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Mono channel in a stereo summing unit
Mono channel in a stereo summing unit
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Old 24th June 2019, 07:39 PM   #1
Robotski is offline Robotski  Sweden
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Default Mono channel in a stereo summing unit

When creating a mono channel in a stereo summing unit it is recommended to do this by splitting the mono source, and via resistors connect to both left and right channels. Are these resistors used only to reduce the level to match the stereo tracks, or are they also needed to not compromise the L/R integrity?

I'm asking because in my design I have summed two mono signals together before they are to be split to the L and R busses, so the level may already be about right. If so, can I omit the usual resistors going to the L/R without inducing any problems?

Would be thankful for some input re this!

/T.
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Old 24th June 2019, 09:06 PM   #2
Mark Tillotson is offline Mark Tillotson
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Stereo summing unit? Can you give details of this please.
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Old 24th June 2019, 09:10 PM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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You don't want to short out the left and right channels or you could do some damage.
So put a couple of 1k resistor in series with each and join the two ends of the resistor together.
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Old 24th June 2019, 09:11 PM   #4
ubergeeknz is offline ubergeeknz  New Zealand
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No need for resistors to split mono to stereo. Only the other way around.
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Old 24th June 2019, 09:14 PM   #5
indianajo is offline indianajo  United States
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Quote:
Originally Posted by Robotski View Post
and via resistors connect to both left and right channels. Are these resistors used only to reduce the level to match the stereo tracks, or are they also needed to not compromise the L/R integrity?
/T.
Major requirement for resistors is to keep the two amplifiers driving the two different channels from blowing each other up fighting to produce different voltages. Resistors must be above the normal minimum resistance of the op amps or channels. 1 k minimum usually, 2 k better, 10 k totally safe.
If internally the two signals passes through any more amp stages, resistors on their outputs must be used to protect those amp stages too.
Voltage dividing to lower volume is an additional parameter not to mess (lower resistance by parallel rule) with the above protection.
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Old 25th June 2019, 06:05 PM   #6
Robotski is offline Robotski  Sweden
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I should have been clearer: this is a balanced 16 channel passive summing circuit that uses 10k input resistors and I will feed it with balanced line signals from EQ's, compressors and DAC's from my studio computer. I want to make a couple of the inputs mono, so this is about what is happening before the signal hits the input resistors of the summing unit.
Since these input resistors are in place there is already no shorting between the L and R channels, right?

I also thought that I could make a simple 3-way pan switch with something like 30% to the L, 30% to the R and a middle position, but by testing this found out that after the split of the mono signal, there need to be isolation between the L and R or I cannot reduce one channel without affecting the other... I think I need transformers for this (I don't want to start messing with OP amps).
What transformers should I get? Balanced signal, will I need four transformers then for one mono channel? Signal(+) and reversed(-) for both L and R side?

Thanks for the input so far!

/T.
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Old 25th June 2019, 06:07 PM   #7
Robotski is offline Robotski  Sweden
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Quote:
Originally Posted by nigelwright7557 View Post
You don't want to short out the left and right channels or you could do some damage.
So put a couple of 1k resistor in series with each and join the two ends of the resistor together.
What's the difference between puttin two 1k or puttin one 2k...?

Cheers!
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Old 25th June 2019, 08:19 PM   #8
ubergeeknz is offline ubergeeknz  New Zealand
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Feed left to one of the channels in your mixer, right to another. Then you'll be fine.

Otherwise use a 10k resistor on each channel with the junction to the "input".

Panning can be done using a single 20k potentiometer.

Feed L to one end, R to the other, output on the moving arm.
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