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General Discussions => Drawing Board => Topic started by: Viitalahde on December 04, 2005, 05:01:39 AM

Title: Time constants in compressors
Post by: Viitalahde on December 04, 2005, 05:01:39 AM
This morning's thoughts:

I started wondering about the basic attack/release time constant circuit used in many compressors.

Code: [Select]

                   
                   R1
IN >---------/\/\/\/-------o--------o-----------> OUT
                           |        |
                           |        /
                      C1  ---       \ R2
                          ---       /
                           |        \
                           |        |
                           |        |
                           o----o---o
                                |
                                |
                               ---
                               |||
                                 


The basic circuit: R1 defines how C1 is charged, and R2 defines the dis-charge. Attack & release.

Now, without doing any maths (yet), I am wondering if there could be a sonic difference between these two circuits?

A) R1 & R2 are large, C1 = small

b) R1 & R2 are small, C1 = large

Both cases designed for same mathematical charge & dis-charge. Case A is obviously a high impedance circuit where B is a low-impedance circuit with high capacitance.

If both circuits act the same way on the paper, I suppose the driver before this could make a difference? Low-Z with high capacitance..  tough to drive.

From somewhere I remember the Fairchild 670 has a beefy (8 watts or so) sidechain driver?

I quess the ultimate question is: how do the designers end up with the R/C combinations in sidechains? They design the rectifier & driver and calculate around that, then listen & tweak?
Title: Time constants in compressors
Post by: Tekay on December 04, 2005, 07:43:33 AM
As I've been messinĀ“ around alot with the N*eve circuits I came to a point where the attack / release acts strange, if the attacktime is slow(high R1 value) and the release fast( small R2 value), C1 won't charge full as R1 / R2 acts like a voltagediver! And as the Ratio switch is before the timing circuit this kind of settings will get you lower ratio?  :?
Title: Time constants in compressors
Post by: CJ on December 04, 2005, 04:18:00 PM
You use a slide rule, fool!

(http://vacuumbrain.com/The_Lab/TA/Fairchild/Rein_Narma/es_3.jpg)

And then, you see those big ears?  :razz:
Title: Re: Time constants in compressors
Post by: bcarso on December 04, 2005, 05:15:03 PM
Quote from: "Viitalahde"
This morning's thoughts:

I started wondering about the basic attack/release time constant circuit used in many compressors.

Code: [Select]

                   
                   R1
IN >---------/\/\/\/-------o--------o-----------> OUT
                           |        |
                           |        /
                      C1  ---       \ R2
                          ---       /
                           |        \
                           |        |
                           |        |
                           o----o---o
                                |
                                |
                               ---
                               |||
                                 


The basic circuit: R1 defines how C1 is charged, and R2 defines the dis-charge. Attack & release.

Now, without doing any maths (yet), I am wondering if there could be a sonic difference between these two circuits?

A) R1 & R2 are large, C1 = small

b) R1 & R2 are small, C1 = large

Both cases designed for same mathematical charge & dis-charge. Case A is obviously a high impedance circuit where B is a low-impedance circuit with high capacitance.

If both circuits act the same way on the paper, I suppose the driver before this could make a difference? Low-Z with high capacitance..  tough to drive.

From somewhere I remember the Fairchild 670 has a beefy (8 watts or so) sidechain driver?

I quess the ultimate question is: how do the designers end up with the R/C combinations in sidechains? They design the rectifier & driver and calculate around that, then listen & tweak?


You probably know this, but for the sake of those who may not:

Before one gets too busy with the slide rule realize that the "IN" port is not a typical voltage source, but has high Z for one current polarity and low for the other.  Sometimes this is done with a diode or diode enclosed in an op amp feedback loop, sometimes with a transistor etc.

Suppose we are sensing magnitude peaks and converting these to positive voltage pulses.  So the voltage at the left of R1 charges the cap with roughly an R1*C1 time constant (R1 << R2), the attack time constant.  

When the voltage falls below the cap voltage the current from R1 ceases, for a drive circuit that acts like an ideal diode.  At this point R1 is open-circuited and C1 discharges toward ground with an R2*C2 time constant---the release time constant.

Having lower or higher impedances for the network shown will, as you acknowledge, have consequences---how hard is it to drive?---how much does the loading by the circuitry downstream of it affect things?---but if those effects are taken into account, there should be no effect on the function.  If you make the cap large you will have to use a 'lytic; too small the R's get big and leakage currents become important, and things could even get a bit noisy.
Title: Time constants in compressors
Post by: CJ on December 04, 2005, 07:25:06 PM
It starts with the music, and what you want to do to it, and what you don't want to do to it.

