A „Vox-Wah“ project, some circuit analysis and measuring results.

[vintagekiki]

crybaby vs wahwah

http://www.voxshowroom.com/ct/misc/wah_record/crybaby.mp3

http://www.voxshowroom.com/ct/misc/wah_record/vox_wahwah.mp3

[ronh]

Surfing the 'net, I found a mod attributed to Mr. Teese. It said you could change the pot taper by making one cap bigger. Really? So I tried it on my Crybaby. Follow the yellow wire to a cap. 0.22 uF. I then tacked on a 0.1uF cap across on the solder side. The transition from treble to bass seems (feels) linear and vice versa. Is it better? I like it, you may not. Simple to reverse. Since we are testing things, This won't change the pot taper. I think it changes the peak shape.

[Helmholtz]

Surfing the 'net, I found a mod attributed to Mr. Teese. It said you could change the pot taper by making one cap bigger. Really? So I tried it on my Crybaby. Follow the yellow wire to a cap. 0.22 uF. I then tacked on a 0.1uF cap across on the solder side. The transition from treble to bass seems (feels) linear and vice versa. Is it better? I like it, you may not. Simple to reverse. Since we are testing things, This won't change the pot taper. I think it changes the peak shape.

The Vox circuit uses 2 internal 0.22µ coupling caps (C4 and C5). Not clear which one you increased.
Increasing either by 0.1µ has no significant effect. Actually, increasing C5 lowers the height of the bass peak by 0.5dB (6%).
Increasing C4 has no measurable effect at all.

G.Teese uses original value 0.22µ caps in both positions, while Roger Mayer uses two 0.1µ caps.

Of course, pot taper can only be changed with a different pot.

[vintagekiki]

Wah Wah Mods

[vintagekiki]

An interesting solution was given by Morley, instead of a conventional potentiometer, it use photoresistor.
That's how avoided the need for a specific potentiometer taper and there are no potentiometer in the signal path to wear out over time.

[Helmholtz]

Wah inductor saturation effects:

Inductors with a ferrous core and especially wah inductors having a high permeability core without an airgap are non-linear, meaning that inductance depends on current.
Above some current the inductance drops and voltage and/or current start to distort (inductor distortion does not cause voltage clipping, though).
Inductor current depends on the ratio of signal voltage to frequency. So with the same voltage across the inductor, a lower frequency is more likely to get distorted.

For some comparison I connected the inductors in series with a 33k resistor to my sine generator and scoped the inductor voltage.

At a voltage of 1Vp-p @400Hz none of the inductors showed visible shape distortion.
It required around 4Vp-p for some kink to show with toroidal core inductors like the Dunlop red Fasel.

The black plastic encased Vox V847 and Dunlop Crybaby GCB-95 inductors behave amost identical to the red Fasel.

All pot core inductors showed less distortion than the toroidal core types.


Now as the inductor voltage is the input voltage of the wah gain stage, 1Vp-p would produce around 25Vp-p at the collector, which isn't possible.
In fact the max. signal voltage the emitter follower can handle before clipping is around 6Vp-p.
(Because of the input voltage divider and as the impedance of the resonant circuit is limited by the 33k resistor, the max. voltage across the inductor is around 30% of the PU signal.)

Means that transistor distortion will start much earlier than inductor distortion.

As the Q1 base current flows through the inductor, there's a little bias. But that DC current is only around 1µA, so it shouldn't really matter - especially as AC current at the peak is much larger.

In my inductor measurements I did not see any signs of asymmetry as reported by R.G. Keen.


So I think it's safe to assume that wah inductor distortion is typically irrelevant.

[Helmholtz]

The Sola Sound wah and some useful mods:

Vintagekiki's post #44 reminded me of my almost forgotten Sola wahs.

I always wondered why they sounded so different from Vox/Thomas wahs and why they used a modified circuit: file:///C:/Users/User/AppData/Local/...hematics-1.pdf (Link provided by vintagekiki).

