Frequency response of different inductors in Crybaby wah

[ElectricDruid]

I don't know if anyone else has looked at this before, but it was brought to my attention recently that the wire resistance of the inductor in the Crybaby wah circuit affects the response quite significantly. The various inductors that have been used over the years cover a fairly wide range (halo, stack of dimes, Fasel red/yellow, etc) - roughly from 10ohms to 150ohms series resistance.

Here's the difference between a 10 ohm inductor and a 150ohm inductor, the two far ends of that spectrum:



Since we're not changing the inductor's value, the other characteristics are the same - same frequency range, same change of resonance across the range, peaky at the bottom, softer and wider at the top. But the two different inductors give a overall picture which is quite different, with one going from quieter to louder, and the other doing the reverse. The difference in the peaks at the low end is almost 10dB. This is a big difference for only 100 ohms or so of resistance!

I thought this was interesting. All credit goes to Richie Burnett (who I know from the SynthDIY list) for bringing this to my attention and providing the LTspice sim. Cheers Richie!

[iainpunk]

wow, that genuinely shocks me, such a difference in only 100 ohms,
i'd get an as low as possible resistance inductor and add a 150Ohm pot in series, and set it to the best sweep.
now i am curious about core losses and stuff like that, would probably have to model the inductor as a transformer, with resistance on the secondary.

cheers

[Rob Strand]

I know I've mentioned that a few times over the years,
https://www.diystompboxes.com/smfforum/index.php?topic=120336.msg1141118#msg1141118

The series resistance affects the low end of the spectrum and the parallel resistance affects the other end.

The inductors people like tend to have more even peaks.   While that's appealing to the eye, at the end of the day it's what is appealing to the ear that matters.

[iainpunk]

i've heard about people putting 330r resistors in series with the inductor, probably to mellow the peak, they say it ''sweetens'' the wah.

cheers

[Rob Strand]

i've heard about people putting 330r resistors in series with the inductor, probably to mellow the peak, they say it ''sweetens'' the wah.

330R is fairly aggressive but different people have different ideas about what they want from a wah.

The Wah heavily modifies the frequency response so we would expect wide variations in people's expectations.   If goal is a flat response there seems to be less tendency to deviate too far from the ideal.   The wah geeks on the other hand seem to narrow things down quite tightly.    They seems to get down to the 1 to 2dB level, at least in A/B testing.

[Elektrojänis]

If you think about the source signal, the lower end of that sweep spectrum tends to have fundamentals and lower harmonics that are naturally louder. Higher end tends to have more harmonics that are probably quieter. So, it might actually be beneficial to tame the peak at the low end to provide more even responce overall.

That said, it will be a matter of taste anyway. Sometimes I've found that especially with overdrive the low end of the sweep on some wahs can get a bit too... honky and muddy while the high end is nice.

So... One thing to consider when I get around building my own wah. Thanks!

[Halkbi]

Nice findings! This might explain why I never seem to be able to replicate the low end response of my old Jen V846 when fiddling with the emitter resistor value on Q1 or the 33k in parallel with the inductor in modern Crybaby/V847s. I'm assuming red is the 150R inductor here? Anyone know what the difference in resistance is between the red and yellow Dunlop reissue fasels compared to the vintage ones?

[Rob Strand]

The inductor has two losses.  The DC resistance and the parallel loss.  The parallel loss is like a resistor in parallel with the 33k resistor.   These affect the apparent gain  but they also affect the Q, which is the pointy-ness of the response.

The effect of tweaking RE is quite deceptive.  It adjusts the overall gain.   The change in the gain affects the frequency range.  The wah is an adjustable capacitor and the gain affects the capacitor.  However it doesn't change the pointyness so much, that's more to do with the inductor.

So here's the observed effect of playing with RE,


However if we correct the overall volume to compensate for the RE gain change we can see the true effect on the *shape* of the response, which isn't much!



I actually should compare put the RE=470 case side by side at different pedal settings along side the effect of RE at low frequencies.



So that proves RE doesn't do much for the response shape at the low frequencies, it's just tweaking the gain.

FWIW, small changes RE can compensate for  transistor gain differences.

[ElectricDruid]

I'm assuming red is the 150R inductor here?

Yes, that's right. Green is 10 ohm, red is 150.