Svart

Time constants in compressors
« Reply #20 on: December 06, 2005, 11:56:00 AM »
oh nothing wrong with VCAs!  i was just wondering about the JFETs as I kinda have a preference for them in other audio areas.  I do like my 1176s a lot and a simplified FET compressor is almost too good for me to ignore!

3rd order distortion is definately a negative aspect of this.. surely there is something to be done about this?

EDIT:

it is my feeling that well designed detection circuit can be scaled to work with most any modifying element..  So should we focus on creating a simple sidechain first and then we can try different elements for the actual attentuation?
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
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PRR

Time constants in compressors
« Reply #21 on: December 07, 2005, 01:46:21 AM »
> use the FET in series with the signal allowing it to flow through the FET or shorting the audio to ground via the FET?

The voltage across the FET for low distortion is very small, say 50 milliVolts.

The level into a limiter is ideally unlimited, and in practice may peak 20 or 30 dB above the limiting level, typically many volts. If you attenuate the input so a hot source can't overdrive the FET, the nominal levels are very low, near noise level.

> put up with the high levels of distortion caused by the difference between the ac potential of the drain (or is it source.... I can never remember) and the gate.

The gate voltage must be compared to the middle of the channel, not just to one end. If your gate voltage is much-much larger than your signal voltage, that may not matter. If your gate voltage were say "-1V" and the two ends of the channel were at 0V and +2V, the effective gate voltage would be -2V; when signal reversed to 0V and -2V it would be zero V. If the gate/gain control voltage should be -1V but varies from -2V to 0V in the course of the cycle, that wave will be very bent. Smooshed on one side and tall on the other. 2nd-order distortion. You can push-pull and get complicated; it actually (for once) works just as well to throw half the signal voltage onto the gate (does about the same thing). That works to kill the gross 2nd-order distortion; the residual high-order products and the inevitable fall-to-pieces at high level still remain. (My impression is that they are not huge, up to that point; but hugeness is in the ear of the beholder....)

Note also that some THD, even 3rd, at and above threshold is not necessarily bad. Limiting the voltage reduces musical dynamics. If it is not just sloppy playing, you may want the dynamics, just not the high levels. A dash of distortion can give the impression of "louder" without actual higher voltages, lessening the squashing effect. The difference between loud shouting and VERY loud shouting is often not a matter of voice power, but voice timbre. Taken to extreme, this explains why the crappy virgin LevelLock in my closet is worth more now than it was back when. It "screams" without clipping the tape/CD.

> use FETs as mute elements

That works because the FET is driven hard-off, and so fast that we don't hear the distortion as it goes from on to off. if we leave the FET part-on, the distortion will be gross.

The only way is to put the FET to ground so that it never sees any more than the limited signal. Of course for that to work, the series resistor must be larger than the FET ON-resistance by the amount of limiting. If the FET can be pushed down to 1,000 ohms, and we want 40dB of excess input, we need a 100K series resistor. When not limiting, this is our noise source. The noise of 100K is about 4uV. Add a uV for post-amp noise, 5uV. Maximum level at this point is the 50mV or 50,000uV that the FET will stand before folding. Maximum signal to noise ratio just below threshold is then 50,000uV/5uV or 80dB. This was ample for tape and LP, may seem small for digi-media. We can fiddle the numbers to look better, but there is no huge improvement with classic FETs. (I have not had a hard look at the super-JETs; my suspicion is that the reduction in ON resistance is matched by a reduction in voltage before distortion, so little or no net gain. One of the FET boys may prove me wrong.) We do know that many people use and love 1176 type FET limiters, and even LevelLock which lacks basic THD-reduction, so a super S/N number must not be the most important thing in life. And the FET limiter is the only great $1 gain-cell. LDRs are fun and natural, but annoyingly inconsistent, proper VCAs cost at least a couple bucks (thank you, THAT!), and a twin-tube gain cell runs closer to $50 even with dubious transformers.

