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Re: MXR - Phase 100 question

PostPosted: 21 Feb 2015, 22:30
by armdnrdy1
zaphod wrote:That article has a pretty good summary of textbook flanger and phaser theory. R.G. Keen writes some great articles. What's interesting is that he seems to be agree with me on some important points.
It turns out that the human ear is not too sensitive to phase shifts...

The most common commercial phase shifters stop with four stages (such as the MXR Phase 90, Univibe) or six stages (MXR phase 100).

...although the effect is noticeable, it's much better if the notches move up and down in frequency.


So while I might gree that maybe there's some benefit in having some extra fixed phase shift stages, it's really not clear how much of a benfit that really is. Maybe it becomes more useful when feedback is switched on...? :hmmm: Even then, wouldn't it make more sense if different RC values were used in each of the fixed stages to enhance the effect...?


The most common commercial phase shifters stop with four stages (such as the MXR Phase 90, Univibe) or six stages (MXR phase 100).

R.G. has been known to make mistakes. MXR advertises the Phase 100 as a ten stage phaser and..the schematic verifies this.

...although the effect is noticeable, it's much better if the notches move up and down in frequency.

If the Phase 100 only incorporated the six swept stages...it would not sound quite the same. The fixed stages reinforce the notches.

zaphod wrote:wouldn't it make more sense if different RC values were used in each of the fixed stages to enhance the effect...?

Actually...just the opposite. Some companies use 1% resistors or "match" the R/C components in phasers.
There are reasons why MXR did what they did. The factory documents state that the vactrols are matched. (matching the resistance slope) Vactrols are not inexpensive...you have to "weed" through quite a few to find "matching ones. Vactrols also take up quite a bit of room. Much more than a few caps and resistors.

Re: MXR - Phase 100 question

PostPosted: 22 Feb 2015, 03:39
by zaphod
armdnrdy1 wrote: MXR advertises the Phase 100 as a ten stage phaser and..the schematic verifies this.

Yes, I've read MXR's description and seen the schematics. But then I suddnly found myself questioning what's really going on. Just call me a cynic if you like :)

armdnrdy1 wrote:If the Phase 100 only incorporated the six swept stages...it would not sound quite the same. The fixed stages reinforce the notches.

And that's possibly where the answer lies. Maybe they want one set of fixed frequency notches with another set sweeping across them.

armdnrdy1 wrote:...just the opposite. Some companies use 1% resistors or "match" the R/C components in phasers.

Yes, I get that. However, that's a somewhat different matter from deliberately using different phase shift filter values to get a wider spread of notches across the frequency spectrum, for example, like in a Univibe, which is what I was suggesting.

Anyway, thanks for your patience in helping me think through what's happening here with the Phase 100. :thumbsup When I get some more time I may play around with switches to try and hear the difference with the fixed stages in and out of the signal chain. Maybe also add a Phase 90 switch option, with only four phase shift stages engaged.

Re: MXR - Phase 100 question

PostPosted: 22 Feb 2015, 04:55
by armdnrdy1
As the number of phase stages increases, so do the number of notches...but...there are limitations to what sounds good.

When the control voltage is varied, it causes a variation of resistance in the R/C...which shifts the notches up and down the frequency spectrum.

Having fixed stages is like having another reference besides the dry signal. If you lift the dry signal..you have a different animal...not a phaser.

Re: MXR - Phase 100 question

PostPosted: 23 Feb 2015, 00:50
by Fender3D
[quote="zaphod"]
Yes, I've read MXR's description and seen the schematics. But then I suddnly found myself questioning what's really going on. Just call me a cynic if you like :)

What's going on is that you have 2 more notches with the 4 added stages...

You can't count how many notches you have only by ear... but you clearly can hear the difference, if you only bother picking up your axe and play (remember F. Zappa?)
You shouldn't worry that 2 or more stages are static, you should look how they inter-react with the moving ones

Re: MXR - Phase 100 question

PostPosted: 28 Feb 2015, 16:28
by zaphod
armdnrdy1 wrote:As the number of phase stages increases, so do the number of notches...

If the RC values are the same for each stage, then I think the number of notches will remain the same, but they will be deeper and more emphasized.

armdnrdy1 wrote:If you lift the dry signal..you have a different animal...not a phaser.

Very true. You need the dry signal mixed with the shifted one to get the notches occuring. Then the LFO lets you sweep those notches to get the swirling effect.

