# General Tso’s Compressor



## K Pedals

This is a really nice compressor...


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## Mourguitars

Another nice build KP !

Mike


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## music6000

Another Tidy Build!
Can you confirm which way is the Blend & Juicy Tggle switch?


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## Barry

Looks good!


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## K Pedals

music6000 said:


> Another Tidy Build!
> Can you confirm which way is the Blend & Juicy Tggle switch?


Yeah Juicy mode is up and Blend mode is down


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## manfesto

Hi! For a drill template did you just use a 4-knob template and drill the bottom-left hole smaller for the switch?


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## K Pedals

manfesto said:


> Hi! For a drill template did you just use a 4-knob template and drill the bottom-left hole smaller for the switch?


Yep


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## BurntFingers

Nice. You went for the opa chip like a baller.


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## K Pedals

BurntFingers said:


> Nice. You went for the opa chip like a baller.


I got a bunch off eBay for cheap ...
idk if they’re real but they sound like the real one I bought off mouser...
But I got them for a little over $1 apiece


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## Chuck D. Bones

Looks good.  I guess if I want a schematic, I'll have to buy a board and trace it.


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## manfesto

Just noticed that there’s a pad labeled “mute” with the footswitch wiring, what’s that for (I’m assuming muting the circuit input)?

Haven’t seen it on other buffered bypass PCBs


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## geekmacdaddy

cool build


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## Mike McLane

I just finished my General Tso and I like that it seems to be more subtle than the non-optical pedal.  Also, it puts a noticeable and pleasant fatness in the signal.  As a single coil guy this is great, particularly when playing clean  The only thing that bugs me is that when in Blend mode I can't blend in a much of a heavily squashed signal without the volume getting out of hand.  Is there something I'm missing about the way this thing works?


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## cooder

Great build, makes me curious...


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## Chuck D. Bones

Mike McLane said:


> The only thing that bugs me is that when in Blend mode I can't blend in a much of a heavily squashed signal without the volume getting out of hand.  Is there something I'm missing about the way this thing works?



That's how it works.  You can have either a Blend control or a Volume control, depending on the Mode switch.  Have you tried the other mode?


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## Mike McLane

Just came back to dealing with this issue. . .maybe its my ears, but after listening to several YouTube demos it seems that mine is not getting the same level of compression (that chickin' pickin' snap) that the demos produce.  I checked all of the component values (except for the pots) and the only thing that's different is that the LDR's that PPCB offered were out of stock so I went with the attached.  Is something wrong with the LDR spec or installation?  I also checked the LED's in a dark room and get modest, but discernible glow when fed a guitar signal (but nothing like the intensity of a the LED power indicator).


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## Chuck D. Bones

A Youtube demo is not the same thing as having the real pedal in you home or studio for comparison.  All sorts of things can happen in the recording or playback process to change the sound.  

I'll qualify my statements by saying that everything I know about the General Tso is based on analysis.  I have never built one or heard one live.  The Tso is not capable of large amounts of compression, so you may be chasing a ghost.  The biggest variable in optical compressors is the LDR.  LDRs vary widely and that will affect circuit performance.  Let's see what we can learn about your LDRs.  Do you have any spares?  If so, measure their dark resistance, and by that I mean in total darkness.  Put the LDR in cardboard box (with test leads attached) and close it up tight.  Next, measure the light resistance in normal room light.  Report the results and we'll go from there.


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## jubal81

Chuck's spot on. 
After building this and monkeying around with the circuit a bit, I have a couple tips. 
To get closer to what you want, you need a different LDR. Look for one with the lowest possible ON resistance. The GL5516 goes down to 300R-ish, so that's a good target. This will get you the subtle optical style with longer sustain - working as intended. If you want ridiculous chicken-pickin', try a superbright LED. The sustain knob will be pretty much useless and the sustain isn't right, but it will give that crazy squish attack. It sounded 'broken' but I had a fun afternoon with it like that.


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## jubal81

Got a more specific update.
Look for an LED with luminous intensity of 100 mcd, point it directly at the GL5516 LDR and make it as light-tight as you can (this is important because even the tiny amount of light from an indicator LED inside the box can affect the LDRs).

Buy this LED from Mouser or Arrow or Newark or an actual, reputable new-component seller. HINT: Not Tayda.


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## Mike McLane

OK.  I got my marching orders.  Thanks!!!!


