using larger/smaller pot values than called for

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using larger/smaller pot values than called for

Postby drewberryelectronics » 17 Dec 2013, 01:10

in general if you use the wrong value pot in a circuit but not by a huge amount (say a 100k or 25k instead of the required 50k)
what effect would that cause generally?

I know in some circuits a common mod is to increase the size of the gain/volume pot to get more gain/volume

is there a way to limit this in order to avoid pedals with too huge a sweep that are hard to find a sweet spot with.

I ask because I'm thinking of whipping up a pedal for my bass player but don't QUITE have the right pot types for what I was planning on building and won't be able to get a Tayda shipment by next week
(the local electronics place doesn't have a great selection)

thanks in advance
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Re: using larger/smaller pot values than called for

Postby Nocentelli » 17 Dec 2013, 07:38

drewberryelectronics wrote:in general if you use the wrong value pot in a circuit but not by a huge amount... what effect would that cause generally?


It's hard to generalise, it depends on the function of the pot, the circuit, and how far from the specified value you are.

To cite some specific examples, the fuzzface volume pot value will affect the highpass filter cut-off point: The bigger the pot, the lower the cut-off and the bassier/woollier the sound of the fuzz. People often replace the 500k stock pot with a 250k or lower to tighten up the response. Here, one could compensate by increasing the value of the output cap if you wanted the same sound (i.e. a 500k pot with a 10n output cap acts like a 250k pot with a 20n cap). The fuzzface fuzz control on the other hand, sets the bias of the second transistor: A 2k instead of the stock 1k might screw up the bias and reduce the gain, but you could alter the value by putting fixed resistors in parallel with the pot lugs to lower it. This in turn would change the taper of the pot, so it wouldn't react the same as the stock pot but the range of control, minimum to maximum, would be the same.

In general, using bigger pots than specified would allow you to reduce the value with parallel resistors to give the stock value, but will alter the taper. Using smaller value pots means you can only add resistance in series to get the original value, which may greatly reduce the range of control you have.

Post up the circuit you're planning to build and which pot values you're planning to use and we can give you a better idea of what might happen.
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Re: using larger/smaller pot values than called for

Postby drewberryelectronics » 17 Dec 2013, 22:22

Well basically I just finished a hotcake/prunesncustard 2in1 build and I really like it and sounds fantastic on bass so was thinking of either building another one for my bass player, or maybe a hotcake/booster hotcake/fuzz. I'm gonna breadboard and tweak the circuits a bit to optimize them for bass anyway so I will play around with some different pot values and see what I can work up, either using parallel resistors or just seeing how they work as is.
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Re: using larger/smaller pot values than called for

Postby drewberryelectronics » 18 Dec 2013, 06:11

ok so everything but the gain knob seems to be happy to be switched out. Schematic calls for 50k for all three knobs, went higher for presence and volume, and lower (20k) for gain.

All I get is a weird extremely gated fuzz. Tried throwing a 27k resistor in series to bring it closer but this didn't help, only adjusted where in the sweep the gate completely cuts out all sound.

Back to the drawing board, might have to come up with my own TL071 based design
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Re: using larger/smaller pot values than called for

Postby DrNomis » 18 Dec 2013, 12:20

drewberryelectronics wrote:ok so everything but the gain knob seems to be happy to be switched out. Schematic calls for 50k for all three knobs, went higher for presence and volume, and lower (20k) for gain.

All I get is a weird extremely gated fuzz. Tried throwing a 27k resistor in series to bring it closer but this didn't help, only adjusted where in the sweep the gate completely cuts out all sound.

Back to the drawing board, might have to come up with my own TL071 based design




Strange, the 20k pot should have decreased the amount of gain avalible at max gain setting..... :hmmm:


If it sounds gated then that indicates that something is a bit mis-biased....... :hmmm:


Check your solder joints, and also check for any un-intentional short-circuits, the 20k pot might be faulty too..... :thumbsup
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Re: using larger/smaller pot values than called for

Postby drewberryelectronics » 20 Dec 2013, 08:51

DrNomis wrote:
drewberryelectronics wrote:ok so everything but the gain knob seems to be happy to be switched out. Schematic calls for 50k for all three knobs, went higher for presence and volume, and lower (20k) for gain.

All I get is a weird extremely gated fuzz. Tried throwing a 27k resistor in series to bring it closer but this didn't help, only adjusted where in the sweep the gate completely cuts out all sound.

Back to the drawing board, might have to come up with my own TL071 based design




Strange, the 20k pot should have decreased the amount of gain avalible at max gain setting..... :hmmm:


If it sounds gated then that indicates that something is a bit mis-biased....... :hmmm:


Check your solder joints, and also check for any un-intentional short-circuits, the 20k pot might be faulty too..... :thumbsup


Cheers will do all that. Any idea what rough voltages should be on the pins? I can measure and check those.

