General Tso Not Compressing / LEDs too Dim

[lumberjack]

Hey gang, just finished a General Tso tonight and everything seems to be working other than the fact that the LEDs associated with the LDR barely engage when playing, so I get no noticeable compression from the circuit. The biggest strum gets a very faint glimmer from them, almost imperceptible, so I double checked the resistors associated with the LEDs and they are indeed 1.5k to ground as called for. If I run a boost in front of the compressor you can see the LEDs light up a bit more, but I'm not sure what a realistic fix would be, or what's wrong. It's like the signal voltage the LEDs "see" from the guitar signal is too low to light the LEDs, perhaps the transistors associated with them aren't doing their job properly? I got them from Tayda and Mouser, and I haven't had any trouble sourcing components with them before.

The op amp seems to be working fine, as the volume knob generates quite a bit of boost, and the "sustain" control increases the volume as well in both modes, but again it just makes everything louder, not compressed. Would this be something as simple as running lower resistors to ground in place of the 1.5k? It seems like something else must be wrong though, as there's plenty of people who have built this and they used 1.5k for the LEDs.

Note: trimmer at right is for power LED brightness, and the tied resistors are just to get the right values.

 

[almondcity]

Have you played it with the lid on?

 

[lumberjack]

Have you played it with the lid on?

Yup, tried that first. I just tried lower resistors too, but it was no better, the LEDs very faintly glimmer even with a straight jumper to ground. I’ve built some other LED/lamp + LDR based pedals, tremolos and univibes, and they got a good bit brighter than these LEDs will.

Not sure what’s up.

 

[Nic]

Could the power led interfere ?

 

[jimilee]

Could the power led interfere ?

Nope

 

[jimilee]

Yup, tried that first. I just tried lower resistors too, but it was no better, the LEDs very faintly glimmer even with a straight jumper to ground. I’ve built some other LED/lamp + LDR based pedals, tremolos and univibes, and they got a good bit brighter than these LEDs will.

Not sure what’s up.

Have you gone through and made sure all your values are correct and reflowed your soldering?

 

[Alan W]

I just built this a few days ago, and had a very similar issue. Especially since I was using a different LDR, that has a higher resistance range, I figured I’d need lower values for those resistors also. So I put in trimmers; first setting them to 1K5. If no boost, just guitar in, very dim. I turned each trimmer about 4 complete turns, and they lit up more like I was expecting. I will measure where I ended up when I pull the board to paint it. All my transistors tested easily within spec.

you mentioned that you tried lower resistors—how low?

 

[MichaelW]

Yup, tried that first. I just tried lower resistors too, but it was no better, the LEDs very faintly glimmer even with a straight jumper to ground. I’ve built some other LED/lamp + LDR based pedals, tremolos and univibes, and they got a good bit brighter than these LEDs will.

Not sure what’s up.

@lumberjack
This might be a silly question but have you built/played any other optical compressors? The compression is much more subtle than VCA/OTA type compressors. The TSO's is my always on compressor but it's one of those pedals that I don't know is on until I turn it off. Which is how I personally use compressors. I've also built a bunch of other optical compressors that tend to be that way. Where as the Boss CS-3 (VCA) clone I built is much more "in your face" as is my retail Keeley Compressor (OTA)

 

[harmaes]

Try to use shrink tube or black tape covering the led and ldr completely. Sustain and balance to max will give you the most compression from this pedal. I use the bottom switch position and normally have all controls in the middle for a nice soft compressed boost. The treble control is great to dial in the highs and I often use this pedal to boost overdrive pedals and add highs on higher gain settings.
It’s not as extremely compressed/squishy as other pedals can but it’s the most musical compressor I’ve used next to the Cali76. with higher ouput humbuckers you can get more squish.

 

[Laundryroom David]

Which LDR's are you using? And have you doublechecked voltages just in case?