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## Mike McLane

Where did you get 100mcd?  I contacted Smallbear and what I have puts out around 30mcd.  I can get the 100mcd from Mouser, though.  I found a data sheet for the Tayda LDR KE-10720 (attached) and it looks pretty darn close.  GL5516 calls for light resist: 5-10K, dark resist - 500K, rise time - 30ms, fall time - 30ms.  This appears to match except for a fall time of 5ms.  Don't know how that would affect performance, but should work until PPCB gets their GL5516's in (which I'm sure will be any day now ?). Thx tons. BTW. . .GUYS LIKE YOU ARE INDISPENSIBLE TO THE SUCCESS OF THIS FORUM AND TO ME IN PARTICULAR. I'M SURE I SPEAK FOR A LOT OF OTHERS AS WELL!!


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## Chuck D. Bones

I don't think you need such a high luminosity. I've only built a few optical compressors, but I never had an issue with not enough light landing on the LDR. It was usually the opposite. I've had to move the LDRs back from the LED, or use a lower brightness LED. The General Tso has two LDRs in parallel with the 100K SUSTAIN control. They work in tandem. The gain of that stage is set by the SUSTAIN control, the two LDRs and a 10K resistor to Vref. If the resistance range of each LDR goes from 10K (light) to 200K (dark), the pedal will do everything it was designed to do. If the LDR resistance goes higher or lower than that, it's ok, but it won't make the pedal perform any better.


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## Pimpoftheyear70

What LEDs do you use in these locations ?
Water clear or diffused style ? 
And why green ??


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## Chuck D. Bones

Most of the LDRs we use have their highest sensitive in the yellow & green part of the spectrum.  They'll still work with any of the other colors, the LED just needs to be brighter to get the LDR down to the same resistance.  I think diffuse is better, for two reasons.
1. Diffuse LEDs will spread the light more evenly across the entire LDR.
2. The LDR position will be less critical since light is going in all directions instead of just straight out the top of the LED.

Here's what I did in my Delegate, it works great.  LED is regular brightness diffuse.  LDR I bought surplus and cherry-picked.  It has a light resistance under 2K and a dark resistance over 1Meg.  That compressor needs a much higher dark resistance than the Tso.  I actually had to move the LDR away from the LED for better performance.  Too much light = slow release.


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## Mike McLane

Hmmmm?   OK, so riddle me this Mr. Peabody (does anybody remember Sherman & Mr. Peabody?).  How 'bout we go back to your suggested "black box" test.  I hook up the my looper and "strum roll" an open E chord and run a continuous drone into the pedal.  Assume silence gets me >= 200K. . . if full volume gets me =<10K  then we're good???  If the schematic indicates that the proper resistance limits are 10K-200K for optimal performance it would only make sense to have to "calibrate" the LED/LDR "relationship" to yield the appropriate resistance ranges.  I'm thinking some guidance on the LED spec should be clarified in the buld docs.


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## Chuck D. Bones

_Sherman & Peabody_... ahhh the Classics.

Sometimes I think it's more like _Coyote vs. The Roadrunner_ where I'm Wile E. Coyote and the tone I'm chasing is The Roadrunner. That would make Tayda the ACME Corp. 

The 10K-200K resistance limits I provided are one edge of the performance space.  Any diffuse green LED should work fine.  Just because I have a tendency to overthink things doesn't necessarily make it a good idea.


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## Dan0h

Chuck D. Bones said:


> Most of the LDRs we use have their highest sensitive in the yellow & green part of the spectrum.  They'll still work with any of the other colors, the LED just needs to be brighter to get the LDR down to the same resistance.  I think diffuse is better, for two reasons.
> 1. Diffuse LEDs will spread the light more evenly across the entire LDR.
> 2. The LDR position will be less critical since light is going in all directions instead of just straight out the top of the LED.
> 
> Here's what I did in my Delegate, it works great.  LED is regular brightness diffuse.  LDR I bought surplus and cherry-picked.  It has a light resistance under 2K and a dark resistance over 1Meg.  That compressor needs a much higher dark resistance than the Tso.  I actually had to move the LDR away from the LED for better performance.  Too much light = slow release.
> 
> View attachment 7167


Hey Chuck, what is that housing around the LDR? Did it come that way or did you rig that? Your work is epically clean man. Cheers.


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## Chuck D. Bones

Thanks.

The LDR came that way.  Bought it from Electronic Goldmine.  They had 'em on sale, 5 for $1.99.  Normally they're $1 ea.

https://www.goldmine-elec-products.com/prodinfo.asp?number=G15176

They have quite the selection of LDRs.  Be warned, they are untested surplus.  Lotta part-to-part variation and a few are duds.  They're inexpensive (when they're on sale) and I don't mind testing them.  

EG has these LED/LDR isolators, but a lot of them turned out to be unusable, so caveat emptor.


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## zgrav

I think the speed of the LDR has more to do with how much snap you do or don't get from the rise in the sound envelope when you hit the strings.   I don't think the speed of the LDR matters in most phaser applications, but it can make a difference in how crisp the attack is in a compressor.


