Ross phaser (with Tonepad schematic) problem

[bizzle]

Good news...replacing the dual op-amp worked!  My Phaser is now whooshing and burbling like new (although I didn't buy it new...must be close anyway).  Many thanks to all who provided guidance.  I wish I had some way to repay the favor, but alas, I think when it comes to electronics expertise, there are givers and there are takers.  I may be doomed to be the latter.

[Mark Hammer]

My experience here is that everyone ends up being a giver at some point, though sometimes it takes a while.  Rest assured that even if you thought you were taking rather than giving, every trouble-shooting case that gets laid out here in sufficient detail, and gets followed up to a successful conclusion, provides someone somewhere with an idea for how to solve their own trouble-shooting challenge.

[bizzle]

Good point.  One more for the archive.

Meanwhile....

I might've spoken too soon when saying the phaser is working like new.  The effect is loud enough now, but there is a minor problem with its sound.  I stress the word 'minor' because this sonic flaw is actually kind of interesting, and I might not want to fix it.  When the phase effect sweeps 'low', a dissonant tone is audible.  It's as if a second slightly detuned note (flat, I believe) were being played along with the source note.  It actually gives a nice warpy sound when Rate is slow and a more watery sound when fast (almost leslie-ish).  Could this be caused by having replaced the dual op amp with a different part number than the original?  Not wanting to wait for shipping or to try to make it to a limited-hours supply store during business hours, I bought the only (low power) dual op amp i could find at my local electronics megastore (Fry's electronics).  Having seen in the Tonepad phaser parts list that "other dual op amps [besides the RC4558] will work", I boldly assumed that most op amps had a generic function.  But, I tend to think that the dual op amp that I replaced doesn't have much to do with the actual sound of the effect since (at least) one side of it served simply to amplify the overall volume of the output. 

I'm guessing this new, more subtle problem will be a bit harder to address, but I thought I'd mention it in case someone had any ideas off the top of their heads as to how I might find its source.  Thanks in advance, as always.

[Mark Hammer]

The LFO has two things that appear to play a role in the desirability of the sweep.  One is the 270k/100k biasing circuit shown on the "left" of the LFO in Francisco Pena's Tonepad PDF file and the other is the 270k resistor from pin 1 to V+ that helps to provide the "hypertriangular" shape.  I've built the circuit successfully a couple of times, and in some builds I too have noticed a certain blip that occurs at the "turn around" (extreme of the sweep).  I wonder if it is a reflection of a) a chance combination of within-tolerance resistor-variation that results in misbiasing, b) within-tolerance resistor-variation that results in the hypertriangular shape being off in some fashion, or c) a combination of the two.

Worth exploring more systematically.

[Valoosj]

I am just about to order the parts for this one. But what are 500KC potentiometers? Reversed?

[Mark Hammer]

Yes.  "C" taper is reverse-log.  Note that, if you don't mind having your speed control go slower in a clockwise direction, you can use a normal log pot.

[Valoosj]

Is that taper part imporant? I'm getting parts for the Neovibe as well, and it needs a 100K dual Reverse log  taper, and I can only get a 100K dual reverse log.
Same with the pots for the phaser, it's without taper. What is this taper thing anyway?

[bizzle]

I should've known...

For anyone who cares, the anomaly in the sound of the phaser pedal was due to (i guess) insufficient voltage from a consumer-grade AC adapter.  Not having any fresh batteries, I used another 9V designed-for-pedals power supply, and the weirdness went away.  Come to think of it, the "faulty" sound of the phaser reminded me slightly of an old DOD flanger I used to own that would get *really weird* when the battery was dying.  Not the intended sound, but it has it's place when you need weirdness.

[Mark Hammer]

Is that taper part imporant? I'm getting parts for the Neovibe as well, and it needs a 100K dual Reverse log  taper, and I can only get a 100K dual reverse log.
Same with the pots for the phaser, it's without taper. What is this taper thing anyway?

You can use ANY taper you want.  The extent to which reverse-log taper will make a difference will depend on which portion/s of the speed range are of most interest to you.  When one uses a linear or log-taper pot and wires it up so that turning clockwise increases speed, you'll find that it is hard to dial in the precise faster speeds you may want because the faster speeds are squished into a very small arc of the pot's total rotation (the last 20 degrees, pretty much).  Using a reverse-log taper will stretch out the last bit of resistance change over a much broader portion of the pot's rotation, making it easier to dial in the different faster speeds more precisely.  If you are willing to have the control go from fast to slow as you turn it clockwise, you can use a normal log taper pot and wire it up in reverse.