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Pre-amp not balance when measures with a DMM

qguy2000

Member
2018-09-24 2:50 pm
Pre-amp not balance when measures with a DMM

When listening to female vocals, the singer is in the middle of both speakers, but when I measured the output from the pre-amp using a 400hz tone, I am getting not equal measurements

Volume Left channel Right channel
9 13mv 10mv
11 49mv 35mv
12 90mv 67mv
1 146mv 136mv
2 154mv 152mv
3 157mv 156mv

should I be concerned or should I just enjoy the music ?
 

rayma

Member
2011-04-29 8:37 pm
Pre-amp not balance when measures with a DMM

It appears this is volume control mistracking, due to manufacturing tolerances.
A 10% mismatch is about 1dB, so you have significant tracking error below 1 o'clock
on the control. If your typical listening volume setting is below 1 o'clock, you could
consider getting the volume control replaced with a better one, but if the acoustic
centering from the speakers seems ok to you, there is no need.
 
I guess the preamplifier have a potentiometer as volume control?

All standard potentiometers have very bad channel balance especially at low volume settings, the famous blue ALPS RK27 can have as much as 3dB unbalance down to an attenuation of 60dB and up to 5dB at very low settings, these are values from ALPS data sheet.
Other potentiometers are not much better, the best I have seen is a TKD that promise 1dB unbalance but even that is possible to hear. To make matters worse it is not uncommon that a potentiometer change balance from one side to the other when changing volume setting.

The obvious solution is to use a stepped attenuator, either with a rotary switch or with relays. With 1% resistors maximum unbalance is about 0.2dB and if resistors are measured and selected much better, (resistors from same batch usually have an unit to unit tolerance that is much lower than the maximum allowed).
 
I guess its “the reason” why a surprisingly high number of audiophiles glom onto rotary-switch based attenuators. 2 pole, 15+ 'position' is essentially acoustically perfect when matched with 2% or better resistors.

Oh, one can also 'hand pick' matching resistors too … with a conventional DVM, a pair of cloth gloves (get rid of one's skin resistance) and some patience with a pile of resistors. Not hard … I do it all the time. And we then have the caché of saying the thing was built out of hand-matched components.

They are nice. The usual problem is that they don't often 'fit' in the allowed space of one's existing preamp cabinet. Sometimes the magic works, and sometimes it doesn't.

Just Saying,
GoatGuy ✓
 

6A3sUMMER

Member
2016-06-07 6:50 am
Make sure your L and R speakers are matched better than 1 dB.

Make sure your L and R speakers are placed such that the reflections of the room boundaries, and furniture, and wall hangings, etc. are all balanced.

Or . . . just enjoy the music.
Do you always have to pick a center seat in the orchestra hall, or at the Jazz gig?
And what instrument sound is that big 350 pound football star in front of you blocking, the Saxophone?
 
Last edited:
"Pot is ok, imbalance is happening after the pot. "

That would have been very useful information to put in your original post!

How do you know the imbalance is after the pot? You have already asserted: "when I measured the output from the pre-amp." Where else have you measured and what's the schematic?
 

qguy2000

Member
2018-09-24 2:50 pm
If you read my original post, the ONLY ask was if I should be concerned about the difference in measurement. There was no ask to identify or fix the cause of the problem. I think most of us are hard wired to offer solutions once they read a problem in a post, in this case is the problem really a problem :)

To answer your question below, I measured the output after the pot, just before it goes in the pre-amp circuit, results is balance. I measured the outputs of the pre-amp circuit and the left channel is weak. Like I said, I am not looking to fix the issue right now, I am wondering if those minute difference in voltage really matter....


"Pot is ok, imbalance is happening after the pot. "

That would have been very useful information to put in your original post!

How do you know the imbalance is after the pot? You have already asserted: "when I measured the output from the pre-amp." Where else have you measured and what's the schematic?
 
qgyy2000,

Yes, I was too unkind and too critical with my dismissal of the problem of channel balance.
I have to apologize for that.

And yes, many stereo recordings will have some cancellations of some signals when you just do a simple Mono combine of the signal (L+R).
But if there are such electrical cancellations in the L+R of the recording, then when you have perfectly balanced signals, amps, and loudspeakers, those L and R signals will also cancel acoustically.

And then there are pan pots, phase reversers, and other trick tools in recording and production techniques.
Not all is perfect.

I used to be very careful about signal balance.
I would monitor the L and R channels on an oscilloscope XY display.
You can learn a lot about what different recording techniques are like (results).
I found out about that when I listened to the music and compared it to the XY display.

Many recordings have mono-like equal amplitude and equal phase of L and R signals,
much more than you might expect. If you have an XY scope, you can see the upward 45 degree slope line (and with very little width to that line).
Then you put on another recording, and the XY display 'blooms' with many signals appearing everywhere within what looks more like a scribbled filled-in circle. You can easily find some classical recordings like this, but other recordings like some Jazz ones.

Our 2 ears and our brain have very powerful analysis capability.
. . . But then I lost the use of one ear.
Now, with just one ear, I found out that when you walk across a line that is perpendicular to the line from speaker to speaker, you hear lots of cancellations, like a variable comb filter as you walk the line.

Now, with one ear, I often just use a mono-block tube amp, and a 2-way loudspeaker.
I only apply either just the L signal from the CD player to the amp, or just the R signal to the amp.
There are no L R cancellations when I only use L 'or' R, versus combining L+R.

I plan on building a switch and matrix to take the CD L and R outputs, and give the following:
Left only; Left and -6dB of R; Left and - 3dB of R; L+R; R and -3dB of L; R and -6dB of L;
and R only.
That way, I can hear both the L and R channels on a mono-block amp and single 2-way loudspeaker, but without the cancellation of a simple L+R mono mix that is a problem of some recordings.
 
Pre-amp not balance when measures with a DMM

When listening to female vocals, the singer is in the middle of both speakers, but when I measured the output from the pre-amp using a 400hz tone, I am getting not equal measurements

Volume Left channel Right channel
9 13mv 10mv
11 49mv 35mv
12 90mv 67mv
1 146mv 136mv
2 154mv 152mv
3 157mv 156mv

should I be concerned or should I just enjoy the music ?

The auditory image is to a large extend determined by the listening room. It is often that the best center image is reached with slight imbalance in signal.

I would just enjoy the music.

Jan