Ghost Dance: can it be done with an op-amp?

[David]

Like the subject says.
I'm intrigued with the simplicity of the Ghost Dance, but in searching the appropriate posts, it appears that others have thought the circuit lacks somewhat in performance.  Out of curiosity, is there a Hollis-type design quirk that makes the design dependent on the use of a 4069, or could a TL071 be substituted -- possibly with at least equal if not superior results?

Is the overall consensus that the circuit lacks in output, or in "quack"?  If it's output, could the CMOS boost be patched in to provide gain recovery, or is this too noisy?

[The Tone God]

Out of curiosity, is there a Hollis-type design quirk that makes the design dependent on the use of a 4069, or could a TL071 be substituted -- possibly with at least equal if not superior results?

There is nothing special really about the 4069. It is a CMOS IC running in linear mode as a inverting amplifier. An opamp can do the same thing. R.G. discusses how to do this and other aspects of wahs in this article:

http://www.geofex.com/Article_Folders/wahpedl/wahped.htm

Is the overall consensus that the circuit lacks in output, or in "quack"?  If it's output, could the CMOS boost be patched in to provide gain recovery, or is this too noisy?

When people say "quack" they are not talking about the output of the circuit so changing the output will not help. The are talking about frequency response. The above mentioned article will explain more.

Andrew

[David]

Out of curiosity, is there a Hollis-type design quirk that makes the design dependent on the use of a 4069, or could a TL071 be substituted -- possibly with at least equal if not superior results?

There is nothing special really about the 4069. It is a CMOS IC running in linear mode as a inverting amplifier. An opamp can do the same thing. R.G. discusses how to do this and other aspects of wahs in this article:

http://www.geofex.com/Article_Folders/wahpedl/wahped.htm

Is the overall consensus that the circuit lacks in output, or in "quack"?  If it's output, could the CMOS boost be patched in to provide gain recovery, or is this too noisy?

When people say "quack" they are not talking about the output of the circuit so changing the output will not help. The are talking about frequency response. The above mentioned article will explain more.

Andrew

I didn't do a good job of explaining myself.  My apologies.  I read the wah article before posting here.  The diagram concerned me because it referenced outdated design techniques and looked like its performance would be less than optimal.  What prompted my post were other postings uncovered by searching for "Ghost Dance" which indicated that the circuit was perceived as being "faint" (which I mistakenly took to mean "not loud enough") and also posts looking for ways to make it "more intense".

[The Tone God]

I didn't do a good job of explaining myself.  My apologies.  I read the wah article before posting here.  The diagram concerned me because it referenced outdated design techniques and looked like its performance would be less than optimal.  What prompted my post were other postings uncovered by searching for "Ghost Dance" which indicated that the circuit was perceived as being "faint" (which I mistakenly took to mean "not loud enough") and also posts looking for ways to make it "more intense".

I played with one a while back and I didn't think it was faint but I think people feel it is not as "complex" sounding as other wah designs which I tend to agree with. Perhaps if you take a look at some of the Morley designs as they are opamp based inductor-less wahs that may help you out.

Andrew

[David]

I played with one a while back and I didn't think it was faint but I think people feel it is not as "complex" sounding as other wah designs which I tend to agree with. Perhaps if you take a look at some of the Morley designs as they are opamp based inductor-less wahs that may help you out.

Andrew

Sorry, Andrew.  Don't mean to be a PITA.  I have a Morley PWV-II.  It works fine, but it's too "muddy" or "bassy", even at the top end of the sweep.  I did a SEARCH for Morley mods and found a few.  I don't actively love this pedal anyway.  I'm really looking to put new guts into a smaller, lighter shell (which I already have).  I thought an op-amp build would be good because it might be simpler and could possibly give me a free op-amp to implement volume recovery (or even boost) if needed.

[Meanderthal]

 Hehe- the thing that bugs you about the Morely is exactly what I like about mine - I play bass. What I don't like about mine is:
The pedal won't stay put where I leave it,
When I shut off the wah it's suddenly and forever stuck in volume pedal mode(defaults to a less than unity volume pedal, no bypass)

Neither one of these things would be a problem if it were in a crybaby shell, without the damn volume pedal mode. This might be a great project for bass... If it handles the bottom end the same way...

[David]

Hehe- the thing that bugs you about the Morely is exactly what I like about mine - I play bass. What I don't like about mine is:
The pedal won't stay put where I leave it,
When I shut off the wah it's suddenly and forever stuck in volume pedal mode(defaults to a less than unity volume pedal, no bypass)

Neither one of these things would be a problem if it were in a crybaby shell, without the damn volume pedal mode. This might be a great project for bass... If it handles the bottom end the same way...

Well, the biggest drawbacks are its large size and how short the pedal travel "feels".  I really like the buffering, volume pedal capability and the fact I don't have to worry about the wah pot wearing out.  I have the schematic and I might have to consider rehousing the guts in a smaller pedal more suitable to my needs.  It just so happens I have a nice Bespeco shell just waiting for new guts...  I also have a list of mods somebody with the handle of Marino posted last year.  A few of those look like they'd push my unit closer to where I want it.  Also, I wonder how it would do with a 5532 instead of the TL072?

[David]

Some REALLY bad puns there.  Sorry!

This topic changed real fast last night.  The Bespeco wasn't ready to become a shell.  I just didn't know it.  Some time back, I had had some kind of power misadventure with it and thought I had cooked the wah circuit.  Then a few days ago, I tested it again about a year and a half after the power incident and nothing...  I was ready to do an autopsy on it.  Well, last night I decided to find out definitively what was wrong with it.  Plugged it in, put a battery in, stepped on it, and WAH?!?!?  Huh? I thought it was dead!  Well, I guess reports of its death were greatly exaggerated.  Turns out the volume was flaky, not the wah (it uses a dual-gang pot and a cheesy SPDT switch).  I cured that fast with a little solder reflowing.  Well, I basically have a Crybaby clone - with that nasal honk and high screech I've been looking for.  Good deal.  This circuit uses an inductor and 2 BC547 transistors.  I'm gonna trace it out and see if the circuit topology is like a Crybaby / Boomerang / Vox type.  If it is, maybe I can do the Doug Hammond retrofit and the Anderton de-scratch mods on it.  It would be cool if I could do the Boomerang volume switch thing on it too.

Well, I guess I'll just hang up the FX-17, Morley and Ernie Ball for a while...  Nice when something works!
Now, about all those builds I've been putting off...      Come here, Mockman!

[gez]

What prompted my post were other postings uncovered by searching for "Ghost Dance" which indicated that the circuit was perceived as being "faint" (which I mistakenly took to mean "not loud enough") and also posts looking for ways to make it "more intense".

CMOS inverters have low gain and high output impedance so aren't the ideal choice of amplifier for filters.  Filters constructed around them sound weak compared with similar circuits using op-amps/OTA's etc, though that doesn't necessarily mean they sound bad just that the wah effect tends to be less pronounced.

If you buffer the outputs of inverters, however, it's possible to get a pretty decent wah sound with a bit of tweaking (I've done this).

[David]

[CMOS inverters have low gain and high output impedance so aren't the ideal choice of amplifier for filters.  Filters constructed around them sound weak compared with similar circuits using op-amps/OTA's etc, though that doesn't necessarily mean they sound bad just that the wah effect tends to be less pronounced.

If you buffer the outputs of inverters, however, it's possible to get a pretty decent wah sound with a bit of tweaking (I've done this).

I know.  Your tweaks were what made me start researching this circuit.  That and its excruciatingly beautiful simplicity...