Turning a bax-style EQ into a Tilt EQ? LTL Pentode Mod

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maedaeburning

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Heyo, I'm pretty new to doing mods/designs that haven't been pioneered or well documented already, so forgive any lack of knowledge on this one. Here's the skinny:

  • I want to take two trim pots and turn them into one, meaning turning beyond center position will increase one value and decrease the other and vice-versa for turning the other way
  • The trim pots on the circuit are both thru-hole 3 lead (pos, grnd, neg) so should be easy to remove/modify
  • There is a resistance difference I measured with the current trim pots- the low cut/boost is 25k ohm vs the high 20k ohm
  • The idea is to take the Bax-style EQ on the LTL Pentode mk2 colour module and turn it into a tilt EQ (similar to the one on their Mister Focus)
For a little more detail, I recently built a couple of Diyre Colour Pallets and grabbed a couple of LTL's Pentode mk2 for them. On the pentodes, there is a bax-style eq you can enable/disable with jumpers and the EQ actually sounds pretty fantastic but you have to remove the 500 card and adjust the trim pots on the pentode any time you want to adjust the level of cut or boost (circled the trim pots in the photo). I want to leave both bands enabled by default and mod the colour palettes by drilling a hole big enough to slip a small pot in (there's one place with about enough realestate for that or a toggle switch)

In theory, all this would require is finding a 6 lead potentiometer of about 20k, wiring it to the existing leads on the pentode PCP properly, and puting a 5k resistor in-between the relevant lead going to the low on the board, right? Or is there more that I need to be thinking about/measuring in terms of the pre-existing pots?

I appreciate all input and resources as I'm pretty new to this kinda thing, thanks so much!
 

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gyraf

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Yes, if you have a baxandall-type tone control, you can turn it into a classic tilt by wiring them both to a stereo potentiometer, attaching one of them in reversed order - turning up highs as you're turning down lows. The usual direction is reversing the low control.

I'm pretty sure that those trimmers don't come in both 20K and 25K versions - they're probably either-or, just with large tolerance. Any 20K or 25K linear stereo pot will do fine.

A schematic would make analysis easier

/Jakob E.
 

maedaeburning

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Yes, if you have a baxandall-type tone control, you can turn it into a classic tilt by wiring them both to a stereo potentiometer, attaching one of them in reversed order - turning up highs as you're turning down lows. The usual direction is reversing the low control.

I'm pretty sure that those trimmers don't come in both 20K and 25K versions - they're probably either-or, just with large tolerance. Any 20K or 25K linear stereo pot will do fine.

A schematic would make analysis easier

/Jakob E.
Thanks for the reply! This does confirm what I hoped in terms of it being a fairly straightforward wire, but after double-checking both pentode modules it seems that the bass eq trimpots hover around 30k and the highs around 23k (so I'm guessing 22k with the tolerance factored in). The bass trimpot has the marking 50k840m on it which would make me assume it's a 50k trimpot but that's not what it ever reads and those markings don't match the manufacturer's coding that I could find.

Piher Spain is the manufacturer and they do do custom orders of the general models of these pots in all sorts of different values, so nailing down the exact values is tough- however it does seem there is a difference in markings between the two on the pentode.

I emailed the guys at Louder Than Liftoff to see if they had the values or schematics or anything to offer and I was told they'd check and get back to me, so we'll see!
 

Gustav

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Thanks for the reply! This does confirm what I hoped in terms of it being a fairly straightforward wire, but after double-checking both pentode modules it seems that the bass eq trimpots hover around 30k and the highs around 23k (so I'm guessing 22k with the tolerance factored in). The bass trimpot has the marking 50k840m on it which would make me assume it's a 50k trimpot but that's not what it ever reads and those markings don't match the manufacturer's coding that I could find.

Piher Spain is the manufacturer and they do do custom orders of the general models of these pots in all sorts of different values, so nailing down the exact values is tough- however it does seem there is a difference in markings between the two on the pentode.