> So should we focus on creating a simple sidechain first and then we can try different elements for the actual attenuation?

No because....

> well designed detection circuit can be scaled to work with most any modifying element..

Aside from the gross differences (positive or negative, milliVolts or tens of volts), every gain-cell has a different control law.

There are BJT cells that are straight linear.

There are proportions for LDRs and tube cathode followers that come close to straight linear over a limited range.

There are BJT cells that are perfectly exponential V/dB.

There are tube cells that are sloppy exponential V/dB with a broad soft bend right where low-pressure engineers may spend a lot of time.

Yes, you can add or take-away an exponential in the sidechain. But this looks to me like a long muddy road. It may be better strategy to pick a path, any open path, and slog along until you reach a happy answer or are forced to abandon it. Certainly we have a big map of limiter-paths that are not dead-ends.

And I think some of the "sound" of some types, aside from the time-constants and residual THD that Tedf discusses, is the "imperfect" control law. While the very-Mu scheme is frustrating if you want a brick-wall limiter, it has a very natural shape that can be easy on the ear, built right into the shape of the vacuum. The BJT gain-cells have that wonderfully exact control law that you can shape as you please.

Also the choice of linear or exponential dictates your time-constant choices. The exponential laws fight the exponential decay of an R-C or I-C network. DBX had an active capacitor that worked inside an exponential sidechain, but I've stared at it for hours and think it is screwy (I don't argue with the results, just baffled by the circuit).

If you think feedforward is always better, control law is vital. Yes, you can trim an imprecise law to give a tolerably constant or curved slope over 10dB or 20dB, but ultimately an FF limiter needs detector and gain-cell very well matched over many dB, which discourages the "sloppy" control laws on LDR, FET, and very-Mu in heavy-limiting and graceful gross-overload applications.

> (circa 1972) was the first one where I saw the 'auto release' circuit, but I believe that its origins are earlier than that.

I first noticed it far later, but it is present in the Fairchild 660 and as NYDave says it was written up long before.

While early claims mention reducing charge/discharge current ratio, it does not give the same result as a single R-C with a honkin-big driver. It does give a very musical result, though "auto" is surely an exaggeration (Narma gives two dual-RC positions, so even with two RCs no single pair of time constants was truly "auto" enough for him, and maybe not for all of us.)

> alter the standing voltage of the bottom 'ground' connection, or even make it dynamic

Yeah, there are a zillion frills and most of them have been tried. "Dynamic" release can include going into Hold for a short time after a big transient, or going into Hold if the current level drops low; these tricks "fix" specific types of pumping, though may also do The Wrong Thing for some program material. (I once watched an old movie on TV that seemed to have been designed to trick the station's level controller every way possible.)

PRR

Time constants in compressors
« Reply #22 on: December 07, 2005, 02:12:03 AM »
> I do like my 1176s a lot and a simplified FET compressor is almost too good for me to ignore!

How about the Shure LevelLock? Its bigger faults can be fixed. Time-constant is the RC at point C and can be elaborated. 2nd harmonic cancellation can be added. The "distance" control is bogus, the in/out impedances are too. You'd need a line-amp and output interface. This and everything except Q6 Q7 could be JFETs or MOSFETs.

As for liking FETs in other uses: just because a rock is a good hammer does not mean it must be a good saw. The way an FET works in the limiter attenuator is not at all the way FETs work in amplifiers. It is sweet, but not because FETs are sweet amplifiers.

TedF

Time constants in compressors
« Reply #23 on: December 07, 2005, 05:23:57 AM »
My joke comment about VCAs was almost serious.... If you are looking for an inexpensive element for compression, for a project, then surely the VCA is ideal; it is predictable, well documented and countless designers have tried to use it as a compression element, made false suppositions believing the art of dynamics is simple, and mangled the audio industry in the process. :shock:
Some application of new thinking, plus some good old 'suck it and see' technology is called for here..... and isn't that better than trying to use a device that was never meant to be a linear attenuator (the FET), and really doesn't do the job very well?
 :wink:

Svart

Time constants in compressors
« Reply #24 on: December 07, 2005, 07:47:32 AM »
the FET is God.

no discussion.



