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Why is there audio out even though personal mixer is unpowered?

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Why is there audio out even though personal mixer is unpowered?

Postby DC-Choppah » Sun Dec 02, 2018 2:33 am

OK, I would appreciate it if you guys could help me turn this experience into a positive.

I give everyone a personal mixer when we record. They can adjust their own
-headphone overall level
-cue mix
-their own monitor level (the 'me ' knob)
-personal sound sources (iphone, metronome etc.)

Everyone has one of these personal mixers but they are not all the same brand.

The drummer complained that his knobs weren't working. He was getting a signal into his headphones, but he could not control any of the levels. When he tried to mute the cue mix for example, it didn't change level. And all the levels were too low so he couldn't get enough signal.

Now in the end (after we spent a long time retracing the whole signal chain) he reached over and plugged in the power cord, and everything works fine and as expected.

Meanwhile, the other little mixers, when they are unpowered don't give any sound at all.

OK, so what gives? Why does this unit: http://www.rolls.com/product.php?pid=MX28
give us output when it is not even plugged in?

Meanwhile, other units like the small Mackie mixers are completely dead when not plugged in?

I am just looking to learn something about these designs so I can turn this wasted time into a positive to learn why some circuits output a signal when off! We did not even think to look for the mixer being unplugged since it was outputting a signal. How do I know in the future when I am dealing when a mixer that will be outputting a signal when unpowered?
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Re: Why is there audio out even though personal mixer is unpowered?

Postby ef37a » Sun Dec 02, 2018 10:09 am

Found a schematic! Did not expect that.

Yes, there is a path around all the op amps because they are all configured with shunt feedback. There is some 25k in the audio feed with no power applied but obviously there is a enough getting through to be audible.

The circuit looks very odd to me and not at all optimized for low noise but I guess Rolls had their reasons?

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Re: Why is there audio out even though personal mixer is unpowered?

Postby Hugh Robjohns » Sun Dec 02, 2018 10:50 am

I've had some rather dubious experiences with Rolls equipment in the past and I'm not at all convinced about their design qualities...

But yes, it's not unusual for there to be some audio bleed through from input to output in unpowered audio equipment via the feedback paths. It will be lossy, but is often audible. If the equipment is also ungrounded that can make the bleed through worse.

H
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Re: Why is there audio out even though personal mixer is unpowered?

Postby DC-Choppah » Tue Dec 04, 2018 4:08 am

Ok Thank You.

I really thought that perhaps something was wrong with it, but I feel better now. Shunt feedback across the op-amp? You get some signal through even when there is no power. Ok.

In defense of this little unit, it is very handy to have control of three stereo source levels and a headphone amp. The drummers especially love having this control when recording.

It sounds fine (when plugged in!) Not really Hi-Fi though.
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