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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Pre-amplifier circuit not working despite attempts to fix it
Pre-amplifier circuit not working despite attempts to fix it
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Old 17th July 2016, 08:29 PM   #1
Rudi1441 is offline Rudi1441
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Question Pre-amplifier circuit not working despite attempts to fix it

I tried to build a preamp circuit for an electret condenser microphone. I used this schematic.

Click the image to open in full size.
Click the image to open in full size.

The problem I have is there is a VERY loud hum that changes frequency when I change the gain in the second stage of the circuit (I switched R5 for a 220k pot) and when I turn the resistance down on that pot, the frequency goes higher and goes above what I can hear when the pot is near 0k.
I used an audio probe to see how far the original signal makes it through the circuit (it makes it to the front of R2, the horrible loud oscillation starts right after R2 and goes louder and softer throughout the rest of the circuit and still sounds the same at the output of the circuit)

I have to mention that I accidentally connected the power supply the wrong way around when I tried it the very first time, and a little smoke came out of the circuit (not sure out of what component) before I realized my mistake

I tried fixing the problem by doing the following:
I changed the electrolytic caps
I cleaned the back of the vero board and all the solder joints.
I re-did every solder joint to make sure none of them are 'cold' connections
I used a wall power supply (properly filtered) and I also tried a fresh battery to eliminate ground loops.
I changed the opamp for a different one to see if that was the problem.

Any idea what could be wrong?
Thanks in advance
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Old 17th July 2016, 08:39 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Place a 100uF 16v cap across R7.
That would be a good place to start. There is no stability in the virtual ground connection.
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Old 17th July 2016, 08:45 PM   #3
Rudi1441 is offline Rudi1441
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Quote:
Originally Posted by JonSnell Electronic View Post
Place a 100uF 16v cap across R7.
Thanks! I will try this and let you know what I find.
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Old 17th July 2016, 09:34 PM   #4
Michael Bean is offline Michael Bean  United States
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Quote:
Originally Posted by Rudi1441 View Post
I tried to build a preamp circuit for an electret condenser microphone. I used this schematic.

Click the image to open in full size.
Click the image to open in full size.

The problem I have is there is a VERY loud hum that changes frequency when I change the gain in the second stage of the circuit (I switched R5 for a 220k pot) and when I turn the resistance down on that pot, the frequency goes higher and goes above what I can hear when the pot is near 0k.
I used an audio probe to see how far the original signal makes it through the circuit (it makes it to the front of R2, the horrible loud oscillation starts right after R2 and goes louder and softer throughout the rest of the circuit and still sounds the same at the output of the circuit)

I have to mention that I accidentally connected the power supply the wrong way around when I tried it the very first time, and a little smoke came out of the circuit (not sure out of what component) before I realized my mistake

I tried fixing the problem by doing the following:
I changed the electrolytic caps
I cleaned the back of the vero board and all the solder joints.
I re-did every solder joint to make sure none of them are 'cold' connections
I used a wall power supply (properly filtered) and I also tried a fresh battery to eliminate ground loops.
I changed the opamp for a different one to see if that was the problem.

Any idea what could be wrong?
Thanks in advance
Two things:
First, as already mentioned, there should be supply bypass cap across R7.
Second, and more importantly, the minimum supply voltage for the NE5532 is 10 volts (or +/- 5 volts), so 9 volts won't work properly.

If you're using 9 volt batteries, just add another in series for 18 volts.

Mike
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Old 18th July 2016, 01:53 PM   #5
Rudi1441 is offline Rudi1441
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It's working!!! All I had to do was add the 100uF cap as JonSnell suggested. I know the datasheets says there should be a minimum of +-5V going into this opamp, but I've actually used many of them with 9V and they work 100%. Also, a lot of guitar pedals use this opamp while running off 9V supplies, so that doesn't make complete sense to me, but at least it works, so I am happy!!
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Old 18th July 2016, 02:35 PM   #6
adason is offline adason  United States
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there are often two 9 volt batteries...creating +/- 9 volts
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