Artifacts are what you don't want.

Remember, the 660/670 were designed not for rock and roll.

Maybe one setting is for a cymbel crash.
Maybe another is for a vocal peak.
And another for a overall rise in volume from an orchestra.

Once you have that, then you pick some times you think might work.

Then you listen. Most of the tweaking done on the 660/670 was those time constants. The rest of the circuit is actually pretty simple.

Balanced transformer coupled audio makeup.
No caps in the signal path.
Then a balanced voltage amp.
A power supply, and those time constants.
Title: Re: Time constants in compressors
Post by: clintrubber on December 04, 2005, 07:45:16 PM
Quote from: "Viitalahde"

A) R1 & R2 are large, C1 = small

b) R1 & R2 are small, C1 = large

Both cases designed for same mathematical charge & dis-charge. Case A is obviously a high impedance circuit where B is a low-impedance circuit with high capacitance.

I quess the ultimate question is: how do the designers end up with the R/C combinations in sidechains? They design the rectifier & driver and calculate around that, then listen & tweak?

Not sure, but is you main question how R and C are chosen given certain attack & release-times (so the RC-products already known) ?
Title: Time constants in compressors
Post by: NewYorkDave on December 04, 2005, 07:54:33 PM
Quote
without doing any maths


Well, do the math and your questions will be answered. :wink:

We can spend a whole lot of words describing what can be summarized in a simple formula.

T=RC

Ask yourself: for a particular value of C, what must be the value of R in order to allow C to charge within a given time (T)?

Then it'll become obvious why compressors designed for a fast attack time use a sidechain driver amplifier with a low source impedance. And it'll also become obvious why an Altec 436C (as an example) cannot achieve very fast attack times.

Also: you have to look at the R-C network in its complete context, wrapped around an amplifier. In other words, be aware that the likelihood for unwanted feedback (motorboating) increases as you reduce the time constants. When I was doing the mod on Soundguy's 436 recently, it became apparent to me pretty quickly why the designers of the 436C chose those particular time constants: if you reduce them more than slightly, the amp motorboats.
Title: Time constants in compressors
Post by: PRR on December 05, 2005, 01:06:07 PM
> Case A is obviously a high impedance circuit where B is a low-impedance circuit with high capacitance.

This is a very real problem.

A "good" limiter will capture a sub-milliSecond peak and hold gain down for a second.

To hold gain down, you hold a negative voltage on the grids. The grids need a DC path to absorb their leakage. If this path is too high resistance, the I*R drop will cause a voltage that screws-up your control voltage.

At the other side, you have to grab enough current and charge in a milliSecond to feed the grid resistor for a whole second. The charging current can be at least 1,000 times higher than the discharge current. This can easily be a Problem.

Tubes usually specify 1Meg max grid resistor. 660/670 had eight grids to feed. 1Meg/8= 125K grid resistor. It takes -50V to get deep limiting. 50V/125K= 0.4 milliAmps grid current, which has to be maintained for a whole second. To capture this much power from a milliSecond transient, we need 0.4mA*1,000= 400mA from the rectifier and its driver. And we need it at 50V, so 400mA*50V= 20 Watts peak power, or a 10W sine-RMS amplifier.

0.1W is easy. 1W isn't too hard. 10W amps are big toys, especially in vacuum tubes.

The lesser tube limiters don't work this hard, it would be too expensive. Just two grids, and cheating the max resistance spec using 1Meg or even 2Meg for two tubes. (Not all tubes need to go to 1Meg max, just the worst-case ones. In pro limiter work, leakers would fail calibration and be replaced.) Also they don't have super-fast attack, no 1,000:1 ratio of attack to release, no 1,000:1 ratio of charge to discharge current. Also lower voltages on the tubes. -20V in 1Meg is 0.02mA, a 500:1 attack/release ratio suggests 10mA charge current, which can be tapped from a 30mA+30mA push-pull stage.

The Fairchild does better than 1,000:1, so Narma cheated the grid resistor up to about 270K(?) to reduce discharge current towards 0.2mA. With the attack times he needed for disk-cutting, this gave ~300mA @ 50V = 15 Watts peak attack power, which could be done with a pair of 6V6 and a 3:1 transformer.