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## Chuck D. Bones

Absolutely.  When I test LDRs, I drive the LED with a square wave and watch how quickly the resistance changes.  Here's an example using one of the TO-18 can LDRs.  Green traces is LED current, 1mA/V.  Blue trace is LDR voltage.  LDR has a 100K pull-up to +9.3V.  At 4.6V, the LDR resistance is 100K.  LED is blue diffuse, LDR is up against the LED, near the top.  At this time scale, the LDR resistance drops almost instantaneously when the LED turns on.  The LDR takes 60ms to get up to 100K and 120ms to get up to 200K when the LED turns off.


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## Dan0h

Chuck D. Bones said:


> Thanks.
> 
> The LDR came that way.  Bought it from Electronic Goldmine.  They had 'em on sale, 5 for $1.99.  Normally they're $1 ea.
> 
> https://www.goldmine-elec-products.com/prodinfo.asp?number=G15176
> 
> 
> They have quite the selection of LDRs.  Be warned, they are untested surplus.  Lotta part-to-part variation and a few are duds.  They're inexpensive (when they're on sale) and I don't mind testing them.
> 
> EG has these LED/LDR isolators, but a lot of them turned out to be unusable, so caveat emptor.


This Website!


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## Mike McLane

Holy crow!  I just checked Mouser and I can get 5mm green LED's anywhere from 1.6mcd to 180mcd.  In the range mentioned here. . . 30, 38, 80, 100.   I've got 30 now so assuming Chuck's right about 100 being too much that leaves me with 38 or 80 as possible alternatives (or a new set of ears).  What's a feller to do?


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## Mike McLane

Just got to tearing into the bugger to check the range on the LDR.  It reads 12K under normal room light. . . 3.5K with my LED reading lamp on, HOWEVER, in complete darkness I only get 28K (spec sheet calls for 500K).  I also checked (however subjectively) that the LED's were operating and they do flash when a signal is put to the unit, ever so small a blink when Sustain is at min, significant reaction when max.  In fact I notice that the LED on one side is more signal sensitive than the other.  Any thoughts?


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## zgrav

Are you getting those readings in circuit?  Are you getting the same resistance ranges of both of the LDRs?   Looks like your 100K sustain pot and the two LDRs are in parallel, so they would all three affect the resistance you measure for either one of the LDRs.  if you are only blocking the light on one of the LDRS the light hitting the other LDR will affect your reading.  

you need to max out the resistance on your pot and then make sure both LDRs are in the dark when you measure the resistance on each of them.  keep in mind that the most resistance you could measure for each LDR when it is in the circuit will be maxed out by the 50K pot.


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## Mike McLane

Ah, yes.  I didn't have the schematic in my build docs, but dl'd the update.  I see what you mean with the SUSTAIN knob. . . I max'd it out and retested and got a reading of 107K (probably the real value of the pot) under total darkness.  I guess that means that the LDR's are functioning properly.


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## zgrav

Sounds like the LDRs are working.  are you testing the effect with all other light shut out from the LDRs?


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## Chuck D. Bones

zgrav said:


> Are you getting those readings in circuit?  Are you getting the same resistance ranges of both of the LDRs?   Looks like your 100K sustain pot and the two LDRs are in parallel, so they would all three affect the resistance you measure for either one of the LDRs.  if you are only blocking the light on one of the LDRS the light hitting the other LDR will affect your reading.
> 
> you need to max out the resistance on your pot and then make sure both LDRs are in the dark when you measure the resistance on each of them.  keep in mind that the most resistance you could measure for each LDR when it is in the circuit will be maxed out by the 50K pot.



If we're still talking about the General Tso, the SUSTAIN pot is B100K.  Pots tend to have a pretty loose tolerance, like ±20%, so 107K is in spec.


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## zgrav

if the pot and LDRs are all parallel, wouldn't that drop the effective resistance across all of them?


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## Chuck D. Bones

Yes, in accordance with this formula:





Let's assume the LDRs are each 2M and the SUSTAIN pot is 115K, then R = 103K


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## zgrav

Or 50K if the resistance on the LDRs is 200K each with a 100K pot, using this calculator.


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## Chuck D. Bones

Yep.  We are in agreement.


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## Mike McLane

Now that I know the LDR's are OK I've gone back and played with this thing for half an hour this afternoon on the notion that my ears and expectations are just plain wrong and this thing is working properly.  I still can only hear a modest compression effect at best. . . to the point that the difference btw off and on is almost pointless.  That's frustrating. . . the Tone, Volume, etc. sound like they're behaving as designed, but the actual squishiness seems AWOL.


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