# This Week on the Breadboard - The Engineer's Middle Finger Compressor



## Chuck D. Bones (Feb 10, 2022)

I had been meaning to breadboard this for a long time, even before PedalPCB started making boards.  It's a very clever and well-explained design by Valve Wizard.  My first compressor was OTA-based: the Electro-Harmonix Black Finger. It used the same design trick of putting the OTA in the feedback loop as the Engineer's Thumb.  Once I've spent a little more time refining it, I'll share the schematic.  It's very close to the Engineer's Thumb v4.  The main difference is I put in a pot to control the brightness instead of a switch.

Controls, L-R: LEVEL - BRITE - RATIO - RELEASE - ATTACK - THRESHOLD
The 3mm red LED at the bottom, next to the EZ-hook, is the compression indicator.  The two grey LEDs just to the left of the green power-on indicator are the input clippers.


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## music6000 (Feb 11, 2022)

Chuck D. Bones said:


> Controls, L-R: LEVEL - BRITE - RATIO - RELEASE - ATTACK - THRESHOLD
> The 3mm red LED at the bottom, next to the EZ-hook, is the compression indicator.  The two grey LEDs just to the left of the green power-on indicator are the input clippers.


Something like This?:


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## fig (Feb 11, 2022)

Engineers have a middle finger? hmpf..I always heard they were removed at birth.


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## Chuck D. Bones (Feb 11, 2022)

I assure you, mine is fully intact.


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## fig (Feb 11, 2022)

Chuck D. Bones said:


> I assure you, mine is fully intact.


Hmm..it wasn't engineers....it was drummers and it's the big toe......or was it impressionist painters and an ear?


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## princeofyams (Feb 11, 2022)

Hey so while it's up there; and we're talking about it in the other thread. How does the voltage affect the Threshold between 9/12/18V?


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## Chuck D. Bones (Feb 11, 2022)

I don't think changing the supply voltage affects the THRESHOLD control, at least not directly.  It might upset the gain control circuit.  If the point of raising the supply voltage is to increase headroom, then there will be a perceived difference in all of the controls.


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## Chuck D. Bones (Feb 11, 2022)

I'm not seeing it, can you explain why the Threshold will change?


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## cooder (Feb 11, 2022)

Yehaaaw looking forward to this one. And I think @jjjimi84 is too...


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## princeofyams (Feb 12, 2022)

Chuck D. Bones said:


> I don't think changing the supply voltage affects the THRESHOLD control, at least not directly.  It might upset the gain control circuit.  If the point of raising the supply voltage is to increase headroom, then there will be a perceived difference in all of the controls.


I was reading there were concerns that the higher voltage was creating signal loss. Figured that you’d then be lowering the Threshold control for the same level in comparison. 

I must have misread. Was only asking after seeing people asking about using higher voltage to eliminate distortion. I guess the more appropriate question is how does the higher voltage affect the effect itself. 

I’m currently populating one, so wondering if I should populate for a higher voltage or not.


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## Chuck D. Bones (Feb 12, 2022)

You can keep your options open and use 25V or higher caps.  I ran some tests just now on my breadboard with 9V power.  The peak detector (IC2.1) and current source (IC2.2) have plenty of headroom and are not affected one way or the other by running the power at 9V, 12V or 18V.  I had previously voiced concerns about needing to alter the value of R10 when running at high voltage, but that turns out to not be necessary.  The peak current into IC3-1 is limited to around 1.5mA by R10 and the limiting diodes D3 & D4.  The peak current is not influenced by R10 except during startup. 

The THRESHOLD control is a bit of a misnomer.  It is a sensitivity control for the peak detector.  It limits how much IC3 can reduce the gain of IC1.1.

By contrast, the RATIO control limits the maximum gain in IC1.1.


