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

[jmwreck]

Wow, it looks wonderful!
Is the text silkscreened?

Thank you. It's UV print from Tayda

[merlinb]

Hi, How would I connect a sinus LFO 0V to 5V to get a sort of tremelo effect? I tried disconnecting the 1K and 1uF and connecting the LFO to the + of opamp C with a voltage divider pot but that doesn't seem to work. I tried other ways to connect to pin 1 of VCA F but with varying results, not really satisfactory.

I would try this as a first instinct

[Hypernurbs]

Hi, How would I connect a sinus LFO 0V to 5V to get a sort of tremelo effect? I tried disconnecting the 1K and 1uF and connecting the LFO to the + of opamp C with a voltage divider pot but that doesn't seem to work. I tried other ways to connect to pin 1 of VCA F but with varying results, not really satisfactory.

I would try this as a first instinct

Thanks Merlin, I'm gonna try it soon. Although I already built your ET compressor and removed it from my breadboard. It's a very nice compressor so kudos to you. I added a 'dry blend' and it sits on my pedalboard right now, it fattens up my tones nicely!

[Hypernurbs]

I got an email right after I posted my previous reply, asking how I did the dry blend. I have added the schematic plus stripboard layout to this post. Maybe it can be optimized by the guru's overhere but for now it's verified by me and works.

It'integrated in the ET circuit so you'll have to tap the 4.5V Vref and replace the 1uF output cap with a jumper. The 1st lug (says 3rd but it's a typo) from the output pot needs to be connected to Vref instead of ground. Same goes for the dry put, it says lug 3 but it's lug 1 but you'll notice if you test the circuit, the volume will be the other way around...

The dry level pot maxed is a little more than unity gain from what I experience.

Opamp is a regular dual opamp.

[bluebunny]

https://www.premierguitar.com/gear/reviews/mayfly-flat-earth

Although the Flat Earth definitely feels like part of this lineage, it's relatively quiet. That's partly thanks to a circuit that uses a LM13700 chip inside the feedback loop.

Now where have I seen that before?   

[merlinb]

I got an email right after I posted my previous reply, asking how I did the dry blend.

II know people love a wet dry blend on compressors, but I keep pointing out that the ratio control on the ET already is a wet dry blend. It performs the same function!

[Hypernurbs]

Yes, I read that in this thread. And I can see what you mean, I think. But isn't it so that with a seperate dry blend, one can set the ratio really high and still blend dry in, for example? With the ratio control alone it's one way or another, it would seem to me.

[r080]

Interesting. I had not, in fact, seen that about the ratio control until today. Helpful information.

Kevin from AionFX just released a board for this project, and his documentation has that note on the ratio control.

https://aionfx.com/app/files/docs/nomad_documentation.pdf

Also interesting, Foxpedal's Refinery pedal seems to use both the ratio control and a separate blend, forgoing threshold control:

https://foxpedal.com/products/refinery

[jtmsrl]

I got a Engineer's Thumb PCB from Musikding, I wanted to add sidechain input by following this schematic from merlin, but the provided schematic seems a little bit different so I'm not quite sure how to do this or even if its possible.

So far, I tried to translate the previous schematic into the one that comes with the PCB, I've added the modifications in blue / red, but not sure if this is the correct way. (I'm assuming a switching jack socket)

https://imgur.com/a/CQSKRkn

I guess these are the the necessary modifications on the PCB, but again, not an expert, not sure if correct

https://imgur.com/a/9EyqXxa

light blue: existing circuit
blue: Sidechain in modification
orange: 4.5v for A opamp

[merlinb]

So far, I tried to translate the previous schematic into the one that comes with the PCB, I've added the modifications in blue / red, but not sure if this is the correct way.

I guess these are the the necessary modifications on the PCB, but again, not an expert, not sure if correct

Yes both of your diagrams appear to be correct

[jtmsrl]

So far, I tried to translate the previous schematic into the one that comes with the PCB, I've added the modifications in blue / red, but not sure if this is the correct way.

I guess these are the the necessary modifications on the PCB, but again, not an expert, not sure if correct

Yes both of your diagrams appear to be correct

worked perfectly, thanks for the verification

I ended up not wiring one of the threshold pins instead of cutting the PCB circuit

https://imgur.com/VxoE1vA

Added a 1M resistor to close the circuit for TL074

https://imgur.com/KUzo7yz

And finally the switching jack with a 10nF film capacitor connected to T, the original line IN connected to TN and GND on S (didnt use SN and it was an stereo switch so didnt use R, RN either)

https://imgur.com/9CWCFof

[patricks]

I bought an Issue 3 PCB about 5 or so years ago and finally got to build it up yesterday. Really happy with it, it's super quiet and very transparent!
I built it with some mods:

• Ratio end-stop resistor, per the Issue 4 web page (4.7k resistor in line with lug 2 of the pot)
• Release pot (100k resistor in line with A500K pot pin 2, and pins 2 and 3 tied together)
• Bright switch per the Issue 3 and Issue 4 pages (same info on each page)
• Guitar/bass switch per the Issue 3 and Issue 4 pages (same info on each page)
• Compression indicator LED per the Issue 4 schematic

Attack and release controls don't make a hugely audible difference, you can tell when there's been a change but my ears are going to have to get used to how it's adjusted best.

