Ross phaser LFO timing capacitor

[jrc4558]

Greetings,

I am facing a usual problem - lack of specific value potentiometer (500kC with knurled shaft) to build a Ross Phaser clone. I do have several 100kC pots that fit the bill, but the LFO speed doesn't decrease to it's slowest, naturally. I wonder, is there a timing capacitor in this particular LFO, or is it purely current-driven? Can anyone perhaps advise on the topology of this particular LFO and formulas to calculate it?
Thanks so much in advance!

Link: http://tonepad.com/project.asp?id=25
File:  tonepad_rossphaser.pdf 255.1 KB-- Ross Phaser, Rev.1.Nov.29.2005

[frequencycentral]

I believe the 3.3uF cap (at pins 10 and 12 of IC4b) is the timing cap. Probably best to socket it and try some higher values. Personally, I'd whack a 10uF in there and see what happens.

[Mark Hammer]

Personally, I've never found the rate control on the Ross pleasing, even with the "correct" taper pot.  Absolutely nothing wrong with having a fast/slow range switch that introduces either a fixed resistance in series with the 100k pot, or else a parallel resistance along with a larger-value (e.g., 1meg) pot.

FWIW, when I converted a Ropez to a Uni-Vibe format, a 50k pot was really all I needed to cover the complete range of usable speeds.

So, no real need to change the cap value.

[jrc4558]

Gentlemen, you have my gratitude. Will try this tonight!

[Mark Hammer]

Constantin,

It just clicked.  I haven't seen you here in a long time.  Are you still in Toronto?

[jrc4558]

Hi Mark,

Yes, although now my stomping grounds are the business area of Mississauga. How are you? Still in Ottawa?