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Old 12th June 2012, 10:11 PM   #1
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Default dual balanced ne5534 schematic

I'm looking for a dual balanced microphone preamp schematic with bootstrapped NE5534 op-amps. I read that the Cohen design is good. I've already built figure 4 from this link with NE5532 chips. It works well, but I should be able to get a much quieter noise floor with the bootstrapped version.

Audio Mixing Console - Part 1

I want to build the bootstrapped version similar to page 46 here:

http://www.thatcorp.com/datashts/AES...ic_Preamps.pdf

Can anyone help me find a tried schematic? I do want a single ended output also.

Thanks,
Nick
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Old 12th June 2012, 11:27 PM   #2
jcx is offline jcx  United States
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the bootstrapping of input Z only improves CMRR in the face of source Z imbalance - not really "noise" improvement - and only important if both of high CM signal needing rejection and the Z imbalance conditions exist together in your application

for phantom mic out's 6.81 kOhms there are fine fet input op amps

Last edited by jcx; 12th June 2012 at 11:31 PM.
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Old 13th June 2012, 03:38 AM   #3
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R2 plus R4 in the Fig. 4 schematic are large enough to effect thermal noise somewhat. Also the 5534's are in series for voltage noise, a 3dB penalty.

The page 46 circuit isn't bootstrapped in the usual sense of the term. R7 and R8 are conventional negative feedback.

All good fortune,
Chris
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Old 13th June 2012, 04:48 AM   #4
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Woops, that page 46 isn't a good example. I couldn't find a schematic that had the proper values for what I was reading about Cohen's design.

The cheep condensor mics I was considering buying are still low-z. I'm just have a couple old dynamic mics right now. I'm just using them for internet chat/podcasting type stuff.

I'm aware of the R2 and R4 being large enough to effect thermal noise. That's one reason I was interested in an alternative. Any suggested schematic would be appreciated. I'm not great with the math, but can follow directions. For some reason I decided to go to school for business rather than electrical engineering.
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Old 13th June 2012, 09:16 PM   #5
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Quote:
Originally Posted by nicksmurf View Post
R2 and R4 being large enough to effect thermal noise. That's one reason I was interested in an alternative. Any suggested schematic would be appreciated.
R2 and R4 could be replaced with small inductors (maybe 100uH or so) or by paralleled inductor and resistor. It's a good circuit except for needing a large electrolytic with no DC across it from pin 2 to pin 2 of the 5534's.

The highest performance in the real world still usually involves an input transformer, but some cash outlay is involved.

All good fortune,
Chris

Last edited by Chris Hornbeck; 13th June 2012 at 09:19 PM.
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