Author Topic: Valve Wizard (merlinb) "Precision Compressor"  (Read 3251 times)

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highwater

Valve Wizard (merlinb) "Precision Compressor"
« on: February 05, 2015, 10:02:22 PM »
While searching for something else, I came across this topic, wherein the apparent predecessor to the Engineer's Thumb is discussed, but all the schematic images are dead links. I've tried to search both this site and the internet as a whole for them, and can only find links to that thread, a few remarks that the ET is far superior and should be built instead, and a myriad of valve-spring compressors for automotive purposes. Also, one partial snippet suggesting how to attach an LED array as a compression-meter, which is actually the only reason I was able to determine the name of the design.

Does anyone still have a copy the schematic? Is there a specific reason why it has been removed, aside from the superiority of it's successor? I can certainly understand why it has been deprecated in favor of the ET - from what I've read, it was JFET-based and even more finicky to bias than the Orange Squeezer. I'd very much like to see it, though - if there's one thing I've learned, it'd be that there's more to be learned from the evolution of a design than from the design itself... also, that anything inferior has the mojo nature whether-or-not that mojo is worth anything.

merlinb

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #1 on: February 06, 2015, 08:42:19 AM »
The ET didn't evolve from the precision comp, they are totally different. The precision comp was more of an experiment in JFET compression, and it was 'just OK'. The ET, by contrast, does everything, better than everything else.
Frankly I didn't want people building a 'just OK' circuit and being disappointed, since the blame would lie with me. Not when they could spand the same money and have an ET that will be a joy forever. So I cut my losses and buried the old FET circuit. I might have a copy somewhere, if you really want to see it.

samhay

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #2 on: February 06, 2015, 09:59:59 AM »
The side chain sounds interesting....
I'm a refugee of the great dropbox purge of '17.
Project details (schematics, layouts, etc) are slowly being added here: http://samdump.wordpress.com

merlinb

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #3 on: February 06, 2015, 02:36:09 PM »
Found it:

highwater

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #4 on: February 07, 2015, 01:41:55 AM »
Awesome, thank you! I see what you mean about it not being worth the effort (complexity-wise - I'll take your word on the inferior performance), but it really shows how elegant of a design the ET is. This looks more like a mutant, feed-forward Orange Squeezer with an ridiculously-complicated envelope detector.

StephenGiles

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #5 on: February 07, 2015, 03:00:23 AM »
Not that complicated really if you look at a studio compressor circuit that is half decent.
Sometimes it is the people no one imagines anything of who do the things that no one can imagine.

samhay

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #6 on: February 07, 2015, 05:57:55 AM »
Interesting - thanks Merlin.
I'm with Stephen - the side chain is not overly complicated, and has some cool features. The threshold controls the gain of U1a and U1b in a similar way to the 'Bluesbreaker' gain control. U1b does a sort-of-log amplification, which I guess gives the compression a better feel. U1c is the envelope detector and is somewhat similar to that used in the ET - Merlin, any comment on the relative merits of the differences in the envelope detectors?
« Last Edit: February 07, 2015, 06:25:00 AM by samhay »
I'm a refugee of the great dropbox purge of '17.
Project details (schematics, layouts, etc) are slowly being added here: http://samdump.wordpress.com

PRR

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #7 on: February 07, 2015, 03:49:11 PM »
The intriguing design decision is the 1K into U1d. The FET works off a node impedance of 909 Ohms. Since we struggle to get FET channel resistance down below 1K, this doesn't allow a lot of gain reduction. (It may be to keep the signal level across the FET low.)

Although the specified J112 will go down to 50 Ohms, which is pretty low.

A more classic approach would make U1d an infinite input NI stage. The same gain-control would be possible.

I'm also looking at the AC-coupling between U1b and U1c, wondering what happens with erratic "real" wave-forms.

I sure do not think the side-chain is "complicated".
« Last Edit: February 07, 2015, 03:52:18 PM by PRR »

merlinb

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #8 on: February 08, 2015, 06:01:51 AM »
Quote
A more classic approach would make U1d an infinite input NI stage. The same gain-control would be possible.
The trouble with that approach is that you have to keep the voltage across the FET below 1Vpp or it will distort. The purpose if tapping it off the input resistance of an inverting amplifier is that it guarentees the voltage across the FET will be less than 1Vpp for all input voltages up to the limit of the preceding opamp.

For reference, this is what I originally posted about the side chain (four years ago!):
U1b is a pseudo-logarithmic amplifier. It conditions the signal prior to rectification. Because of the non-linear characteristics of the FET I found it necessary to add the log amp so as to get a soft knee on the compression characteristic. Actually it does more than one job: it provides gain so the threshold can be adjusted, shapes the control signal, prevents large negative swings from hitting the rectifier (which would otherwise cause it to hit the rail and invert, which is something TL07x opamps do), and it provides a convenient way to filter the control signal if desired- hence the treb/bass/normal switch.

It was an interesting experiment, but like I say, the ET is a far better comp and vey easy to tweak.
« Last Edit: February 08, 2015, 06:04:14 AM by merlinb »

Aslem

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #9 on: November 30, 2017, 09:14:54 AM »
Hello guys, anybody built this compressor and it worked? I want built...

vigilante397

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #10 on: November 30, 2017, 02:39:15 PM »
Hello guys, anybody built this compressor and it worked? I want built...

Welcome to the forum :)

As Merlin mentioned, you will probably be much happier with the Engineer's Thumb compressor. There's A LOT of information about it in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=97123.0

Good luck!
"I'm not sure what "serious design flaws" you see. Does it explode or poison your dog?" - PRR

"Tayda's Whipping Boy"

http://www.sushiboxfx.com

Transmogrifox

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #11 on: December 01, 2017, 11:53:32 PM »
The value in that circuit is pedagogical :)

If you take time to study the circuit and try to guess at the design considerations put into making it what it is, you learn a lot.  Who cares if it actually sounds good. Who builds pedals because they sound good?  If I want a good guitar sound I'll listen to somebody who has it in his fingers.
trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.

Transmogrifox

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #12 on: December 01, 2017, 11:57:15 PM »
Who builds pedals because they sound good?.
Admittedly tongue-in-cheek

If I want a good guitar sound I'll listen to somebody who has it in his fingers
More truth than joke here...
trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.

Aslem

Re: Valve Wizard (merlinb) "Precision Compressor"
« Reply #13 on: December 06, 2017, 06:50:55 AM »
Hello guys, anybody built this compressor and it worked? I want built...

Welcome to the forum :)

As Merlin mentioned, you will probably be much happier with the Engineer's Thumb compressor. There's A LOT of information about it in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=97123.0

Good luck!

Thanks bro, I will test with voice and instruments,i wanted to build some home studio equipment,