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

[Chuck D. Bones]

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.

 

[music6000]

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?:

 

[fig]

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

 

[Chuck D. Bones]

I assure you, mine is fully intact.

 

[fig]

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?

 

[princeofyams]

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?

 

[Chuck D. Bones]

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.

 

[Chuck D. Bones]

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

 

[cooder]

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

 

[princeofyams]

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.

 

[Chuck D. Bones]

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.

 

[princeofyams]

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.

 

[Dan M]

LM13700 and 2x TL072?

 

[Chuck D. Bones]

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.

 

[Chuck D. Bones]

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.

 

[fig]

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?

 

[Chuck D. Bones]

Touché.

 

[Chuck D. Bones]

I found another clever compressor design. Hmmmmmm.

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

 

[cooder]

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

 

[Chuck D. Bones]

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.