Odd variable resistor symbol in EQ schematic

[Karlsbad_kontradiction]

Hi All,

I'm planning to hack the EQ section from an Allen & Heath GL-series mixing desk into a pedal-type-thing, and I've got it mostly sussed but for the circled symbol. It's definitely a dual-gang pot, but I can't quite get how it's supposed to be connected. All 4 of the bands use this as the frequency control, but the lines with arrows apparently don't actually connect to the circuit. Which is confusing because that's the bit that does the selecting... And on the high- and low-pass sections, one of the gangs appears to be unconnected. Which is also confusing.

Does anyone know how to interpret this?

[marcelomd]

The arrows not touching the box seem just a particularity of the graphical design of the symbol.

That part about unconnected gangs is confusing but it's just that. One of the sides is not connected. Why use a dual gang pot and not a single? Probably it makes sense in a manufacturing or accounting context. Probably it is cheaper to buy, handle and account for 2x dual gangs than 1x dual gang plus 1x single gang.

[marcelomd]

Now I see what you meant. The lack of dots connecting the arrows to the center of that part? Looks like it makes some kind of variable resistor with a center tap.

[iainpunk]

its an old trick to make a pot have a C type taper, which is more sensitive around the maximums and less sensitive in the middle, you can buy the Ctype pots but a dual linear pot in this configuration is almost the same at a lower pricepoint.

[marcelomd]

I always thought C was reverse log. Like this:

[anotherjim]

It depends where in the world and manufacturer. Pot taper letter codes are not an international standard. "C" can be a certain % of normal log taper or an anti-log. There are even some that use A for Linear and B for Log.

The designation of those pots "204C" are probably 200k log, but I wouldn't swear to it! I only think that a dual gang anti-log would be very rare as a stock item. The other "203B" pots probably are 20k linear

I would wire those 204C pots CCW ends together as the centre. The wipers are joined together (as the dotted line) and need no other connection.

[Rob Strand]

I would wire those 204C pots CCW ends together as the centre. The wipers are joined together (as the dotted line) and need no other connection.

That's what I was thinking when I looked at the schematic but when you look at the range of frequencies it implies 200k across the terminals at the max resistance setting.

I think I get it now.  In effect it *is* a 200k pot reverse log pot.  One gang is not used: the part on the left for HF and the part on the right for LF are just left hanging.   The pots are actually dual but they have internal connections to save pins.  I've seen those 3-pin dual pots on mid-freq pots of semi-parametrics.   I trace a circuit once and it didn't make sense until I realized what was going on.

[Karlsbad_kontradiction]

its an old trick to make a pot have a C type taper, which is more sensitive around the maximums and less sensitive in the middle, you can buy the Ctype pots but a dual linear pot in this configuration is almost the same at a lower pricepoint.

So, just to make sure I have it clear in my head, it's essentially putting one gang in parallel with the other, giving a specific response pattern and a max resistance of ~100k?

Economics makes people do really strange things...

EDIT: typos

[Rob Strand]

So, just to make sure I have it clear in my head, ot's essentially putting one one gang in parallel with the other, giving a specific response pattern and a max resistance of ~100k?

No.  The simple answer is just use a single 200kC pot for LF and HF) and a dual 200kC for the mid bands.  With that you will get the 10 to 1 adjustment range in frequency (and 30:1 for the mid-bands).

I think the schematic is drawn wrong.  The two pot wiper connections should connect to the midpoint line when they cross over the wire where the two pot gangs join.

The way the circuit is drawn doesn't make sense (especially for the mids).  At the cw position the resistance would be Rpot/2 and in the ccw position the resistance is only Rpot.   That will give less than 2 to 1 adjustment in frequency.  The only way I see that making sense is *if* they decided to change the range of frequency to 2:1 and only for LF and HF.

FYI, the part is listed in the GL3300 spares,

https://www.allen-heath.com/media/GL3300-SPARES.pdf

[Karlsbad_kontradiction]

Yeah, I thought it was drawn wrong too, even though it came directly out of the GL3300 repair manual. Thanks for the infor on ybe tapers, though. I ended up working with a schematic that can do 3 bands with only 2 opamps, but its response sounds like it definitely needs C-taper pots.

[pinkjimiphoton]

its drawn like that to show its a dual gang pot

[Rob Strand]

Yeah, I thought it was drawn wrong too, even though it came directly out of the GL3300 repair manual. Thanks for the infor on ybe tapers, though. I ended up working with a schematic that can do 3 bands with only 2 opamps, but its response sounds like it definitely needs C-taper pots.

This circuit is pretty good and uses minimal opamps.  You will see it popping up in many semi-parametric circuits (eg bass amps and preamps).  The  Q an the amount of boost is linked.
https://www.diystompboxes.com/smfforum/index.php?topic=95483.0

You can use linear frequency pots provided you reduce the frequency range to about 4:1.   For home stuff you can use a dual log pot in reverse but you would want a reverse log for any professional stuff since it's a bit unintuitive having low frequencies at the clockwise setting.