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I'm trying to simulate a Wien network-based equalizer circuit in LTspice. The circuit, from "Small Signal Audio Design", should have a +/- 15 dB response and a frequency range of 220 Hz to 2 kHz. My simulation shows a 16-17 dB response and a frequency range closer to ~105 Hz to ~750 Hz. Also, the simulation's response range unexpectedly decreases across the frequency range.

I've triple checked the component values and circuit diagram. I can get closer to the textbook response curve by tweaking values but I'm worried there's a fundamental issue with the simulation. I can get a bit more range of frequency response by tweaking the curve of the reverse-log pot but the filter responses still aren't what I'm expecting.

Are there any common LTspice pitfalls that might cause this? Is it normal for the simulation to diverge this much from the book? I'm wondering if there's an assumption about the op-amp's performance in the textbook that I didn't encode. I'm using this NE5532 model for the op-amps and the potentiometer_standard library for the linear and reverse-log pots.

Textbook

Textbook Wien network-based EQ

My attempt

Simulated Wien network-based EQ

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  • \$\begingroup\$ What op-amps were used in the first circuit and, what op-amps were used in your simulation? \$\endgroup\$ Commented Jan 25 at 21:02
  • \$\begingroup\$ @Andyaka The book doesn't say explicitly, but should be either TL072 or 5532. I'm using this 5532 model. \$\endgroup\$ Commented Jan 25 at 23:15
  • \$\begingroup\$ I know that error is not related to GBW product, rather it needs \$G~BW~Q^2\$ although Q seems low \$\endgroup\$ Commented Jan 28 at 17:40
  • \$\begingroup\$ Can you share the .asc (zip)? \$\endgroup\$ Commented Jan 29 at 13:00
  • \$\begingroup\$ @TonyStewartEEsince1975 sure, here's a zip with the .asc and relevant libraries. \$\endgroup\$ Commented Jan 30 at 4:46

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It's probably where you have the wipers set and that is a place where you could have a common pitfall. Make sure the wipers are correct and verify the resistance (you could do this inside of another sim). Especially the RV2 wiper since that sets the frequency.

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  • \$\begingroup\$ Thanks for taking a look—in the simulation I'm actually sweeping RV2 from 0 to 1 to see the behavior across the whole range. I was thinking the curve might affect the behavior, and checked that, since that should be an audio taper/reverse log pot, and that extends the range a bit more in the upper frequencies, but I still have unexpectedly resonance behavior at the top of RV1. I did also check the range of RV1 values and get boost/cut as expected (when cutting, I'm getting -17/16 db as well instead of the expected 15db). \$\endgroup\$ Commented Jan 25 at 20:09
  • \$\begingroup\$ What is the Rtap=10k, tap=0.5 in the RV2 potentiometer parameters? \$\endgroup\$ Commented Jan 25 at 21:42
  • \$\begingroup\$ Hey Carl, those are related and determine the curve of the reverse log taper. "Tap" is the ratio of travel of the pot when "Rtap" is reached (so for Rtap=10k, tap=0.5, at the halfway point of travel the pot hits 10k). \$\endgroup\$ Commented Jan 25 at 22:24
  • \$\begingroup\$ Okay, but you don't really need a taper for the simulation \$\endgroup\$ Commented Jan 25 at 23:36
  • \$\begingroup\$ You're right in that I could use a linear pot and step through the simulation logarithmically to get measurements at the same points, but that part of the circuit expects a audio taper variable resistor, so why not just use what it calls for? The circuit has variable Q depending on the selected frequency so I need to measure it at various points along the logarithmic sweep somehow if I want to confirm how the Q varies. \$\endgroup\$ Commented Jan 26 at 0:20

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