opinions on HIFLI - Diode based phasing ?

[Rene Schmitz]

At 13:58 15.04.1997 -0400, you wrote:
>> I once made a tremolo for a guitar, using two diodes as a variable gain cell,
>> and I didn't notice any noise. It may be that diodes have a high THD, but
>since
>> we are not dealing with high levels anyway (>20mV), I would say it is OK to
>use
>> diodes for a phaser. (I made some spice simulations about this, but I didn't
>> focus on THD or SNR but just whether it works.)
>>
>> Bye René
> -- End of excerpt from Rene Schmitz
>
>Rene,
>
>You wouldn't mind elaborating on this circuit and concept
>would you?  How can two diodes act as a variable gain cell?
>
>steve

Its actually not a gain cell, but a variable attenuator.
The concept is to vary the current thru the diodes.
Therefore the dynamical (ac) resistance varies.
It was a very simple schematic. Just hook up two diodes, and a resistor.
The whole thing works as a current contolled voltage divider.
Just a resistor at the input, one diode to ground, and one to a transistor
that feeds the current.
But it had some drawbacks, since it is so simple, the CV feeds thru.
I simply filtered it away using a low pass filter. 
It worked (somehow) for a guitar. (They are happy with 30 dB SNR ;-)
I made it way back when I didn't know about OTA's or diff. transistor pairs.
Quite unelegantly!

Bye René