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scwhiteley 21st March 2012 06:53 PM

Isolating Left & Right Channels, before mono amplification
This is a bit of a strange one, but worth a shot:

On the current project I'm working on, I'd like to keep the left and right channels of the signal isolated for separate VU meters, but as there's only room for one speaker on this enclosure, I'm combining the signals and amplifying the one mono signal.

I already tried just using a simple diode on each channel before combination (assuming that it wouldn't work due to the AC properties of the signals) and it clearly was not the solution.

Now I realize that the difference in the left and right channels in most music is probably negligible in most cases, meaning that the VU meters would look pretty much the same anyhow. But in the rare case that whatever I'm playing exhibits substantial right-left asymmetry, I'd like it to be represented on the VU meters.

Maybe you guys/gals can help?


P.S. - In retrospect, I think I would have rather tried to fit two smaller speakers in this project and amplified the channels separately, but too late for that now. That being said, I could still amplify the signals separately and just connect the 2 amp outputs to the one speaker, that seems like a suspiciously bad idea to me though.

qusp 21st March 2012 07:56 PM

you do realize youve posted this in the digital source section yes?

Mooly 21st March 2012 08:06 PM

Problem with simple VU meters is that they are passive and you have to overcome the volt drop of the diodes before anything happens. Germanium diodes are much better but the real answer is active circuit using an opamp for example where the diode/s can be incorporated in he feedback networks and the volt drop eliminated.

Boscoe 21st March 2012 08:25 PM

Run the two channels through two op-amps then join them after? I don't know whether that would work just a suggestion. I think you can do it with two resistors alrthough i would guess cross talk would be high.

scwhiteley 21st March 2012 10:26 PM

I've got some UA741 and LM324 op-amps around here somewhere. Maybe I can pull it this off with some of those.

scwhiteley 22nd March 2012 06:23 AM

1 Attachment(s)
Okay, I am trying the op-amp approach.

I used two UA741 op-amps for buffer amplifiers. I used the "textbook" absolute-simplest approach (for want of a more inspired application) shown here exactly.

Good news: it did keep the signals isolated from one another as displayed by the VU meters.

Bad news: the amplifier produced only a very scratchy faint bit of sound.

So I suppose that my conclusion is that while the op-amps did keep the signals separate, they are not faithfully passing the signals through to the amplifier (in this case: TPA3110D2 class D chip). Perhaps there is some configuration of these op-amps that will work for me?

Thanks again!

Mooly 22nd March 2012 08:16 AM

Your opamp configuration is correct as far as it goes. It sounds as though you haven't biased it correctly. Without seeing the circuit details it's hard to advise but pin 3 must be at 0 volts.

The 741 isn't the best choice these days but it will work very well non the less.

VU And PPM Audio Metering

scwhiteley 22nd March 2012 04:45 PM

Hmm. I was using pin 3 as the straight-up, hardwired input from the mp3 audio source. Are you suggesting that I tie pin 3 to ground and try an inverting buffer setup? I'm not opposed to the idea, hopefully I'll have some time this afternoon to give it a go.

As for the use of the UA741, I'm just using it now because I have some sitting around to play with, but I have no problem putting in another order to Digikey before I finish the project. What op-amp would you suggest? (and why?)


DF96 22nd March 2012 04:58 PM

How did you bias pin 3? You did provide bias, didn't you? I am not clear why you are introducing an op-amp anyway. Your earlier mention of diodes suggests you don't quite know what you are doing.

Feed the two signals to your two VU meters. As a separate issue, use a simple resistor pad to combine them into mono (a search on here will show you how to do it - people seem to ask every few weeks).

scwhiteley 22nd March 2012 05:48 PM

To say I don't know what I'm doing is a bit of an understatement. (I do, however, intend to enroll in some electronics courses next semester). In the meantime, I will look into this biasing and this resistor pad you speak of.


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