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?
>My first thought would be. "cheap design, bad SNR", but maybe it's not
>as simple as
>that.
>The famous EMS HIFLI (for which people pay insane prices at the
>moment) obviously
>uses dides as voltage controlled resistors, but not only one or two
>per stage. There are
>20 (twenty !) diodes connected in series for each stage, making a
>total of 240 diodes
>in the 12-stage phaser secton. Insane ?
>There must have been some reasons to do it that way. The LFO and ramp
>generators
>in the HIFLI are full of CA3080's, so it would have been likely to use
>them for the
>all pass stages as well. Can it be that the diode version is superior
>to an OTA design ??
>What I can *imagine* (but far from being sure) is that 20 diodes in
>series lift the
>maximum signal level (for tolerable amount of distortion) out of the
>opamp's
>noise floor somehow.
>And maybe using a large amount of diodes gets rid of doing selections
>on the diodes,
>as there would be some averaging of tolerances across 20 diodes in
>series.
>(The phaser is organized in 6 stages x2. The diode strings of 6 stages
>are directly
>connected in parallel, each battery driven by an opamp buffered CV at
>the top end,
>and by an opamp inverter at he low end.)
>
>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  (;->)
>
>(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?
>
>Any input -to these questions, or general comments about the HIFLI,
>is
>highly welcome!
>


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