in their parlance) while monitoring the output with your DVM on AC volts. Don't go above 500Hz though as few meters can measure that high a frequency with any accuracy.
Never operate your amplifier without Either a Load Resistor, Or a Loudspeaker !
If you leave the amplifier unloaded, Your Mileage May Vary.
If the amplifier breaks, there will be no more mileage.
Load the amplifier outputs.

Test Routine to determine the cause of the un-balanced problem:
Use a test tone, or music as your signal.
Get a double RCA combiner adapter (One Female RCA connector and two Male RCA connectors).
Connect The Left channel signal to the Female RCA combiner connector.
Connect one Male combiner connector to the amplifier Left channel RCA input.
Connect the other Male combiner connector to the amplifier Right channel RCA input.

A. If the sound of the left channel and right channel sounds (or scope waveforms) are un-balanced, then, one or more of the following is true:
1. It is the Amplifier.
2. It is one of the Speakers.
3. It is the Room, and/or speaker placement
4. You have reversed the connections of one of the speaker cables (putting the channels out of phase, which can have multiple strange audible effects).
5. One of your ears has lost some hearing.

B. If the sound of the test above Is Balanced, then the problem is:
1. The Left channel versus Right channel of your Signal source is un-balanced.

That is all.
Have fun fixing and listening !
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Connect a mono source to your amplifier and your DVM
on AC volts to the 2 + outputs of your amplifier
with the speakers or load connected.
If you have a reading that is not near zero, you have a
problem. If not check youur speakers or speaker wires.
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Checking now, 22h00 here !

I was wrong, it is the preamp that is affected : Yamaha CX2 of the 90s or the late 80s ! Maybe a mismatcj of the alps carbon pot or the agging of the caps....

I wish the advices are the same to check the pre....,A lot of work if correxct. The yamah has a very relaxed sound, also due to the old Elna caps, none of the newest come close imho !

I have another pre, I will test tomorrow.

Thanks again for the nice help provided.