Re: Op-amp mystery

On Thu, 12 Sep 2024 00:59:58 +0200, Lasse Langwadt <llc@fonz.dk>
wrote:

On 9/10/24 16:47, john larkin wrote:

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.

>
until all the inputs are low, then it only works until the bypass cap is
discharged ;)
>
seem to remember seeing a boost converter made from just a cmos gate and
an inductor, using the output as input and the supply pins as output via
the ESD diodes, but I can't find it again, but I found this
>
https://www.edn.com/efficient-voltage-doubler-is-made-from-generic-cmos-inverters/
>
>

This will drive a blue or whatever LED from a low voltage supply.

https://www.dropbox.com/scl/fi/w3km774qoq5979eeh0uhg/LED_boost.JPG?rlkey=mvar3ydo8pjsgd6f04ggybfls&raw=1