THE ENGINEER'S THUMB... At last, a better compressor!

[blackieNYC]

Question - for the four-knob builds.  Is yours behaving a little oddly at shorter release times?  Interesting thing going on.
   If I position the release pot (1M) at around 400K, I get a bit more desired "pop" on each of a sequence notes - as I would expect, because the release lets go quickly and the attack reveals itself.  This is definitely a compressor-as-effect setting.

But the output level is hunting around, skipping around in level. I realize the short release will change things, but this seems excessive. (I've breadboarded this from scratch twice.)    It sounds to me like the circuit is responding to beat frequencies from the chord.  Low ones - 3Hz, 7Hz, I dunno.  My MXR doesn't seem to do this. So I've attempted to filter off the lows from when the input is split off from the audio path to the side chain at p[in 5 of U1B.   
  It appears to be working a bit.  I think?  It makes sense in theory, a little. If the amplitude of the beat frequencies is enough to trip the compressor. Don't you think?  I used a 100K to bring Vbias to pin 5, and a .022uf cap from pin 3 to pin 5.  Does this make sense?
   So does yours do this when the release pot is under 500k or so?

[merlinb]

But the output level is hunting around, skipping around in level.

I think we need a recording to tell if this is just normal compressor behaviour or a fault.

[Koldman]

Hi I just built the two knob from the provided PCB. I'm new to compressors and I'm not sure if the way its working is normal. If I play firmly it compresses but then switch to soft picking it takes a while to bring up the volume. Have I made a mistake in construction or is this how it works?

[highwater]

Depends on what you mean by "a while".

If it's "a couple seconds" or longer, you might have a bad solder joint around the 1meg "decrease for shorter release" resistor (the number depends on the schematic, it's R12 in Merlin's PDF). That's what happened to me when I put that resistor in the wrong hole on a stripboard build - still more-or-less worked after I fixed my other mistakes, but it took several seconds for the volume to come back up.

Otherwise, might be normal, might not. Somewhere in the last 20-some pages, someone measured and posted the release times with a few different resistor values... I can't remember who/when/what the numbers were. The graph on Merlin's website looks to show about a 0.7second release time with what I assume is a 1meg resistor.

Unless there is an obvious-in-hindsight soldering issue, try swapping that 1meg resistor for something smaller; I tried 1meg and 470k before deciding to just box mine (eventually) with a release knob, and the 470k was still (as far as my tin ears can tell) a much longer release than an Orange Squeezer (the only other compressor I've used with a guitar).

---

BTW, Merlin, I absolutely love this thing. I can't imagine wanting a guitar compressor that's squishier or quieter... and it'll only get quieter when I get a box for it and it's not a bare circuit board hanging-off my breadboard.

Also, I didn't want to bump the thread for what I'm guessing is a silly question, but I'm thinking about adding a sidechain input when I box it up. I assume that I can just separate the non-inverting inputs to U1a and U1b (numbers from the web-page), add a second 1meg bias resistor, and wire-up a switching jack to connect them together when there's no plug there... anyone see a problem with that approach?

[Koldman]

I tried to record it, but it didn't come out for some reason, but you can hear the swell when I stop playing at the end.
https://youtu.be/FUnqxVv3jmc

[highwater]

Youtube's compression is masking the effect on the notes I suspect... in any case I can't hear whether that's correct or not, but the swell at the end sounds more-or-less the same as mine. Probably normal.

You could take R12 out and replace it with something smaller -- the lowest setting on the 4-knob version is 220k.

[Koldman]

I put in a 220k resistor and a 416k trimpot in but I didn't find it hugely useful. I think this just isn't the compressor for me. Thanks anyway!

[merlinb]

I put in a 220k resistor and a 416k trimpot in but I didn't find it hugely useful. I think this just isn't the compressor for me. Thanks anyway!

In that case you won't find any compressor for you, since all compressors have a release time! 

[jfrabat]

See, I thought I was building my final pedal, and you all force me to look at this thread!

On a serious note, where's the best place to get the LM13700?  I have not been able to find it locally in Costa Rica, si it is off to the internet!

[davepedals]

I'm trying to make a PCB of this, two knob version.  Don't want to use the PDF file because I want to add other PCBs to my transparency sheet. The PDF will not let me copy the PCB image.  Does anyone have an image file of this PCB?
Thanks!

[digi2t]

I'm trying to make a PCB of this, two knob version.  Don't want to use the PDF file because I want to add other PCBs to my transparency sheet. The PDF will not let me copy the PCB image.  Does anyone have an image file of this PCB?
Thanks!

How about screen shot and paste to Paint? Then resize to the proper dimensions? Give me a minute though, I'll see if I can extract it with Nitro.

Here you go chief...




You can also click on the image and go to Photobucket and download it from there. Let me know if there's a problem.

[davepedals]

I appreciate your efforts... I've tried this before however and upon resizing, the image is degraded.  I'm really hoping someone has the original image file correctly sized.  Thanks for the effort.

[merlinb]

I appreciate your efforts... I've tried this before however and upon resizing, the image is degraded.  I'm really hoping someone has the original image file correctly sized.  Thanks for the effort.

The plain image file is at the bottom of this page:
http://valvewizard.co.uk/engineersthumb.html

You can paste it into a word processor or whatever and set the size.

[davepedals]

Thank you, Merlin, that's exactly what I was looking for, a large image that would be scalable without degrading. Can't wait to try his one, I definitely need a new compressor from a rig. This looks like the right one... kudos for the compressor design!
Once again, digi2t,  I do appreciate your effort, however like I said I tried the same thing and resizing may damage too fuzzy.

[digi2t]

No problemo miho. I was just about to tell you to PM merlin, but I guess his ears were burning.

[bean]

Hope this isn't too much of a derail: I have a version of the Thumb I made for myself which uses an LM13700 and has external pots for attack, release and threshold. Build is verified with a couple minor caveats. Anyway, I have 5 extra boards I don't need and I'm happy to just give them away. If anyone wants one lemme know.

Build doc is here: http://madbeanpedals.com/projects/Thumb/Thumb.zip

[davepedals]

I'd certainly take one off of your hands if it's not too late! I'm definitely going to make a PCB for the tuna version, however I would like to fool around this other version as well . I'll PM you my address . Thanks!

EDIT:   tuna version?  Jeeze... I meant 2 knob version. Ha ha!

[davepedals]

Hey, Merlin ...


This PCB I got from you... could I just add the ratio and the level and leave the others blank?  I'm just curious really, as I think I'm gonna add the others anyway , nice to know though! And thanks again!

[digi2t]

Quick Q...

I was looking at the GTI version with the bar graph, and was thinking of trying it. Can someone confirm if the LED driver arrangement works?

[adam193]

Im looking to build one of these on stripboard.

The layout is on 20 row, can this be built on 19 row?