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Old 17th March 2013, 01:09 AM   #1
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Default Want to build a simple microphone mixer

I'm new to diyAudio, so I'm not even sure I'm posting in the right forum. I've come up with an op amp-based design for a 4 or 6 input microphone mixer, and I'm looking for any tips on the design. I'm attaching a schematic.

The mic input stages are differential amps with 40 dB gain, and these are combined in a summing stage with unity gain per channel. For the microphone level output, I'm planning to use a Jensen transformer, the same one that's used in DI boxes.

I didn't use any caps in the design. Should there be a blocking cap before the summing stage, or maybe before the transformer?

Is the overall gain appropriate for a mixer with mic-level output? Any ideas will be appreciated.


Thanks!
Attached Files
File Type: pdf MicrophoneMixerSchematic.pdf (623.0 KB, 54 views)
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Old 17th March 2013, 01:59 AM   #2
sreten is online now sreten  United Kingdom
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Location: Brighton UK
Hi,

Your mixer or this :
Behringer Xenyx 502 Mixer at Gear4Music.com
What would you choose ?

Click the image to open in full size.


or this :

Click the image to open in full size.

or this for 4 balanced mic inputs :

Click the image to open in full size.

rgds, sreten.

Yuor a lot better off buying and then modding if quality really matters.
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Last edited by sreten; 17th March 2013 at 02:10 AM.
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Old 17th March 2013, 02:10 AM   #3
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I would put a capacitor after r9 to r12 otherwise you will be amplifying any dc offsets.

I don't know about the inputs, it depends if the mic's give out any dc offset.
I have always put coupling capacitors on the inputs and outputs of my mixers.
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Old 17th March 2013, 02:13 AM   #4
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Google 'differential amplifier'
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Old 17th March 2013, 02:18 AM   #5
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Quote:
Originally Posted by counter culture View Post
Google 'differential amplifier'
Whoops sorry forgot to add about adding a resistor to make inputs differential.
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Old 17th March 2013, 03:53 AM   #6
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Thanks Nigel. I see that I am missing the resistors from the non-inverting inputs to ground on the differential amps. I'll add those, and also appropriate capacitors to block the DC offsets.
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