Potentiometers: what do you suggest??

Gene & Debby Stopp squarewave at jps.net
Wed Jun 11 18:32:39 CEST 1997


This is the drawback to this type of ADSR design - when the time constant
pots are directly used as the R in the RC equation. I intentionally
designed it to get down to the low-single-digit millisecond attacks and
decays, but that does mean that costlier pots are needed for precise
control.

With all this talk of finding good pots I'm embarrased to admit that my
usual pot-buying routine consists of scavenging around the surplus bins,
usually at All Electronics in Van Nuys, where the pots are usually around a
buck apiece. I buy huge bunches and then pick through the box at home until
I find one I like. It turns out that my machines are usually somewhat
quirky in their behavior and definitely no two are alike. But heck no two
viloins are alike either and these are musical instruments, right? (I think
I'm subconsciously trying to impart a more "human feel" to the machines by
randomizing each individual component decision!)

I do like the inifnite slope attack sometimes, for that Jan Hammer "poppy"
sound... but it shouldn't be annoying. Is it a loud "thump"? Perhaps your
VCA has some offset that needs to be trimmed with the trimpot.

Now I really want to dust off the ASM-1 test bed in the garage. I'm at home
now for good - I turned in my Fibermux badge on Monday and the severence
check is forthcoming. Most activities have involved the inevitable
housework that comes when you get large chunks of spare time but in the
next few weeks I should be able to sneak in some synth work (the workbench
situation in the garage is now like a small manufacturing organization).
Check out my new email address - my wife set up our accounts, and she told
me "You're account is called "SquareWave" and I said "Cool". She made that
up for me.

BTW I did not have time to un-sub from Electronica or Bode - advice in that
matter would be most appreciated since my email archives at work were
killed in a server crash.:(

- Gene
squarewave at jps.net

----------
> From: Christopher_List at Sonymusic.Com
> To: synth-diy at horus.sara.nl
> Subject: Re: Potentiometers: what do you suggest??
> Date: Wednesday, June 11, 1997 12:57 AM
> 
> The pots you are referring to are "Xicon @ Alpha" pots - it says it on
the
> top of the catalog page. I have only these Mouser pots in my ASM-1. The
> pots say "Mouser" on the back, but they are made by Xicon. Now I don't
know
> if they're the same as the RS parts, but come on, they're $1.25 each!!!
You
> can't expect a lot from that kind of pot.
> 
> I was, in fact, just working on "the dead zone" problem last night. Seems
> the 1M audio taper pots I have for the attack and decay are at 0 ohms for
> the first few fractions of a turn, then suddenly jump to about 3.5K! -
OK,
> I know that 3.5K isn't alot relative to 1M, but it's noticable when the
> attack goes from about .1ms to 10 or 15ms!
> 
> I don't much care for the pop created by a superfast (.1ms) attack, and
10
> - 15ms is too slow, so I was trying to figure out what to do. The way I
see
> it, my options are;
> 
> 1. Put an 800ohm resistor in series with the pot so that the min attack
is
> around 4ms. The down side is that this is intentionally degrading the
> performance of your synth - something the synth gods frown upon. I could
> put a bypass switch in, but this is a waste of real estate for something
> that'll rarely get used...
> 
> 2. Lower the value of the cap in the ADSR so that when the pot jumps to
> 3.5K, the attack time only jumps up to 5ms or so. This results in shorter
> max attack - again degrading synth performance.
> 
> 3. Get a perfect pot. Must be 1M, audio taper, 1/4" shaft - these are
hard
> to come by. I could go with a linear pot (like the RV4), but I'm almost
> always in the lower 50% of the pot travel when it comes to attack - so
this
> would be bad. Actually, this is a good reason to go with option 2!
> 
> 4. Put a LPF on the CV for the VCA to filter out pops. Again degrading
the
> performance of the synth - I'm also not sure how to do this given the
fact
> that the summing amp for the CV in the VCA doesn't have a feedback
resistor
> and is current output....
> 
> - Any other ideas?
> 
> - CList
> 
> 



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