Phase resistor swing

[armdnrdy]

I've been playing with the Phase 100 circuit which uses Vactrols for the op amp phase stage variable resistor.

I commandeered some factory documents which state that the Vactrols for assembly are pre matched.

Well....my Vactrols didn't come pre matched so...off I went on another electronics safari!

I set up the Phase 100 LFO on the breadboard to use as some sort of reference point. I recorded the min/max resistance readings from (77) VTL5C3/2 that I had laying around in the walk in cigar humidor, right next to the wine cellar entrance, just off of the 5000 sq. ft. garage where I keep my rotating stock of luxury cars. (OK...they were in a box next to my work bench! A man is entitled to  dream now and again isn't he?  )

After I compiled all of these measurements, I looked at them all and cried out loud....."What does it all mean!!!" (then my neighbor came over and asked if I could keep it down)

Here are the measurements:



Next, I quietly punched up an online parallel resistance calculator to see where some of these measurements would be in relationship to the 100K resistor in the Phase stage.

Here are a few examples that I worked out for reference:

Vactrol #1
In parallel with 100K....254K =71K
In parallel with 100K....2.7M =96K  (25K resistance swing)

Vactrol #2
In parallel with 100K....46K =31K
In parallel with 100K....749K=88K  (57K resistance swing)

Vactrol #5
In parallel with 100K....24K =19K
In parallel with 100K....400K=80K  (61K resistance swing)

Vactrol #27
In parallel with 100K....580K =85K
In parallel with 100K....6.4M=98K  (13K resistance swing)


So...what does all this mean?

The information that I could gather from R.G.s "The technology of phase shifters and flangers" article,
http://www.geofex.com/article_folders/phasers/phase.html

and Fred Ridder & Michael Beigel's "More Than You Ever Wanted To Know About Phasors" article,
http://www.beigeltech.com/phase/phse.html

states that "modulating the phase resistor sweeps the phase induced notch up and down the frequency spectrum."

I understand that the frequency change can be limited by the LFO and if there is a limited amount of resistance swing, I believe this would also result in limited frequency sweep.

With all this being said....I guess my question would be....
I would be looking for a resistance swing that starts fairly low and goes fairly high for the optimum phaser sweep. Then I would match the Vactrols as closely as possible for the rest of the phase stages.
Most Vactrol based phasers have a form of adjustment (trimmer) to "fine tune" the resistance sweep to the most usable range, so a matched "set" doesn't have to be tied down to one exact range.

Am I thinking along the right path? or have I strayed too far off the footpath and become lost?
Opinions, always welcome.

[PRR]

All those numbers look high. Vactrols should swing well below 10K. And the low resistance zone will be more consistent, since it gets away from LED and LDR leakage.

Figure if you can slam say 10mA into the LEDs.

See if that gets you lower more-consistent resistance.

Measure also at 1mA. Does that give you resistances consistently higher by the same ratio?

[armdnrdy]

Hey Paul,

Those numbers are kind of flawed. the Phase 100 uses three VTL5C3/2 with the LEDs connected in series. Also, I just thought that these numbers might be good for rough matching but not for the parallel resistance calculations.....they are dual Vactrols...I just measured the outside legs....like I said for ball park matching. I think I got a little ahead of myself!

[armdnrdy]

Well I just connected 3 Vactrols in series as they are in the Phase 100 and got the same min/max readings from the outside two legs of Vactrol #2.

I took measurements from the outside legs to the center tap leg as well:

MIN    MAX
21K    254K
23K    295K

Vactrol #2
In parallel with 100K....21K =17K
In parallel with 100K....254K=71K  (54K resistance swing)

In parallel with 100K....23K =18K
In parallel with 100K....295K=74K  (56K resistance swing)

[armdnrdy]

This morning I was thinking about what Paul said :

"All those numbers look high. Vactrols should swing well below 10K."

I took another measurement of Vactrol #2 from the center tap to one of the outside legs.
I was able to get "well below" 10K for the minimum resistance reading by adjusting the Phase 100 LFO center frequency trimmer.

Note: This setting wasn't the lowest resistance I could achieve with the trimmer.

Sample reading Vactrol #2

MIN    MAX
8K      68K

Vactrol #2
In parallel with 100K....8K =7K
In parallel with 100K....68K=40K  (33K resistance swing)