Fender3D wrote:You shouldn't worry that 2 or more stages are static, you should look how they inter-react with the moving ones

Yes, that's in fact how I'm beginning to understand it, with the help of the good people on this thread. :)

Fender3D wrote:... but you clearly can hear the difference, if you only bother picking up your axe and play (remember F. Zappa?)

This is why I think a little experimentation with switching the fixed stages out and in should be worth trying. I'm also keeping in mind that the phasers that most people rave about are true four-stage phasers, with no fixed frequency stages. So it might be neat to be able to switch between Phase 90 and Phase 100 modes. The PCB is a more recent rev though, and all the components are really packed in compared with an original script logo pedal. So it looks like it will be a bit of a PITA to work with.

Not quite sure about the Zappa reference though. I'm afraid I only remember a couple of his songs.... :oops:

Re: MXR - Phase 100 question

PostPosted: 28 Feb 2015, 20:10
by Fender3D
zaphod wrote:If the RC values are the same for each stage, then I think the number of notches will remain the same, but they will be deeper and more emphasized. ...


Nope, the number of the notches depends on the stages number or type (two 1st order filters make 1 notches such as one 2nd order filter does), if you check usual phaser schematics, you'll notice they use the same Rs andCs each stage..

zaphod wrote:...This is why I think a little experimentation with switching the fixed stages out and in should be worth trying...

Look for LTSpice and learn how to simulate stuff. CTRL+C and CTRL+V are really usefull when you need stages multiplying :)

zaphod wrote:...Not quite sure about the Zappa reference though. I'm afraid I only remember a couple of his songs.... :oops:


Lol
He said:" Shut up 'n' play yer guitar"

:mrgreen:

Re: MXR - Phase 100 question

PostPosted: 01 Mar 2015, 15:59
by zaphod
Fender3D wrote:.... the number of the notches depends on the stages number or type (two 1st order filters make 1 notches such as one 2nd order filter does)

Aha, now that makes a lot of sense from an EE perspective! :idea:

Fender3D wrote:if you check usual phaser schematics, you'll notice they use the same Rs andCs each stage...

Yes, I've pretty much studied them all, and seen that. :) However, I had always assumed the idea was just to get deeper notches. Now I need to rethink what a Univibe is doing.

Fender3D wrote:Look for LTSpice and learn how to simulate stuff. CTRL+C and CTRL+V are really usefull when you need stages multiplying :)

I already have LTSPice, but ultimately I want to hear how it sounds. :)

Fender3D wrote:Shut up 'n' play yer guitar"

I couldn't agree more! :lol:

Re: MXR - Phase 100 question

PostPosted: 02 Mar 2015, 18:35
by Fender3D
zaphod wrote:Yes, I've pretty much studied them all, and seen that. :) However, I had always assumed the idea was just to get deeper notches. Now I need to rethink what a Univibe is doing.


Remember, we're talking about ALLPASS FILTERS.
Raising the filter's order will raise the Group delay, then, if you cascade several filters, the higher delay will lead to a different notches position (when mixed back with dry signal).

You'll obtain deeper notches by selecting and matching caps and resistors.

By shifting the filters' fc you'll add different Group delays leading to a more complex notches position.

That's why I suggested LTSpice...

check this:
http://www.ti.com/lit/ml/sloa088/sloa088.pdf

Re: MXR - Phase 100 question

PostPosted: 05 Mar 2015, 03:23
by zaphod

Heh, reminds me of Circuit Theory classes from long ago. Most of the intervening years I've been working more on the dark side, with "1"s and "0"s, rather than poles and zeroes, although I've also been designing and building tube amps in my spare too. :-)

Re: MXR - Phase 100

PostPosted: 18 Apr 2017, 15:59
by swisher
Can I use different value of 15uF tantalum capacitors? 10uF or 22uF?

Re: MXR - Phase 100

PostPosted: 18 Apr 2017, 16:10
by Fender3D
2 out of the 3 15uF caps are just DC filters, use whatever value you have (higher is better).
the third 15uF sets the LFO speed: rise it and LFO will "swoosh" slower, the opposite if you lower it.
Find your best compromise here.

BTW electros value tolerance is usually so high that you may just pick the nearest value...

Re: MXR - Phase 100

PostPosted: 18 Apr 2017, 16:53
by swisher
Thanks! :wink: