Re: Op-amp mystery

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From	Lasse Langwadt <llc@fonz.dk>
Newsgroups	sci.electronics.design
Subject	Re: Op-amp mystery
Date	Thu, 12 Sep 2024 00:59:58 +0200
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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/

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Thread

Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-10 10:31 +0100
  Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-10 05:27 -0700
  Re: Op-amp mystery john larkin <jlarkin_highland_tech> - 2024-09-10 07:47 -0700
    Re: Op-amp mystery Jan Panteltje <alien@comet.invalid> - 2024-09-10 15:05 +0000
    Re: Op-amp mystery Lasse Langwadt <llc@fonz.dk> - 2024-09-12 00:59 +0200
      Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-11 19:00 -0700
      Re: Op-amp mystery john larkin <JL@gct.com> - 2024-09-11 19:10 -0700
      Re: Op-amp mystery piglet <erichpwagner@hotmail.com> - 2024-09-12 08:27 +0100
        Re: Op-amp mystery Jan Panteltje <alien@comet.invalid> - 2024-09-12 10:12 +0000
    Re: Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-12 13:21 +0100
      Re: Op-amp mystery piglet <erichpwagner@hotmail.com> - 2024-09-12 14:34 +0000
        Re: Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-12 18:03 +0100
  Re: Op-amp mystery John Robertson <jrr@flippers.com> - 2024-09-10 12:38 -0700
    Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-11 12:15 -0700
      Re: Op-amp mystery liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2024-09-11 21:35 +0100
        Re: Op-amp mystery Don Y <blockedofcourse@foo.invalid> - 2024-09-11 18:37 -0700
  Re: Op-amp mystery legg <legg@nospam.magma.ca> - 2024-09-11 09:29 -0400

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