OK, change the ~1Meg 1uFd discharge side to 10K 100uFd. In the old days, you could not trust a 100uFd electrolytic to leak less than a 10K resistor, but today that's fine. If you want milliSecond attack, you need a 10K/1000= 10 ohm attack resistor. If you use 20V max control voltage, you could need 20V/10Ω= 2 Amps attack current, at 20V is 40W peak 20W sin-RMS.

The attack amp never really delivers the 10W-20W long-term, only on the big peaks. In transistor work, we can rely on that to use a small heatsink. But a linear tube amp generally has to idle at fairly large current so the Fairchild's 20Wpk amp is as big and hot as any 10W sin-RMS amp.

You are almost always caught between the highest resistance your grids can tolerate, the peak power needed to meet your Attack/Release ratio goal, and your budget. The resistor values are usually FORCED on you, especially with an all-tube sidechain. Transistors change the economics at both ends. A $5 chip can replace the P-P 6V6. We have good negative-swing DC amps now, so we could use a 22Meg release resistor and buffer it down to a low impedance at the grids. We could even do the time constant at -5V and amplify it down to -50V for about $2 (plus a negative rail).
Title: Time constants in compressors
Post by: Viitalahde on December 05, 2005, 01:50:57 PM
Bcarso, Dave, PRR and the rest - thank you!

I wasn't asking about calculating the time constants T=R*C, but was just curious about the relationship between R and C. High or low R or C?

I pretty much got now - it really is a matter of the rest of the circuit & economics. Sidechain driver, the "port" the CV is dumped to.. So when you have your limiter topology and you know its charasteristics (FET, remote-cutoff triodes etc) you are given a "window" to calculate your time constants to.

When I posted my question, I did not actually think of charging currents, but it makes perfect sense. A few consumed watts on the attack period starts to raise sweat on the designers forehead. :razz:

http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=4478

This link Crusty2 posted is a good one and gives me some thoughts. The problem of a small release resistor<->big capacitor versus huge release resistor<->small capacitor is an obvious one. Huge charging currents vs. hard-to-buy potentionmeters.

Of course, in normal recording/mastering use, release times of over 1-2 seconds are rare, so it's not a real problem in this use. Looks like release is easier and attack is harder, design-wise? If the driver amplifier can't deliver all the current the attack period needs, it translates to slowed-down attack?

I wonder if anyone has used either a FET or an LDR as the release resistor? Those would offer interesting opportunieties for auto release experiments.
Title: Time constants in compressors
Post by: bcarso on December 05, 2005, 02:11:12 PM
FETs and LDRs are pretty poor-tolerance resistors in general, as especially so at high values, although matched pairs with one side dedicated to a servo could be workable, depending.  But I doubt that one would try to use them as a variable element in this application.

One circuit I've used for pulse-stretching that has the appeal of simplicity is a simple emitter follower with a diode in series with the base and the emitter going to the capacitor.  You get the beta current gain so it allows the preceding stage to run at relatively low power.  It has the disadvantage of two diode voltage drops, and the diode has to be a low-leakage type.  If you stay below the emiiter-base breakdown voltage of roughly 6 volts you can skip the diode, and the typical below-breakdown leakage of a decent bipolar is very low.

Then buffer with a JFET, or a CMOS input op amp.

For the sand-averse, some of the supertriodes like the 6H30 have peak currents in the ampere region.  Of course you wouldn't be running them class A  :razz:

Another technique for really fast capture with long hold is to cascade a fast peak detector with a slower one.
Title: Time constants in compressors
Post by: kvintus on December 05, 2005, 02:26:43 PM
Very good information!  :thumb:  :thumb:

/Anders
Title: Time constants in compressors
Post by: PRR on December 05, 2005, 06:01:51 PM
> release is easier and attack is harder, design-wise?

No. Consider if Narma had penciled-in a nice 10Meg release resistor. He could use a smaller cap(s) and much less attack power, lovely. BUT the grid current of eight grids in 10Meg would cause several volts unpredictable error in the control voltage. He could have used a cathode follwer buffer, but that adds error and needs a negative power line. Part of his solution is to run the tube grid resistance a little higher than the per-tube spec, knowing that he won't get 8 worst-case grids and if he does, the budget includes calibration which will weed-out the worst leakers. Buying a few extra mini-tubes, which can be re-sold to radio makers, was cheaper than adding another pair of 6V6 or going up to 6L6.

> raise sweat on the designers forehead.

