True "variable inductor" wah...

[gmoon]

I'm very impressed with your variable inductor wah! I must say that it is much easier to get some results compared to the variable capacitor. If the variable inductor is too small you can just use a bigger capacitor. In the variable cap case you would have to go to 100H inductors (!!!) or use a Miller multiplying circuit which works only to a certain extend. Mine started oscillation at high amplification.

Where do find the variable inductor? I have a hard time finding a source for them.

Thanks! It's improving.

Using really large inductors might lead to selectivity problems, too (Not to mention their size and cost).

I encountered a lot of oscillation trying to implement a NFB version with BJTs. I got one version to work by "tuning" the emitter resistor (a POT), but it was noisy and inferior. Using op amps eliminated the problem.

I'm not a "wiz" with op amps--the last time I used one was probably 15 years ago playing with C. Anderton's compressor designs. It's odd to me, for instance, that my op amp circuit works so well with DC coupling between the stages...

Try searching the web for Wurlitzer organ parts. Someone here indicated they are available (I listed the part number somewhere in the thread). Maybe it was "Morelock's Organ Parts." It's a place to start, anyway.

[brett]

Hi
Because large value variable inductors are large (ie a pain in the butt), and the resonant frequency depends on LC - I wonder whether a fixed inductor and variable capacitor would be easier/better?  e.g. the old ganged plate contraptions in radios (around C=500 pF).  If that were "Millered" (or otherwise converted) up to a higher capacitance, a small fixed inductor would work well.  It could be a DIY wound inductor, around 1 mH.
cheers

[gmoon]

Because large value variable inductors are large (ie a pain in the butt), and the resonant frequency depends on LC - I wonder whether a fixed inductor and variable capacitor would be easier/better?  e.g. the old ganged plate contraptions in radios (around C=500 pF).  If that were "Millered" (or otherwise converted) up to a higher capacitance, a small fixed inductor would work well.  It could be a DIY wound inductor, around 1 mH.

I don't think size is really an issue. Most tuning capacitors I've seen are a lot larger than my Wurli inductor. Of course those little handheld AM radios had much smaller variable caps--maybe one would work. Those are somewhere in the 50 to 150pF, I think. Vari caps do have one advantage--they are rotary devices.

With a 1mH inductor, you'd have "Millerize" the cap up to the 40-10uF range for a wah. That's a pretty high capacitance-multiplication factor. Probably better to stick with a standard Vox inductor. Or go with the larger inductance (1-5H) of a small choke.

Would I like to see/hear a vari-cap wah built? Yeah, I would.

Personally, I'll stick with this project for now. It's different, and it works. Never could multitask very well...

[jasperoosthoek]

Vari caps do have one advantage--they are rotary devices.

Hmm, I would call that a disadvantage. So a wah is normally based on a rotary device and still you need gears to convert a semi-linear motion into a rotary motion...

With a 1mH inductor, you'd have "Millerize" the cap up to the 40-10uF range for a wah. That's a pretty high capacitance-multiplication factor. Probably better to stick with a standard Vox inductor. Or go with the larger inductance (1-5H) of a small choke.

My thoughts too. I would say pretty high is an understatement, it would be around 40,000. Meaning that if you have a 100mV signal on one side of the cap you need a 4kV signal on the other side... Any multiplication factors above about 30 are impractical and are very sensitive to oscillation.

Would I like to see/hear a vari-cap wah built? Yeah, I would.

The circuit doesn't work yet. Any thoughts would be appreciated. I tried various values for the ?? resistor. 1k didn't have a notch and at high values it oscillated at audio frequencies... A haven't put too much effort into it but will probably do some simulations the coming days.

[jasperoosthoek]

I did some simulations and this circuit seems to work. It is pretty much a crybaby circuit with all the impedances a factor of 10 higher.



Vary the 30pF cap between 10 and 500pF to get some nice looking frequency sweeps. It might need some higher amplification factors in the second opamp stage.

[jasperoosthoek]

Try searching the web for Wurlitzer organ parts. Someone here indicated they are available (I listed the part number somewhere in the thread). Maybe it was "Morelock's Organ Parts." It's a place to start, anyway.

I sent a mail to Morelock's Organ Parts if they have the 550mH inductor (500805 SECO). They have lots of them! Price including shipping, handling and insurance is $5. But they ship to US post offices and want me to send them my credit card details by email or fax. Hmm, I rather not by email and I don't have a fax except at work.

