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Old 14th September 2012, 03:08 PM   #1
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Default Electret opamp gain problem

Using the TL072 after another TL072 unity gain buffer, there is limited amplification regardless of what combination of resistors are used. I wish at least a gain of 10.

It will not go past about 2:1. I have to scream into the omni-directional element to get about 180mV. The output of the buffer is good. Power supply is -+ 12v. Several different amps were tried to eliminate any damaged units to no avail. Another copy of the same element was tried also to no avail. The output of the element at the blocking cap is about 3mV as always.

Here is the wiring used:Non-inverting opamp wiring.jpg

Can someone help me out of the Twilight Zone?
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Old 14th September 2012, 03:31 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Can't see the point of the unity gain buffer, have you biased the capsule ?

rgds, sreten.

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Simple Stereo Electret Mic Preamp
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Old 14th September 2012, 06:53 PM   #3
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Quote:
Originally Posted by sreten View Post
Hi,

Can't see the point of the unity gain buffer, have you biased the capsule ?
The buffer was left over from an attempt to drive the resistors in a differential stage being fed from two elements, and is not needed at present.

The 2-terminal capsule is biased through a 2.2k resistor on the pos side (3v.) with the other grounded, and a DC blocking cap just before the output term. as per mfr's. recommendations.

As I see your circuit, the input is fed rail voltage. Then there is a cap blocking the 4.5v from the divider, which I guess is added because the amp is single rail. I don't understand why rail voltage is added to the input, or what the cap after it is doing.

The feedback circuit is rather complex. The 33k resistor is in parallel with a cap, and the rest of the resistors are not directly grounded, but instead go through another cap. I was wondering why all the components. If you could shed light, I would appreciate it. The switch is obviously a range selector. The rest of the circuit is rudimentary.
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Old 14th September 2012, 08:34 PM   #4
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The switch needs to be closed to get decent gain.
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Old 15th September 2012, 01:09 AM   #5
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Thank you everybody for your answers. It has come to me that at least the cap that is in parallel with the one feedback resistor forms a bandpass filter.

Well, I will build the circuit and see how it performs and eventually figure out (maybe) what the series cap does. I plan on using the ADI SSM2019 self-contained preamp which provides differential, anyway.

nigelwright7557 Speaking of gain, I trust you have Dimarzios on that axe!

Tally-ho....
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Old 15th September 2012, 01:39 AM   #6
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Originally Posted by ITPhoenix View Post
nigelwright7557 Speaking of gain, I trust you have Dimarzios on that axe!

Tally-ho....
It is a completely original Peavey Patriot, I have had it for about 20 years and it still plays really well.
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