Seamoon Funk Machine Pot

[Rochey]

Heya folks,

I'm having a real hard time finding 5Mohm Linear pots to use on this pedal. Has anyone an alternative, or another source in the UK?

many thanks,

R :wink:

[Paul Perry (Frostwave)]

If you can find a 2.5M dual gang pot, you can wire the two sections in series, to fake an adjustable 5M.

[Mark Hammer]

Personally, I wouldn't recommend anything much greater than 1meg for that pot.  If you want to get nasty, stick a 1meg resistor in series with the pot and wireup a SPST toggle in parallel with the fixed resistor so you can select a 47k-1047k range or 1047k-2047k range.  Trust me, though, you won't like the upper range all that much.  Note as well that it gooses the gain, so you will definitely want to stick a 100k log pot on the output, after the .0047uf output cap, to be able to trim level for volume balance.

As with the Dr. Q, reducing the 100R resistor value will speed up attack time and make the unit appear more sensitive.  Increasing it will provide a softer, slower attack, but make the unit seem less sensistive.  I wouldn't go much below 47R or much above 1k.  If you get an objectionable raspberry-like trill during the decay of notes, increase the 10uf cap after the 100R resistor to 22uf.  This will add some lag and make the filter sweep decay more slowly.

If the unit is generally not sensitive enough, increase the 2.2M feedback resistor to 2.7M.

The filter range is set by the pair of .005uf (.0047uf, really) caps.  If your intention is to use it for bass, change these for .01uf to drop the range by an octave.

[Rochey]

Mark,

I'm hoping R.G won't mind me borrowing the schematic from his site just for a bit (i'll remove it at request). I just want to double check if I've understood the mods you recommend.



I've circled what I've understood from your message. Please double check for me? I've already made the PCB's so I'm hoping most mods will be painless  

Cheers

R

[Mark Hammer]

Yeah, generally 98% correct.  The small value resistor that adjusts attack time has a side effect of reducing sweep width and apparent sensitivity, since it also restricts current.  I have a short document ( http://ampage.org/hammer/files/fast-slow.png ) indicating how to easily switch from "fast" to "slow" in envelope-followers like the one used here.  If you used a DPDT togle, you could simultaneously change the value of the feedback resistor in the gain stage just ahead of the diode so that gained was boosted a little when using a larger (say, >390R) value series resistance for slower attack.  That would give relatively constant sensitivity/sweep in fast and slow positions.

[Rochey]

can you confirm the pedal name for each pot? I'm still trying to work out what does what..

cheers

R

[Mark Hammer]

The *suggested* 5meg pot sets the gain of the filter around the centree frequency, hence it can be called the Q or Resonance control.  Lots of Q can sound great in some filters, but not in this one.

The 100k pot on the output is, you guessed, it, an output level pot.

The resistance that can vary between 47R and 1k (though a 47R in series with a 500R pot is probably optimal) is the Attack Rate pot.  The reduced sweep is simply an annoying side effect that is coipensated for by cranking up the 50k pot, which is the Sensitivity control..

[Paul Perry (Frostwave)]

Is the bias right on the bottom op amp??

[Mark Hammer]

It is identical to the biasing on the original Dr. Q.  The only thing that is really different between them is the filter type, the value of the sensitivity pot, and the way the transistor is handled. In the DQ/NQ, there is a trimpot between the envelope follower and transistor, plus an emitter resistor.  In the FM there is a bizarre little voltage divider made from a 10k and 13k fixed resistor tied to the base.  I wonder if that could be improved a little.  It may also account for why I found the use of feedback resistances in the >2M range so appalling.  Having a means of tuning the transistor might be the key to an optimal sound.

[Paul Perry (Frostwave)]

Well, my suspicion is that it will behave very differently depending on what op amp is used! Hope someone builds it & puts a socket in :wink:

[Mark Hammer]

I'd rather put a socket in than a sock in it. :wink:

I made mine a while back, went "Okay" and cannibalized the parts.  I honestly forget what I used for op-amp, but chances are it may well have been a 1458.