Help - anderton frequency booster oscillating

[blackieNYC]

http://www.muzique.com/news/frequency-booster/
Maybe someone can help me with this problem:
I have a freq booster on the breadboard, followed this schematic with .01u caps. I have a test tone app on the phone HP output ( very low impedance) and the booster works fine.  Ranges from pretty flat to a peak of around 20dB. Fairly steep rolloff.  Everything seems great.  Then when I plug in the guitar, it oscillates at any gain setting.  With no cable on the input it oscillates.  With the scope i find the oscillation absolutely everywhere except the Vref point in the 100k/100k voltage divider or ground.If I ground the input it stops.  If I remove the input cap it stops. Yesterday I coulda swore,  well it doesn't go away when I ground the input or pull the input cap. It goes away if I ground the output side of the input cap, which is part of the FB anyway.  I would expect RF but the thing is putting out a screaming 900hz to 1.2khz at 8v p-p. ( using these caps and the tone generator  i measure a center freq of 1700 hz). The frequency changes with the gain pot a little.  if I unplug the guitar, the oscillator drops to 45 hz.  My bias voltages and pin voltages look good when it is working with the iPhone source, or with the input shorted.  The headphone jack is almost a short of course. My breadboard wires are pretty short.  I've been shutting off lights and unplugging wall warts all over the apt.
The other half of the op amp is a completely separate working circuit for some other pedal.  If I ground it's input there is no change.

[MaxPower]

Do you have any caps on the op amp's power connections (Vcc, Vee or whatever the hell they're called)? Sometimes op amps like to oscillate. I see a lot of recommendations for a .1uF and 1uF Cap in parallel from each power pin (Vcc, and Vee) to ground as a means to prevent oscillation from the op amp.

I'm still a newbie so I don't know if that is the problem and solution, but it's worth a try. And the sizes of the caps may need adjusting for all I know.  Just google for examples.

Hopefully R.G., Paul, or someone else who knows their stuff will give you a proper reply.

[MaxPower]

My previous post pertains to a dual power supply. I expect the caps would go from the power connections to Vref in your case.

And maybe instead of Vcc and Vee, it should be +Vs and -Vs for the op amp's power supply pins?

[blackieNYC]

I have 100ufs on Vbias and +9. Is there such a thing as too big?  I'll try reducing them. I have a scope, and the Vbias is about the only place I don't see the massive square wave, but I did see a 10yr old thread on "motorboating" where RG and others mention caps and stages.  
Tried smaller filter caps.  No change.
just swapped the TL072 for a 5532.  Same oscillation.

[Mark Hammer]

In the original GP project article, Craig A has a 10k fixed resistor going from the inverting pin of the op-amp to Vref, rather than 10k + 10uf going to ground.  In principle, that ought not to make a difference, but try it out anyway.

[GibsonGM]

This schematic WORKS, with the exception that I used the version drawn by Paul Marossy in 2005.  I had the same trouble with the ORIGINAL Anderton version (the one using the dual power supply).   My original oscillated so as to be garbage, but going to this 1-opamp style setup worked great, and I've used many times now. 

Yup, that 10K returns to Vref as Mark suggested.  Also, Paul used 10K resistors in the Vref network with a 470K in place of the 1M (scaled resistor values) - don't know if that would do anything - I'd think returning the opamp inv. pin back to Vref would do more.   

One last thought is that with .01u, you might be pushing the upper limits before feedback occurs - IIRC, I did reach a place where decreasing the caps too much brought the FB back, perhaps because breadboards are just too inductive and "antenna-like" (coupling)....try making C1/C2 something like .1u if moving the resistor to Vref doesn't do it, see what happens....

[Mark Hammer]

This schematic WORKS, with the exception that I used the version drawn by Paul Marossy in 2005.  I had the same trouble with the ORIGINAL Anderton version (the one using the dual power supply).   My original oscillated so as to be garbage, but going to this 1-opamp style setup worked great, and I've used many times now.  

I'm looking at the scanned page from the June 1982 issue of GP right now, and "the original" is most definitely a single-ended 9v power-supply with floating ground from a 10k+10k resistive divider.  I wonder if some confusion arose in the conversion from single to bipolar?  I might also note that the AMZ schematic Alan links to shows a 100k+100k resistive divider, and a 1M bias resistor to Vref on the input.  Anderton originally had a 470k there.  So I'm wondering, is there some issue as to the bias current needed to achieve stability in some op-amps?  Note that Anderton used an LM353 in the GP project article.  What are you using, Alan?

[The French connection]

This one  from Marossy's schemas works top nutch.


and this modified version also:


Personnaly the Boost control is useless or maybe the taper must be changed because it's effects is only audible at max.

Good luck!

[Mark Hammer]

I've built it without problems on multiple occasions.  But what op-amps are people using in each of these builds?

[blackieNYC]

For several if the above questions:
I have used a TL072 and a 5532.  I have anLF353 I can try.
Changing the .01 caps is worth experimenting with, though it is smack in the middle of the recommended center freqs listed on all the schematics.
I'll try the Vref instead of 10u cap to ground. 
Funny thing is, this oscillation related to the frequency for which the circuit is tuned.  The pitch varies with the boost pot just a little, but varies more with the presence or absence of a guitar or guitar cable on the input. 
I'll try a few of these suggestions and get back. 
BTW, at the risk of losing any further assistance due to this wacky idea, I'm going to try using this freq booster to drive an uglyface.  (Too much treble already, you say.)  I hope to get a little more sustain out of higher notes vs. the low ones.  Then roll some treble off later.

[Mark Hammer]

I doubt it needs an LF353.  I was just wondering aloud if the 353 behaved better with the original biasing values, while other op-amps behaved well with what Jack shows.

