JFET as VCA...cont.
« on: August 17, 2004, 12:44:21 PM »
Hi all! :cool:

PRR wrote:


The bidirectional audio voltage appears across the FET channel. A bidirectional audio current flows. The ratio of voltage and this current can be controlled by gate voltage.

When you say "gate voltage", do you mean the audio Voltage (AC) or the CV voltage (DC)? So do you mean that it is the CV that changes this "ratio of voltage and the current" by going more nengative or positive? Why is this ratio relevant?

I feel pretty comfortable with the theory and operation of the JFET as a normal amplifier, but I find it really hard to understand the relation between Vgs, Id, Vds and R(JFET) when used as a VCR.

I find it all pretty confusing, especially when some people say that the Vgs doesn't really change neither Vds nor Id, just the relation between them, while others say that an increased gate voltage decreases Id. I think what is confusing is to know if they mean the audio voltage or the CV, when talking about the Vgs.

Does, for example the audio Voltage (Vgs) affect the overall resistance of the channel at all, or is this just the CV?

In the output characterisics graphs, when showing different values of Vgs, are these values of the CV (DC voltage) or the Audio voltage (AC)?

PRR (or anyone=)...I would highly appreciate if you could come up with one of your super clear and mind blowing explinations=) I read the one you wrote about Vari-mu tubes...great stuff!=)

Please!   :roll:

What I would like to know is what happens when a audio voltage is applied to the JFET, with the CV voltage constant. How does this change in audio voltage affect the Vds? What happens with Id? And is the change in audio signal going to affect the channel resistance? Or is the channel resistance related to changes in the DC control voltage?

And is the Vgs in the output characterisics graphs demonstrating the CV voltage (DC) or the audio voltage (AC)?

Thank you!!!  

Jules  [/b]


JFET as VCA...cont.
« Reply #1 on: August 17, 2004, 12:51:37 PM »
Hi Jules,

Are you by any chance also Thomas Juth?


Jakob E.
..note to self: don't let Harman run your company..


JFET as VCA...cont.
« Reply #2 on: August 17, 2004, 12:57:27 PM »
Yepp, well...Julian is my middle name...



JFET as VCA...cont.
« Reply #3 on: August 17, 2004, 12:59:27 PM »
What about Mattsson? Is that your last name?

If you're from finland (or sweeden), how come that your IP adresses are from UK?


Jakob E.
..note to self: don't let Harman run your company..


JFET as VCA...cont.
« Reply #4 on: August 17, 2004, 01:03:58 PM »
I am Finish, but live in England at the moment...


Take Care!


JFET as VCA...cont.
« Reply #5 on: August 17, 2004, 02:04:55 PM »
> some people say that .... while others say that

FET amplifier operation and vari-loss operation are completely different modes and circuits. Do you have a schematic we can share for discussion?


JFET as VCA...cont.
« Reply #6 on: August 18, 2004, 05:13:32 AM »
Quote from: "Julian22"

Because Mattson is from Sweden according to the profile.

And because (please forgive me if I'm wrong!) I think that you keep asking questions without really seeming to be interested in the answers. I've noticed this on the parallel thread on gearslutz/geekslutz also.

Again, forgive me if I'm wrong, but it looks a bit like trolling - that's why I checked your three aliases..

If I'm wrong - which I hope I am - I think you should try to get a basic grasp on discrete amplifier and compressor design first. There are a lot of good literature out there if you google a bit. It's not really as hard as you make it look...

Best regards,

Jakob E.
..note to self: don't let Harman run your company..


JFET as VCA...cont.
« Reply #7 on: August 18, 2004, 09:42:38 AM »

-Jakob in 'sleuth' mode~!!!  :wink:

I've said before that I am distinctly uncomfortable with calling a vari-loss FET setup a "VCA", not because of any semantic distinction between attenuation and amplification, but since the term "VCA" has been absorbed into the vocabulary as a 'module' with predictable behaviour. (for example; 'x'dB/V gain change) and FET attenuators are not so simple nor predictable, nor linear.

It is exactly because the discussions tend to follow these trends that I prefer to jump in early and say "stop thinking of a FET as a VCA substitute" so predictably! (and possibly annoyingly!  :wink: )

I think that the questions posed in the first post are a littel too specific for general answers, and would need to be answered with reference to how the FET is connected... I think PRR said it more succinctly though.

"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.


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