The compression indicator LED required some 3D shenanigans bending LED legs and lifting a resistor leg from the PCB, but I was able to get the LED to line up with the hole I'd drilled in the enclosure for it, so I'm pleased about that.
It wasn't lighting up at first, even though the circuit was working, which had me flummoxed for a while until I realised I had to cut the trace between pins 1 and 2 of the TL074 (be brave!) and run a flying lead to connect pin 2 of the IC to the LED cathode. It's working now, my ears say there's compression going on before the LED lights up but it's nice (and pretty) to have the visual indicator.
Does the brightness rating of the LED make a difference to how soon it turns on, out of interest? Mine's a 10,500 mcd clear blue, ratings for super bright LEDs vary from a few thousand mcd to over 30,000 mcd.

Wiring up the bright switch and guitar/bass switch together required some planning, with flying leads and point-to-point 3D shenanigans.
The bright switch adds some extra sparkle without making it super trebly, the guitar/bass switch is similarly subtle but there's definitely a slight muddy change to the sound when playing a guitar with it switched to bass.

If anyone wants details of which component leads to lift, what's attached to switches, etc. to help with the mods let me know, I don't imagine there are many Issue 3 boards around still tho

[merlinb]

Does the brightness rating of the LED make a difference to how soon it turns on, out of interest? Mine's a 10,500 mcd clear blue, ratings for super bright LEDs vary from a few thousand mcd to over 30,000 mcd.

Yes it will do. The brighter the LED, the more light you get for a given current, so the tiniest currents should become that bit more visible.

[patricks]

Fantastic, thanks Merlin
If I get super enthusiastic I'll replace it with an even, even brighter one

[micmac1]

Hi all,

I recently built an ET 4.1 from Merlin (using the Gerber files on his web site). I think it worked out good and it sounds great. But when I checked the IC voltages one pin seemed a bit strange. Here's the Merlin PDF: Link

It says that I should read 3.9V on pin 14 of U1 (TL074). I do measure 3.9V, but after a few seconds it goes up to a little bit over 8V. I realized that if I start strumming the strings then the voltages immediately goes back to 3.9V. And if I then silence the strings, after a few seconds (the release time?) the voltage goes up again to ~8V. And the background noise increases a bit.

Is this "normal"?

Kind regards,
Seb

[bartimaeus]

Does anyone know the resistance values for different ratio settings? Or a good way to calculate it?

I'm guessing that at minimum (4.7k) it's a ratio of 1:1, and at maximum (1M) it's a ratio of infinity:1. But what resistance gives a ratio of 2:1 or 4:1?

[Rob Strand]

Does anyone know the resistance values for different ratio settings? Or a good way to calculate it?

I'm guessing that at minimum (4.7k) it's a ratio of 1:1, and at maximum (1M) it's a ratio of infinity:1. But what resistance gives a ratio of 2:1 or 4:1?

This post has a detailed analysis. 
https://www.diystompboxes.com/smfforum/index.php?topic=130630.0

The compression ratio changes by shifting the compression profile left/right, essentially changing the threshold.   Some calculations are given.

[merlinb]

Does anyone know the resistance values for different ratio settings? Or a good way to calculate it?
I'm guessing that at minimum (4.7k) it's a ratio of 1:1, and at maximum (1M) it's a ratio of infinity:1. But what resistance gives a ratio of 2:1 or 4:1?

This may help (ignore the grey lines, they are for a Dynacomp). It's not a pure ratio control, rather it softens the knee which creates the impression of a ratio change in the region where the signal mostly lies, even though it still hits limiting if the signal is large enough.

[merlinb]

It says that I should read 3.9V on pin 14 of U1 (TL074). I do measure 3.9V, but after a few seconds it goes up to a little bit over 8V.

Possibly that is only happening because your voltmeter is attached, allowing current to leak through the transistor while the opamp is desperately trying to turn it off.

[bartimaeus]

Does anyone know the resistance values for different ratio settings? Or a good way to calculate it?

I'm guessing that at minimum (4.7k) it's a ratio of 1:1, and at maximum (1M) it's a ratio of infinity:1. But what resistance gives a ratio of 2:1 or 4:1?

This post has a detail analysis. 
https://www.diystompboxes.com/smfforum/index.php?topic=130630.0

The compression ratio changes by shifting the compression profile left/right, essentially changing the threshold.   Some calculations are given.

Incredible, super interesting stuff!!

This may help (ignore the grey lines, they are for a Dynacomp). It's not a pure ratio control, rather it softens the knee which creates the impression of a ratio change in the region where the signal mostly lies, even though it still hits limiting if the signal is large enough.

Exactly what I was looking for, thanks so much!! Even if it's not ~technically~ a ratio control, sounds quite useful!