 

[lumberjack]

Hey guys, thanks for all the input! Much appreciated. I checked parts and reflowed all my connections, and replaced the LED resistors with trim pots wired as variable resistors. The lights seem to light up better now, although even with the trim pots (2k) shunting to ground they don't get tooooo too bright, and possibly the weirdest part is that I don't see almost any brightness difference between the trim pots full up at 2k and all the way down. I wrapped the LDR/LED up in gaffe tape and closed the box just in case.

With these changes though, I've noticed the circuit compressing in a very studio-esque fashion, i.e. its a very subtle effect; at full compression and 90:10 ratio (juicy mode) it brings up your quiet/soft picking a lot but doesn't effect your peaks. So you can strum full strength with it on and off and not notice any difference, but with lighter picking the signal stays really strong when the pedal is on, and in that sense it is functioning very much like an ideal compressor, even perfect in the sense of limiting dynamic range, but other demos of the pedal give a much stronger "compressor effect pedal" sound that this build doesn't give.

One question I had for this circuit is whether my LDRs are "real", they're GL5516 I got from amazon, and their dark measurements seem to be all over the place, from around 800k to 3M and beyond. I have a few other types of LDRs from Small Bear and Tayda and I have trouble getting any of them to give consistent dark measurements too though, so I'm not sure if the GL5516 I got from amazon are real or not, but I *have* used them in tremolo circuits and they worked perfectly in that.

Overall a very confusing build so far. This isn't my first optical comp build though, so I know they can be much more subtle, and the pedal is doing SOMEthing, I just hear stronger comp sounds from other Fat General/General Tso demos which leaves me wondering. I did a Diamond clone a while back that I absolutely love, and it's optical as well, and you can get a much juicier/compy sound out of it.

 

[Sebastian_H]

Hello together

Same here, the pedal really works fine, except it's not compressing at all... the knobs change the volume and the tone, but no compression whatsoever happening. At first I heatshrinked the LEDs but now I removed the heat shrinks to see what was happening and the LEDs only glim fairly dark when I strum with full force... I bought this as a kit via musikding in Germany and I was really careful building this pedal (and I am a bit proud of the design of the enclosure, as a forum member encouraged me to paint the enclosure by hand via his YouTube channel
Nonetheless, it doesn't work... would placing some sort of trimmer like lumberjack did would be a possible solution to the problem? I and thankful your for your answers Sebastian

 

[lumberjack]

Hello together

Same here, the pedal really works fine, except it's not compressing at all... the knobs change the volume and the tone, but no compression whatsoever happening. At first I heatshrinked the LEDs but now I removed the heat shrinks to see what was happening and the LEDs only glim fairly dark when I strum with full force... I bought this as a kit via musikding in Germany and I was really careful building this pedal (and I am a bit proud of the design of the enclosure, as a forum member encouraged me to paint the enclosure by hand via his YouTube channel
Nonetheless, it doesn't work... would placing some sort of trimmer like lumberjack did would be a possible solution to the problem? I and thankful your for your answers Sebastian

Hi Sebastian, I did change the GL5516 I got from Amazon out for KE-10720 LDRs from Tayda, that and the trimmers lowering the resistance to the LEDs seems to have done the trick. The pedal is compressing, but if you're comparing it to most other compressors the effect is still subtle. I thought I knew "subtle" cause I had used and built other optical comps like the Diamond comp, but that is a much more dramatic effect and imparts more EQ and feel changes by a long shot. I thought the Diamond was subtle because I was comparing it to Ross style comps like the Wampler Ego and the Keeley, but comparing the Diamond to something like the Engineers Thumb makes the Diamond seem far from transparent.

