Re2: opinions on HIFLI - Diode based phasing ?

[Rene Schmitz]

At 12:50 14.04.1997 +0100, you wrote:
>Hi!
>
>Have you ever seen a phaser that used ordinary diodes as voltage
>controlled resistors?

>I am looking for opinions from two different sides:
>(1) What do you electronic experts (Don, Joachim, Gene, synth-DIY
>members, ...) think of this
>circuit concept? Would there really be some advantages in SNR,
>compared with
>a OTA design ? I'd like to hear some opinions before I start soldering
>a whole
>drawer of 1n4148's onto a large veroboard, for some reason  (;->)

As I mentioned in the earlier posting, I did once some simulations 
(MicroCap V, nice program!) about it.
Today I started simulating an allpass filter (one stage)
and four diodes connected in series about +/- CV.
And I could reach an input level of 1 Vp-p with very low distortion.
Only the 3rd harmonic was a bit higher than others.
So I guessed that with 20 diodes it should be possible to raise the
input level to 3Vp-p with still pretty low THD.
Thats what I did this afternoon, and then I hooked up 20 (simulated) 
diodes, in a similar manner to what you describe. I didn't buffer the CV
but just connected a battery over the diodes, grounded both ends of the
battery via 10k 
It seem that it *has* an advantage concerning SNR.
At a 3Vp-p input level I measured 2.8 - 2.9 Vp-p for the fundamental,
and 0.065 - 0.008 Vp-p for the 3rd harmonic, CV varied from 0.1V to 10.1V.
Thats -32dB for the 3rd harmonic. (if memory serves my right)
The other harmonics are even lower.
So I would say that THD is lower than with an OTA, and you have the
advantage of 
higher input levels, thus lower SNR.

I'll give it a try making my old elektor phaser (6-stage) voltage controlable.
(It had LED->LDR variable resistors, but these are too sloooooow).

Two questions remain: -I would find it interesting comparing this to a FET based
                       phaser.
                      -Can one trust simulation programs ??
                        
>(2) 	Are there any *users* of the HIFLI out there ? I know it's ultra
>rare, but as
>I haven't heard one myself, other than on records, I'd like to know
>how it sounds.
>Is the phasing *clean* (in therms of Noise and distortion), or is it
>just an interesting
>"dirty" distorted phasing sound ??
>I'd also like to know about the tracking of the suboctave generator.
>Does it
>work as intended, or are there tracking problems?

Gee, I would like to see some schematics !!!

Fly High!
  René