opto compressors

[SPAZ]

Where could I find some good stompbox opto compressor schematics?

[Sweetalk]

You have the DOD 280 compressor, in Tonepad it's the layout and the schematic... I didn't tried yet, but take a look at it...

[SPAZ]

Never played through the 280. Has anyone got a tone report ?

[oldschoolanalog]

Where could I find some good stompbox opto compressor schematics?

Layouts gallery. First UTSF. Lots of choices.

[SPAZ]

Thanks guys, I'll check it out.

[markm]

Check out the Flatline Compressor, it has to be the best Optical Comp around IMO. 

[cab42]

Mark, you beat me with app. 10 seconds 

There is also John Hollis' Flatline. You can find a layout (incl. schematic) for that in the layout gallery.

Regards

Carsten

[MartyMart]

Flatline is simple and works great, recommended 100%

MM.

[SPAZ]

On a scale of 1-10 how transparent is it ? Or say compared to the diamond. Thats about the best opto I've played through.

[markm]

On a scale of 1-10 how transparent is it ? Or say compared to the diamond. Thats about the best opto I've played through.

I've never heard nor played through the Diamond but, The Flatline certainly is what I would consider "transparent".
Marty?
Whatdya think?

[SPAZ]

I never until last weekend in a studio. I was absolutely flattened, no pun intended. They are made in Canada.

[MartyMart]

On a scale of 1-10 how transparent is it ? Or say compared to the diamond. Thats about the best opto I've played through.

I've never heard nor played through the Diamond but, The Flatline certainly is what I would consider "transparent".
Marty?
Whatdya think?

I used to use the word "transparent" but now it seems to be a " no no " along with "flabby bottom end" "creamy" and
several other "choice" visual descrriptions of       " S O U N D "
The word police start whole threads about  " Dude what do you mean by the word smooth "

IMO all those words are vital to "descriptions" of sound and their meaning is obvious, even to the complete noob.

(  Sorry  .... rough morning here .... more coffee )

YES I THINK THE FLATLINE IS TOTALLY TRANSPARENT   !!!!

It compresses very well, you will hardly know it's there until you hit a  B I G chord and it doesn't change your tone one bit  :D

It also can serve as a pretty MEAN booster too  !!  ( MEAN -- verb/descriptive -- slang for "Loud' or "Menacing" / large volume increase / valves will like it )

MM

[scaesic]

On a scale of 1-10 how transparent is it ? Or say compared to the diamond. Thats about the best opto I've played through.

I've never heard nor played through the Diamond but, The Flatline certainly is what I would consider "transparent".
Marty?
Whatdya think?

I used to use the word "transparent" but now it seems to be a " no no " along with "flabby bottom end" "creamy" and
several other "choice" visual descrriptions of       " S O U N D "
The word police start whole threads about  " Dude what do you mean by the word smooth "

IMO all those words are vital to "descriptions" of sound and their meaning is obvious, even to the complete noob.

(  Sorry  .... rough morning here .... more coffee )

YES I THINK THE FLATLINE IS TOTALLY TRANSPARENT   !!!!

It compresses very well, you will hardly know it's there until you hit a  B I G chord and it doesn't change your tone one bit  :D

It also can serve as a pretty MEAN booster too  !!  ( MEAN -- verb/descriptive -- slang for "Loud' or "Menacing" / large volume increase / valves will like it )

MM

"colouring" isn't a choice word to describe sound, it's a self evident term meaning what you get in is changed tonally. Transparent follows on from this, "transparent" means it has a "flat" response, and does not colour the signal.

thats just how i feel about those terms.

"flabby" and "warm" are much more subjective.

[MartyMart]

"colouring" isn't a choice word to describe sound, it's a self evident term meaning what you get in is changed tonally. Transparent follows on from this, "transparent" means it has a "flat" response, and does not colour the signal.

thats just how i feel about those terms.

"flabby" and "warm" are much more subjective.

Flabby = Too much bottom end = "Mush"
Warm = reduced top end = rounded tone like a "Creamy" tube amp

Easy, how can that mean anything else ? 

MM

[petemoore]

I used to use the word "transparent" but now it seems to be a " no no " along with "flabby bottom end" "creamy" and
several other "choice" visual descrriptions of       " S O U N D "
The word police start whole threads about  " Dude what do you mean by the word smooth "

IMO all those words are vital to "descriptions" of sound and their meaning is obvious, even to the complete noob.
  Grains of salt...
  Soundclips can do pretty good, pics not so much...at describing 'thing's/sounds around here.
  Other than that...Words are what we have!!
  *Describe it in detail, without the use of *descriptive words.
  Words can be mis-placed, used inappropriately or inaccurately, but still convey messages that only they can convey.
  They are sometimes messy, un-clinical, I find it easier to read and write when words are used, limiting them to non-descriptive types is...limiting IMO.
 
