Slugging pots with negative impedance - Page 1
Forum section: electronics / projects

author: ilya
author_rank: Contributor
timestamp: September 02, 2018, 11:41:06 am
content: I have an audio equalizer circuit that calls for dual log variable resistor. Unfortunately, I can't get a correct curve with conventional slugging methods (I can only get a rev log curve). But if I add a negative resistor parallel to the pot I get a very good curve that is close to log. Now, this worked fine when I tested the concept with a simple non-inverting op-amp. However, in the eq circuit this negative impedance gives some strange results and I can't figure what's screwing up the performance.The eq topology is bridged-t.I'm attaching the circuit. Arrows show the signal flow. Negative impedance resistors are circled in blue (one is "ideal", another is a standard NIC circuit). Basically what happens: with an ideal neg resistor everything is fine, when I substitute it with a op-amp NIC, I get a large widening of the EQ curve (Q drops a lot) and gain is also a lot smaller (7-8 dB less). This can be fixed (sort of) by replacing 10K resistors around NIC op-amp to 1Meg ones. This keeps the Q correct, but the gain is still not right, although much closer to the "ideal" resistor (1-1.5dB).Can anybody point me to the direction of possible solution?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2018, 12:17:30 pm
content: Consider the effect of finite GBW, and of having a reactance (hmm, not sure if positive or negative capacitance or inductance; think it depends on series/parallel equivalent, too) with your negative resistance.Tim

author: macboy
author_rank: Super Contributor
timestamp: September 04, 2018, 06:16:14 pm
content: Instead of shorting the wiper to one end of the pot, try using a resistor between wiper and that end. The value will likely be between 0.5 to 0.1 of the pot resistance. This gives a really nice pseudo-log for volume control, but I can't say how well it will work here.

author: Lee Leduc
author_rank: Regular Contributor
timestamp: September 04, 2018, 06:27:49 pm
content: Changing pot tapers with resistors. Interesting read. Google "changing pot tapers" for more.http://www.geofex.com/article_folders/potsecrets/potscret.htm

author: ilya
author_rank: Contributor
timestamp: September 04, 2018, 06:58:41 pm
content: Quote from: macboy on September 04, 2018, 06:16:14 pmThanks for a suggestion. This was the first thing I did. Unfortunately it didn't give me the right curve. I need a log curve and it's possible with voltage dividers only which is not the case here. I think I found a promising solution by making a neg resistor truly floating though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2018, 04:44:42 am
content: Example, start with an ordinary volume control circuit. Input to top of potentiometer, output from wiper, bottom to ground. The transfer curve (angle of rotation to output voltage) is linear.By connecting some resistance in parallel with the bottom half (wiper to bottom), the curve gets skewed, not much at the very bottom (the wiper is nearly touching the bottom, so the resistance is small, which dominates over the added resistance), quite a lot in the middle (where the Thevenin resistance of the pot peaks to R/4), then not a lot at the top (where again the pot's resistance to one side is small, and dominates).One final observation we have to make: a log pot cannot truly go to zero. That would be absurd! The log of zero is negative infinity*. Rather, we have to define some minimum ratio we can adjust down to. This will then set how close of an approximation we can make.*For certain kinds of log.So, to limit how low the pot goes, we simply put a resistor in series with the ground return path. The parallel resistor remains in parallel with the pot wiper and bottom terminals, then the bottom terminal goes to ground through this extra resistor.Here are some typical results:Code:Atten.Range (dB)bcPeak Error (dB)400.042220.00432439200.179620.0911468.3100.336590.438051.760.42290.980530.5630.508932.40030.13The parallel resistor has value b * R, where R is the nominal (total, top to bottom) resistance of the pot. c * R is the value of the bottom series resistor.Obviously, the error is best for small ratios, where the nearly-linear scale matches up closely with a small segment of the log(x) curve. The peak error is gross at large ranges (err ~= range) because the output amplitude rises sharply for the first little turn, despite the loading resistor.Tim