Budda - Chakra Compressor  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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johnnyg
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Post by johnnyg »

BuddhaComp.jpg
BCCjgFSB.jpg
First optical compressor I've used and a nice change from DynaComp type I've only previously experienced. Also first attempt at publishing a full circuit trace; hopefully it's right.
BuddhaComp.pdf
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roseblood11
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Post by roseblood11 »

have you measured the LDR?

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johnnyg
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Post by johnnyg »

Dark resistance in circuit was c. 250k. But any other parameters (like on and off times) I've no idea. I would guess, like most circuits that use an optocoupler, one would have to plug in different types of LDR or vactrol to find something that works.

BTW the circuit reminded me of Rod Elliott's compressor designs and Rod provides some explanation of the circuit (like the 100uf cap across the LED: "some LDRs may give distortion at low frequencies. A 100uF cap in parallel with the LED will probably help if this is a problem").

http://sound.westhost.com/project92.htm
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Post by dv8r601 »

I'm really on the hunt for a non ota comp and I want to build this one correctly. So, the 2w10 diode bridge has a max Vf of 1.1v, could I just sub that with rectifier diodes? Or do I just need to pony up and order one? I know what a bridge does and is I'm just curious if the diode type matter as much.

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Post by bmxguitarsbmx »

Hey dv8r601,
In my estimation, this schematic is missing some connections.

Here is the link from above, that is now dead:
http://sound.whsites.net/project92.htm

Wish I could be more helpful, as it looks like it could be an interesting circuit.

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Kram881974
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Post by Kram881974 »

any updates on this?

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mr coffee
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Post by mr coffee »

Not to throw water on this design, but...

This circuit is fighting the virtues of optocompressors. I don't know who designed the circuit, but they are probably fairly new to design of an optical compressor circuit and didn't know much except that it used a photocell instead of a JFET as a voltage control element and just stuck a photocell in where you you would have used a JFET in a circuit they were familiar with. Here's the deal...

All photocells have a faster drop in resistance (milliseconds) with exposure to light and a much slower increase in resistance (hundreds of milliseconds) as light level decreases. Therefore, since compressors nearly always seek to have a relatively fast "attack time" (aka reduction in gain in 1 to 150 ms) and a much slower "release time" (aka the increase in gain after a reduction in gain has occurred), the photocell is almost always used in a circuit to take adavantage of this, i.e., either the photocell is set up to act as the lower element in a passive voltage divider or as the feedback element in a variable gain op amp circuit, NOT as the resistor to ground in the divider controlling the gain of an op amp, as in this circuit.

If you want an opto compressor sound, this ISN'T the circuit to copy.

FWIW.

Don't mean to insult whoever designed this, just the facts. Check it out, guys.

mr coffee

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Post by mictester »

mr coffee wrote: 30 Nov 2021, 03:34 Not to throw water on this design, but...

If you want an opto compressor sound, this ISN'T the circuit to copy.
You're exactly right! My simple compressor uses the LDR in the negative feedback loop around an op-amp for exactly the reasons you outline in your post. Incidentally, I've recently found a convenient little opto element - the LCR0202 - which is a little 4-terminal plastic box containing an LED and an LDR with just the right characteristics for the circuit. The datasheet can be downloaded at http://pliki.aksotronik.pl/lcr0202.pdf, and you can buy them quite cheaply from lots of suppliers. Incidentally - if you want to use two of them for a stereo processor of some kind, it's easy enough to match them - if you buy a bag of 10 of them you can easily get 5 accurately matched pairs!
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mr coffee
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Post by mr coffee »

Hi Mictester,
Thanks for sharing your experience with the LCR0202.

Datasheet looks wonky, though... Hmmm

I take it the datasheet is showing the attack/recovery characteristics on the 3rd graph which mislabels the horizontal scale as "ma" instead of "ms"?

Presumably also missing having a different Recovery time scale on the top of the plot of the Resistance/Time graph like the old Vactec datasheets used? Odd that they show two different curves for both attack and recovery times, presuming I am seeing through the misinformation fog of their datasheet.

Makes me wonder if they just copied a graph from the Vactec catalog datasheets, cutting off the seconds time scale at the top and putting "ma" instead of "ms" on the end of the horizontal scale???

mr coffee

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george giblet
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Post by george giblet »

The schematic can't work as shown.

Also, if you look at the parts on the PCB they don't quite add-up with the schematic.
It's too hard to fix or rebuild the schematic from the top side of the PCB.

To me it looks like John Hollis's Flatline compressor except:
- buffer added at input and output
- attack pot added, perhaps in series with the 330R resistor
- ge diodes replaced with mini silicon bridge rectifier

You can see the similarities and most likely fixes by looking at the Flatline schematic,
john hollis - flatline compressor rev_1.jpg

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Post by mr coffee »

Hi George,
Makes MUCH better sense than the schematic posted initially.
I think you're right about the likely circuit being a flatline with buffers, ... and maybe a little tone shaping if the pots are rearranged correctly with some of the excess capacitors shown.

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Post by george giblet »

With the addition of one wire things change a lot.
BuddhaComp - possible sch fix GG_2023_06_22.jpg
The things hanging would be the 47k on the opamp and the precise wiring of the Attack pot.
As drawn the pot isn't really attack but could be perceived as affecting the attack.

It would help a lot if we could assign the actual part designators on the PCB to the schematic positions.
That would help pin down the 10k vs 47k issue.

On the PCB I can see 3x10k and 3x47k. The 3x47k weighs in favor of 47k on the opamp;
also the fact a 47k is near pin 2 IC2.
But even with that change we still need 4x10k on the schematic.
Parts under the pots are unaccounted for.

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