 :green:

OK in all seriousness.. PRR your explaination and discussion was educational and appreciated once again!  TedF, I do like the VCA too but i have a rack full of VCA compressors and only a few others types..


PRR, I'm still trying to get to a point to proto up the new version of the Varimu that we were discussing.  I REALLY like that compressor on an overall mix  and I NEED another one for cymbals/overheads because the natural compression curve you talk of sounds wonderful.  Your point of FETs being good at one thing but maybe not another is duly noted, I was just making a broad statement in hopes of clarification, which was fully satisfied.

Thanks everyone!

 :guinness:
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

TedF

Time constants in compressors
« Reply #25 on: December 07, 2005, 08:54:03 AM »
OK Svart,
I give in!
Try your FETs, then get back to real compressors.... optical ones! :grin:

Svart

Time constants in compressors
« Reply #26 on: December 07, 2005, 09:01:16 AM »
TedF do you use LDRs?  Do you match them?  or have you figured out another opto-part to use?  I wonder if transistor or diode based optocouplers have been researched..
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

TedF

Time constants in compressors
« Reply #27 on: December 07, 2005, 09:25:58 AM »
Yes, I use conventional LDRs.... not ORP12s, they are on the slow side, although they have the lovely non-linear rise and fall characteristics.  The ones I use are Protech MLG5527. They have a faster attack time, but the penalty is a high percentage of rejects, and the need to run a simple matching process.  They are not really suitable for simple compressors because the resistance change range is not as great as the old ORP12.

I prefer to use discrete LDRs rather than the packaged types, it gives me  freedom to vary LED types and distances.

Yes, I have looked at all sorts of devices, but always returned to the LDR; it's a perfect choice as so much of the complex attack and release work is done for you.

alk509

Time constants in compressors
« Reply #28 on: December 07, 2005, 12:26:46 PM »
Every time PRR opens his mouth, it's like being beaten with a learning club! Thank you, Paul! If I ever get a chance to buy you a drink, I'll buy you two. You truly are the proverbial "man". :thumb: :sam:

Peace,
Al.

Lest laziness get the best of you!

Viitalahde

Time constants in compressors
« Reply #29 on: December 08, 2005, 01:30:00 AM »
Quote from: "alk509"
Every time PRR opens his mouth, it's like being beaten with a learning club!


Agreed.  :thumb: Thank you (also the others!) and you have another set-o-pints waiting in Finland.

 :guinness:  :guinness:  :guinness:  :guinness:  :guinness:  :guinness:


PRR

Time constants in compressors
« Reply #30 on: December 08, 2005, 02:33:27 AM »
> VCA is ideal; it is predictable, well documented and countless designers have tried.....

Agree. When VCAs were VERY expensive and just beginning to get good, other schemes were interesting. Nowadays, they are "the obvious choice".

> the FET is God

It is a handy tool. One disgusting problem is that JFETs are going out of style in mega-production, so there are not that many types in-stock at distributors. One of my old reference handy-FETs is no longer stocked at DigiKey.

I have a half-baked thought, so I will put this link here while I think:
Philips PMBFJ620 is a dual-JFET that is in recent production and stocked by DigiKey at $0.75. datasheet

What happens if you port the twin-tube vari-Mu gain-cell idea to FETs? I don't know. A hasty sim says it don't suck, and the practical logistics are much better than tubes. I'm not sure I trust my sim: I'm sure it "works", but maybe not as good as it sims. Yet down to a point, the sim model "must" be correct. Very low currents may not correctly model the knee in an FET's Vgs/Gm plot; but that may not matter. It is clear that the JFETs won't have the soft slope near zero grid bias that most tube limiters have, though we can soften the sidechain and maybe fake it close enough for government-work. Especially if the FET is God.