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## princeofyams (Feb 12, 2022)

Chuck D. Bones said:


> You can keep your options open and use 25V or higher caps.  I ran some tests just now on my breadboard with 9V power.  The peak detector (IC2.1) and current source (IC2.2) have plenty of headroom and are not affected one way or the other by running the power at 9V, 12V or 18V.  I had previously voiced concerns about needing to alter the value of R10 when running at high voltage, but that turns out to not be necessary.  The peak current into IC3-1 is limited to around 1.5mA by R10 and the limiting diodes D3 & D4.  The peak current is not influenced by R10 except during startup.
> 
> The THRESHOLD control is a bit of a misnomer.  It is a sensitivity control for the peak detector.  It limits how much IC3 can reduce the gain of IC1.1.
> 
> By contrast, the RATIO control limits the maximum gain in IC1.1.


Thanks Chuck! That’s exactly what I’m going to do. I appreciate the clarification, my apologies for the confusion. 🤣


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## Dan M (Feb 12, 2022)

LM13700 and 2x TL072?


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## Chuck D. Bones (Feb 12, 2022)

Yup.

I guess it's time to share the schematic.  I won't claim it's 100% done, but it's close.  I didn't change much.  As Valve Wizard says, running the two halves of the LM13700 in parallel cuts the noise by 3dB.


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## Chuck D. Bones (Feb 17, 2022)

I am still struggling with this bitch.  Everything works except it *POPS *when RATIO and THRESHOLD are set above noon. I don't know if it's a case of Bill's Law biting me in the ass, an artifact of the breadboard's less than perfect grounding or what. Everything I've tried so far was to no avail. I'm not giving up. Engineer's Middle Finger indeed.


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## fig (Feb 17, 2022)

Chuck D. Bones said:


> I am still struggling with this bitch.  Everything works except it *POPS *when RATIO and THRESHOLD are set above noon. I don't know if it's a case of Bill's Law biting me in the ass, an artifact of the breadboard's less than perfect grounding or what. Everything I've tried so far was to no avail. I'm not giving up. Engineer's Middle Finger indeed.


Perhaps you could post some pictures? Do you have an audio probe?


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## Chuck D. Bones (Feb 17, 2022)

_Touché._


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## Chuck D. Bones (Feb 21, 2022)

I found another clever compressor design.  Hmmmmmm. 

https://www.diystompboxes.com/smfforum/index.php?topic=122568.0


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## cooder (Feb 21, 2022)

^ well I'm looking forward hanging on tenderhooks which one gets the biggest nod from you then...! Nice find.


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## Chuck D. Bones (Feb 21, 2022)

Right now, a slightly modd'ed version of the Pellucid is on my breadboard.  Once my left middle finger finishes healing, I'll give it a proper workout.  I fiddled a few component values and added a BRIGHT control.


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## Feral Feline (Feb 21, 2022)

I looked at that Pellucid, ‘twould be grand to have a Boneyard Edition!


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## music6000 (Feb 22, 2022)

Chuck D. Bones said:


> I found another clever compressor design.  Hmmmmmm.
> 
> https://www.diystompboxes.com/smfforum/index.php?topic=122568.0


Have you stop struggling with the other You Know What compressor before you start this one?


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## Chuck D. Bones (Feb 22, 2022)

For the moment, yes, but I will come back to the Engineer's Middle Finger.
The problem I was having with the EMF was that at high RATIO settings, the attack was too pronounced.  My thinking was that the peak detector was not tracking a rapid rise in the envelope quickly enough.  The Engineer's Thumb uses a half-wave rectifier in the peak detector circuit and tracks only the negative peaks in the guitar signal.  This can cause a 1/2-cycle delay in the peak detector.  Other compressors, such as the Delegate & General Tsao, employ a full-wave rectifier that tracks both peaks.  The Pellucid also uses a full-wave rectifier in the peak detector.  I wanted to see how much difference that made.  The Pellucid's full-wave rectifier does improve the peak tracking and the attack is smoother, however it's my observation that aggressive picking with the bridge pickup creates an attack so abrupt that there is still somewhat of a "pop" on the first cycle of the note.  I'm starting to think that I had an unrealistic expectation of how well a compressor can control such an abrupt attack.  The other thing I'm looking at is the difference in release between a feed-forward circuit like the ET and a feedback circuit like the Pellucid.  The ET measures the signal volume coming into the compressor and uses that to adjust the gain.  The Pellucid measures the signal volume coming out of the compressor and uses that to adjust the gain.  Both gain control methods have advantages and disadvantages.


