Gyrator Wah - Should Work?

[GM Arts]

Hi all,

Here's a schematic of what I think should be the equivalent of the Cry Baby / Vox wah, using opamps and a gyrator instead of an inductor.



I've built this circuit a couple of times over the last 15 years, and both times, it didn't work.  It models OK, and the only real difference I can see is the gyrator has a series R of 220 ohms, compared to the typical 50 ohms of the real inductor.

I build lots of pedals, so please don't ask me to check my soldering ;-)

Anyone else tried this, or have an idea why it doesn't work in practice?  Maybe the output impedance of op-amps?  Beats me!

rgds all,
GM Arts

[Paul Perry (Frostwave)]

How didn't it work? Did a signal come out, but no wah? or no sig, or distorted, or..?

[gez]

I did something similar to this once.  Used a non-inverting amp and put a gyrator from negative terminal to ground and a resistor from output to -ve input.  I think that's right, gyrator might have gone to V ref (can't remember but I could check schematic).  In this way mid-frequencies got a boost as the gyrator had low resistance to them.  Range was controlled by a pot.  Had to divide down the input to prevent clipping.

It actually worked but was noisy as hell (should have used an input buffer and lower value resistors for the divider).  The reason I didn't continue with it is I couldn't get the Q high enough, it wahed but really weakly.

I seem to remember someone here saying these type of circuits have been used in comercial pedals, so someone out there knows what they're doing (not me!).  If you are one of those designers and are reading this, fancy sharing the secret?  

[R.G.]

I'm not sure why your version doesn't work. John Hollis' version, the Phantom Wah, did, and was a similar approach. The Phantom Wah had the problem that it had very high intrinsic hiss, possibly from the form of the gyrator circuit that he used, which was different from yours. You can see his circuit at
http://www.hollis.co.uk/john/phantomwah.jpg

I did a layout of it, and build one. It wah'ed, but it also hissed.

Your gyrator simulates OK when mixed into his gyrator circuit, which differs from yours in that yours seems to run voltage mode and his current mode to get the polarities correct and get a resonant response.

I would bet it's some silly mistake that you'll slap yourself in the forehead for. I can't see any of Mother Nature's laws you've violated.

Yes, I am a subscriber to the mantra that you can never, never trust a circuit simulator, but a lot of times they do show you what's happening correctly enough.

Build it again, take it in sections, and make sure your wiring works before closing the loop, I bet it works.

[GM Arts]

Many thanks all, for the input and suggestions.

I should have elaborating by saying I did,'t get any tring wah effect, just some subtle tone change, as you found also, gez.

Looks like I'll have to go for 3rd time lucky, and build it again  

GMA

[gez]

I should have elaborating by saying I did,'t get any tring wah effect, just some subtle tone change, as you found also, gez

It was more than just tone change there was a definite wah sound, just very weak which made it unusable (virturally nothing on the lower strings and some wah on the upper strings).  I had the same problem RG mentioned, loads of hiss.

[R.G.]

I did,'t get any tring wah effect, just some subtle tone change,

If you don't get the polarity right around the feedback loop, it will indeed be a not-very-interesting lowpass filter. But the circuit you have as shown does get the polarity right.

Your circuit has a lowish Q, perhaps a result of that 220 ohm resistor. Is there some chance you got the value wrong on that (I'm slightly colorblind, so I mix up red and orange bands a lot) and the Q is really low? That might account for the no-wah sound.

I guess that I would separate fragments - input buffer, forward gain amp, active attenuator, and gyrator - and then test each one for proper operation before closing the feedback loop with that 1.5K resistor. If each block was working correctly and I still didn't have wah, I'd flip the phase on the feedback loop.