Presence Pot- 5k vs 2k

[ToneMerc]

With all else being equal in the 390/1uf circuit, what's the tangible difference between a 2k and 5k pot?

TM

[erwin_ve]

With all else being equal in the 390/1uf circuit, what's the tangible difference between a 2k and 5k pot?

TM

Raising the pot resistance to 5k will result in more negative feedback in the PI comparing to a 2k pot at the same level.
Soundwise it will mean that it's duller at the same level compared to a 2k pot.

IMO you just shift the setting of your pot setting. If the sweet spot is 11 '0 clock with a 2k pot, with a 5k pot that sweetspot maybe at 3 'o clock.

[martin manning]

With all else being equal in the 390/1uf circuit, what's the tangible difference between a 2k and 5k pot?

TM

Since the pot is wired as a variable resistor, there is no difference when the presence is turned all the way up (resistance = 0, maximum shunting of high frequency NFB to ground). From there up to 2k, where the network becomes a low-pass shelving filter, the two give the same result, but the 5k will be capable of further increasing NFB of highs, but only by about 1dB. The problem is that the usable range of the control will be reduced. If you want to restore it, just put a 3.2k shunt across the two terminals of the pot that are in use and all is well. That's what I'm doing... I couldn't find a 2k pot to match all the others ;^)

[ToneMerc]

Thanks guys

Tm

[Structo]

I used a NOS Allen Bradley 2K5 pot for the presence.

I start hearing the brightness at around noon-1:00.

I also changed the NFB resistor from the 4K7 to a 10K after some members reported liking that.
I like it as well.

[ayan]

I used a NOS Allen Bradley 2K5 pot for the presence.

I start hearing the brightness at around noon-1:00.

I also changed the NFB resistor from the 4K7 to a 10K after some members reported liking that.
I like it as well.

The use of a linear pot here makes the control pretty lousy, IMHO. The best solution I have ever found was to use a reverse log pot... but the only ones commercially available, as far as I know, would be the ones used by Fender in their reverb amps, and they're not a good match (value-wise) for the Dumble's presence circuit needs.

You may want to explore this tool:

http://www.diystompboxes.com/analogalchemy/emh/emh.html

You can enter the starting (linear) pot value, say 10K, the final desired pot value (2K), and the desired taper, and let the tool suggest resistor values to put across the pot's lugs. I have never tried it myself, but perhaps someone can play with this and come up with a cool alternative.

Gil

[martin manning]

The best solution I have ever found was to use a reverse log pot... ...You may want to explore this tool:

http://www.diystompboxes.com/analogalchemy/emh/emh.html

You can enter the starting (linear) pot value, say 10K, the final desired pot value (2K), and the desired taper, and let the tool suggest resistor values to put across the pot's lugs. I have never tried it myself, but perhaps someone can play with this and come up with a cool alternative.

I did some playing on my own in Excel. The result is I don't think you can get a reverse log-type characteristic if what you need is a variable resistor (not a voltage divider).

In the attached plot the various resistances are shown as a function of rotation for a 5k pot shunted with 3.33k across 2-3, which are the lugs in use for the presence control (1, 2, and 3 represent CCW, Wiper, CW lugs respectively, and Rs is the shunt).

What is needed is a resistance that decreases quickly with CW rotation, and from what I can tell there is no way to get it using one, two, or three shunts (1-2, 2-3, 1-3). Notice the voltage divider trace has the right kind of shape, but that doesn't help here. R2-3 is what counts.

[Structo]

Pretty cool Martin.

Is the R1-2 the actual curve of the pot without any shunt?

If it is, that would seem like the ideal curve wouldn't it?

Just a gradual change?

Or perhaps the yellow trace would be more ideal?

For me, if the presence control started adding audible brightness around 10:00 and increased linearly all the way up, that is what I would like.

On mine, I don't hear much change until about 1:00.
Of course my hearing is damaged so take that for what it's worth.

[phsyconoodler]

I play my amp and set the presence where it sounds good.I don't care where it is.

[martin manning]

Pretty cool Martin.

Is the R1-2 the actual curve of the pot without any shunt?

If it is, that would seem like the ideal curve wouldn't it?

Just a gradual change?

Or perhaps the yellow trace would be more ideal?

For me, if the presence control started adding audible brightness around 10:00 and increased linearly all the way up, that is what I would like.

On mine, I don't hear much change until about 1:00.
Of course my hearing is damaged so take that for what it's worth.

You want the resistance to decrease as you turn the pot CW, so it's the resistance between the wiper and the top (R2-3) as a two-terminal variable resistor that matters. The yellow trace is what you'd like to have, but that is only available if the pot is wired as a voltage divider, which doesn't work for the presence control.

The pot in the plot is shunted across 2-3; notice R1-2 (which is un-shunted) looks just like a 5k linear pot, which it is.