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Old 22nd December 2014, 09:21 PM   #1
weroflu is offline weroflu  United States
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Default ina217 mic pre schematic help

First project, trying to make a very small battery powered stereo mic preamp

-trying to keep the board small
-microphones are battery powered, unbalanced output, no phantom needed
-microphone impedance is 500ohms, max output .7V
-input/output on two 3.5mm board mounted mini jacks
-intended connection to a portable recorder, input impedance 22k
-ICL7660S with boost pin connected, hopefully will keep ripple unheard
-trying to keep this powered by a single 9v
-fixed gain of 10 (20db), might change to a trim later

I'm unclear on a few points:

-do I need input coupling caps since there is no phantom involved?
-since the input is unbalanced I'm unclear how to handle pin 2 on both ic's. I left pin 2 unconnected now on ic2 until I get some advice.

Thanks in advance for helping.
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Old 22nd December 2014, 09:33 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by weroflu View Post
-do I need input coupling caps since there is no phantom involved?
-since the input is unbalanced I'm unclear how to handle pin 2 on both ic's. I left pin 2 unconnected now on ic2 until I get some advice.
Pin 2 should be grounded on both, not left open. The other inputs should have a proper load resistor to ground for the mics,
usually around 1k - 3k.

Last edited by rayma; 22nd December 2014 at 09:36 PM.
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Old 23rd December 2014, 02:33 AM   #3
rayma is offline rayma  United States
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Also, both pins #5 should be grounded. These are part of the diff amp. The C3 and C6 are reversed.

Last edited by rayma; 23rd December 2014 at 02:37 AM.
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Old 23rd December 2014, 08:52 AM   #4
weroflu is offline weroflu  United States
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Thank you Rayma.

Attaching revision with corrections.
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Old 23rd December 2014, 04:35 PM   #5
weroflu is offline weroflu  United States
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Can I get away with smd 6.3v caps for c4 and c7 or do they need higher voltage rating?
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Old 24th December 2014, 10:47 AM   #6
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You are using 7660 for a mic preamp for negative supply...
I suspect it will be noisy.

Gajanan Phadte
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Old 24th December 2014, 02:10 PM   #7
weroflu is offline weroflu  United States
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7660S

With the boost connected it is supposed to operate well above audible frequency, something near 60khz. Have you used the 7660S before or are you referring to the 7660?
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Old 24th December 2014, 02:54 PM   #8
weroflu is offline weroflu  United States
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edit: more like 35khz, the Maxim 1044 might get near 60khz?? nominal frequency is 7-10khz, boost is 6 x nominal.

New Page 1

I'd like to know if anyone used the icl or max successfully with audio. If they are no good then time for redesign.
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Old 25th December 2014, 07:34 AM   #9
weroflu is offline weroflu  United States
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Cleaned up the schematic.
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