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## lrgaraujo (Feb 22, 2022)

Hey Chuck, should tr1 and tr2 be as well matched in the Pellucid rectifier as they should in an octave circuit, for example?


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## Chuck D. Bones (Feb 22, 2022)

Good question.  Transistor matching is not critical in this circuit.


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## music6000 (Feb 25, 2022)

Chuck D. Bones said:


> For the moment, yes, but I will come back to the Engineer's Middle Finger.
> The problem I was having with the EMF was that at high RATIO settings, the attack was too pronounced.  My thinking was that the peak detector was not tracking a rapid rise in the envelope quickly enough.  The Engineer's Thumb uses a half-wave rectifier in the peak detector circuit and tracks only the negative peaks in the guitar signal.  This can cause a 1/2-cycle delay in the peak detector.  Other compressors, such as the Delegate & General Tsao, employ a full-wave rectifier that tracks both peaks.  The Pellucid also uses a full-wave rectifier in the peak detector.  I wanted to see how much difference that made.  The Pellucid's full-wave rectifier does improve the peak tracking and the attack is smoother, however it's my observation that aggressive picking with the bridge pickup creates an attack so abrupt that there is still somewhat of a "pop" on the first cycle of the note.  I'm starting to think that I had an unrealistic expectation of how well a compressor can control such an abrupt attack.  The other thing I'm looking at is the difference in release between a feed-forward circuit like the ET and a feedback circuit like the Pellucid.  The ET measures the signal volume coming into the compressor and uses that to adjust the gain.  The Pellucid measures the signal volume coming out of the compressor and uses that to adjust the gain.  Both gain control methods have advantages and disadvantages.


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## Chuck D. Bones (Feb 25, 2022)

music6000 said:


> View attachment 23512


Nice, but now it has 4 knobs.


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## fig (Feb 25, 2022)

Chuck D. Bones said:


> Nice, but now it has 4 knobs.


One is invisible...it's all the rage.


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## music6000 (Feb 25, 2022)

Chuck D. Bones said:


> Nice, but now it has 4 knobs.


I should have Known!
Version 2 Prototype :


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## Chuck D. Bones (Mar 1, 2022)

OK, I've taken this circuit about as far as I can.  I'll see if any of the tweaks I did here can be applied to the Engineer's Thumb board.  Latest changes:
1. Put one of the LM13700's internal buffers (U2D) to work in the gain control part of the circuit.
2. Added a RELEASE control.  In the CS-3, it's called ATTACK, but it's the same control.  It governs how quickly the gain rises as the note decays.
3. Put the LM13700's linearizing diodes to work (pin 2).  I have no idea why no one else has done this.  Since the LM13700's inputs are connected in parallel, one set of linearizing diodes serves both sets of inputs.  Adjusted the value of R9 accordingly.
4. Put clipping diodes (D4 & D5) on the output to limit the initial overshoot.  These diodes only conduct during the first few milliseconds of the note attack. They don't add any distortion because U1A is hitting the rails during that time.
5. Put in C3 to minimize the DC offset on U1A.
6. Reduced C2 to preserve the brightness at high SUSTAIN settings.

I tried TL072 and NE5532.  Both work well.  NE5532 might be a little quieter.  The only noise I can hear is coming from the guitar.  Don't max out the SUSTAIN with single-coil pickups unless you like listening to hum.

With the linearizing diodes in place, the only source of distortion (after the first few ms of the note attack) comes from ripple on the gain control voltage (+ side of C8).  A few mV is all it takes.  The distortion is mostly 3rd harmonic.  The only time you hear it is during the tail end of the note decay when the gain is high and there is low freq content.  If you stay off of the low E-string, it's barely noticeable.  The Boss CS-3 does the same thing.  It's a consequence of using a VCA (voltage-controlled amplifier) for fast response time.  Turning RELEASE up above noon makes the distortion go away, but at the expense of a slower response time.  Optical compressors have the advantage of the LDR's slower response filtering out the control ripple.  It's a trade-off of distortion vs. response time.  If you're running a dirt pedal or a cranked amp, then that tiny bit of distortion doesn't really matter.

