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Old 13th November 2004, 11:17 PM   #1
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Default comparator

Hi, not being a whiz when it comes to circuit design - can somebody point me in the right direction?

The problem: I run the signal from my dcx 3way crossover through a six gang poti/per channel for volume control (all balanced I/O), and from there to the amps.
As the potis are not joined for the left and right channel and not remote controlled ( I could not get a decently inexpensive good six gang motorpot), I hate to get up again after the volume setting, having found the balance being off.

My idea to eliminate this problem: Can I install a comparator circuit (where do I find one?) with high input resistance - so not to effect the signal - between the two midpass pot's and an instrument that reads zero when the voltage output is equal on both left and right pot's?

Or any other ideas?

Any help appreciated for a couch potatoe

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Old 14th November 2004, 10:05 PM   #2
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OK, as nobody wants to help maybe this way -
what if I use a LF 311 op amp, and feed it with the rectified signal from the pot's?
Do I need more amplification in front of the LF 311?
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Old 14th November 2004, 11:33 PM   #3
azira is offline azira  United States
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A comparator won't do what you want it to do. They work how you're thinking but it's nearly impossible to use a straight comparator for this purposes, less than a millivolt difference will cause it to slam the rails.

I don't quite follow what you're trying to balance and what your set up is. If you can explain a little more maybe I can help you out with your circuit.
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Old 15th November 2004, 12:35 AM   #4
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Default Re: comparator

Quote:
Originally posted by audio-kraut
. . . when the voltage output is equal on both left and right pot's?
Since the voltages at the pots are the varying audio signals, each channel will be different from the other most of the time so the idea won't work.

Can't you put some dial markings on the front panel?
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Old 15th November 2004, 02:58 AM   #5
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Default I'm just curious...

Why not use a single stereo control before the crossover? To maintain the balanced connection you would need six op amps (three duals). A differential at the input, followed by the volume control, and then finally a buffer and inverting buffer at the output to make it balanced again. See attachment.

I don't think that it's a good idea to use a passive multi-ganged attenuator on a balanced connection. The tracking error between the sections of the potentiometer would ruin the CMR.

If you must have the volume control post-crossover, you should consider VCAs. I believe a company called THAT makes good quality ones. Then you would only need a single mono pot to supply a control voltage to the VCAs.
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File Type: gif volume_control_balanced.gif (8.4 KB, 93 views)
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Old 15th November 2004, 05:36 AM   #6
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The problem is that i enter all signals - digital and analog - into a sample rate converter. The whole digital processing chain _ src 2496; deq 2496;dcx 2496 works best when digital - and analog - signals are not attenuated before entering the chain. The pots from a german company - THEL - are of fairly high quality and guaranteed to be within <1dB.

Sure I could put markers on - but up where I live and on the net I couldn't find any white letraset for my housing. And - I want to be a bit more accurate than that as i am quite sensitive when it comes to differences in balance - more so than differences within the spectrum of each crossover range.

Plus it is more fun to build something - if I could find out how to.

Anyway, thanks to all who responded to a couch potatoes cry for help.

I still might go the route of a digital pot with remote - thanks for that tip too. I just am a little doubtful of the quality of those.
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Old 15th November 2004, 05:43 AM   #7
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maybe other ways to skin this cat?
do you have any metering in the path
can you feed a common test tone at the inputs?
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Old 15th November 2004, 05:56 AM   #8
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for some reason I like this comparator thingie- kinda like an old
fm tuning meter

I think you would need filter stages before the comparator to smooth
/average the signal, otherwise any phase difference between channels would drive the comparator to the rails.

a window comparator with deadband hi and low led works in my mind

maybe introducing feedback within the comparator loop to reduce gain
and sensitivity?
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Old 15th November 2004, 06:44 AM   #9
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The comparator idea would work if you installed a momentary mono button. That way, any volume difference between channels would be due to unequal volume control settings.

Set the volume on one channel to where you want it, press "mono" and hold it while you adjust the other channel until an led went out. When you released the mono button though the led would start blinking like crazy so you'ld want to do something about that and also the noise the comparator switching might introduce.

Come to think of it, an extra pole on the mono button could provide power to the comparator so that would solve the "blinking like crazy" and "comparator noise" problem.

Thinking even further, you could make one of the volume knobs "touch sensitive". That could turn on a relay which put the system into mono mode and powered the comparator.
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