Some help with guitar active controls OpAmp circuit

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Dr.Obscure
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Some help with guitar active controls OpAmp circuit

Post by Dr.Obscure » Wed Dec 03, 2014 8:38 pm

Just finished the active controls guitar build project in the October issue (HotRod your electric guitar...), and have been experiencing some problems...

It seems to work ok in general, but when the guitar strings are played hard/loud,, the circuit output fades as if over driven and 'squelched', returning in 1/2 second or so... sometimes accompanied by pops and clicks at the start of the fade...

I put a scope on the front end being driven from the guitar pickups, which are delivering up to +/- 1 V max when strummed hard/loud.. which seems high for an opamps input... ??

The TL072 opamp specs reference a Vio of 10mv, so wondering if the 1v input is the problem, and then what to do to correct it... perhaps a voltage splitter across the input??

Any ideas would be greatly appreciated.. I need to get this guitar back in operation for a buddy :)
-thx
-Dr. Obscure

sghioto
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Re: Some help with guitar active controls OpAmp circuit

Post by sghioto » Thu Dec 04, 2014 4:56 pm

I've read the article and seen the schematic. One volt input should not be a problem, in fact the maximum input is limited to the supply voltage which in this case is + and - 9 volts. The design seems to be able to handle a 1 volt input with out clipping at full boost. I suppose I would check the batteries first then double check the wiring at this point.

Steve G

Dr.Obscure
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Re: Some help with guitar active controls OpAmp circuit

Post by Dr.Obscure » Thu Dec 04, 2014 7:22 pm

Thanks for the reply Steve...
The batteries are new, and verified.. the circuit works ok with low input, but cuts out a the 1v pp area...
If it was clipping would I expect to hear serious distortion, as opposed to a complete fade/loss of output?

I'll throw the guitar back up on the bench tomorrow and see what the output shows on the fade -- clipped or reduced waveforms..

Thanks again,
-chane

sghioto
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Re: Some help with guitar active controls OpAmp circuit

Post by sghioto » Fri Dec 05, 2014 6:16 am

If it was clipping would I expect to hear serious distortion, as opposed to a complete fade/loss of output?
Yes you would. Something else to consider, does it cut out regardless of where the controls are set? From the circuit description the gain is flat with the controls at midpoint. Should easily handle a 1 volt signal.

Steve G.

Dr.Obscure
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Re: Some help with guitar active controls OpAmp circuit

Post by Dr.Obscure » Sat Dec 06, 2014 9:08 am

Steve happy to report the issue has been resolved...
To answer your queries, yes it was happening regardless of control settings, or which set of pickups was engaged, etc.. was all very perplexing..

Anyway, in the process of setting things up on the test bench, I used a signal generator (so I wouldn't have to pluck the strings :) and traced with the Oscope.. somewhere in that manipulation and moving a part of two for access with the probe, the problem went away.. all the solder joints look good (all glossy, no obvious cold joints) so I'm assuming one of the components, likely a capacitor, was touching something it shouldn't, and caused the failure.. this is a small perf board assembly, stuffed into an area it was never designed for, and this is expected at times :)

Thanks again for your interest and help with my problem... opamps have always been my nemesis.. someday.. :/

sghioto
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Re: Some help with guitar active controls OpAmp circuit

Post by sghioto » Sat Dec 06, 2014 6:34 pm

Your welcome, glad you found the problem :grin:

Steve G.

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