The designer can spec any part he wants; the production engineer sweats the cost. (Obviously a designer of things that can't be produced at profit has a short career....)

Plot the cost of both ends. A milliWatt of drive power is cheap, 100mW not much more, a Watt is money and 10W may be big money. 1K to 900K release resistor is all the same, over 1Meg you may have to sort-out leakers, over 10Meg you have to find super-clean tubes or add a buffer. So you have a curve that is high at both ends but pretty flat between. If the width of the low-cost flat-spot is wider than your Attack/Release ratio, you can pick anywhere in that area. In practice, you don't get a lot of leeway, especially for a high-performance limiter. Your Release resistor will be the highest value your gain-cell (or its buffer) can stand, and you put money onto the drive amp until you meet your Attack spec. In some cases this is negligible added cost, especially for moderate Attack speed: you can tap enough audio power from an existing stage to do it. And in other cases you just go overboard, like the Fairchild and the big GE.

Using JFET attenuators does raise the allowable grid resistance; often you can't readily buy a resistor big enough to upset the FET. 22Meg is often no problem. 100Meg might reduce cap size required Attack current even more, but 100Meg resistors cost more. And adjustable 100Meg pots don't exist (which is where Crusty's link goes; for a lousy 1Meg audio, I'd just look in guitar amp supply sites, or fudge to 500K, but for higher resistances that levered resistor idea may be useful).

Using transistor drivers or buffers sure reduces costs, but also spoils us: we expect low cost.
Title: Time constants in compressors
Post by: TedF on December 06, 2005, 06:59:36 AM
The original question does remind me of some original thinking about attack and release times and shapes from many years ago.... It's true that there can be a conflict; my method of overcoming a possible inflexibility was to introduce a 2 or even multi-step circuit, where a second R/C combination sits on top of the main one. The attack time of the upper small capacitor can be extremely short and doesn't need high current to fill it. This arrangement also provides a form of auto release... for transients the upper capacitor charges and discharges rapidly, for normal signals, the main capacitor charges, and discharges slowly.

The character of the sound of a compressor is very much a combination of the attack and release times and shapes, and the distortion characteristics of the audio during the compression process.
Title: Time constants in compressors
Post by: Viitalahde on December 06, 2005, 07:26:13 AM
Ted, my next question was going to be about these R/C stacks (auto release).  

I quess the basic theory is that the smaller capacitor on the top reacts to smaller transients while the lower one doesn't really react (charge) at all? Then the big hairy drummer does his tom fill and that makes the big caps work too.  :twisted:
Title: Time constants in compressors
Post by: TedF on December 06, 2005, 08:41:40 AM
I couldn't have put it better! :cool:
Title: Time constants in compressors
Post by: Svart on December 06, 2005, 09:23:35 AM
great! i was wondering about parallelling those circuits..
Title: Time constants in compressors
Post by: TedF on December 06, 2005, 09:48:07 AM
It's 'seriesing' them really!
Just stand one of them on top of the other; and then for extra interest, alter the standing voltage of the bottom 'ground' connection, or even make it dynamic, changing with the musical content.... but then it's starting to get complicated!
The Audio and Design F600 compressor (circa 1972) was the first one where I saw the 'auto release' circuit, but I believe that its origins are earlier than that.
Title: Time constants in compressors
Post by: NewYorkDave on December 06, 2005, 09:48:42 AM
Here are some small excerpts from literature that references dual time-constant sidechains in compressors. I cannot post more than these excerpts because the works are covered by copyright.

PDF (http://groupdiy.twin-x.com/albums/userpics/10031/dualtimeconstant.pdf)
Title: Time constants in compressors
Post by: Svart on December 06, 2005, 10:29:37 AM
series.. ok for some reason i envisioned them as parallel.

If they are series, which one should come first, the transient stage or the low freq stage?

I guess we could look at the high freq stage as peak detection and the low freq as more of an RMS detection?

do you sum the output from each stage and how do you do it, through simple resistor summing?

EDIT: NYD that is great stuff!  

time for a groupDIY discrete compressor I say!


In anyone's opinion, If I were to use a JFET as the level modifying device, would it be better to use the FET in series with the signal allowing it to flow through the FET or shorting the audio to ground via the FET?

