decreasing the value of a standard pot ?? (forssell comp)

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matthias

Well-known member
Joined
Jun 4, 2004
Messages
768
Location
germany / frankfurt
Hi,

Is it possible to decrease the value of a pot by adding a resisor in parallel??

The ratio control of the forssell opto compressor needs a 5k rev. log pot.
My supplier can only deliver a 10k neg log pot...

So I thought, I could decrease the value by adding a 5k resistor in parallel.... (2 resistors for one pot)

Will this work fine, or maybe cause any problems or change the curve??

thanks,
matthias
 
(sigh!) I think we need a potentiometer law/reverse log meta, or more use of the 'search' function...

Yes, Rafa is right, -this doesn't work as a log or reverse log variable resistor. -we've covered this a lot and it just doesn't work.

Secondly, think about this for a second... wiring a resistor in parallel with a 10k pot to try and make the maximum resistance = 5k (i.e.10k in parallel) will screw with your law... you'll end up with a whacked-out control. The 10k resistance will have little effect when the control reaches mid point (about 1k at mid point... the middle resistance should be 10% of the maximum resistance)

But think about this some more... if the law of a reverse-log pot is such that it increases tenfold with 50% rotation, then a two-fold increase overall is a tiny step by comparison.

One option is to just use the 10k INSTEAD of the 5k and forget about it. The difference in gain range is likely to be in the order of 6dB... if this is for a mic pre as an example. then insted of a 15-60dB gain range, you might get a 15-66dB gain range... on an EQ frequency sweep, you might have a slightly different problem, you'd get almost another octave of sweep range, which would turn a 5-octave range into a 6-octave range... as long as you're not replacing a pot in a previously-labeled device, this is not grave... you could alternatively double the value of the R/C componentry in the filter (about 3 or 4 components) and use the same frequencies.

But there's no way to make a 5k reverse log out of a 10k reverse log, and have it stay reverse log. Not unless you get a dual gang 10k reverse log, and join the three sets of pins, CW end to CW end, wiper to wiper, CCW end to CCW end.

That's it.

There's no other way.

Trust me.

Keith
 
Youre right. I should have thought before I posted the link in my response. I had only seen this on webpages before and assumed it would work. I just put together an Excel spreadsheet to simulate the response and it's not reverse log at all. It's a log type response but increasing more rapidly as you get away from the max value of the pot. I then tried it out of curisity in a mic pre circuit and it's worse than a linear pot........

Oh well, I always thought I could fall back on that circuit if I neded an obscure rlog pot. Oh well.....

Brian
 
There´s a way.

Use a log pot with backward rotation, or a mechanical interface to reverse the rotation. This has been covered here somewhere.

Yamaha use log pots wired backwards in the PM series mixer... You will turn the pot at the same direction, from left to right, but instead of turning above, you turn below. Sorry for the bad english. Hope you understand.
 
[quote author="BuzAllen"]I then tried it out of curisity in a mic pre circuit and it's worse than a linear pot.[/quote]

Yeah, this is due to the fact that the whole resistor-in-parallel trick actually gives you a backwards logarithmic law - exactly the opposite of what you want!

Peace,
Al.
 
This frigin' article keeps popping up over and over... Has anybody ever said anything to the Geo FX guy? Just curious.

Peace,
Al.
 
> I should have thought before I posted

Buz- no reason to feel silly. This difference (between loaded-pot and loaded-rheostat) is NOT obvious and has fooled many minds, even when it is pointed out.

What we often want in a Mike-Amp (I dunno what the Forssell compressor is doing) is a variable resistor that is 10K at one end, 1K in the middle, and 100Ω at the other end, to give gains of 60dB 40dB 20dB. Common audio-taper pots, wired as a rheostat, do that pretty well. Audio-taper evolved in AM radio where you need about 40dB range of gain from big/local to far/weak stations, and audio-taper gives 30dB or 40dB of usable change. Except the "10K" (mike-amp lowest gain) end winds up as the clockwise end of rotation, and most audio knobs have low-gain at the counterclockwise end of rotation.

BTW: look at most kitchen stoves. "Big flame" is usually COUNTERclockewise, and "turn down" is a clockwise rotation. So if you want to turn-up the fire and turn-up the kitchen-radio, you turn the two knobs in opposite directions. Maybe all we need to do is use kitchen-stove knobs on our straight-audio mike-amp Gain pots?

Ah, can't break soundman hand-habits. So we want a reverse-audio taper. There are only two uses for these: reverse-James feedback tone controls with gain (some Quad and Dynaco), and our vari-gain mike amps with gain control in the shunt side of the feedback network (to minimize noise resistance). Dyna is dead, Quad is either dead or far beyond where they were in 1964, and the world's demands for aftermarket generic mike-amp-pots is very small (mixer-makers special-order big cartons of pots for production and repairs).

For some unknown reason, 10K reverse-audio is often available. It will work in many mike-amps, even if they call for 5K. The difference is in the "9" to "10" range: the 5K will have 100Ω at "9", the 10K will have 200Ω at "9". What we want is something like 50Ω at "10" and maybe 100-150Ω at "9". A 50Ω fixed plus a 5K reverse audio-taper can give a smooth shift over this range, a 10K will be very "jumpy", sudden large change of gain in the "10" to "9" range. For folks who use only hot-mikes on loud acts, and never go near "10", this may not matter. For my dynamics on choir, the high gain I need is right in that crack where the gain is too "jumpy" to set accurately.

You could scale the whole network higher, to suit the available pots. But in transformerless amps, we want the shunt resistance very low, lower than mike impedance. That leads to impossibly high currents, so the compromise is to use low resistance at high gain, high resistance at low gain where noise is less important. (Even so, a 10K pot can give audibly excess noise at medium and low gains.)

And loading a rheostat just does not do what we want. The mid-point is wrong, and wiper loading does not change the curve at the extremes of rotation where we need some control. As a POT, OK, but not as a rheostat.

FWIW, I fought this concept for weeks once, trying to soup-up my parents' low-fi console stereo. I didn't have a good audio pot, so I loaded and series-resistored a linear pot every which way. Took a lot of failure-based calculation to prove it to myself.

> said anything to the Geo FX guy?

As a POTentiometer, loading can and does give a useful shift in the mid-rotation ratio. Mid-rotation on a linear pot is -6dB, and you can make it -10dB or -15dB if you want. But "1" (on "0"-"10" marking) is -20dB on a linear pot, and pretty much -20 or -22dB on a loaded-linear pot, unless you load it SO heavy that it gets pretty useless.
 

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