I emailed the guys at Louder Than Liftoff to see if they had the values or schematics or anything to offer and I was told they'd check and get back to me, so we'll see!

Are you measuring the trimpot in-circuit? If so, thats your problem with the measurement vs printed value. Its 50K if it says 50K.

If its a bax circuit, you can adjust resistors in series with the pot for uniform gain on your high and low end. Getting a special-value double stacked pot is unrealistic, so best to try and solve it with a few, easy design-tweaks (if need be), and a standard pot value.

Isnt there a scheamtic for this? Looks like a DIY build, so should be one out there, right?

Gustav
 

TwentyTrees

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Isnt there a scheamtic for this? Looks like a DIY build, so should be one out there, right?
The Louder Than Liftoff "Colour" modules are some of the few that aren't DIY kits - as far as I'm aware they're only available assembled, certainly I couldn't find schematics when I looked a little while back.

I do like the idea of modding the Palettes to add utility to the modules though. Keep thinking about doing this with their Implode module, which is apparently most of an 1176 in sub-credit card size format. Keep us posted!
 

maedaeburning

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The guys at LTL got back to me: the highs are on a 100k pot, the lows 50k.

Are you measuring the trimpot in-circuit? If so, thats your problem with the measurement vs printed value. Its 50K if it says 50K.

If its a bax circuit, you can adjust resistors in series with the pot for uniform gain on your high and low end. Getting a special-value double stacked pot is unrealistic, so best to try and solve it with a few, easy design-tweaks (if need be), and a standard pot value.

Isnt there a scheamtic for this? Looks like a DIY build, so should be one out there, right?

Gustav
As mentioned, there's no schematic available unfortunately. LTL stuff isn't diy, but most of the format tends to be. I was totally measuring it in-circuit, thank you for clearing that one up for me though! Pretty new to all this. So I have a question, in terms of using resistors in parallel, would I just throw a 50k on two leads (+ and -) in-between the pot and the board for the highs?


Unfortunately finding a multi-gain 9mm pot seems very unlikely, and custom ordering one would be ridiculously expensive from what I could find.

I have space for 1, *maybe* two 9mm eurorack-style trimpots if I push it but probably just the one.

If anyone happens to have any ideas for somehow using a dualgang pot but compensating for the resistance difference, I'm all ears. From what I can tell it's just a VR circuit not a divide (and pots are marked on the board as VR).

I may either a) order some pots for one of the bands, see the size and see if I can squeeze one more on there, or b drop this in favor of a 3way toggle switch with pre-dialed in EQ if I can find one in a similar footprint. Or c I guess use one of the existing ones with a tiny lil hole that a small philips head could sneak thru, but I have worries about turn life.

I like the 500 format, but man, sometimes that real-estate limitation... The alternative is having stuff hanging out of the module and that would just be nasty yo. Then again, I mainly like it cos I don't have that much money or space for a million power supplies lmao.
 
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maedaeburning

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A quick update, I found this on aliexpress. I have some concerns that it would be slightly too big, but otherwise it would be perfect and actually give me control over both EQs individually as opposed to tilt-style. Will continue to look, I'd hate to wait a month on something from china only to figure out it won't fit lol.
 

gyraf

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I think that one is a balance/volume pot - a stereo/dual logaritmic volume pot at the outer shaft, and a single linear at the inner

/Jakob E.
 
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maedaeburning

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I think that one is a balance/volume pot - a stereo/dual logaritmic volume pot at the outer shaft, and a single linear at the inner

/Jakob E.

Uh oh, I don't think I totally understand- I know understand the difference in curve, but would one pot being dual gang get in the way of using it in place of a 3 lead trimpot? Or the difference in log/lin prevent use in some way? I figured the as long as the resistance matches it wouldn't be a big deal.
 