Guessing this wouldn't have anything to do with increasing the input cap to 47nf to let a bit more bass into the circuit right?
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Re: using larger/smaller pot values than called for

Postby drewberryelectronics » 20 Dec 2013, 10:58

Ok the circuit is fine, it's the pot. Only just realised now it's a 20k 'W Taper' which I think is like a weird backwards taper.
Wired it the other way around and that makes it have no gain.
Subbed in a 500ka (the only other pot I have lying around) and that makes it have insane fuzz like gain (sounds kinda good actually) but there is no control over it really. Difference between full on and 9 oclock was barely audible, then in the lower portion of the sweep it dissapears to an inaudible whisper really quick.

So looks like a trip to the local jaycar tomorrow to try and hack something together before xmas.
Have made it footswitchable between two different gain pots (basically and overdrive and a fuzz control for the same circuit) so I will try and get a 50k something pot and maybe a 100-250k for the fuzzy one but somehow need to stop it from bunching up in the lower quarter of the knobs travel.

I have a feeling this has to do with lug 1 & 2 being wired to each other, is there a way to wire this so it won't do that?
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Re: using larger/smaller pot values than called for

Postby induction » 20 Dec 2013, 18:37

A couple of notes for you:

The gain knob on the hotcake is a variable resistor, so the value matters. (If you don't understand the different ways to wire a pot, then read this.) I disagree with Dr. Nomis here. If the hotcake schematic I'm reading is correct, the op-amp is wired as a non-inverting amplifier so that reducing the resistance of the gain pot (turning the knob clockwise, since the operative lugs are 2 and 3) increases the gain, and increasing the resistance (turning the knob ccw) reduces the gain. You can't go below zero resistance here, so any pot should give you the same maximum gain. Using a larger pot will give you a lower minimum gain, and using a smaller pot will give you a higher minimum gain. Adding a resistor in series will reduce the maximum gain.

If you reversed the pot and noticed a big difference in the maximum or minimum gain, then something is wrong with your wiring or the pot is broken.

Wired as a variable resistor, having lug 1 and lug 2 wired together should not be any different from leaving lug 1 floating, unless the pot wiper fails.

There are two ways to interpret 'wiring the pot backwards' with a variable resistor:
a) instead of lugs 1 and 2 jumpered (or 1 floating), you jumper 2 and 3 (or leave 3 floating) - this will reverse the operation of the pot and alter the taper, or
b) keep 1 and 2 jumpered (or leave 1 floating) and swap the connections to the circuit (ie lug 3 connects where lugs 1 and 2 used to connect and vice-versa) - this should have absolutely no effect on the circuit.

So depending on what you meant when you said you reversed the pot, you should have noticed either no difference or simply reversed the operation of the pot. If one way gives more maximum gain or lower minimum gain than the other, something is wrong. 'W' taper means reverse log from 0% to 50% rotation and log from 50% to 100% rotation (look here for reference). A type 'a' reversal of this kind of pot in will only reverse the rotation, the taper will remain the same.

As noted above, the 500k pot should give the same maximum gain as any other pot, but the minimum gain will be much much lower because 500k = 10 x 50k, and more resistance equals less gain. Any 50k pot will work for maximum and minimum settings, but the hotcake calls for a reverse log taper, so as you turn up the knob, resistance decreases quickly at low settings and decreases more slowly at higher settings. Reverse log pots can be hard to find, especially in stores (you could probably find them online, but I understand time is short for this project), so if you can't find one your best bet is a 50k linear pot. This will change the taper so that the gain increase is a little more bunched up at the top end of the pot rotation compared to the original. You can get a similar taper by reversing (type 'a' above) a 50k log pot, but the pot rotation will be backwards - as you turn up the pot, gain will go down. (You can simulate a 50k reverse log variable resistor with a 250k linear pot and a 62k resistor in parallel, but this only works when the pot increases resistance with rotation, ie the operative connections are 1 and 2, instead of 2 and 3 like in the hotcake. I don't know of a way to simulate a reverse log pot that will work here.)

Using a switch to select between two different gain pots will not give you a higher maximum gain in either one (because the highest gain is found at zero resistance), it will simply allow you to choose between two presets. If one pot is bigger than the other, it will allow that pot to go lower in gain, not higher. Alternatively, you could use a switch to short the gain pot, which will bump you up to maximum gain regardless of the pot setting, or to add a series resistor, which will drop the gain, or to add a series resistor to the 100k in the feedback loop, which will increase the gain, but might cause oscillation at high gain settings.

Hope this helps.

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Re: using larger/smaller pot values than called for

Postby drewberryelectronics » 20 Dec 2013, 22:57

wow thanks that does help. Still doesn't explain why the 500k pot gave ridiculous amounts of fuzz like gain.
Gain that I've NEVER heard coming from a hotcake. I'm gonna swap the volume and presence controls to 50k ones cause I think the larger pots there might be loading down the circuit or something to cause these changes.
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