fet sugestions for compressor/limiters

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wilebee

Well-known member
Joined
Jun 15, 2004
Messages
390
I like to know what one should look for when choosing a fet for a gain reduction stage along with some families of fets

Thanks Wilebee
 
I want to design from scratch an fet style compressor/limiter ala compex,1176ish . I was looking for a starting point with the fet like whats being used in the 1176 or the fx760 or the pye comp lim. I was inquireing as to sugestions people might have. :idea:

WIlebee
 
You could try a VCR2N from Siliconix/Vishay. Designed to be a "voltage controlled resistor". I recently got a couple of VCR11N's from Interfet, which are monolythically matched, but cost ~16.00US each!

Others you can use: 2n5457, MPF102.

All have different characteristics, so they wouldn't be plug n' play.

Good Luck!
 
[quote author="buttachunk"][quote author="Winston O'Boogie"]Winston aka "The Master" (better watch out Butta :evil: :wink: ) aka John[/quote]


Ha !!!! We meet again !!!

snip to save bandwidth...
[/quote]


Oh, bugger! Where's Davros when you need him?
By the way Doctor, I just happen to have stolen your sonic-screwdriver :twisted: so I believe the battle has already been won before it has even begun :twisted: :twisted: :twisted:
 
Ahh, I see you have an original Tardis! I had one for a while but like an idiot traded it in for a newer model. My reissue is great, same look, does everything the originals did, still, not as nice as an original. Don't ya just hate dem Daliks? Oops, now everyone is going to know how I get my NOS tubes.
Kelley
 
It would be a good idea to choose a FET that is factory-selected to specific cut-off voltage ranges, like the BF245A/B/C. This means that you'll have an easier time matching your compressor's specific needs. Buy a standard FET, and the specs are all-over.

Jakob E.
 
For the VCR2N, follow the link. The spec's are fairly tight...


http://www.vishay.com/docs/70293/70293.pdf


I've measured lots of them, and they come alot closer in spec than most others. Of course, YMMV, but unless you're ordering a finite range from the manufacturer (read $$), you won't get closer tolerances.
 
I just took a look at the BF245 data sheet.

IMO, and experience, the Vgsoff spread of that device is not only very wide, but the lower end 0.25 - 3 volts or so, would be not very usefull in a gain reduction circuit. It will work, but distortion will be much higher than a FET with a larger Vgsoff. While the entire spread is 0.25 - 8 volts, only the top half would be usefull and would require selection beyond what the manufacturer offers.

The VCR2n on the other hand has a Vgsoff spread of 3.5 - 7 volts. Any device in this range would be an excellent candidate for a low distortion compressor or limiter.

For more info, download this from Vishay re: FET's voltage controlled resistors.

http://www.vishay.com/docs/70598/70598.pdf
 
This is great! I love questions that inspire engineering brilliance. thank you everybody I think you sayed me at least a week of research

P.S. I like the idea of a simplified Pye limiter Who's Game?

Wilebee
 
[quote author="jdiamantis"]I just took a look at the BF245 data sheet.

IMO, and experience, the Vgsoff spread of that device is not only very wide, but the lower end 0.25 - 3 volts or so, would be not very usefull in a gain reduction circuit. It will work, but distortion will be much higher than a FET with a larger Vgsoff. While the entire spread is 0.25 - 8 volts, only the top half would be usefull and would require selection beyond what the manufacturer offers.

The VCR2n on the other hand has a Vgsoff spread of 3.5 - 7 volts. Any device in this range would be an excellent candidate for a low distortion compressor or limiter.

For more info, download this from Vishay re: FET's voltage controlled resistors.

http://www.vishay.com/docs/70598/70598.pdf[/quote]




I've successfully used a BF245 for my 1176 clone.
It's true, these ABC groups don't help - the variance is huge even
within one group. Also, someone told me the Vgsoff of the FET in
the original was 2.1V (or was that 2.6V ?), and it was hard to find a specimen that fit this value. Most BF245As were too low, most BF245Bs
were too high. But in the end I found two that fit (two for a stereo version).

Why does a higher vgsoff mean less distortion ??
I see that the wrong Vgsoff (too high or too low) would f**k up the whole threshold and loop gain settings, but I wasn't aware that higher Vgsoff would also give a wider linear range in VC resistor mode. Is this really true?

JH.
 
I would like to measure the characteristics of the FET's in my 1176 and various ADR compressors and make a record so that if I ever get a failure I can match the new FET. Can anyone tell me the most essential specs I should be measuring? I can then post them here for reference.
 
jhaible said,

"Why does a higher vgsoff mean less distortion ??
I see that the wrong Vgsoff (too high or too low) would f**k up the whole threshold and loop gain settings, but I wasn't aware that higher Vgsoff would also give a wider linear range in VC resistor mode. Is this really true?"

In checking the following document, under "distortion causes", we find:

http://www.vishay.com/docs/70598/70598.pdf

"Where large signal-handling capability and minimum
distortion are system requirements, the feedback neutralization
technique for VCRs is an important tool in achieving
either or both ends.