Here's the frequency response:





Most important differences to the Vox circuit are:

1. Missing emitter resistor R4,
2. Missing 33k impedance limiting resistor (R7),
3. An added 50% output voltage divider,
4. Input series resistor 100k instead of 68k.

The frequency response shows very large peaks (note the tripled voltage scale), especially a huge and broad (low Q) high peak.
I could confirm that this is due to considerable transistor distortion/limiting with an input signal of 0.1Vrms (=0.28Vpp).

After some thought, I think I understand why they needed to modify the original Vox circuit - though there's room for improvement.


TBC.....

[Helmholtz]

Sola Sound wah continued:

Here's the Bode plot with added 33k across the inductor (smaller black and green peaks):


Effect as expected. I consider it an improvement. But peaks still huge and some signal distortion.


Now I added Q1 emitter resistors (note voltage scale changed back to "original"):



This brought down peaks to more "useful" levels and did away with the distortion effects.
But it reveals the problem, the Sola designers must have had with original Vox circuit:
With their different gear mechanism and the linear 100k pot, the lowered gain with the emitter resistor didn't give access the desirable lower peak frequency of around 440Hz.
As mentioned earlier, the Sola gear mechanism only allows to use about 40% of the pot taper.

Now it is interesting to note that the Sola wah requires reverse connection and reverse rotation of the pot.
That makes no difference with a linear pot, but.....
(First want to try out my idea, before I post/recommend it.)

[Helmholtz]

Sola wah continued:

Ok, I realized this needs a pot that allows to vary gain by more than 40% between 0 and 40% of the taper. And that is a log pot.
Mind you that this only works because of the reverse operation of the pot in the Sola wah. It wouldn't work like that with a Vox type wah.

For the following measurements I used a 220k log pot, that allowed to vary gain from 0 (essential for upper peak frequency above 2kHz) to almost 80% by only 40% accessible taper usage.





The low height of the "bass" peak is due to the very high DCR (79R) of the inductor (small pot core).

Note: For all my Sola wah measurements I disengaged the output voltage divider.

[Helmholtz]

Here's the frequency response of a Dunlop Cry Baby 535Q at min. (low peaks) and max. (high peaks) settings of the "Q" control.
As noted earlier, the "Q"control is a variable 1k resistor in series with the inductor (red Fasel).


With max. series resistance (1k), the low frequency peak (green curve) is barely noticeable.

[SoulFetish]

Yeah, but no one seems to be addressing the biggest problem that every Wah pedal seems to present, (ultimately making them unusable in live situations, regardless of pot taper or inductor tolerances... )
which is the embarrassment that I can't stop my mouth from making the "wah" movements in unison with the Wah pedal. It won't stop doing it.

[g1]

[Helmholtz]

Here's the frequency response of my later (1977) Sola wah:




The circuit differs from the older one by:

- increased input series resistor (470k), seems they tried to reduce the distortion,
- instead of the output voltage divider, there is a 100k resistor wired in parallel with the 100k pot,
- the "Stack of Dimes" inductor has a lower DCR of 33R.

As a consequence of the 470k input resistor and the roughly doubled gain (compared to Vox), this wah has noticeably increased (thermal) noise.

Generally noise is an issue with wahs, especially when used before a distortion device. So selecting the first transistor for low noise might make sense.

I like this one even less than the older Sola.

Both Sola wahs are rather microphonic, i.e. pick up handling noises. Adding glue to the coils didn't help.
Maybe the cheap stamped steel housing is involved.


Actually I think, the original Vox circuit (and gear) is just about perfect and very well designed.
Didn't find anything to improve with it.

[vintagekiki]

Vox Wah OS/158



https://www.voxsupreme.org.uk/vox_wah_wah_pedals_1967.html

Vox Wah OS / 158 uses inductor 0.25H (1/4 H)

[vintagekiki]

thomas wah april 1967