The control loop is very stable and does not overshoot or ring.  It is normal to hear "breathing" when you play chords because the peak detector senses everything and the low freq content is the loudest.  Any beating between notes or feedback will cause the volume to go up and down and the gain control will try to track that.  Turning up the BRIGHT control helps a little by shifting the emphasis away from the lower frequencies.  Turning RELEASE up also helps by slowing down the response time.  It's a balancing act.


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## music6000 (Mar 1, 2022)

Like This:


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## almondcity (Mar 1, 2022)

Boneyard Edition when???


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## Chuck D. Bones (Mar 5, 2022)

OK, *now *it's done.  I'm still going to take another crack at the original ET, but this version which uses the output signal to control the gain is finished.  I changed how the SUSTAIN control works.  Like the CS-3, the SUSTAIN control limits how high the automatic gain control can set the gain.  The lower the voltage on C8, the higher the gain.  Turning SUSTAIN down raises C8's minimum voltage.  I put the other internal buffer in the LM13700 to work as a voltage clamp. To ensure that the SUSTAIN setting doesn't drift, Vref is now regulated at +5V.  If you don't have a B5K for the SUSTAIN control, then use B10K and raise R21 to 10K.  I used tantalum for C8, although aluminum will work.  I rearranged how the BRIGHT control works.  Instead of raising the treble gain, it lowers the bass gain.  Has the same effect on the tone, but this way it's less likely to run out of headroom.  With BRIGHT at zero, the freq response is flat.  A good initial setting is all controls at noon.  I changed D4 & D5 to red LEDs.  The only time they conduct is during the initial overshoot, which lasts a few milliseconds.  The max output with LEVEL dimed is about 1.5V p-p.  Unity is around 10:00.  If you want it to drive your amp harder, stick a booster after it.


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## fig (Mar 6, 2022)

I see U2B took up residence between SUSTAIN and RELEASE. Is that acting as a buffer to both?


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## Chuck D. Bones (Mar 6, 2022)

U2B serves two purposes:
1. It buffers the DC voltage coming from the SUSTAIN pot.
2. It acts like a diode and only sources current into C8.  If Q1 & Q2 want to raise the voltage on C8, U2B does not interfere.

When there is no guitar signal, C8 discharges thru R18 and VR4 until U2B starts conducting.  U2B sets the minimum voltage on C8.  The voltage on C8 feeds U2D (another buffer) which feeds current to U2A & U2C via R19 & R20.  The current in R19 & R20 set the gain of U2A & U2C.  U2A & U2C control the feedback around U1A.  The overall effect is when the voltage on C8 rises, the gain of U1A goes down, and vice-versa.  By setting the minimum voltage on C8, the SUSTAIN control is setting the maximum gain.  I stole the SUSTAIN control idea from the Boss CS-3.  The CS-3 uses an opamp & a diode between the SUSTAIN pot and capacitor; I used a Darlington emitter-follower (U2B).


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## fig (Mar 6, 2022)

Kinky!
I've got a crisp new breadboard itchin' to be poked.


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## Chuck D. Bones (Mar 6, 2022)

I'm also cooking up an optical compressor based on the Hollis Flat-Line.


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## fig (Mar 6, 2022)

I printed that out and showed it to my wife. I asked "Do you know what this is?" acting all smart, she looked it over, handed it back and snipped "My good printer paper" and walked off.


