English Channel 250K-A Cut pot

[Bill Bergman]

Would a 100k or 500k pot work here? I don't have a 250k.

Also on all or most  the amp sims at ROG there are master volume controls as well as volume controls. Are both needed or does one act as a gain control?

[Peter Snowberg]

If you want to "make" a 250K pot from that 500K, try adding a 750K from end to end and then add 1.45M (or 1.5M this is Rock-n-Roll ) resistors from each end to the wiper. That gets pretty close. Wiper error is roughly 4.3% and total resistance error is roughly 8.5%. These errors happen most at the center of rotation, with a clean taper off to the ends. At the ends of rotation the values are almost perfect with less than 1% error.

The actual numbers are like this:

Code Select Expand


1            2            3

!  Rpot*2.9  !  Rpot*2.9  !
+--/\/\/\/\--+--/\/\/\/\--+
!            !            !
!            V            !
+--------/\/\/\/\/--------+
!           Rpot          !
!                         !
+---------/\/\/\/\--------+
         Rpot*1.5

The actual resistors should be chosen based on your pot (and what you want your pot to become). I did a lot of graphs using a PHP program to figure the 1.5 and 2.9 coefficients out. I got to them by trial and error, watching the graphs shift. The numbers are about as close to the desired results as I could get.

If you want to cut a pot value in 1/4, use 0.47 and 1.1 intead.

Take care,
-Peter

[B Tremblay]

You're correct, the Volume controls are just Gain under a different name.

The Master Volume is needed if you want to have the option of full gain at lower volumes - just like an amp.

100k or 500k are fine for the Cut control, but you may want to adjust the cap value.

[Bill Bergman]

Cool, thanks.
I got the EC and the Tweed boards populated. Just got to wire'm up. Might do the Chief also.

[Bill Bergman]

Peter, does your suggestion apply to a linear or audio pot?

[Peter Snowberg]

Whoops! :oops:  That's for linear. I have no idea how it would look on an audio pot.

[Bill Bergman]

Thanks Peter, I'm a little rusty.

Brian,
what's your favorite of all the amp sims you guys have come up with?

[B Tremblay]

I'd have to say that my current favorite is the Supreaux.  It has a great range of sounds that come from a slightly different angle than the Professor Tweed.

[Lonestarjohnny]

I've built all but the Matchbox and I've got it perfed, waiting for the Switch to come in to finish it, I call each one my favorite, the Guy's at ROG ROCK, the Supereau is indeed a good pedal, But I love The Professor,    
JD

[Bill Bergman]

Peter, are the two 1.5m resistors to the wiper ther to make the linear pot act like an audio pot or what do they do?

I put a 560k resistor on lugs 1 and 3 on a 500k linear pot with out the 2 other resistors you show in the diagram and got about 260k.

[Bill Bergman]

Finished the EC and it sounds great!!!  
Here's some pic's (2 and 3). http://www.freewebs.com/elroy2/

How much effect should the cut control have? It seems to cut the harshness a little when the treble is high but not a whole lot.

I tuned the trims to the suggested drain settings although my Vin was @9.56v instead of 9.1v. I did not check the source and gates.
I'm impressed, now I can't wait to build some of the others(tweed is done, just needs off board wiring). Think I'll try the Uno next.

[B Tremblay]

I'm glad to hear that you like it!  Thanks for the report.  The photos are beautiful!  Would you be willing to share the PCB layout?

Did you build the Vox amp sim (the one with the non-disclosure agreement) as well?  If so, how do they compare?

The Cut control should function as you describe, shaving off the extreme highs.  The Treble pot sets the baseline and the Cut allows for fine tuning the upper freqeuncy content.

If your power supply is higher than 9v, just set the drains to 1/2 of the supply voltage.  I'm using a regulated supply with a 9.1v output, so I set mine for 4.55v or so.  A half volt shouldn't make too much difference.

[Bill Bergman]

I'd be glad to share the pcb layout. In fact I done one for the EC,PT,Uno and TC.  Where can i send them for you to post.

Yes, I made the other Vox sim. I haven't compared them yet, sided by side. Picture 4 is the other Vox sim.

[B Tremblay]

Check your e-mail...