At least I have a source for them. I'm going to send them an email if they ship to continental Europe and accept paypal.

[gmoon]

Yep, that must be the source he mentioned. Not expensive, either--less than a custom-taper wah POT.

Have you seen this from Ax84? (PDF in link):

http://www.ax84.com/media/ax84_m58.pdf

It's a tube-based, variable-capacitor wah. Uses a 60H choke with a three-gang variable tuning capacitor. They list the availability of the cap as "excellent." I can't find it w/google, but it appears to be three 400pF sections, so a similar tuner with 1200-1500pF would work. "New" ones don't appear to cheap, though.

It's very similar to the approach I'm using--LC notch filter returned as negative feedback (which becomes a bandpass filter on the output). The good news is there's some "prior art" to vindicate your idea. You know it works.

Any luck with the multiplier? I'm having a little trouble following the second schematic, partly 'cause the two batteries aren't in a bipolar setup (but the op amps are the more "ideal" type without power input, so maybe that's why), and the drawing is a little confusing. Which sim are you using? Also, on the first schematic, shouldn't Vb be used as the signal ground throughout? (input and filter).

EDIT: Oh, yeah--rotary vs linear (arc, really) motion: the advantage to rotary is simply that existing petals already do that conversion, so using an old wah shell is a viable option...

[PRR]

> I haven't been completely asleep. ... uses an LM324 op amp

LM324 is not a great audio amp. It distorts badly for loads under 100K unless you add a pull-down resistor (which spoils its low power consumption). The input current is higher than we might like.

TL072 would IMHO be a better pick.

The input impedance of the inverter is 100K (||1Meg) which is on the low side for some guitars. Also the 100K (and the 47K!) inserts noise. I guess you inverted because the second stage inverts; absolute phase is not needed here. Use non-inverting with 1K and 470 NFB resistors. The dummy resistor on the other input pin is usually not needed in audio, and not with JFET TL072.

The 0.01uFd against the ~~90K input is a heavy bass-cut. The 0.02u at the output could be too, depending what comes after.

The simulated inductor should have series resistance; it matters enough that you want to include it. I'd guess hundreds of ohms. If you have an inductor in hand, just put the ohm-meter on it.

If you are going to model the 5H of the pickup, you should add 30pFd per foot of cable, 200pFd-1,000pFd, AND the 250K pot most guitars have. (Sweep this alone before you run it with other circuits; the peak/fall response is quite severe. I would assume some perfect source, also assuming that the pickup-winder designed for desired response.)

[gmoon]

LM324 is not a great audio amp. It distorts badly for loads under 100K unless you add a pull-down resistor (which spoils its low power consumption). The input current is higher than we might like.
TL072 would IMHO be a better pick.

It's what I had on hand (other than a couple TLC274s). The TL07x would be my first choice. I'll have to order a few...

The input impedance of the inverter is 100K (||1Meg) which is on the low side for some guitars. Also the 100K (and the 47K!) inserts noise. I guess you inverted because the second stage inverts; absolute phase is not needed here. Use non-inverting with 1K and 470 NFB resistors. The dummy resistor on the other input pin is usually not needed in audio, and not with JFET TL072.

Yeah, I was planning on using one of the additional stages as a noninverting buffer, I think I mentioned that... I'm aware of the noise issue and the input impedance of ~100K (why the 100K is there in the first place). With a non-inverting buffer (stage 1) I'd drop the 2nd inverting gain stage values but use Rf/Rin ratios so I can get 2-3X gain. yes, gain's lower now, but I've discovered it's a little below unity.

The 4.7K resistor between the stages is essential, at least for the LM324. I tried removing or changing the value, and it makes a difference. I still feel weird about DC coupling, I didn't expect this to work at all.

I'd like to keep the overall circuit noninverting, but I'm not stuck on it. I have limited op amp experience, and I always appreciate the extra input (pun intended). Simple is better.

The simulated inductor should have series resistance; it matters enough that you want to include it. I'd guess hundreds of ohms. If you have an inductor in hand, just put the ohm-meter on it.

Entered in the sim parameters for the inductor from the start of the proj...

The 0.01uFd against the ~~90K input is a heavy bass-cut. The 0.02u at the output could be too, depending what comes after.

