Question about tonepad CE-2

[ethrbunny]

I built the CE-2 from tonepad. Went together easily and worked on the first time out.

I started doing mods with the 'vibrato'. The sound is quite a bit more 'binary' than I would have hoped for. Does that make sense? Rather than a nice shift between the two pitches its pretty abrupt. Is this the expected behaviour? Has anyone else tried this mod?

[Mark Hammer]

I'm going to take a shot in the dark and suggest that you swap C21 (.01uf) for a larger value.  Try .22uf.  That should round up the waveform a bit at speeds of around 3hz or so and up, which is the modulation range people will use for vibrato.  It worked nicely for my Ropez.  It may work out for your chorus.

Please let us know if it made any improvement.  I'm curious.

[Fp-www.Tonepad.com]

The ropez vibrato is one of my favorite sounds. I have yet to build one with the added phase stages.

Fp

[ethrbunny]

(sorry about the delay) - I did the cap change and it made a world of difference. Thanks!

I didn't have a .22 so I used a .33 instead. Can you tell me what I'm doing by altering this value from 0.01 to .22 and on up?

[Mark Hammer]

If you look at the schematic, R35 and C21, while they may be in there for other reasons, ALSO form a simple one-pole lowpass filter.  I suppose it is hard to imagine that there is somehow "treble content" in the LFO, but in some respects there is a higher frequency element that may need to be removed the same way it needs to be removed when using wall current that has that "nasty 60hz treble content".  If the hum (which is, "treble" when compared to DC) from an adaptor is objectionable, the solution is often to lowpass-filter the otherwise rectified (and supposedly, but not purely, DC) wall current with a bigger cap or with a low value resistor and big cap. 

We're doing exactly the same thing here.  The existing component values - 220k / .01uf - form a lowpass filter with a shallow rolloff starting around 72hz.  Increasing the value of the cap moves that rolloff frequency downward.  A .33uf cap gives a rolloff starting just above 2hz.  With such a low rolloff, it is unlikely that any sharp edges of the LFO waveform (and a triangle has a "point", or abrupt turnaround time) would be preserved.  What you've done to the LFO waveform is exactly the same as taking the "bite" out of your guitar signal with the tone control, just at a MUCH lower frequency.  Just note that even though the rolloff is low, it has less impact on much slower sweep speeds,which kind of works out to our advantage.

Make sense?

[Khas Evets]

Hmmm...

I would have thought that since the LFO is producing a triangle wave at a variable frequency, as you speed up the LFO, you are attenuating the higher frequencies. I'm not sure what the actual frequencies are here, but probably 1Hz to under 20Hz? What I was imagining was as the Hz of the triangle wave increases, the amplitude is reduced. I thought this would merely act like the depth pot.

Now that I've spiced up the LFO, I can see that the effect on the triangle wave is a softening of the peaks and slight amplitude reduction at higher frequencies. Using a 10uF cap (in the spice sim) makes it look close to a sine wave, at the expense of amplitude.

I think what I was overlooking is the harmonic content of a triangle wave, focusing solely on the root frequency. As usual, Mark was spot on.

[ethrbunny]

Thanks Mark! Ill study the schem and your answer. I appreciate your time.

[Mark Hammer]

Khas,

I expect you do attenuate the LFO output a bit too - hard to imagine how the peaks could be rounded yet still maintain the same ampitude.  The attenuation and rounding probably get perceptually blended with each other.

Actually, this is a very recent insight for me, having come entirely out of accidentally looking at the schem for the first Small Stone and the Ross Phaser close together in time.  I saw a 10uf cap to ground in kind of an analogous position for the Small Stone which was missing on the Ross.  The SS seemed to have a more pleasing sweep at high speeds.  A thread was B=born, and the ensuing comments identified the function of that 10uf cap for me.

As luck...and theory...would have it, what was good for the phaser goose, seems to have been good for the chorus gander as well!

[Khas Evets]

This is a great insight Mark. I'm going to try to apply it a boss BF-2 to see if I can coax a VB-2 out of it (w/o the ramp feature).

[Fp-www.Tonepad.com]

Not much to say here, other than I have enjoyed reading this thread!

You guys ROCK!