Help understanding a circuit (NIC) that creates an S/W taper from a linear pot - Page 1
Forum section: electronics / projects

author: Nick_Denison
author_rank: Contributor
timestamp: July 08, 2025, 04:31:00 am
content: I've been working on a graphic EQ circuit that has gyrators in place of inductors, the type that typically bunches the effect of the EQ slider up to the ends of the pot travel. Typically, commercial designs seem to employ slide pots with an S/W taper, where the ends of taper are shallowed, and the centre is steep. This gives a more linear feel to the cut/boost control, which otherwise feels like it does nothing until the last 10%.I found this old reference on stackexchange for using a negative impedance convertor to force such a taper from a linear pot. From the info provided, I haven't been able to make sense of where my pot ends and wiper affix to said circuit, nor am I clear on where the 'synthetic' wiper is taken from (it's not clear to me whether "output" refers to the test point "Pr", or the op amps actual output, which is junctioned with R5 and R6). In my EQ circuit, the pot ends go to a summing amp's cut/boost nodes respectively, and wipers go to the gyrator filters.I know S/W taper pots exist, but they seem much harder to source than the other garden varieties, so I'd love to have a workaround such as this, if this circuit works at all. If anyone can shed some light on this setup, I'd be most grateful! Circuit, and desired taper pic attached.Cheers!

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2025, 12:04:18 pm
content: Would aLTspicesimulation of that circuit, with the potentiometer and the output explicitly shown help? Resistor names and values match your diagram, which states Rnic=-Rpot/2 and Rf=-2*Rnic, which gives Rf=Rpot. The potentiometer in your diagram is represented by R1 and R2.It appears to do what's claimed if you stick to a simple DC sweep analysis, but goes very strange if you attempt a time domain analysis and seems non-functional if you substitute a more realistic OPAMP model (e.g. LTspice's opamp2, with +/- 15V supplies). Even if you scale all the resistors to get the OPAMP output current down to something reasonable it rails most of the time.Seehttps://www.eevblog.com/forum/beginners/simulating-potentiometers-using-ltspice/for an explanation of the potentiometer model used.Zipped LTspice sims attached. Unpack in new folder and open either .asc file with LTspice. "SimSWtaperNIC.asc" is the original circuit with a near-ideal OPAMP. "SimSWtaperNIC2.asc" attempts to make a practical implementation, but fails - see above.

author: Nick_Denison
author_rank: Contributor
timestamp: July 10, 2025, 07:12:00 am
content: Thanks Ian. I really appreciate you taking the time to put that simulation together!So, I had hooked it up right after all, but was getting the same railing with the AC sim I was running. I didn't run a DC sweep at that point....Looks great on the sweep, then fails utterly in more real-world testing! darn. I guess the original poster of that circuit never tried it thoroughly.I actually found some generic W taper sliders. I purchased them by accident when they were labelled as "linear". It's only now I have legit linear Bourns in circuit that the filter behaviour is apparent. I'd use the generic ones with W taper, but can't find ROHS documents for them.... *sigh*Thanks again. I'll keep poking around and see if I can find a sensible source of W taper slide pots.

author: Ian.M
author_rank: Super Contributor
timestamp: July 10, 2025, 08:14:47 am
content: OTOH an actual negative resistance (possible in SPICE, not in the real world) between the wiper and half the input voltage works just fine with no apparent instabilities! It even worked OK when I gave the pot an AC input signal. Therefore the problem must be in the implementation of the OPAMP NIC, not the basic concept.

author: Ian.M
author_rank: Super Contributor
timestamp: July 10, 2025, 11:26:33 am
content: Thinking about it a little harder, I came up with a possible practical implementation. It DC sweeps OK, and the TRAN analysis has no problems even with a sinewave input signal. It works by the OPAMP doubling Vwiper (with respect to Vin/2) so positive Rf can act as if it was -Rf connected to Vin/2.It doesn't rail at Rf=Rpot/2 and can actually work to a limit of Rf>Rpot/4, though as you approach that the pot 'law' becomes increasingly distorted and the risk of railing increases. Resistor matching for R1, R2 & R3 should be non-critical, I expect 1% will do well enough and 5% should be usable.Vin *MUST* be a low impedance source that cansinkcurrent. If necessary add a unit gain OPAMP buffer to meet this requirement.With the modelled 0-1V wiper voltage range a 5V or even 3.3V single supply RRIO OPAMP will do fine. For AC signals without a DC offset it needs symmetrical OPAMP supplies.As with my other sims, it needs the potentiometerVC component included with my first sim or from the link in reply #1.

author: Nick_Denison
author_rank: Contributor
timestamp: July 11, 2025, 06:03:24 am
content: Thanks Ian, it does look fantastic in simulation. Alas, my attempts to implement it into my EQ topology, which isn't voltage divider based, doesn't seem to jive unfortunately. The topology uses the pot to mix cut/boost, with neither end going to ground. Seems that throws a spanner in the works. My apologies for not having given clarity on that from the get go. I've attached a spice mock up of the basic functioning of it. In reality the op amps are discrete, but this stripped down version conveys the idea. Thank you again, I really appreciate it.-Nick

author: Ian.M
author_rank: Super Contributor
timestamp: July 11, 2025, 06:59:08 am
content: Yes, I can see your issue. The trouble with the NIC S/W taper approximation is that it requires *both* ends of the pot track to be a low impedance to ground, so that at the limits of the wiper travel, the max. injected current from the negative resistance has no effect on the wiper voltage. Its really only suitable for simple attenuators, mixers (with both signals buffered) and control voltages, not circuits like yours where both end of the track are fairly high impedance nodes.I suspect you will have to continue yours search for pots with the correct taper. However it may be worth considering digipots, with a MCU reading control voltages from however many linear pots and mapping that to the desired taper to command the digipots.