Intentional. Partly personal taste, partly the specific amp, partly due to the bass-heavy nature of the circuit, which was much worse with the BJTs. Maybe with a higher-impedance input I can increase the cap.

I don't generally care for bass-heavy or high-gain... Certainly not during testing or development. Just looking for a nice (subjectively) linear sound.

If you are going to model the 5H of the pickup, you should add 30pFd per foot of cable, 200pFd-1,000pFd, AND the 250K pot most guitars have. (Sweep this alone before you run it with other circuits; the peak/fall response is quite severe. I would assume some perfect source, also assuming that the pickup-winder designed for desired response.)

Noted, that would be an easy improvement. Regardless, I take LTspice sims with a grain of salt. They make a great starting point, though.

[PRR]

> I think I mentioned that...

Apologies. Between server up/downs, doctor date, brother driving up tonight, I've only skimmed the recent posts.

> Entered in the sim parameters for the inductor

Ah. My sim aint that smart. (I now have two here, and both have remarkable stupidities.)

> I still feel weird about DC coupling, I didn't expect this to work at all.

No, it should work. That's one thing op-amps can do well. FAR bigger systems are totally DC-coupled. A dozen DC-coupled vacuum-tube opamps can get a little hairy, trying to trim #12 and get a result before #1 drifts out of your error band. But in audio huge DC "error" is acceptable.

> The 4.7K resistor between the stages is essential

Yes. Without it the gain is "always infinite". In real-world it comes down to parasitics and clipping.

It's a parallel tank. Impedance is infinite for one freq and zero for faraway freqs. So gain looks like zero/zero which is a funny number. LF gain actually has coil resistance, but 333/zero is another funny number. With the 4.7K in front all the gains are readily calculated (readily but laboriously, so we either plot 3-5 easy points or we ask the computer to do some work).

If you don't mind >= unity gain away from resonance, you can do hi-Z input and resonance in a single opamp. Non-inverting, ~~~2K "-" to ground, LRC from Out to "-".

Yeah, get some TL072. I'm bitter about the LM324 because when it was very new, I needed a buffer FAST for a paying gig. Solder-up, passes signal, leave it running while packing.... some nasty there?? 'Scope says yes. Then I actually read the app-note and discovered the deep-B output stage.

[jasperoosthoek]

The simulated inductor should have series resistance; it matters enough that you want to include it. I'd guess hundreds of ohms. If you have an inductor in hand, just put the ohm-meter on it.

It's even written on my inductor... Doh! http://www.entertonement.com/clips/pwmlfkvdfx--HomersDoh-MP3

Yep, that must be the source he mentioned. Not expensive, either--less than a custom-taper wah POT.

Good news! They accept paypal and will ship to Europe for $8. Giving the current $US/Euro exchange rate they are very cheap indeed. Especially because they replace both the wah pot and the inductor!  

Have you seen this from Ax84? (PDF in link):

It's a tube-based, variable-capacitor wah. Uses a 60H choke with a three-gang variable tuning capacitor. They list the availability of the cap as "excellent." I can't find it w/google, but it appears to be three 400pF sections, so a similar tuner with 1200-1500pF would work. "New" ones don't appear to cheap, though.

Funny that you mention that one, on page two of this thread I wrote I remember seeing a wah circuit years ago with a variable radio tuning capacitor. I think it was tube based. I cannot find it anymore. Sounds familiar? That might be cool too. I saw it about 7 or 8 years ago. At the time my understanding of electronics was in it's infancy. Right now I'll be able to tackle that one a lot better.

There are dozens of variable capacitors on ebay, a couple are in that range. All taken from old radios etc. For example http://cgi.ebay.nl/Triple-3-Gang-500pF-Variable-Radio-Tuning-Capacitor-/150490943445?pt=UK_ConsumerElectronics_SpecialistRadioEquipment_SM&hash=item2309f58fd5

They are not cheap though. Mine came from a tube radio I converted into a guitar amp about 5 years ago. The complete radio was somewhere between 3 or 5 euros...

It's very similar to the approach I'm using--LC notch filter returned as negative feedback (which becomes a bandpass filter on the output). The good news is there's some "prior art" to vindicate your idea. You know it works.

Only it is not my idea actually. That was one reason why I bothered. I knew someone had built it before.

