A Polytone twist to allpass stages

[Mark Hammer]

I'm looking at this schematic for a Polytone amp: http://music-electronics-forum.com/attachments/26061d1383070140-polytone-lead-bass-preamp-schematic.pdf

A quick search for demos of the amp show that the vibrato has a pleasant warble.  But perhaps you see what I see in those two allpass (phase-shift) stages used to produce the vibrato: there are two caps in series, going to the non-inverting input of the op-amp, with the JFET to ground coming from their junction.

What the....?  I have NEVER seen that in my life.  What could it possibly be doing?  Any guesses?  And what, pray tell, is a "diddle pot"?

[Kipper4]

Sorry Mark can't help you with the other but i Like the idea of a diddle pot.
Maybe its for when rock stars make those 20min widdly widdley diddley videos.

[R.G.]

What most phasers use for a phase shifting stage is a first-order all pass filter.

There exists a second-order all pass that I've seen some references to. I suspect this is one of those.

I'll dig through the schemo a bit when I get time.

[tubegeek]

And what, pray tell, is a "diddle pot"?

It's how you adjust your diddley bow, of course.

[Mark Hammer]

What most phasers use for a phase shifting stage is a first-order all pass filter.

There exists a second-order all pass that I've seen some references to. I suspect this is one of those.

I'll dig through the schemo a bit when I get time.

I've seen schematics of 2nd order allpass stages, and this doesn't really look like that.  On the other hand, if I think about it, I guess this sort of meets the criteria since the .05uf cap and 470k resistor form one highpass pole and the .01uf and JFET form another.   But the pole formed by .05 and 370k is pretty dang low as highpass sections go.  On the ther hand, since the goal is vibrato and not phasing, applying phase shift across the entire spectrum is not an entirely foolish idea.  The .01uf/JFET pole simple adds a swept pole on top of a fixed one.

Certainly makes me curious to try this out.  If you think of it, bridging the .05uf cap simply places the 470k in parallel with the JFET, making the allpass section a lot more familiar to most of us.  What this suggests is that - if having two poles makes for a nicer-sounding vibrato (and assuming this circuit doesn't rely too heavily on any quirks in the LFO circuit) - one could use a 3PDT toggle to convert a P45 from phaser to nice little vibrato.  On set of toggle contacts lifts/engages the dry signal, while the other two sets of contacts bridge/insert that extra cap in each stage to make a pair of 2-pole allpass sections.

What appeals to me about this is that 2 single-pole allpass stages tends to make for a rather ild vibrato, even as it makes for a nicely retsrained phaser.  If a conversion like this allows it to be both a pleasant vibrato AND phaser, then that's win-win.

[PRR]

> what, pray tell, is a "diddle pot"?

Sets the bias on, with possible offset between, the two JFET Gates. Simple "matching".

You set each, then both, for good mid-range nulls. The LFO sweeps either way from there.

> There exists a second-order all pass

Surely is one of them; but the second C-R doesn't vary.

Phase (one stage) varies from 180 degrees at DC to zero at infinity. Sticking a "wrong" JFET in the sim and diddling the Gate gives 90 degree points from 50Hz to 80KHz. For equal increments of gate voltage, they are all bunched-up at higher frequencies.

I think it is a cheap trick to save a couple FETs/opamps yet get an "almost four C-R network" effect.

Note also that the LFO is a bridge-Tee hammered by a square-wave. Moreover that last chip must be odd because it has the SPEED pot hanging from its "output" to ground. And if that's really a '301, two diodes hanging on its Compensation pin (which IS a documented trick for '301).

The "47" at U3A is surely 47K. I bet there's more typos. I wonder if the SPEED pot really connects that way.

[PRR]

The fixed 0.05u+470K C-R pole is at {EDIT} 6.8Hz. Not sure what that means.

[amz-fx]

Mark,

I made a prototype phaser about 6 or 7 years ago with a similar gain stage. The idea was that you could make one with a single opamp...  one section as as the input buffer and the other opamp section as the phase delay stage. It looked something like this:



As I recall, it worked okay (not great) but I never got around to doing anything further with it. I'll see if I can dig out the full schematic.

