Re: The Wien Bridge Oscillator problem (your Sunday ruined part1)

On a sunny day (Mon, 21 Oct 2024 09:38:21 +0100) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1r1rup0.62nful1573k8wN%liz@poppyrecords.invalid.invalid>:

Jan Panteltje <alien@comet.invalid> wrote:
>

On a sunny day (Sun, 20 Oct 2024 20:17:58 +0100) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1r1qtlq.zz9rd16lqyaiN%liz@poppyrecords.invalid.invalid>:
 

piglet <erichpwagner@hotmail.com> wrote:
>

Cursitor Doom <cd999666@notformail.com> wrote:

Gentlemen,
 
How critical is the choice of thermistor for a WB oscillator?
Stability is desired; we know that much (along with minimal
distortion of course). The usual problem with WB oscillators is they
are not amplitude stable and either die out or saturate. However, by
means of a non-linear negative f/b arrangement, they *can* be made
stable. One common solution is to use a thermistor, usually one with
a positive temperature coefficient. The issue is, that it must be
'nimble' enough to regulate the output level without being *so*
'nimble' as to respond to peaks and troughs of the output signal's
cycles. I'm going to call this 'nimbleness' "tau" for the time being.
I know it's not the right Greek letter, but no doubt some helpful
soul will point out the correct one. So the question is, how the hell
do you select a thermistor with the optimum 'tau' for any given
wideband WB oscillator?
 
Your pal,
 
CD.
 
 

 
I think they should all get there in the end but a longer tau thermistor
will take longer to settle.

>
You could always use a large bulky thermistor which does not self-heat
and then heat it with a resistor fed from a power amplifier.  That would
trade off distortion against settling time.

 
 
Servo controlled potmeter?

>
It would be constantly hunting unless the resolution was infinely small
and there was absolutely no mechanical backlash..

Yes, I should have written 'resistor' perhaps
Imagine a temperature controlled wire pulling on the arm of a pot:


power amp -->- heated_wire -> ground

                                 |
                                [ ] 
 ------0----------------------->[ ] resistence element
(     bearing                   [ ]
)                                |
( heated wire on power amp


Thing will point up and down at the resistor element as the wire cools or heats up, contracts or expands..
Or perhaps modify an old moving coil volt meter to point at some resistor element from an old potmeter?
Plenty space for playing / experimenting.
Not sure it will work or is very practicle.

So I will stay with Linux and 'sox' on my Rapberry Pi for now ofr audio.
or with that on my big PC that has 2 good soundcards.

Or for higher frequencies the FPGA and DAC I have.
Or all the other stuff I already mentioned.