LDRs are a ton of fun too. Impossible to characterize so you can design blindly, and too inconsistent to plug-n-play without testing and sorting-out, but pretty magical musical devices. And they can be VERY simple, or as complicated as you can stand. (Just don't ask one to protect your disk-cutter.)

rafafredd

Time constants in compressors
« Reply #31 on: December 08, 2005, 06:15:48 AM »
Quote
A hasty sim says it don't suck


Can you post a schem PRR?

PRR

Time constants in compressors
« Reply #32 on: December 09, 2005, 01:25:02 AM »
> post a schem

The very basic tube gaincell, only with JFETs:

aurt

Time constants in compressors
« Reply #33 on: December 09, 2005, 08:47:21 PM »
Quote
What happens if you port the twin-tube vari-Mu gain-cell idea to FETs?


I've wondered for a while if you could make remote cutoff FETs. Specifically, would varying the depth/length of the channel across the width produce a response similar to a remote cutoff pentode? It seems to me that the bias would turn more or less of the channel on or off, giving a variable charachteristic. I'm probably off my rocker, but it's been niggling at me.
"Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read." Groucho Marx

PRR

Time constants in compressors
« Reply #34 on: December 09, 2005, 09:02:39 PM »
> I've wondered for a while if you could make remote cutoff FETs.

I'm pretty sure I can't make FETs, remote or other, at home on the kitchen stove.

But I know there are folks with access to semi-labs, and a fat little audio FET is not a very complicated gizmo.

Off the top of my head (or rocker), I guess a tapered gate electrode would be similar to a variable-pitch grid winding, but I'm not sure the geometry really makes sense. AFAIK, production FETs all follow the same law so well that it isn't necessary to specify more than Vp and Idss; all else can be calculated from those two points.

Kingston

Time constants in compressors
« Reply #35 on: March 13, 2006, 01:31:44 PM »
Apologies for digging up an old thread, but this FET version of the PRR vari-mu compressor looks extremely interesting.

Did anyone ever get it past the simulation stage, or design something similar?

If so, how was the sound and general characteristics?

ulysses

    Mississippi River, halfway between the North Pole and the Equator
  • Posts: 113
Time constants in compressors
« Reply #36 on: March 13, 2006, 07:28:01 PM »
I'm glad you did dig it up, or I never would have seen it.  Interesting stuff.

PRR, I'm curious about the dual FET you mentioned, the PMBFJ620.  Certainly is inexpensive, but the noise figure looks rather high compared to, say, the 2SK389 or now the LSK389.  But it can be tough to compare noise specs on the data sheets when they tend to be specified differently, under a different phase of the moon, and so on.  Have you used these guys and found them to be reasonably low-noise?  Well-matched?  Promising as an input pair for a discrete op amp?  The little SMT package would put a lot of this crowd off, but it looks like fun to me.
Justin Ulysses Morse
Roll Music Systems
Minneapolis, MN

www.rollmusic.com
Think outside the box.

CJ

Time constants in compressors
« Reply #37 on: March 13, 2006, 07:49:31 PM »
Whats the latest on the new 170, didi anyone get samples yet?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
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JustinS

Time constants in compressors
« Reply #38 on: March 13, 2006, 10:57:30 PM »
I filled out their online form for a set of samples and got an automated reply... but so far... nada

Justin.

(It was only 3 weeks or so ago though)

BradAvenson

Time constants in compressors
« Reply #39 on: March 13, 2006, 11:59:06 PM »
Quote from: "CJ"
Whats the latest on the new 170, didi anyone get samples yet?


I got a few of the LSK170's and they seem to work just like the toshiba's in my DI box.  I couldn't measure any difference in that circuit.  

I had better responce for samples by making a quick call.  (EDIT: may be costly from overseas.)  Seemed like a nice bunch over there.


 

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