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Old 7th March 2011, 12:40 PM   #11
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Quote:
Originally Posted by mickeymoose View Post
Oops! 10 blind mice!

As drawn the circuit should not work at all. No split power-suply and all is ground referenced. E
My thoughts, exqctly. The negative half-cycle of input signals will drive the op-amp's inputs below ground, no doubt violating the common-mode range specification for the device. I don't know if this is causing the noise problem which you believe the circuit has, but as a test, try creating a bipolar supply by making ground the node mid-way between the four batteries. Disconnect mic power from the circuit and use a separate battery string to power the mic, not forgetting to then connect that second battery string to the circuit ground.
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Old 7th March 2011, 04:05 PM   #12
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Ouch! You're so right!



Fixed it and noise is gone completely. Sounds much better now without the negative halfwave cut off.




Somehow I thought it was possible to operate an opamp with a single supply and the output at half the operating voltage after I saw this schematic:
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Old 7th March 2011, 05:37 PM   #13
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I removed R7 and R8 because they did nothing than to cause way too high LF rolloff.
C8 and C9 are way too high. 10pF is more than enough and still cuases a bit HF rolloff.
Now I think the mic preamp is usable. For sure not the best but usable.

Thanks to all who helped me.
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Old 7th March 2011, 06:23 PM   #14
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Quote:
Originally Posted by Lee Knatta View Post
Ouch! You're so right!



Fixed it and noise is gone completely. Sounds much better now without the negative halfwave cut off.




Somehow I thought it was possible to operate an opamp with a single supply and the output at half the operating voltage after I saw this schematic:
Good for you. It IS possible to run opamps on single supplies - even this mic preamp - but it's so much easier on dual supplies I don't know why anybody would waste time on it. I suspect it's the 'hollow state' mentality coming from tubes where the only negative supplies (maybe) used for bias.

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Old 7th March 2011, 06:37 PM   #15
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S46: The Glassgods will go after you and punish you severely for inflammatory statements like that! E
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Old 7th March 2011, 06:42 PM   #16
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That simple balanced input will neither A) give optimum noise figures; where the mic's impedance is low, it can't 'eat" the resistor noise, since there is always a resistor in series, and there is a √2 X theoretical minimum, nor B) give optimum common mode rejection, as the impedance is not balanced (although the more gain it has, the closer it gets. It's fine for line inputs, but doing anything like classical recording you're adding both hiss and electromagnetic pickup.

Not that there aren't commercially available consoles based round the same theory; but one can do better.
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Old 7th March 2011, 06:50 PM   #17
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True. There are better designs available. But it is only a $60 microphone! E
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Old 10th March 2011, 12:55 AM   #18
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Two things: 1. Do you really need that much gain? If you're throwing away gain later in the signal chain, you should instead reduce the gain of the circuit. 2. The mic itself has a fairly high self noise (ECM8000 - in the neighborhood of 22-23 dB). If you're using it in a classical situation, you may need a mic with a substantially lower noise figure.


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Old 10th March 2011, 05:36 AM   #19
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Yes, I need that much gain. I'd want even more gain but 40dB is sufficient.
Yes, the mic is extremely noisy by itself. A better preamp might be a waste of resources.

However, I read a bit what Self wrote about this and came to the conclusion that the instrumentation amplifier is what I'd prefer as a fixed gain preamp. The variable gain preamps he discussed fit more universally.

The 1k resistors in series with the input are definitely not an issue noise wise since you can't do any better but the low input impedance of 1k is not good. Thus the instrumentation amp with non-inverting buffers / amplifiers that offer high input impedance almost noise free before the differential amp built as is would be an improvement.

I'm very space constrained inside the box but think about rebuilding the amp circuit this way. I used a cheap prototype pcb anyway and would reuse the valuable parts thus there's no significant loss.
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Old 13th March 2011, 10:34 PM   #20
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This is how I plan to modify the preamp.

What do you think about the schematic?
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File Type: png mic-preamp_inst.png (22.9 KB, 90 views)
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