It has also been shown that FETs with high pinch-off voltage
require larger drain-to-source voltages to produce
drain current saturation. Therefore, FETs with high
VGS(off) will have a larger dynamic range in terms of applied
signal amplitude, while maintaining a linear resistance.

It is advantageous to select FETs with high VGS(off)
compatible with the desired rDS value if large signal levels
are to be encountered."

By large signals, this means reasonable and usable signals in a compressor/limiter. The value you chose, -2.0V is about minimum for good results. Anything lower and you could run into noise/distortion issues.

In designing and building several FET limiters, FETS with higher Vgsoff were more linear, and produced lower distortion for equivalent audio input. I have recently received some dual FET's with Vgsoff of -11Volts! It will be interesting to see how these test vs. other types I've used.
 
Thanks for the info, jdiamantis!

That's interesting. I wasn't aware of that.
So maybe I should build all my other FET-as-VCR-applications with
BF245C instead of A,B types (FET-based Phasers, whatever.)

I wonder why the 1176 has used a FET with rather low Vgsoff then?

As I said, the info I got called for something like 2.x Volts. (Can
anybody confirm this?)



[quote author="jdiamantis"]jhaible said,

"Why does a higher vgsoff mean less distortion ??
I see that the wrong Vgsoff (too high or too low) would f**k up the whole threshold and loop gain settings, but I wasn't aware that higher Vgsoff would also give a wider linear range in VC resistor mode. Is this really true?"

In checking the following document, under "distortion causes", we find:

http://www.vishay.com/docs/70598/70598.pdf

"Where large signal-handling capability and minimum
distortion are system requirements, the feedback neutralization
technique for VCRs is an important tool in achieving
either or both ends.

It has also been shown that FETs with high pinch-off voltage
require larger drain-to-source voltages to produce
drain current saturation. Therefore, FETs with high
VGS(off) will have a larger dynamic range in terms of applied
signal amplitude, while maintaining a linear resistance.

It is advantageous to select FETs with high VGS(off)
compatible with the desired rDS value if large signal levels
are to be encountered."

By large signals, this means reasonable and usable signals in a compressor/limiter. The value you chose, -2.0V is about minimum for good results. Anything lower and you could run into noise/distortion issues.

In designing and building several FET limiters, FETS with higher Vgsoff were more linear, and produced lower distortion for equivalent audio input. I have recently received some dual FET's with Vgsoff of -11Volts! It will be interesting to see how these test vs. other types I've used.[/quote]
 
[quote author="jhaible"]Thanks for the info, jdiamantis!

That's interesting. I wasn't aware of that.
So maybe I should build all my other FET-as-VCR-applications with
BF245C instead of A,B types (FET-based Phasers, whatever.)

I wonder why the 1176 has used a FET with rather low Vgsoff then?

As I said, the info I got called for something like 2.x Volts. (Can
anybody confirm this?)"


2+ volts will work fine. I'm sure part of the changes in 1176 schematics after the A & B versions not only had to do with noise, but with linearity of the FETs, so perhaps lower Vgsoff devices could be used. This would certainly have reduced the "reject" devices if they were originally striving for higher numbes. Also, that 2 volt range would be essential to keep the threshold, and side chain gain the same so the G/R ratios would not have to be recalibrated.

You could use a FET with a Vgsoff of 1 volt, but you'd have to reduce the signal voltage into the circuit to achieve the same distortion, and G/R linearity of a higher Vgsoff unit.

If we are building from scratch, we have the luxury of selecting the optimum device for our use. :green:
 
>2+ volts will work fine. I'm sure part of the changes in 1176 schematics >after the A & B versions not only had to do with noise, but with linearity >of the FETs,

I don't have an awfully great overview of the different versions, but the extra circuit that is often referred to as "low noise mod" looks like the usual trick of adding 1/2 of the signal voltage to the CV at the gate voltage. Only that most other circuits really derive that signal directly from the drain or source, while the 1176 with LN mod uses a fraction of the amplifier output signal (part of the feedback loop). The trimmer just sets the right fraction of this voltage to get the lowest THD.
So I look at the mod as a THD reduction mod in the first place, which allows for a larger input signal, and thus improoving SNR. So for the same THD level as before, it's a low noise mod indeed. At least that's how I understand it.


>Also, that 2 volt range would be essential to keep the threshold, and side >chain gain the same so the G/R ratios would not have to be recalibrated.

Absolutely. Once you choose the gain and bias voltages, resistor divider chain etc., you have to stick to a narrow range of Vgsoff.

>If we are building from scratch, we have the luxury of selecting the >optimum device for our use.

Ok, so the real question is: Why didn't they use higher Vgsoff FETs when they built the original 1176 from scratch ??

JH.
 
>If we are building from scratch, we have the luxury of selecting the >optimum device for our use.

Ok, so the real question is: Why didn't they use higher Vgsoff FETs when they built the original 1176 from scratch ??


It looks as though they used the 2n5457, or one from that family as the FET. Compared to a FET with a higher Vgsoff, like the VCR2n, the cost differential is 10:1 or more. If they specified a higher Vgsoff with the 2n5457, it would limit the amount of usable units in any group buy.
 
pye_schem_1.jpg
 
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