Re: Op-amp mystery

On 2024-09-10 2:31 a.m., 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.
 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.
 

A cuppa Tea solves EVERYTHING!
John :-#)#
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