Nothing new under the sun 

Best regards, Jack

[Mark Hammer]

The fixed 0.05u+470K C-R pole is at 6.8KHz. The action of the variable pole is enhanced about a decade either side of there. For guitar there is no decade-up from 6KHz, it's all about enhanced action 600Hz-6KHz. Which of course is where we want it.

Are you sure?  By my reckoning 1 / (2 * pi * .05 * .470) = 6.8HZ

Mark,

I made a prototype phaser about 6 or 7 years ago with a similar gain stage. The idea was that you could make one with a single opamp...  one section as as the input buffer and the other opamp section as the phase delay stage. It looked something like this:

http://www.muzique.com/misc/amz_phaser.gif

As I recall, it worked okay (not great) but I never got around to doing anything further with it. I'll see if I can dig out the full schematic.

Nothing new under the sun 
Best regards, Jack

Hi Jack, nice to see you here.    (not to Hi Jack the thread, or anything....   )

In this particular instance, Polytone used it ONLY to produce vibrato.  The sound-sample I heard in the Youtube video was pleasant.  Would it have been AS pleasant or striking if clean had been mixed in to produce phasing?  I have no idea.  I know that Mike Irwin showed me a design for a phaser years ago that employed multiple-pole allpass sections (sworn to secrecy on the details), and it sounded terrific.  The notches were also more closely spaced than is typical, so it was a more resonant sound, with a very defined movement as opposed to diffuse swoosh.  Those sections were NOT like these sections, however.

I'm pleased this has piqued interest.  Sometimes, there are interesting ideas just sitting there out in plain view, and we walk right past them. 

[amz-fx]

I made a prototype phaser about 6 or 7 years ago with a similar gain stage. The idea was that you could make one with a single opamp...  one section as as the input buffer and the other opamp section as the phase delay stage.

Good to hear from you too, Mark!

My memory is bad... I looked on my hard drive and the drawing I could find for it had two of those 2-pole phase sections in series driven by a transistor buffer.    It might be worthwhile to test the single 2-pole phaser section, which apparently I never got around to doing.

regards, Jack

[Fender3D]

Would it have been AS pleasant or striking if clean had been mixed in to produce phasing?  I have no idea.

Have a look at TC XII first stage... 

[R.G.]

I did a bit of digging and refreshed myself on two-pole all passes.  Then I found a decent write-up on line. Here's a good, succinct version:
http://www.analog.com/static/imported-files/tutorials/MT-202.pdf

From what I read, I think that the circuit you found is not a second order, but rather and attempt to make two cascaded single order circuits and save some hardware.

The two capacitors, each with resistor to ground after it, amount to two highpass sections. The opamp does the job of adding the dry signal to the highpassed signal seamlessly. I think the idea is that you save the normal buffer/splitter between the two highpass sections. This ought to work kind of - it works for frequencies where the two highpass sections don't overlap much and the second doesn't load the first much, and the frequency is low compared to the turnover point of each section.

This last is easy to see - at high frequencies, the impedances of the resistors doesn't matter much and the caps become just two series caps.

It would work much more reliably with a buffer between the first R-C and the second R-C. Of course, if you've done that, you're only two resistors away from a classical two-stage sequence of single pole stages.

Still, there may be some interesting stuff there. The phase response, and maybe the magnitude response will be idiosyncratic, and might turn up something interesting. A simulator would tell some interesting stories. In the polytone version, the second phase section is fixed, with a fixed resistor for the second section.

[PRR]

> Are you sure?

My head-cold got into my slide-rule. A day later, I thot "that can't be right!", but you already caught me.

At 7 Hz, I dunno what it can do phase-wise up in the audio (especially overtones) zone. Maybe it's just a DC-block, though the stage sure should work without DC blocking.

[Mark Hammer]

> Are you sure?

My head-cold got into my slide-rule. A day later, I thot "that can't be right!", but you already caught me.

Coll, now you're only 5718 correct inferences up on me.  I'll catch up eventually! 

Again, I remind that there is no intent of the circuit to produce notches.  It is a vibrato-only circuit, so adding phase-shift to the entire signal, not just a select chunk of it, may be part of it.

Give a listen, and see if the sound of it sparks any ideas about the role of the extra sections: http://www.youtube.com/watch?v=JItTZNJfdZQ