EP Pre help - Alternative Value for Volume Pot

[karis12]

Hello everyone, I've been planning on building myself an (alleged) ClinchFX EP Pre with a transparent shaft potentiometer so I can put the LED indicator inside. Problem is that the only pot value I can get right now is 100k, which is different from the 500k that this schematic requires:



Simulating this circuit in LTSpice, I found that simply switching out the 500k pot for the 100k one results in reduced volume and bass response, as well as a slight phase shift, which I found I can remedy by changing out the value of the output cap (C4, from 100nf to 220nf) and the resistor before the pot (R6, 100K to 4.7k).

Now, however, I'm worried that this changes something else I couldn't see in the simulation, that Mr. Clinch intended for his pedal design. I like the sound very much as it is, and I'm just really so stubborn as to insist on using the 100k pot in order to make a cooler looking pedal  Can anyone share their opinions on my modifications - whether they'd be suitable for my purposes or I ought to stick to the original values? Thank you in advance!

[Cozybuilder]

Have you looked at Small Bear's stock? They have over 800 available in both A500K and B500K.

http://smallbear-electronics.mybigcommerce.com/alpha-clear-shaft-pots-w-bushing/

[karis12]

Have you looked at Small Bear's stock? They have over 800 available in both A500K and B500K.

http://smallbear-electronics.mybigcommerce.com/alpha-clear-shaft-pots-w-bushing/

I have seen Small Bear's product page and found that they do sell the pot in the correct value, but I am currently in a position where I am unable to order from them. My local parts supplier does carry the pot in values from 10k to 100k though.

[Gus]

schematic link will help

[karis12]

schematic link will help

All I have is the photo of the schem from this page, sorry: http://www.tdpri.com/threads/ep-pre-circuit.337372/

[slacker]

If the pot is wired like a normal volume pot, one outer lug to ground, the other to R6 with the output from the wiper, then you can probably change the circuit slightly to use a 100k pot instead, The pot and R6 are in parallel with R5, so the total resistance to ground after the output cap is about 160K, 220k in parallel with 600k, so if you remove R5 and replace R6 and the pot with a 100k pot in series with 60k of resistance then that's the same as far as the rest of the circuit is concerned.
If you made R6 about 27k and put 33k between the bottom of the pot and ground, that would limit the maximum volume to about the same as the original circuit, it would mean the volume wouldn't go all the way to silence but it's a booster so that's not really a problem. It wouldn't be exactly the same as using a 500k pot but I doubt it would make any real difference.

[karis12]

I think I'll be able to make it work with slacker's suggestion. Thanks!

Peter Clinch's design for the circuit, I think, originally emulated the effects of long cable runs on the sound. But as long as I can keep the phase shift thing intact for this, I think I'll be satisfied.

[thermionix]

I've never seen those before, pretty cool.  Does the LED go in the bottom of the pot?

[PRR]

> pot value ..100k, which is different from the 500k that this schematic requires

There is no pot, or 500K, on that schematic.

100K on the front is a significant load on a naked guitar, reduces brightness (top resonance).

100K on the back is a mild load on the 22K drain resistor. Gain will be down a bit, but do you have to hit some Exact Specification?

If the pot is after the resistor R6, yes, 100K is a significant load on the 22K+100K total source impedance, but you can change things. Throw-away R6. Put the 100K pot instead of R5 fixed. 0.1u driving 100K is far below the audio band. Even if we assume a next-box impedance of 100K, it still passes full guitar bass.

> a slight phase shift

The ear does not hear "slight phase shift" at the ends of the audio band. Counting all the bass-cuts in amplifier and speaker resonance there is much more phase shift in the path than this box can add.

FWIW: R1 C1 seem to un-do about what C3 R3 Q1 do.

[robthequiet]

Maybe a separate frame of reference might help, might not... 

[karis12]

I've never seen those before, pretty cool.  Does the LED go in the bottom of the pot?

Yes, they do! I plan to have a 2 in 1 pedal where the each knob lights up in a different color.

[karis12]

> pot value ..100k, which is different from the 500k that this schematic requires

There is no pot, or 500K, on that schematic.

100K on the front is a significant load on a naked guitar, reduces brightness (top resonance).

100K on the back is a mild load on the 22K drain resistor. Gain will be down a bit, but do you have to hit some Exact Specification?

If the pot is after the resistor R6, yes, 100K is a significant load on the 22K+100K total source impedance, but you can change things. Throw-away R6. Put the 100K pot instead of R5 fixed. 0.1u driving 100K is far below the audio band. Even if we assume a next-box impedance of 100K, it still passes full guitar bass.

> a slight phase shift

The ear does not hear "slight phase shift" at the ends of the audio band. Counting all the bass-cuts in amplifier and speaker resonance there is much more phase shift in the path than this box can add.

FWIW: R1 C1 seem to un-do about what C3 R3 Q1 do.

I have to admit that I have a very rudimentary understanding of how the circuit works, or what the original designer's intentions were, but I have built it already and am liking what I'm hearing using the schematic I provided. So I was thinking if I were to build it again I'd like to have it sound as close as possible to the first version I built. A kind forum member here offered to send me the correct value pots in a few weeks (thank you very much, Cozybuilder!), so I think I'll wait for those to arrive instead of trying to change anything from my schematic.

[karis12]

Maybe a separate frame of reference might help, might not... 

Hmm, that's odd. Q5 doesn't seem to have a bypass capacitor, unlike what my schematic calls for. Must be why there's a high-end spike around the 7kHz region in my SPICE simulation. The actual build, though, doesn't sound very harsh.