Balanced stereo to mono summing box, with PAD and phase. Help needed - diyAudio
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Old 3rd November 2015, 12:43 PM   #1
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Question Balanced stereo to mono summing box, with PAD and phase. Help needed

Hi everybody,
I'm quite new to the world of electronics and I'd like to build a simple audio circuit which would also suit my needs in live applications, especially when input channels on mixers are limited.
What I want to do is a balanced XLR stereo to mono summing box, which I can use, for example, for keyboards.
In addition, I'd like to add a phase switch on one of the two inputs (DPDT switch?) and a PAD on just one input aswell.
This way I could use it as a snare top/bottom combiner (using two mics), with the pad used to tame the bottom snare.
I'll start off saying I have quite few understandings of the huge world of electronics, after searching the web I have a rough idea of how to start but i still have some doubts.
I think it would be good to start with this circuit http://www.rane.com/n109fig3.gif
placing the phase switch just before the points where the two sources join.
Regarding the pad switch, should I integrate a H PAD?
I found a calculator T-Pad / H-Pad Calculator - Resistor Values for Attenuation
from my poor understanding, I suppose that the summed input and output resistors on the PAD channel should match the values of the ones on the unaffected channel (correct me if I'm wrong) .
But what about the behaviour of that bridge power load resistance once the Pad kicks in? Will it affect the channel input impedance, hence the two inputs have now different impedances? Or is that negligible?
Will keeping its value as low as possible minimize its effect (again, if this is really something undesirable)?

Sorry for asking a lot of questions, but as I said I'm quite new and inexpert and I don't know directly any electronics expert
Thanks a lot to anyone who will respond and will give me an help about this.
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Old 3rd November 2015, 01:24 PM   #2
AndrewT is offline AndrewT  Scotland
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For live performance I think you should keep your balanced impedance connections to minimise the risk of interference.

But your "effects" equipment will be much simpler if you convert bal to unbal inside your effects box and then after the effects convert back unbal to bal for the output.
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Last edited by AndrewT; 3rd November 2015 at 01:28 PM.
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Old 3rd November 2015, 01:36 PM   #3
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I'm sorry, I don't get you... I'm not talking about fx, I need a summing box (balanced, i never wanted to make it unbal for obvious noise reasons) for instruments coming from the stage...
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Old 3rd November 2015, 02:18 PM   #4
AndrewT is offline AndrewT  Scotland
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summing box and attenuator to allow for some phasing. That is an "effects" box.

You heard my suggestion.

and why, because balanced impedance connection rely absolutely on exactly balanced impedance to maintain the interference rejection.
That balanced impedance must take account of both resistance and parasitic capacitance.
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Old 3rd November 2015, 02:58 PM   #5
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so the "problem" is that pad, am I right?
And to convert bal to unbal and then back to bal, I need active circuitry, right?
PS: I would need that pad switch only when using two mic sources (snare top/bottom) so I don't care about crosstalk or interference between the 2 sources if that's what you're talking about)
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Old 3rd November 2015, 03:04 PM   #6
AndrewT is offline AndrewT  Scotland
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yes active.

B.Putzeys has put together an excellent article on bal to unbal and back to bal just to implement a volume control.
Read it.
It will be of a quality that suits professional and live performance.

But the estimated price we are seeing exceeds $200 for the two channels.
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Old 3rd November 2015, 03:14 PM   #7
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ok, thanks for the hints. I think I'll just stick to passive summing box with phase for now, leaving the PAD out...
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