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## Chuck D. Bones (Mar 17, 2022)

Final update:
I changed C7 (tant), C8 (film), R18, R21, VR2, VR3, added R23.  78L05s can be a little noisy and R23 kills the noise.  C4 can be 22nF or 33nF.  33nF adds a little more midrange when BRIGHT is turned up.  With compressors, it's a trade between speed and distortion.  Fast ones can make distortion and low distortion ones have a slower attack.  Setting RELEASE below noon will produce some distortion on the lower strings, but the initial pop when SUSTAIN is dimed has been minimized.  Not a bad compressor, but not as good as the Delegate IMO.  Next up: one more look at the Thumbsucker.


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## Barry (Mar 17, 2022)

You really need to change the title on these to "This Month(s)"

Edit: Though I jest, I really appreciate the time you take to tweak these circuits and explain the changes and their effect


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## Chuck D. Bones (Mar 17, 2022)

LoL.  I'm glad you guys are deriving some benefit from my ramblings.


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## music6000 (Mar 17, 2022)

Chuck D. Bones said:


> Final update:
> I changed C7 (tant), C8 (film), R18, R21, VR2, VR3, added R23.  78L05s can be a little noisy and R23 kills the noise.  C4 can be 22nF or 33nF.  33nF adds a little more midrange when BRIGHT is turned up.  With compressors, it's a trade between speed and distortion.  Fast ones can make distortion and low distortion ones have a slower attack.  Setting RELEASE below noon will produce some distortion on the lower strings, but the initial pop when SUSTAIN is dimed has been minimized.  Not a bad compressor, but not as good as the Delegate IMO.  Next up: one more look at the Thumbsucker.


To Sum it up, Do you think it is worth building or not???


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## Chuck D. Bones (Mar 17, 2022)

I'm not going to take it beyond the breadboard stage.  So, in summary, no.  It's given me some ideas on tweaking the Thumbsucker and also caused me to reconsider optical compressors.


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## fig (Mar 17, 2022)

Chuck D. Bones said:


> I'm not going to take it beyond the breadboard stage.  So, in summary, no.  It's given me some ideas on tweaking the Thumbsucker and also caused me to reconsider optical compressors.


Well, at least I don't feel as bad about not finishing it


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## Chuck D. Bones (Mar 22, 2022)

Back to the Engineer's Thumb...





This version is sounding pretty good.  Some of it is easy to kludge onto the Thumbsucker board, other parts not so much.

The ATTACK control was in the wrong place.  When it was inside U2A's feedback loop, it did almost nothing because U2A would just drive harder and longer to overcome the delay caused by the ATTACK control.  Located outside the feedback loop, the ATTACK control can actually slow down the attack response.  I wanted a solid Vref, so I used a 78L05.  Vref2 is very clean, Vref1 is not quite so clean but can source more current.
This freed-up U1B, so I configured it to drive U3 differentially.  Makes for lower distortion, although the difference is small.  R202 biases the linearizing diodes for very low distortion.  R200 & R201 balance the currents into pins 1 & 16 of U3.  I consider the THRESHOLD control to be useless at best, so I replaced it with R207 & R208.  C200 was added to prevent oscillation in U2A.  The BRIGHT switch was replaced with a BRIGHT control.  It can add up to 8dB treble boost.  C201 was added to improve DC bias stability.  D6 & D7 absorb some of the initial overshoot when RATIO is maxed.  R206 was added to smooth out the bottom-end of the RATIO control's rotation.  R210, Q200 & D200 provide an option level indicator.  Use a superbright LED for D200 and adjust R210 to vary the maximum brightness.

As Valve Wizard says in his write-up, this is a compressor, not a limiter.  It reduces the dynamic range but does not flatten it.  The RATIO control sets how much the dynamic range is reduced by limiting the maximum amount of gain available.

This works very well as drawn.  I'll see if I can simplify it a bit to make modding the Thumbsucker easier.

One more thing: C5 & C6 must be film for minimum leakage.  Aluminum electro is too leaky and even tantalum is borderline.


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## Dan M (Mar 23, 2022)

Pros and cons vs The Delegate?

(Up above on the previous version you commented “not as good as the delegate”)


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## Chuck D. Bones (Mar 23, 2022)

Now that I have the EMF working the way I want, I'll run another comparison with the Delegate and report back here.


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