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Re: Op-amp mystery

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On Tue, 10 Sep 2024 10:31:37 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>It's always the stupid things that cause the most trouble:
>
>I was working on a fairly straightforward experimental audio circuit
>using TL074 op-amps laid out on Veroboard, which is prone to inter-track
>shorts unless you are very careful with your soldering.   The gain of a
>virtual-earth stage was too high, so I altered some resistor values to
>cut it down.  

Show us a pic of your breadboard!

>
>As this was a stereo system, I experimented with one channel first and
>then switched off and replaced the equivalent resistors in the second
>channel (one of the other op-amps in the same chip).
>
>The input signal was a 6 Kc/s sinwave at about -10 dBu.  After the
>modification, the volume control, which was in a feedback loop, didnt
>seem to make much difference.  Then I realised that applying the signal
>to the left channel gave a distorted version of it in the right channel
>and vice-versa.  Obviously I had shorted two tracks somewhere - but
>where?
>
>Things got even worse when I realised that the input signal was
>appearing on the virtual earth input of each op-amp, this definitely
>can't happen!  I decided it was time to switch off the power supply and
>go and ponder the problem over a cup of tea.
>
>That was the point where I discovered I hadn't switched the power supply
>on in the first place.

Sometimes people note that a CMOS circuit works properly with no power
supply, because some ESD diodes are rectifying a logic input.

I never use those dreadful Veroboard or plastic block plugin things.

Try this:

https://www.dropbox.com/scl/fi/f3redjl7umja5v8tsi2p5/Z382_1.JPG?rlkey=epclqs4lkx2cmdakne390bbae&raw=1

https://www.dropbox.com/scl/fi/ydvcds95zvzjq56bzeimr/Z412_Proto.JPG?rlkey=hyejukxbbnk3573engf0if4zt&raw=1

Get artistic!

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Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-10 10:31 +0100
  Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-10 05:27 -0700
  Re: Op-amp mystery john larkin <jlarkin_highland_tech> - 2024-09-10 07:47 -0700
    Re: Op-amp mystery Jan Panteltje <alien@comet.invalid> - 2024-09-10 15:05 +0000
    Re: Op-amp mystery Lasse Langwadt <llc@fonz.dk> - 2024-09-12 00:59 +0200
      Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-11 19:00 -0700
      Re: Op-amp mystery john larkin <JL@gct.com> - 2024-09-11 19:10 -0700
      Re: Op-amp mystery piglet <erichpwagner@hotmail.com> - 2024-09-12 08:27 +0100
        Re: Op-amp mystery Jan Panteltje <alien@comet.invalid> - 2024-09-12 10:12 +0000
    Re: Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-12 13:21 +0100
      Re: Op-amp mystery piglet <erichpwagner@hotmail.com> - 2024-09-12 14:34 +0000
        Re: Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-12 18:03 +0100
  Re: Op-amp mystery John Robertson <jrr@flippers.com> - 2024-09-10 12:38 -0700
    Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-11 12:15 -0700
      Re: Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-11 21:35 +0100
        Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-11 18:37 -0700
  Re: Op-amp mystery legg <legg@nospam.magma.ca> - 2024-09-11 09:29 -0400

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