The "Weak Joe" compressor: PCB vs schematics

[Nordskov]

I am going to build a compressor for use with my bass.
At first I thought of cloning the Carl Martin compressor/limiter, but after some research on the internet, I found a promissing design: The "What compressor".
But because of the lack of controls like attack, threshold etc, I moved on to the "Weak Joe" mod:
http://www.geocities.com/m_natsume/what_compressor.html

Now after having investigated the details I found many inconsistencies between schematics and PCB layout. Different components and values.
The author claims the PCB to be "verified as 100% functional."

The question is; do I proceed following the PCB layout, or do you have any comments before I start?
Any tips will be appreciated.

Regards,
Henrik Nordskov

[Nordskov]

Heh, I have to do some more thorough research before I post 
I did a search on Weak Joe and found that people just followed the PCB layout and components.

But one thing is still a bit unclear. How much voltage do the circuit need
Some say 15V, the original What Compressor schematics states 12V.

Another place I read that the input and output electrolytic capacitors (1uF and 33uF) should be bipolar, not polar.
What do you think about that?


Thanks,
Henrik Nordskov

[Coriolis]

Hi Henrik
I don't see why you couldn't go with 15V. Those chips can handle 18V bipolar I think (but check datasheet).
Only thing to look into might be R32 - some sort of bias trimmer coming from +15V supply. It looks like it adjusts the minimum current going to D6 (an indicator LED to show gain reduction perhaps?). I could be wrong...but I don't think it is critical. Since it IS a trimmer you could always...well...trim it!

Let us know how it works out - I've been wanting to build this one too...

C

[Nordskov]

Thanks for your reply, Coriolis.

I will stick to the 15V then.
I'm not sure whether to make it a stompbox or a rack effectbox.
I don't like too many wires and boxes on scene. Maybe if I could squeeze the PSU into the chassis I would go for a stompbox.

The trimmer will be trimmed  ... but do you have any opinion on the capacitor issue?

I will upload pics when finished, don't worry. Do never miss an opportunity to brag a little 


Regards,
Henrik

[slacker]

You're correct the input and output caps should be non polarised. I would just use the biggest film caps you can find instead of using non polarised electrolytic, 1u or 2u2 should be fine.
Considering how much he goes on about using "high quality" opamps, I'm surprised he used electros in the first place.

I could be wrong but it looks like the LED in the vactrol and D7 are the wrong way round on the schematic. Q1 is connected between +15 and ground so I don't see how it can turn the vactrol LED on, because it's output would need to go negative with respect to ground to do so. I've probably just misunderstood something though.

The other thing that's probably worth mentioning is that the input impedance is pretty low at 10k so if you plug a passive bass straight into it you might get some tone sucking.

[Nordskov]

slacker, I was thinking the same about the electrolytics. But what's the function of the in and out capacitors? DC decoupling? Then wouldn't 0.1-0.2uF be sufficiant?
I think you're right about Q1. Let me think out loud.
As I see it, D7 follows the vactrol by app. double the intensity (1k vs 470 ohms) indicating how much gain the vactrol recieves. The positive lugs on the vactrol and D7 is connected to the emitter of Q1 allowing it to control them both. The BC327 is a PNP transistor and that's why collector should be connectet to positive power rail.

Another thing. The myriads of 1uF capacitors keeping the ICs from occilating seems to have a mark on the PCB layout. Are these polar?

Please explain to me how can you tell the input impedance is 10k? The shortest way to ground is via R1, R2, R24 and vactrol. Am I missing something?
I do play an active bass.


Henrik

[Nordskov]

"Bump"

Will someone please answer the questions I have about this layout, so I can order the parts and get this baby up and going 

1. Is the vactrol and D7 connected wrong compared to Q1?

2. Are the op-amp decoupler capacitors polarised? They seem to have a tiny mark on the PCB layout.


Thanks,

Henrik

[slacker]

1. Is the vactrol and D7 connected wrong compared to Q1?

I was wrong before the LEDs are the correct way round. If you check against the original Joe Cheep schematic on that page you'll see that the collector of Q1 should be connected to -15 volts not +15. It looks like the layout is correct, it's just an error on the schematic.

2. Are the op-amp decoupler capacitors polarised? They seem to have a tiny mark on the PCB layout.

Yes they are polarised, the faint red marks show the positive end.

About the input impedance, I don't understand the technical side of it, but as far as I know for inverting opamp stages the input resistor (R1) sets the impedance.

[Nordskov]

Thanks slacker. You have been of great help.

The reason I questioned the input impedance is that I just finished a tube pre-amp where the input impedance is set by the 1M resistor from signal to ground - or maybe I am wrong again, it wouldn't be the first time 

Now I can order the parts and get going.

Just can't wait...


Henrik

[Ashurbanipal]

Hey Henrik,

Did you build this and how did it turn out? I'm looking at building it myself.

[Nordskov]

Hi Ashurbanipal.
No I did not proceed with the compressor - yet. My friend who has helped me make PCBs before hasn't got time to help me on this. And I haven't got the guts to do the etching myself so it's on a stand-by.

But I've got a good feeling about this project, so best of luck.
Please get back with a review when you're done.

Regards,

Henrik

[swilliman]

7 years later..

Fashionably late to the party chaps, but did either of you finish the Weak Joe?

Just about to embark upon it myself. My electronic knowledge is poor, but thinking of adding a VU meter with a driver as a buffer, would this work?

Also, are you 100% sure you don't need polar caps for I/O?

Cheers,

Will

[StephenGiles]

Worked for me on veroboard in rack case I made!