EDIT: Oh, yeah--rotary vs linear (arc, really) motion: the advantage to rotary is simply that existing petals already do that conversion, so using an old wah shell is a viable option...

That's true of course. But the problem is that they don't fit in wah shells. Especially the chunky 1200pF ones that are about 4 inches long... So indeed rotary is an advantage because it fits existing technology. But some years ago I wanted to build a way using studio faders. The problem was that the only faders with adequate quality were too long. The short ones were crappy.

Any luck with the multiplier? I'm having a little trouble following the second schematic, partly 'cause the two batteries aren't in a bipolar setup (but the op amps are the more "ideal" type without power input, so maybe that's why), and the drawing is a little confusing. Which sim are you using? Also, on the first schematic, shouldn't Vb be used as the signal ground throughout? (input and filter).

I have learned a lot from the simulations. But I lost an hour again because stupid PSpice doesn't recognize 1M resistors (thinks it is 1 ohm). But it does recognize k(ilo), n(ano), p(ico), even f(emto) but no M(ega). :S

Remember that I said earlier on page 5: I have one question about the Kay: the output resistance of the transistor is 330 in parallel to the collector impedance. How can such a small load resistance generate a good Q?
I found the answer, you cannot get a Q with the 330 ohm load resistor. The answer is the very low 0.033uF 'coupling cap' that connects to the LCR circuit in the Kay. The cap is smaller than the filter cap! A suitable coupling cap should have a higher impedance than the circuit it loads. This was no design error but the cap itself has the high impedance that makes it work. If it were much larger the peak would be very broad (the simulation confirms this). Think of the Kay wah as a voltage divider with two inductances Z1 on top and Z2 on the bottom: Z1 is the 330 resistor in series with the 0.033uF cap, Z2 is the parallel LCR.

For good measure I did some simulations on the passive part of the Kay circuit. I didn't include the transistor because I can only use high gain silicon trannies. Still it is good to get a rough idea.

The two simulations were done with a 3H and 0.1H inductor.

I always thought that the wah has a flat frequency response left of the peak:

Taken from R.G.'s wah bible http://www.geofex.com/article_folders/wahpedl/wahped.htm.

Well, this is not what actually happens: The vox/crybaby wah does not have a flat response left of the peak, it actually has a cutoff. It's because of the small size 0.01uF coupling cap. This means that for low frequencies the 0.01 coupling cap cannot load the rest of the circuit leading to a band pass. (The inductor can be seen as a short and the coupling cap needs to load the 1.5k resistor.)

Anyway, I thought that my variable capacitor was too small to get a good wah. But the existing tube design doesn't use Miller amplification as present conventional wahs! It is very similar to your design where you use the bare LCR circuit in a feedback loop. The biggest difference is that they used a tube and you used an opamp. If I adapt the crybaby circuit to 10x higher impedances (caps 10x smaller value, resistors and coils 10x larger value) I only need a 1000pF variable cap. I don't know how big mine is but Millered up bit more might work without becoming unstable. I'll try to build the circuit I posted above with R12 as a 3.3k trimpot. That way I can easily adjust it to the value of the capacitor I have.

[jasperoosthoek]

Nice, I've finished a new design that I tested with an oscilloscope and a block wave generator:


It produces all the right curves on the oscilloscope! The resonance frequency of the circuit is between 380Hz and 2.8kHz. R16 and C8 form a 20kHz low pass filter necessary to avoid oscillation at 500kHz when the variable cap is at its highest value. They might not be needed on a PCB. I'll test it later today with my amp .

[jasperoosthoek]

It's noisy as hell on my breadboard but it works . I have to look at the oscillation problem though but with suitable 20kHz lowpass filters here and there it should work fine. Putting in on a piece of perfboard and in a box should kill most noise.
The variable cap is as long as my wah shell is wide so I cannot just pop it in.

[moonbird]

Hello all -

FWIW - This may be off point at this time but many of the upright styles of organs use a 3in square box that appears to contain a variable capacitor -- I have never measured it. It is connected to a pedal on the organ not sure what is does. This might be a source for a pedal ready vari-cap. I will take some pics this weekend and try to post.

[jasperoosthoek]

By all means, please take pictures! I cannot use my standard wah enclosure with my vari-cap. It fits inside but the gear is in the middle making it just too big.

[gmoon]

Lotsa new stuff here; I'll try to keep my response < epic.