Question for those who may understand the workings of the circuit a little better than I.  Can wider and narrower bands of boost be attained by adjusting the values of the two range-setting caps?  For example, if C2 is a larger value than C1, will that result in a wider chunk of the spectrum being boosted than if C1 = C2?

[PRR]

This circuit is not fussy about the opamp.

This circuit will NOT work right with non-zero source impedance. Therefore the one-opamp plan depends on getting signal from a WELL-buffered source, not guitar, not even all buffers. The two-opamp plan such as in Dan Letourneau's image is the way to build it.

It does not matter if the 10K+10K from out to -in goes to a solid Vref or to ground (through a cap if there is DC).

Resistor values may be fussy. Don't go "nearest available value".

If the resistance in the "Boost" leg goes to 5K, it WILL oscillate. (Heath used a similar thing and sold it as an oscillator.) This is actually 10K/G where G is the gain in the opamp, nominally 2 because of the 10K+10K NFB resistors. Anderton's 5.6K stopper covers 10% of slop in resistors.

ANY source resistance (even 100r) will really foul the action at low Boost resistance (high Boost setting). With 50K it does the opposite of what is claimed: when it tries to boost, the input impedance of the Bridge-Tee goes way low and sucks-out more than the boost puts in. So dedicate a buffer to it.

TL072 5532 LF353 should be entirely ample.

Given *correct* resistor values, howling is either wrong-connection or really bad layout (output leaks all over input). Obviously, because Anderton Jack and Dan all got it working (and some of them without even buffering).

[Mark Hammer]

Many thanks for that very helpful info.

So if boost-leg R approaching 5k creates oscillation, is there any value to putting a trimmer in place of the 5k6, and adjusting it for the point where the howling stops?  In other words, can more boost be achieved is one "walks the edge" of oscillation a little closer?

[GibsonGM]

The schema posted by French Connection is the one I was talking about....seemed to work AWESOME for me, perhaps because of the buffer issue?  I tend to use TL072's or 4558's, with no trouble.

I came up against the same 'wall' Mark is talking about, and look forward to seeing how this plays out - there IS a 'forbidden zone' with these, they ARE asking to become oscillators but are really cool )

[blackieNYC]

Tried lots of stuff.  Any of the listed caps work for the center freq.  tried tl072, 5532, lf353.   Tried different caps on the 9 and the 4.5v.  I tried putting a pedal in front as a buffer (mxr custom comp) and the oscillation just swept around with the output level pot. 
Hoping to avoid two op amps because the other half is taken.  And the purpose for which I want this is merely an accessory.  Maybe this is overkill. Maybe it deserves its own pedal.  Tried the Vref instead of ground on the 10k (-) leg, w or w/o cap.
It's possible that if I boxed this up ins metal enclosure it would work, but that experiment is not an option.
I've rebuilt this several times and I'm afraid Paul is quite right and the two stage design being It.  I'll rant about the viral spread of useless tainted schematics in another thread.  And the publication and sale of kits that have 10 years worth of customer corrections buried in the company's own forums that go unheeded by the company, dooming us all to find out about the distorted, bass-sucking phaser for ourselves.  (I digress.  Lets take that elsewhere).   I'm sure Mr Anderton had it right.  And I hope the Matrix agents leave him alone. 
I'm going to recuperate in Florida for awhile, my wife and kids wont let me bring the breadboard, and then probably build a different filter.  I'm going to go with "Paul, with his definitive answer re the single op amp design."    Tell you what, just for the closure, and in thanks for everyone's help, I will breadboard up the two op amp version just to see it work, and rule out some unseen mistake on my part. If it is the perfect solution for the uglyface, I might even stuff that extra op amp in the box.

[samhay]

I'm not quite sure what you want the tone control to do, but may this will be of interest?
http://www.diystompboxes.com/smfforum/index.php?topic=104233.0
(It doesn't oscillate at least).

[blackieNYC]

Thanks!  Looks promising.
What I'm trying to do:   The uglyface has more sustain on the lower notes than the higher ones. Not unusual but I thought I'd try to fix it. For freaky solos of course.  Boosting (flat) helps add sustain, and i recommend that for the UF,but it adds to the  sustain ofthe lows and highs.  I want to see if I can boost the highs not so much in perceived level, but in terms of sustain,  to stay above the threshold and keep the highs pumping thru the uglyface without getting prematurely cut off. I know designing a boost for some notes/strings and not others is challenging.  Your circuit looks good in that it is a rather steep rolloff, and to change the freq I don't need to change caps.  I'll be trying it next weekend.  ( and if you happen to have an UF and this booster in enclosures, maybe you can see if it can give the highs a sustain similar to that if the lows by putting itin  front of the UF. Sounds like I'm asking you to do the legwork, but this could be most rewarding if you are an uglyface fan.)  Think this will work?  To overcome this audio-thru-logic-chip threshold issue? The Escobedo PWM Schmidt fuzz thing might benefit from this.

[blackieNYC]

I built the full two-stage Anderton frequency booster, one stage being the input buffer, and it worked immediately. Thanks all.  Forwarded these findings to Jack at AMZ, who will add the low impedance clause to the schematic at his site. Thanks.
Samhay's link is a very cool variable frequency boost I will use another time.
But I did not succeed in improving the sustain of higher notes on the uglyface.  A compressor in front is the best I could do.  I've run out of t*rd polish on something that's already a perfect.. one of those.
Now I'm trying to pre-filter my scrambler, to sound like the guitar tone is rolled back.  Tried the above filter and it didn't quite cut it.  See "guitar tone rolled off" circuit thread.