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

[Hatredman]

Hey, Merlin, did I miss something?


merlinb

[patricks]

Try this one: http://www.valvewizard.co.uk/engineersthumb.html

[chuckd666]

Howdy gang, having issues with my circuit. I'm getting no output signal, it's dead quiet. I built it on Merlins PCB layout and the etch turned out quite well.

Images are below.

Voltages as follows (from 9.37V~ supply):

U1
1. 4.68
2. 4.68
3. 2.34
4. 0.00
5. 2.34
6. 4.69
7. 4.69
8. 9.37

U2
1. 4.2
2. 4.68
3. 2.35
4. 0.00
5. 3.13
6. 4.69
7. 4.69
8. 9.37

U3
1. 1.08
2. 0.00
3. 4.69
4. 4.69
5. 4.69
6. 0.00
7. 0.00
8. 0.00
9. 0.00
10. 0.00
11. 9.37
12. 0.00
13. 0.00
14. 0.00
15. 0.00
16. 0.00

Q1
e. 4.68
b 4.19
c 1.1

(Substituted 3mm LEDs for 5mm, BC327 for 2n3906, 47u power supply filtering is tantalum, 100n and 4.7n caps are ceramics due to tight PCB spacing, ICs are soldered because I'm an idiot )

[garcho]

there's not that much of audio signal path, but have you tried ye olde audio probe, starting at the input jack, moving along each junction? the voltages seem ok at a glance

[chuckd666]

Yeah hmm, I will give ye signale proybe a go later (at least properly). It seemed to stop before the transistor, at last check. Also it's very distorted at pin 5 of the LM13700 and cleans up when I turn the ratio down, hm. I get nothing at the level pot which rules out the millenium bypass being an issue (at this stage anyway). I couldn't find 'proper' voltages anywhere online, but if you say they seem ok that's good!

[garcho]

by "audio signal path" i mean from the input jack to the 1kΩ resistor, the 10nF capacitor, the non-inverting input of U1A, the output of U1A, that 10µF capacitor to the following 1kΩ resistor to the 10kΩ pot, to the output jack. not all that much point in audio probing the rectifier. that does sound kinky, though.

[chuckd666]

Yeah I didn't think it had anything to do with the audio path... lol. Ok. I'll look at it tonight after work. Thank you my friend 

[Beo]

not all that much point in audio probing the rectifier.

Heh, that's a good one!

[chuckd666]

I'll bloody probe your rectifier if you're not careful. JUST KIDDING. Thank you for the advice and yes, I am a noob.

I just got the point of rectifiers.. they convert your guitars AC signal into DC so it can change the gain or something right?

[PRR]

> the point of rectifiers..

Audio swings both ways, constantly.

The gain-control element wants a one-way signal, not changing (well, changing slowly with loudness, not with every individual swing of the string).

Rectifiers can convert the both-ways audio into a one-way signal.

There's a capacitor somewhere to smooth-out the individual string-swings into a steadier gain change signal. (Actually two smoothing caps.)

[chuckd666]

So the 1uf caps remove the AC-ness to give it steady DC.

U1A is audio path amplifier, and U1B is for the gain control part. Ok I found your walkthrough of the circuit from page 2! Very helpful Mr PRR.

[merlinb]

Voltages look OK, and I can't see any incorrect parts in the photo. This suggests to me it may be something that is not in the photo. Input/output wired up correctly?

[chuckd666]

Hello everyone! I have worked out my issue. As always, it was a few weak traces which were not giving continuity. Duh. I think it's all good now, very compressy! At full output I'm just under unity - is this normal?

Thank you all for your help so far! You're all very lovely and patient.

[electricco]

Hi, I built it (5 pots and 1 switch) but has an issue.

There is a ramp into the output volume  (from low to hi).
It's happened with any pots regulation...
I've already changes all the IC but nothing.

What I should look at ?

[merlinb]

There is a ramp into the output volume  (from low to hi).

Can you describe this symptom in more detail?

[287m]

hi folks
i just chat with friend, and in middle conversation he say want engineer thumb (again)
he build 2 knob from merlin pdf, he just want add knob but dont want build GTI version
so he redraw schematic like this, no treble boost

so, before i make layout and buy part. just need correction and suggest mods. hehe
Thanks

Febri
===
oh i love edit function in this forum
assume above schemo is right, then he good solderman, and i am big fans of sijosae
here the prev for onboard pots and led in perf https://1.bp.blogspot.com/--Rr1afqCEUo/Vwb9D1LhGuI/AAAAAAAAAJM/sbAj_egsspAI3le6g5Bpfpl4gYX09TC5A/s1600/et.png
the moral today, dont say 'I Can' as fast before think when your friend request all onboard in 1590A!

[Hatredman]

There is a ramp into the output volume  (from low to hi).

Can you describe this symptom in more detail?

I guess he meant that the attack is reeeeeeeeeal slow.


Sent from my iPoor using Tapatalk - now Free because I'm poor

[midwayfair]

Someone on Madbean was having trouble with an added threshold control, and I noticed that the usual suggestion seems to be to replace the 1M with a pot wired as a VARIABLE RESISTOR. There's a bunch of reasons that doesn't work well. I popped over here to suggest, or clarify (Merlin's intention wasn't entirely clear from the graphic on the original schem) that it should be a VOLTAGE DIVDER instead.

It looks like 287m, or his friend who re-drew the schematic he posted, got it right. Just to clarify, if it's a variable resistor instead of the voltage divider, the input impedance drops and the only reason you get a lower threshold is because you're losing signal.

Still probably not worth including the threshold control given the way the design works, but I thought it was worth pointing out.

[Yazoo]

I built this a while ago using the Valve Wizard pcb, the original two pot version Unfortunately, I have not been able to get this working properly. I get a drastic volume drop when the effect is on with the ratio pot turned fully up. I have been trying to debug it but I can't find any mistakes, though I must have done something wrong.

These are my voltages:

U1
Pin 1 4.5 V
Pin 2 4.47 V
Pin 3 3.9 V
Pin 4 0
Pin 5 3.9 V
Pin 6 4.47 V
Pin 7 4.55 V
Pin 8 9 V

U2
Pin 1 3.89 V
Pin 2 4.45 V
Pin 3 3.9 V
Pin 4 0
Pin 5 4.17 V
Pin 6 4.48 V
Pin 7 4.48 V
Pin 8 9 V

U3
Pin 1 1.19 V
Pin 2 0.73 V
Pin 3 4.48 V
Pin 4 4.48 V
Pin 5 4.48 V
Pin 6 0 V
Pin 7 0 V
Pin 8 0 V
Pin 9 0 V
Pin 10 0 V
Pin 11 9 V
Pin 12 0.6 - 1 V
Pin 13 0.7 - 0.86 V
Pin 14 0.52 V
Pin 15 0.77 V
Pin 16 = V

Q1 C 1.18 V
     B 3.9 V
     E 4.45 V

I used a signal generator and oscilloscope to try and trace the signal through. The output on U1 pin 7 to the two diodes looks clipped and healthy but there is no signal after D1 and I can't see anything going into pin 3 of U2.

Any advice would be gratefully received, thanks.

[patricks]

Still probably not worth including the threshold control given the way the design works, but I thought it was worth pointing out.

Hi Midway,
Just wondering what it is about the design that means the threshold control isn't really worthwhile. I'm planning to build the gti version and was going to add a threshold control; I most likely won't now, but I'm interested to know more about the circuit.
Cheers
Pat