TwentyTrees

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If the circuit is designed for linear pots, then substituting log is going to drastically change the control law. For example, if you're looking at an EQ control with say a +/-15dB range, a linear pot might give you a "0" point in the middle of its rotation, at 12 o'clock. A log pot might give you a "0" point at 90% of its rotation, at around 4 o'clock - pretty unusable.
 

maedaeburning

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If the circuit is designed for linear pots, then substituting log is going to drastically change the control law. For example, if you're looking at an EQ control with say a +/-15dB range, a linear pot might give you a "0" point in the middle of its rotation, at 12 o'clock. A log pot might give you a "0" point at 90% of its rotation, at around 4 o'clock - pretty unusable.
This makes perfect sense, thank you. I hadn't thought about how that could potentially affect things heh.
I do not understand what you mean by multi gain or 3 lead pot, but I have 9mm dual 20kLIN with 6mm D shaft..
So the trimpots on the board have 3 pins, but I wasn't sure if connecting them to a multi-gang pot would cause issues or whatever. The main issue is the difference in resistance- if I wanted a tilt EQ, I'd need to find a dual-gang linear pot with one part being 100k ohm and the other 50k ohm which has proved itself unlikely. (I'd also opt for like a concentric style, but again finding that linear with the specific resistances that would fit the required footprint...)
 
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Richard James

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This makes perfect sense, thank you. I hadn't thought about how that could potentially affect things heh.

So the trimpots on the board have 3 pins, but I wasn't sure if connecting them to a multi-gang pot would cause issues or whatever. The main issue is the difference in resistance- if I wanted a tilt EQ, I'd need to find a dual-gang linear pot with one part being 100k ohm and the other 50k ohm which has proved itself unlikely. (I'd also opt for like a dual axis style, but again finding that linear with the specific resistances that would fit the required footprint...)
I think you mean a multi deck pot. Each deck is essentially independent (with 3 contacts: In, Out and ground), but are controlled by the single shaft.
 

maedaeburning

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I think you mean a multi deck pot. Each deck is essentially independent (with 3 contacts: In, Out and ground), but are controlled by the single shaft.
The amount of stuff I need to learn heh.

Unforunately there's only really room enough for me to get one pot on there- I may end up getting a 9mm and checking around if there's anywhere else I could reasonably stick another, but that's sort of why I wanted to combine the two controls into one in the first place.
 

Richard James

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NM..."Gang" and "deck" are interchangeable terms... I read the previous post saying "multi-gain"...

Anyway, I don't even think there's enough room for a 2 gang pot anywhere on the Colour Palette. Also, you'd probably need a different pot than what I linked(assuming the leads are pitched 90 degrees...you might be able to bend them). I know you can get 2 micro SPDT toggles, might be able to get 2 single gang pots on there. Just look for ones with a really small footprint.

Why not just mount the controls on a blank 500 series panel. DIYRE sells them for only $7.50. That would make this much easier.
 
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maedaeburning

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Ah ok, I thought so. I'm outta room on my lunchbox heh. I did consider a DPDT, and may opt for that in the end- dial in the EQ, either have the highs or the lows engaged or nothing at all. Would ultimately still extend the usage of the module. But after extensive measuring there is a chance I could squeeze a 2gang on there.

Edit: May have found a supplier willing to do a reasonably priced sample run of a couple that would fit my needs (i.e. 9mm trimmer sized, but dual-gang with linear taper and proper resistances). Will post on here if that's what happens.
 
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maedaeburning

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After getting two manufacturers to duke it out for my love, I have placed my order. It'll be a tight fit, but if I planned everything out properly what I'll end up with is a tilt EQ in the end- I'm not sure if there's room enough on the board, but I'm hoping to get a couple latch switches in the same style as the ones included with the palette kits to switch bias and turn the EQ on/off. Peterson at Diyre offered to send me some of the caps they use for the latch switches included with their kits :) Was hoping I might be able to have a switch built into the pots but no dice on that one.

I'm gonna wait on that one to see if the pots fit, first. If they don't for some cursed reason I'm going to decide what to do re:switches at that point.
 

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