Re: Op-amp mystery

On 11/09/2024 23:59, Lasse Langwadt 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/
  

It is possible you were remembering my posting this in April 2016:
<https://www.dropbox.com/scl/fi/8n4fz5o0r79rkmbn05j1a/micropowerBoost.pdf?rlkey=v1w9vm0his1pzd3lwd93dsm3v&st=5x8irpk6&raw=1>
All power flows through the inductor. The ESD diodes are only involved during startup, once oscillator running the output stage n-fet is the boost switch and the p-fet is the synchronous rectifier. A version built using 74HC132 was in a successful product which needed micropower 5V from 2-3V input.
piglet