It's noisy as hell on my breadboard but it works . I have to look at the oscillation problem though but with suitable 20kHz lowpass filters here and there it should work fine. Putting in on a piece of perfboard and in a box should kill most noise.
The variable cap is as long as my wah shell is wide so I cannot just pop it in.

Alright!--cool. I just noticed my project is somewhat noisy with other amps, but sounds great with the other. Yeah, shielding will help.

Good job. Now I can see RG's Vox-to-op amp circuit in your schematic, too.

@PPR

>> I think I mentioned that...

>Apologies. Between server up/downs, doctor date, brother driving up tonight, I've only skimmed the recent posts.

>> Entered in the sim parameters for the inductor

>Ah. My sim aint that smart. (I now have two here, and both have remarkable stupidities.)

No apologies necessary. We've had my 88 yr-old Dad as a house guest the last few days (and a good chunk of my time lately is spent cleaning out his house).

Anyway, I hadn't entered the serial resistance in the sim after all--that was in my earlier BJT sims before, but I forgot. Of course, entering it really mucks up the high-freq response, which I've been dealing with from the beginning...  There's no doubt that op amps are better, but ideally a variable-inductor with < 75 ohms of DC resistance would be my wish--or a vari L in the 5H-.5H range. Hey...kinda like the Kay original...

If you don't mind >= unity gain away from resonance, you can do hi-Z input and resonance in a single opamp. Non-inverting, ~~~2K "-" to ground, LRC from Out to "-".

Simmed it, and that does work. Maybe not quite as well as two, but it's pretty cool.

So here's what I've come up with:
--first stage: non-inverting with Rin/Rf of 1K/1K, for a gain of 2, and the high input-impedance expected of a non-inverting stage.
--second stage: as before.

One more addition: between the op amp output and the LC tank I've added a series resistor. It doesn't effect the Q, or the height of the resonant peaks, but it does raise the "floor" of the rejected freqencies. Something up to 2.2K or so; 1K works well. It does, of course, increase the subjective volume, 'cause it's raising all the rejected frequencies by 10 or 20dB. But even with 1K inserted, the high-end peak is 15-18dB or so above the floor (and the low-end even more). This can be adjusted to taste, of course. Makes the effect a little less "reedy" (which again is a taste issue).

Yes, it's inverted overall, but so was the Kay and my first transistor-based models. No biggie.

I'll post a schematic and clip whenever I get a chance. Might be at least a couple days, for the clip, anyway...

I'm near to "boxing" this one, but I'll wait until I order and test with TL072s.

By all means, please take pictures! I cannot use my standard wah enclosure with my vari-cap. It fits inside but the gear is in the middle making it just too big.

Thanks to moonbird for bringing this up. The one I'm using is a re-purposed organ pedal. And I've got two more organ "swell" or expression pedals. I could post a couple photographs, too.

[gmoon]

Organ expression pedals from my basement junk pile. These are resistive, not capacitive, though. I've never run across one of those...

#1: LDR with bulb/cell beside the can caps.
#2: POT, mech coupled with those strap/friction wheels
#3: multi-layer resistive strips that deform when the pedal is pressed.

1:




2:




3:

[PRR]

> stupid PSpice doesn't recognize 1M resistors (thinks it is 1 ohm).

Older pSpice uses the no-letter-case parser. "M" and "m" are ambiguous, mega or milli. You MUST write "meg", "Meg", "MEG". "M"/"m" alone should parse as "milli". If you are very frustrated, write "1000K".

[Lurco]

Thanks guys for bringing this up. My clone wah scratches a lot.  I should have bought the slightly more expensive pot. This might be scratch free.

Maybe this might be done with an old single coil pickup. Just remove the magnets and mount some ferrite material to the pedal.  

I remember seeing a wah circuit years ago with a variable radio tuning capacitor. I think it was tube based. I cannot find it anymore. Sounds familiar? That might be cool too.

Eric Barbours big L small variable C wah: http://smg.photobucket.com/albums/v324/caseyseffectpage/Wahs/?action=view&current=svetlanafuzzwah.jpg

[moonbird]

Hi there -- exactly!! great photos - these are the type of peddles I am fiddling with. The one I have -- that I CANNOT find (yet) has a variable capacitor that is controlled by the peddle movement. Hope to find it this weekend while cleaning the garage (ugh).