 

[petemoore]

IMO all those words are vital to "descriptions" of sound and their meaning is obvious, even to the complete noob.
 and/or especially for the complete newb.
 Flabby = Too much bottom end = "Mush"
Warm = reduced top end = rounded tone like a "Creamy" tube amp

Easy, how can that mean anything else ?  
 Grain of salt, dude...please don't feel
 Besides soundclips, Words are what we have.  I suggest: use words but sparingly/appropriately.

[Mark Hammer]

The Anderton EPFM compressor, and its cousin the M.A.P. Whisper (co-designed by Jack Orman and Thomas Henry) are also decent optical compressors.

I might also add the WHAT and Joe Cheep compressors ( http://dt.prohosting.com/hacks/what.html ) to the list if they hasn't been added already.

The hypothetical advantage of optical compressors is that:

• LDRs are not directly subject to distortion
• LDRs themselves do not add that much noise
• LDR have a certain lag time built in which can further reduce any residual envelope ripple

In principle, this means they would be the basis of a quieter form of compression with no coloration stemming from some of the various sources of distortion.  Before "optical" is treated as a panacea, however, consider that:
1) There are other potential sources of distortion and noise in many compressors whic LDRs might not add to, but certainly don't subtract from.
2) Optical elements vary widely in their speed of response, and absence of coloration can sometimes be a function of using one that responds VERY fast.

So, consider the "Joe Cheep" ( http://dt.prohosting.com/hacks/joecheep_schem.gif ).  Here, the audio signal encounters one fixed resistor between input and output jacks.  An LDR is used as a second resistance to ground, forming a voltage divider in tandem with the fixed resistor.  That's it.  Doesn't get much purer than that, or closer to the idea of a compressor as "automatic volume pot" either.  All the rest of the circuitry is level detection, support circuitry for indicator LEDs, etc.  The LDR (vactrol, actually) is selected for speed, and the envelope follower/rectifier is designed around the properties of that LDR.  Does it add any distortion?  Nah.  Does it add any noise?  If you can find it then you can afford the sort of measurement equipment generally not owned by DIY types.  Of course, since it is ONLY an attenuator, that means output level will always be lower than input level so you have to provide the input/output gain stages, etc., and THOSE might yield some distortion or noise if not designed right.  But apart from that, this comes about as close tothe ideal optical compressor as you can get.

The Craig Anderton compressor from EPFM II is also a pretty decent compressor, but note the following:
1) The signal passes through 2 op-amp gain stages, and the LDR simply piggybacks with an existing resistor to set the gain of one of those op-amps.  Whatever noise and or distortion is inherent in the op-amps themselves, and the surrounding passive input/output components, will not be reduced by the LDR.
2) The "recommended" optoisolator uses an LDR (CLM6000) with a quick-ish rise time (3msec from cold start to min resistance) and sluggish (500msec to return to max resistance) fall time.  It (the LDR) does NOT distort, in the pure sense of the word, but the sort of in-and-out-quick response typical of so-called "transparent" compressors is not going to happen because the design relies on something with a naturally slow gain recovery rate.  Guitar players like this because it maps onto the decay properties of guitar strings quite nicely, and also reduces the audible envelope ripple that often comes during that decay phase, but it may well be unsuitable for other applications without making them sound "colored" and "unnatural".

The WHAT compressor ( http://dt.prohosting.com/hacks/what2.gif ) uses the same general approach as the Joe Cheep, but provides all the necessary buffering and gain recovery before and after the optical voltage divider.  So, sort of an amalgam of the Joe Cheep and EPFM.

So, here we have three different optical compressors, all of which sound decent from all reports, but clearly have somewhat different properties, and are optimized for different situations.

In the same way that a multi-stage solid-state amp, dripping with consumer-grade op-amps, is not going to sound like the highest quality tube amp simply because the manufacturer stuck a 12AX7 somewhere in the bowels of it, inserting an LDR as the control element is not going to instantly result in cleanliness, transparency, and quiet flexible performance.  It certainly won't ADD noise and distortion, but neither can it take away from whatever else might be elsewhere in the entire circuit.

All of that being said, since LDRs do not distort the same way that FETs or OTAs can, the good news is that you can often drive optical compressors with a hotter signal without suffering the ill effects of doing so.  That can help to maintain optimal S/N ratio along the signal path.