Re: Op-amp mystery

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!