phase shifters

[lightningfingers]

Ive playing around with phasers lately, mainly my (diy) smallstone and easyvibe. Basically trying to make them sound like each other, and i'm wondering, what is responsible for that "swoosh" sound in the small stone? I thought it was feedback, but no matter how much feedback an easyvibe has it will never swoosh too well, even with a uniform frequency response.

Similarly with the small stone, even with the feedback loop eliminated, and the staggered caps, it still "swooshes" rather than the "sway" the easyvibe is recognised for.

Its probably been well discussed before but my search of the archives was inconclusive.

TiA

[puretube]

the hyperbolic (aka hypertriangle) LFO waveform is another plus for the SS.....

[Mark Hammer]

The individual allpass stages in a phase shifter result in a shift of phase that is inconsistently distributed across the audible spectrum.  The amount of phase shift is always greater for higher frequencies than lower ones, though, and in an orderly fashion.  When you have a bunch of allpass stages creating the same amount of phase shift with the same distribution, their cumulative effect is to create zones where there is enough overall phase shift (at a given point in the sweep cycle) that what comes out the other end is antiphase with the clean signal it gets mixed with, producing notches.  Regeneration of the signal results in an emphasis of that effect.

In the case of the Univibe, the uneven cap values means that phase shift is not distributed across the frequency spectrum in the same manner.  When one simulates the effect of this, you see that: a) the dips in response are not that steep or focussed (think shallow, broad notches, rather than narrow steep ones), and are spread farther apart, rather than closer together.

The resulting effect is that where a conventional equal-value-caps phaser focusses attention on specific parts of the spectrum as it sweeps up and down, a Univibe distributes emphasis less focally.  Some of Mike Irwin's (Vsat) graphs in this thread (http://www.diystompboxes.com/sboxforum/viewtopic.php?t=25403&postdays=0&postorder=asc&start=15) illustrate the effect of having equal vs unequal value caps.

The swoosh is largely a product of the extent to which each device focusses on a specific part of the spectrum.  Not sure if this answers your question, though.

[Lazy_Jarl]

I'm a newbie, but could the fact that the small stone uses OTA's to phase(and that the OTA stage is swept via the 'gain control' [that little double circle thing]) and the easy vibe uses regular op-amp stages have something to do with it?

[puretube]

not in that sense...

[Mark Hammer]

I'm a newbie, but could the fact that the small stone uses OTA's to phase(and that the OTA stage is swept via the 'gain control' [that little double circle thing]) and the easy vibe uses regular op-amp stages have something to do with it?

Good and legitimate question.

Answer: Not really.  There ARE differences in signal handling of the OTAs vs LDRs vs FETs, and whatever noise and distortion might be introduced, but having made a Univibe out of the OTA-based Ross Phaser, I can assure you that swooshiness vs wateriness is a product of notch/phase-shift distribution and not the specific control elements used.  Though they might not sound identical in every respect, you could vibe-ify a Phase 90, a Small Stone, or any LDR-based pĥaser and the same transformation would occur.  I will also add that the regen control which makes the Ross such a nice phaser, does absolutely diddley squat when you change the caps to vibe-ify it.  This is why I emphasize that the effect of phase-shift redistribution supercedes just about all other aspects of the design.

Think of it this way.  Imagine a big white screen with 9 big blue dots in the middle bunched together.  Now make every blue dot move a few inches to the left, to the right, up and down, all at the same time.  It looks like a "thing" moving around.  Okay, switch to 25 smaller blue dots spaced equidistantly but far apart on the same screen.  Move them up, left, right, and down by the same amount.  Will it look like some "thing" is moving, or will it just appear to vibrate in place?  Same thing happens with a Univibe, it distributes your attention differently, like the smaller blue dots do, so the perception is different.

There are times when it pays to know more about psychology than I do about electronics.   :wink:

[puretube]

psychotronixx... isn`t that what we`re all building?

[Mark Hammer]

I like to think of it as psychoacoustic leger-de-main or prestidigitation.

You thought the musician was playing in a small bedroom?  Behold!  He sounds as though he were playing in a large hall!

You thought there was only one musician?  Behold!!  It sounds as though there were two!

You thought the instrument was in an upper range?  Voila!  It sounds as if it were in a much lower range!

[StephenGiles]

That sounds slightly Biblical Mark!
Stephen

[jrc4558]

Now let's discuss the "justification of an effort" in the case why we like diy more than commercial and think that it sounds better.  :wink: