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

[POTL]

merlinb

Thank you so much
I do not have all the components in my store, your explanations helped me a lot.
Regarding the volume control, I can set the output buffer to avoid the effect of the volume control on the output impedance.
The volume controls of the 100K classic in many distortion pedals, I thought it was a classic and logical denomination for the regulator, maybe it's worth it in the future to try to decrease the volume control's rating in its layouts.
Once again, thank you very much!


If I correctly understand the circuit input resistance 10M // 1M (Threshold Potentiometer) = 1M
It turns out that if the threshold setting is changed, the input resistance will change.
I'm right?

[POTL]

> Transistor BC327

"Any PNP". The specs are nearly unimportant. I can't think of any PNP that won't work. >4V, >4mA, hFE>1, fT>0.1KHz.

You do NEED to know the pin-out. If you get the Base wrong it can't work. I suspect C and E could be swapped and 9 of 10 times it would "work", maybe less-good.

I still do not understand the principle of operation of such effects as  noise gate and compressor
So I ask such questions, I gradually study the effect behind the effect, but until noise gate and the compressor has not reached yet

[merlinb]

The volume controls of the 100K classic in many distortion pedals,

Yeah, pedals from the 1970s! 

It turns out that if the threshold setting is changed, the input resistance will change.

No the input resistance stays the same. The wiper moves up and down the track but the total resistance when looking into the top of the pot does not change.

[POTL]

The volume controls of the 100K classic in many distortion pedals,

Yeah, pedals from the 1970s! 

It turns out that if the threshold setting is changed, the input resistance will change.

No the input resistance stays the same. The wiper moves up and down the track but the total resistance when looking into the top of the pot does not change.

80s and 90s too =)
Do you recommend changing the volume controls to A10K?
I have 2 projects based on Marshall Blues Breaker and Fuzz Face / Tone Bender (the default setting is generally 500K)

[samhay]

10k is a good value for a volume control connected to the output of an op-amp.
The Fuzz Face doesn't, AFAIK, have an op-amp at the output.
The Blues Breaker does use op-amps, but the volume control comes after the tone control, etc, so is not directly connected to the output.

Ultimately, this decision requires some understanding of impedance...

[Elijah-Baley]

Quicky question. I have none 1N4148, just some other diodes. Can I replace temporarily the original diodes 1N4148 with some of these:

DIODE         N.
Bat41         4
Bat46         2
1N914         1
1N60P         3
1N34A         1
1N270         2
1N4733 zener      1
1N4005         1
1N4007         4

Thanks!

[blackieNYC]

1N914 is close enough, and I'm sure the 400X would work.  Ge and schottky and LEDs would probably change operation a bit. Should avoid the 1N60, 1N34, the BATs, and the zener.
Looks like you really only need three for the basic circuit.

[Elijah-Baley]

Thank you. In case I'll do some tests with the 1N4007.

[POTL]

Quicky question. I have none 1N4148, just some other diodes. Can I replace temporarily the original diodes 1N4148 with some of these:

DIODE         N.
Bat41         4
Bat46         2
1N914         1
1N60P         3
1N34A         1
1N270         2
1N4733 zener      1
1N4005         1
1N4007         4

Thanks!

1n914 - uniquely, these diodes often figure as a replacement for 1n4148 and vice versa

[Elijah-Baley]

Too bad, I have just one 1N914. Anyway, I already have in the cart the three 1N4148.

[jonny.reckless]

@rankot you might not need the 100pF feedback capacitor around op amp A if you use an externally compensated op amp such as the NE5534. A 68pF compensation capacitor is pretty heavy compensation for that amplifier, and should be unconditionally stable for all gains depending on the stray capacitances and inductances in your particular build. I found I didn't need it. And yes, you just parallel up the OTAs for a 3dB improvement in noise figure. The 1k resistors are there just to make sure the current mirrors in each half of the LM13700 get approximately the same current into them.

Merlin did a great job here. It's rapidly becoming my favorite compressor. One other small mod I have done to mine is to add a J112 as a common drain (source follower) buffer on the input stage, so I could use a lower impedance 100k pot for the threshold control, which I found also improved the noise level a small amount.

[rankot]

Merlin did a great job here. It's rapidly becoming my favorite compressor. One other small mod I have done to mine is to add a J112 as a common drain (source follower) buffer on the input stage, so I could use a lower impedance 100k pot for the threshold control, which I found also improved the noise level a small amount.

You mean something like this?

[jonny.reckless]

You don't need an input cap, or to bias the gate of a J112 to 4.5V, just connect a 1M from input to ground, and a series gate stopper of 1k to 10k into the gate. The drain goes to 9V, the source has a resistor of around 4k7 or so, and it will bias somewhere between 2 and 5 volts due to the VGSoff off the J112.

Something a bit like this. Apologies for the shitty hand drawn circuit. I'll try and sketch up something a bit better.

[rankot]

Thanks, I am building this!

[rankot]

Maybe stupid question, but if I increase C8 (treble boost capacitor), will it decrease treble boost threshold frequency or make it bigger?

[samhay]

^ increasing the 4n7 cap will reduce the corner frequency. If you want more gain, decrease the value of the 10k resistor in series with this cap and then re-scale the 4n7 resistor appropriately - e.g. 4k7 + 10n.

[rankot]

Is it calculated the same as on this page?

http://www.muzique.com/schem/filter.htm

[samhay]

^yes, that will calculate the high-pass corner frequency.

[CC-TOP]

https://dl.dropboxusercontent.com/u/57831278/TheEngineersThumbGTI.pdf
Are these still available? Dropbox says the file is not found

I have fixed the PDF link, but I can't seem to find the bitmap at the moment...

Is the pdf still available somewhere? I would like to include the LM3915 LED indication in my build

[rankot]

I've built Fencepost noise gate with success few days ago, and since it is also using LM13700 OTA as it's main engine, I thought that it could be nice to combine it with Engineer's Thumb, so to have a single Gate/Compressor pedal. This is what I have in mind, but I would be happy to have some suggestions from experienced DIYers. I've ommited switches (treble boost and gate/expander) from schematic, because I don't think there would be enough place for them on a 1590B pedal, and I also added compression indicator LED as proposed here by PRR, although I never made it work good enough.