I forsee the general issues with allowing the audio to flow through the FET but how much of a problem do you think it really poses?  I've seen many large name consoles use FETs as mute elements, so would you say that the issues involved are worse as less than a full gate ON situation?
Title: Time constants in compressors
Post by: TedF on December 06, 2005, 11:00:26 AM
Hi Svart, I'm juming in on the FET discussion....
Operating the FET as a series element is difficult; there are problems caused by both the drain and the source operating at the audio voltage, and with wildly varying impedances as the compression acts.
Using a FET as a shunt device is relatively easy; as long as you can 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.  This can be overcome to some extent by applying a degree of AC to the gate, but there is still a significant non-linearity left that shows itself as a quite nasty 3rd order distortion.  The 1176 puts up with the distortion, as do the old Audio and Design F600 and F760 series.
But what's wrong with VCAs? :twisted: (joke)
Title: Time constants in compressors
Post by: Svart 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?
Title: Time constants in compressors
Post by: PRR 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.)
Title: Time constants in compressors
Post by: PRR 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.
Title: Time constants in compressors
Post by: TedF 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:
Title: Time constants in compressors
Post by: Svart 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:
Title: Time constants in compressors
Post by: TedF 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:
Title: Time constants in compressors
Post by: Svart 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..
Title: Time constants in compressors
Post by: TedF 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.
Title: Time constants in compressors
Post by: alk509 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.
Title: Time constants in compressors
Post by: Viitalahde 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:
Title: Time constants in compressors
Post by: PRR 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 (http://www.semiconductors.philips.com/acrobat/datasheets/PMBFJ620_1.pdf)

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.)
Title: Time constants in compressors
Post by: rafafredd on December 08, 2005, 06:15:48 AM
Quote
A hasty sim says it don't suck


Can you post a schem PRR?
Title: Time constants in compressors
Post by: PRR on December 09, 2005, 01:25:02 AM
> post a schem

The very basic tube gaincell, only with JFETs:
(http://headfonz.rutgers.edu/FET-comp.gif)
Title: Time constants in compressors
Post by: aurt 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.
Title: Time constants in compressors
Post by: PRR 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.
Title: Time constants in compressors
Post by: Kingston 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?
Title: Time constants in compressors
Post by: ulysses 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.
Title: Time constants in compressors
Post by: CJ on March 13, 2006, 07:49:31 PM
Whats the latest on the new 170, didi anyone get samples yet?
Title: Time constants in compressors
Post by: JustinS 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)
Title: Time constants in compressors
Post by: BradAvenson 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.
Title: Time constants in compressors
Post by: ulysses on March 17, 2006, 04:53:12 AM
Every time I've called them, I think I've talked to the same guy.  Very helpful, and very willing to work with the little guys.  I think they're little guys too, as far as semiconductor manufacturers go.  I bought 100 pieces of the TO-92 LSK170, and used some in an OEM module alongside some Toshibas without incident.  The Toshibas aren't any more expensive or difficult to get yet, but they might will be some day, so the good 2nd source is... good, I guess.  I've also dicked around with some samples they sent me of the surface mount LSK170, and also the various forms of the LSK389 which is finally available.  Kind of expensive, but if you consider the time it takes to match singles (time is a production cost), I think it's worthwhile.  The original 2SK389 is getting hard to find, and expensive lately (since they haven't been made in like a decade and a half or something).
Title: Time constants in compressors
Post by: bcarso on March 17, 2006, 07:13:06 AM
Justin, do you have any data on the real offset voltage distribution of the SK389s?  I suspect they are much better than the listed spec.
Title: Time constants in compressors
Post by: ulysses on March 17, 2006, 01:35:35 PM
I've only used about five 389s so far, in prototypes.  They seemed to be about as close as the 170 pairs I hand matched, maybe better.  But it's hard to tell because the mismatch in the input pair is not the most significant source of offset in my particular circuit.  The only way for me to be more sure would be to install a 389, measure it, remove it, install it backwards, and measure it again.  The surface-mount packages do allow this (the original through-hole 2SK389 doesn't really); but I'm still squeamish enough about soldering SMT chips that I haven't been willing to try it.  Maybe once I get some more appropriate tools and more practice, I'll give it a try.
Title: Time constants in compressors
Post by: bcarso on March 17, 2006, 05:54:19 PM
Reason I ask is I would like to use them in a d.c.-coupled woofer EQ.  One can always put in a trim adjust for offset voltages, but when much trim is required the temp drift is usually not so good.

What I'm probably going to do instead is use one half of a dual as a temp sensor and heater, and bias the other in-circuit device close to its zero-tempco operating point, which for these low threshold devices is close to zero Vgs.