AGC (was: RE: [sdiy] quadrature sine wave oscillator question

[jhaible at debitel.net]

> I always wonder why jfets are used for AGC,
> and not LDRs.
> Perhaps someone came up with a jfet servo loop
> and all the others just copied it?

FETs are faster.
There are some classic opto compressors which have
a "special" sound that may partially benefit from
the LDR's limited speed. But if you want to have
the full range of useful attack / release times,
The FET just dosn't has a speed limit like a LDR.


> The distortion the LDR causes should be way lower
> than those of the jfet.

Depends on the voltage level. Lower level, no more distortion
problems - now your enemy is noise.
But I was pleasantly surprised how well a simple FET 
can work for dynamic reduction, if you have a low noise
(discrete) amplifier to deal with the low signal levels.
(See my 1176 clone schemos for details.)


> Also diode forward conduction
> problems simply do not exist.

Maybe this is why the control circuit of the 1176 has such
high resistor values at the FET base? 


> Perhaps the useable range of the LDR is too small?

I can't see why it should be smaller than a JFET's.
Only that the LDR gets even slower on one end of its range ...


> Applies for quadrature as well as Wien bridges.

Ooops, your still talking about AGC for oscillators,
not about general dynamic reduction.

Well, I guess most sine oscillators have *thermal*
AGC - an incandescent lamp that serves as PTC. 
FWR and tru-RMS built-in, and the thermal
capacity provides the time constant. (;->)

JH.

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