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NNTP-Posting-Date: Tue, 10 Sep 2024 14:47:47 +0000
From: john larkin <jlarkin_highland_tech>
Newsgroups: sci.electronics.design
Subject: Re: Op-amp mystery
Date: Tue, 10 Sep 2024 07:47:46 -0700
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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!