Reducing pot value by wiring parallel resistor

[Locrian99]

Another question from the noob.

I am planning on building a eq/boost pedal and for the treble knob it lists a 250kb, I can do a 500kb with a 510k as I don't have any but I have read it makes the taper weird and basically is only correct at max resistance?   Is this true?   Don't want to solder a resistor on a new pot if it's not going to work well on a pot that I'm going to obviously want the full range on. 

Thanks

[GGBB]

It will depend on the circuit - not all treble controls are the same. Please share the schematic.

[Locrian99]

I'll be doing a vero version but here is the pedal pcb schematic

https://www.pedalpcb.com/docs/BoxAndAll-PedalPCB.pdf

[Rob Strand]

I'll be doing a vero version but here is the pedal pcb schematic

https://www.pedalpcb.com/docs/BoxAndAll-PedalPCB.pdf

The pot needs to operate symmetrically in that circuit so it needs to stay linear.

What you can do is add a 270k resistor across pins 1 and 2 of the treble pot and
another 270k resistors across pins 2 and 3 of the treble pot.

What's that do:

When the treble control is set flat the treble pot looks like a 250k pot set (actually 260k) to the flat position.

When the treble control is advanced to full the treble pot looks like a resistor 500k in parallel with 270k ie. 175k.
For the case where the treble pot is an actual 250k that value would 250k.

The case which has the most impact on the response is when the treble control is set to the mid position, as it affects
the response a lot when the mid control is advanced.  When the treble control is advance the added 270k's have very little effect.    The choice of 2x270ks gives the best match to the overall behaviour and response.

Compared to the 250k pot, you will need to do is advance the treble control a little more to get the same amount of boost.  Mid position and extremes are virtually identical.

[Locrian99]

K thank you.   Not sure how I missed that pot when I made my order. 

[m_charles]

You might be fine just leaving the 500K pot alone. Try that first. If it's weird, just put the right value in later.

[GGBB]

You might be fine just leaving the 500K pot alone. Try that first. If it's weird, just put the right value in later.

The filter circuit is designed around the 250k value for the treble pot. It will "work" with 500k, but it will work differently a.k.a. sound different.

[m_charles]

You might be fine just leaving the 500K pot alone. Try that first. If it's weird, just put the right value in later.

The filter circuit is designed around the 250k value for the treble pot. It will "work" with 500k, but it will work differently a.k.a. sound different.

Yes, of course. But different isn't always bad, haha. Was mainly letting the OP know that as far as getting the circuit up and running, the pedal would still operate with the 500K.

[GGBB]

Never mind - I'm an idiot. 

[antonis]

Never mind - I'm an idiot. 

You definately are not, Gord..
(just an eluded detail like pot's value irrelevancy to filter High/Low freq calculation formulas..)

[GGBB]

Never mind - I'm an idiot. 

You definately are not, Gord..
(just an eluded detail like pot's value irrelevancy to filter High/Low freq calculation formulas..)

Thanks Antonis - I said that as an edit to a response I posted which had nothing to do with the post - because I had completely misread it. 

[Rob Strand]

You might be fine just leaving the 500K pot alone. Try that first. If it's weird, just put the right value in later.

The filter circuit is designed around the 250k value for the treble pot. It will "work" with 500k, but it will work differently a.k.a. sound different.

Yes, of course. But different isn't always bad, haha. Was mainly letting the OP know that as far as getting the circuit up and running, the pedal would still operate with the 500K.

It helps to start with the original intent of the design before making untried changes.   The change from 250k to 500k is noticeable.

I've put hundreds of hours into EQ design and my advice is not to expect much from single opamp three-band EQs.  The bandwidth of the mid control is very wide.   The bass control tends to operate at quite low frequencies and it's not possible to push the frequency up because it affects the mid control.   You can't tweak the response much because the controls interact too much.   IMHO, far better is a two-band Bass+treble followed by a one-opamp mid control.

[Locrian99]

I was actually thinking of using that circuit as kind of a preamp with one of those tda2050 modules that you can get on Amazon.   Just not super happy with the ruby as my test bench amp.   It sounds pretty decent, but not quite clean enough for me.  Unless I turn the volume and gain way way down.   There's a few other eq pedals I've looked at for my actual board but I will probably end up doing a pcb one. 

[Rob Strand]

I was actually thinking of using that circuit as kind of a preamp with one of those tda2050 modules that you can get on Amazon.   Just not super happy with the ruby as my test bench amp.   It sounds pretty decent, but not quite clean enough for me.  Unless I turn the volume and gain way way down.   There's a few other eq pedals I've looked at for my actual board but I will probably end up doing a pcb one.

Sometimes adding a "bridge-T" circuit to provide a slight fixed mid-cut can bring amps to life.  Check out some of the old Gibson amps.    The bridge-T is sort of like a Fender/Marshall stack with a fixed setting.   I suppose you could even just use one of those tone stacks, many small amps use it, even Roland Cubes.

The only thing to be careful with is the LM386 has a 50k input impedance.  Not what a normal Fender/Marshall stack likes.   You would have to scale down the impedances a bit.



Here's a Marshall-ish LM386 design,
https://www.diyaudio.com/community/threads/marshall-ms-2-redesigned-for-lm386.68823/

[antonis]

IMHO, far better is a two-band Bass+treble followed by a one-opamp mid control.

I'd dare to split MIDs into MID Low & MID High and arrange them as two-band per amp (Treble & MID Low and Bass & MID High..) with side-effect benefit of EQ non-inverting output..

[Rob Strand]

I'd dare to split MIDs into MID Low & MID High and arrange them as two-band per amp (Treble & MID Low and Bass & MID High..) with side-effect benefit of EQ non-inverting output..

That idea works.  (also helps reduce interaction for gyrator designs)

[ElectricDruid]

I've put hundreds of hours into EQ design and my advice is not to expect much from single opamp three-band EQs.  The bandwidth of the mid control is very wide.   The bass control tends to operate at quite low frequencies and it's not possible to push the frequency up because it affects the mid control.   You can't tweak the response much because the controls interact too much.   IMHO, far better is a two-band Bass+treble followed by a one-opamp mid control.

+1 another agree. My experience is also that you're better off breaking up a more complex EQ into separate stages. I mean, op-amps are amazing 'n' all, but we can't expect them to work *absolute miracles* Maybe giving the poor things a chance and only one or two things to do at once is a better way to go, right?!?

[antonis]

My experience is also that you're better off breaking up a more complex EQ into separate stages. I mean, op-amps are amazing 'n' all, but we can't expect them to work *absolute miracles* Maybe giving the poor things a chance and only one or two things to do at once is a better way to go, right?!?

Although I agree with the practice, I don't presume it's an op-amp's issue (in the mean of device's inadequacy..)
IMHO, it's more of bands frequencies interaction for respective pots settings other than for 0 dB gain..

[ElectricDruid]

Although I agree with the practice, I don't presume it's an op-amp's issue (in the mean of device's inadequacy..)
IMHO, it's more of bands frequencies interaction for respective pots settings other than for 0 dB gain..

Fair enough. It amounts to the same thing, practically. Let's keep different EQ sections away from each other and we'll have much better results.

Maybe I was blaming the poor op-amps unfairly? For that I wholeheartedly apologise! Op-amps have my undying support and affection! It's *our* fault if we put them in such inadequate circuits that the end result isn't the perfect result we hoped for!

[StephenGiles]

I'm on my iphone so I may have missed something, but why don't you buy a 250k pot? Isn't it easier to mess with the pot's taper than its resistance?