The General Tso still compresses considerably, it's just more subtle. It's more transparent than a Diamond for instance, but still ads its own thing compared to something like the Engineers Thumb, which was specifically designed to be more "ideal". I was so used to characterful comps that the Engineers Thumb felt simply boring; it was too perfect, if that makes sense, and didn't seem to add anything whatsoever to the signal other than the compression. That's what some people want, but it took building and playing one for me to realize I like comps that ad their own sound as well. The Engineer's can be set up to compress the heck out the signal, it just does so without changing the sound much. The Tso compresses much more lightly and adds a little bit of its own character, so I'd put it between the Diamond and the Engineer's, and way under Ross-style comps in terms of squashing power.

 

[DeadAirMD]

Try to use shrink tube or black tape covering the led and ldr completely. Sustain and balance to max will give you the most compression from this pedal.

Anything past 12 on Blend/Sustain and my Tso squeals and oscillates like crazy. Built the pedal a month ago and just start doing that this weekend. Such a shame because I absolutely love this pedal and have no idea where to begin fixing it.

 

[jubal81]

I've had good results with the NSL-32 in this circuit. I don't like much of Thorpy's design choices, though, and made my own version that uses a similar side chain to the Fat General.

A great alternative is the Boneyard Delegate comp here on PedalPCB.

 

[jubal81]

Also:
1. You can't go by eyesight on LED and LDR pairing. The LDR begins reacting to the LED output before it's visible to the eye.
2. LDRs are far more sensitive than you think. The light from the bypass LED will cause "light pollution" inside a pedal. Instead of elaborate shrouds over the Optocouplers, I'd recommend an enclosed housing for the bypass LED.

 

[manfesto]

The best version of the General Tso’s that I’ve built, I used some actual VTL5C4s like the Dinosaural version did. The compression action was *exactly* what I wanted and expected.

Was annoying having to drop a relatively large amount of coin on the optocouplers, but it sure beat all the trial-and-error-ing I was doing with LEDs and my baggie of GL5516s.

 

[lumberjack]

I've had good results with the NSL-32 in this circuit. I don't like much of Thorpy's design choices, though, and made my own version that uses a similar side chain to the Fat General.

A great alternative is the Boneyard Delegate comp here on PedalPCB.

Curious for the sake of learning, you don't have to reveal your secret sauce on this circuit, but when you say you don't like some of the design choices, can you explain what they were and why you didn't like them? Compressors are a mystery to me in terms of circuit design; all I understand from a design standpoint at this point in my electronics studying life is overdrive/distortion/fuzz, tremolo, some simple delays and the things you can do with them, e.g. make a chorus. I would love to learn more about compressor circuits, particularly optical, as those have been the ones I have enjoyed the most.

 

[lumberjack]

The best version of the General Tso’s that I’ve built, I used some actual VTL5C4s like the Dinosaural version did. The compression action was *exactly* what I wanted and expected.

Was annoying having to drop a relatively large amount of coin on the optocouplers, but it sure beat all the trial-and-error-ing I was doing with LEDs and my baggie of GL5516s.

Did you just drop two VTL5C4 into this pcb and that's it? No other changes? I've used the VTL5C4 on madbean's clone of the Diamond Comp and loved it, built a couple more of those clones "just in case", ha. I still have a few VTL5C4 vactrols lying around and I have another pcb for the Tso actually, might build that version with the vactrols and see what the difference is like if its as simple as chucking them in there.

 

[manfesto]

Did you just drop two VTL5C4 into this pcb and that's it? No other changes? I've used the VTL5C4 on madbean's clone of the Diamond Comp and loved it, built a couple more of those clones "just in case", ha. I still have a few VTL5C4 vactrols lying around and I have another pcb for the Tso actually, might build that version with the vactrols and see what the difference is like if its as simple as chucking them in there.

Yup, just dropped them in with no other changes. The OG Dinosaural schematic has 2.2K current limiting resistors in that part of the circuit instead of 1.5K like the Tso, and I wasn’t sure if those would need to be changed too (such a small change but I wondered if Thorpy picked 1.5K for a reason). I didn’t feel like I needed to even experiment after testing it just changing out the vactrols; it sounded great.