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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Thu, 24 Oct 2024 21:56:58 +0100
Organization: Poppy Records
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Arie de Muijnck <noreply@ademu.nl> wrote:

> On 2024-10-22 15:11, Liz Tuddenham wrote:
> > piglet <erichpwagner@hotmail.com> wrote:
> > 
> >> Yesterday in the thread "Random thoughts on sinewave oscillators" I
> >> posted a LT Spice wirelist showing amplitude levelling without fet, ntc
> >> thermistor or ptc filament instead using current steering long tail pair
> >> - kinda poor mans multiplier.
> > 
> > How about using a lamp as the gain-controlling element at a low signal
> > level, so it doesn't self-heat, then heating it with superimposed D.C.
> > from some sort of voltage comparator/rectifier which is separate from
> > the signal path ?
> > 
> > It might have to be in some sort of bridge circuit, so that any noise on
> > the heating current doesn't appear in the signal.
> > 
> > 
> 
> How about the 'good old' lamp (LED) + LDR method? But I cannot find any
> linearity data of the LDR resistance itself (at constant illumination),
> important for harmonic distortion. Most searches end up in the
> audiophoolery corner.

My recollection is that they were good enough for amateur use but not up
to professional quality (hence professional mixing desks not using them,
despite how useful they might have been).  I seem to remember they had
distortion levels around 0.5%, but that might not be exactly right.

The photoconductive cells had a rather strange recovery curve which
exhibited two time constants.  When they were plunged from light into
darkness their resistance rapidly increased to a percentage of the dark
value, then slowly crept upwards to the final dark value over tens of
seconds or even minutes.  

If rapid control was needed, a pair of photoconductive cells was wired
as a potential divider so that the much faster decrease in conductivity
of one cell, when it was illuminated, would short-circut the other one
while it recovered slowly after being darkened.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk