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Old 12th April 2011, 11:37 AM   #1
diyAudio Member
Join Date: Jan 2011
Default Breadboard amp = echoes + highpass filter?

Heya guys, I'm not sure where to post this, but its really frustrating me to no end.

I have a breadboard (shoot me) and I currently have a very basic push-pull amp on it.

Now, the amp works perfectly fine as long as I have the oscilloscopes ground connected (the probe makes no difference) to it.

If this is not connected then the amp will do two things:
-Sound echo'y
-Lose lower frequency audio (inc voices)

I tried this circuit with both batteries and a 12DC supply form the wall, both have the same result.

Now, I had a PWM infra red transmitter on it previously and this experienced the same problem with the receiver missing audio frequencies!

I checked the signal and it seems to experience this change (when disconnecting the. oscilloscope) right after the opamp providing voltage amplification (inverting opa2227). (though it still happens without the op amp)

Thanks for any suggestions, I'll try another breadboard soon!
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Old 12th April 2011, 12:08 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
It seems the scope probe (earthing clip) is providing an "earth" that is somehow missing.

Have you missed out a return route for one or more of your currents?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 12th April 2011, 12:50 PM   #3
diyAudio Member
Join Date: Jan 2011
You're right, I had another talk with my Dad about it and I found the problem.

It was the 3.5 mm connector I was using. I was only using one channel of the stereo, but instead of ground and one of the channels, I was listening to the difference between left and right.

This explains the echoing and the grounding.

Thanks for the help.
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