Analog Alchemy - about the D. Comp. article - calling Joe...

[Phorhas]

or any one else who experimented eith this idea... will different type of diodes give a differant sonic behavior? (e.g. use a FET or MOS configured as diodes or an LED etc...)

BTW, will any Si diode work?

[brett]

Hi.  I'm no expert, and I just read the article and looked at the Vulcan article for the first time today, but I'll make a couple of guesses to get the ball rolling.

RE Will there be differences between diodes?
I suspect lots.  Because the principle is that more signal (ie more conduction) will expand the depletion zone of the diode.  Some diodes have narrower "gaps" than others.  A 1N4148 is a very leaky diode compared with a 1N400X.  A FET should be fairly similar to a 4N00X.  I'm not sure how Schottkys compare.  Ge diodes, like a 1N4148, leak a lot,  but I don't know how the growth rate of the depletion zone would compare with Si.

The Vulcan schematic specifies a 1N914 (=1N4148), which has about 25nA leakage at about 20V.  It is quite possible that other diodes won't leak enough to work.  But you can always try!!

[Phorhas]

thanx, I'll ghive it a shot

[brett]

Hi again.  Just had a look at the leakage of rectifiers (1N4001 to 4007).  It seems that leakage is a function of the rated reverse voltage, so a 1N4001 is a lot leakier than a 1N4004.  Maybe leaky enough to work.

[Phorhas]

so a ge diode or ge tranny configuerd as one should work really good, right?

[RobB]

This technique is so new and unusual to me that I haven’t quite been able to understand how it works.  

My initial guess was that there is a quiescent forward running current through the diode.  An audio signal comes along and tries to push this DC current forwards and then pull it backwards on the other half cycle.  When it tries to push it against the direction of normal flow of the diode the signal is compressed.

I really hadn’t considered how leakage would affect this.  This is even more food for thought, maybe the key to fully understanding this.

During the week I lobotomised my Highway 89 build, replacing the treble boost input section with one of Joes Vulcan modules.  For the first time it started making Highway 89 type sounds (I was one of the few who couldn’t get that project to sound right).  I’ve been so impressed with the difference this diode compression technique made that I’ve altered the layout and made a new PCB for my 89 build.  Also been thinking about the following possibilities for a couple of days.  

I wonder what it would sound like if two diodes were used?  Would the compression be softer?  Would it sound less compressed/more dynamic?  

Could the diode and associated baising network be turned around so that it would compress the negative side the signal (for the following cascaded section of a Vulcan type circuit)?  

Any answers?  Guesses?