The Ross Phaser...again; a compendium of ideas and concerns.

[Mark Hammer]

Attempted liftoff on a 4/8 stage Ross phaser using the Tonepad "Ropez" layout.  The 4-vs-8 stage difference is quite apparent, and there are times when the 8-stage setting sounds almost chorus-like.  One of the things I hadn't expected was that regeneration with 8 stages (where the signal is returned to the 2nd of 8 stages and passes through 7 stages repeatedly) is more prone to oscillation than regen with 4 stages (where is only passes through 3 stages repeatedly).  In some respects, it seems like it might be worth it to use a 3-pole toggle switch to go back and forth between 4 and 8 stages.  The 3rd pole would be used to switch in an additional series resistance on top of the existing 27k return resistor (although most likely it would be used to shunt an additional series resistor - e.g., 27k+1k8 - used to limit the maximum regen when in 4-stage mode).

What I'm wondering about, though is whether the 10k current-limiting resistor that sits between the LFO and all the Iabc pins of the 8 OTA sections is an appropriate value.  Looking across a variety of schematics, one tends to see that the resistor is generally larger in value when the current source is feeding fewer OTAs and smaller when it feeds more of them.  So, I'm wondering whether that resistance should be different for 4, 8, or 12-stage modes.

One of the other things I seem to get is a kind of "bounce" at the sweep turnaround point when in 8-stage mode.  This does not happen in 4-stage mode, so I'm wondering if anyone else has experienced it.  Keep in mind that what works in the original 4-stage design may be nonoptimal for extrapolations of the basic Ross design to more stages.  This is not a potential flaw of any kind in either the Tonepad board, or even the basic idea of adding stages.  Rather it may be simply something that needs tweaking of a component or two when one attempts extensions beyond the basic Ross 4-stage design.

A second aspect I'm wondering about concerns the phase-filter mod.  I originally suggested this mod based on similarities between the Blacet Phasefilter from the late 1970's, and the guitar-oriented adaptation here: http://hammer.ampage.org/files/Device1-6.PDF to the design of the Small Stone and other similar OTA-based phasers.  What I suspect I neglected was that the original Blacet design was aimed at synths whose signal would have much greater spectral content, and which would also have a manual tuning control so as to avoid the lowpass sections being too low in their cutoff frequency.  After firing up a 4-stage phase-filter modded Ropez last night and playing with it for a while, it struck me that maybe smaller value caps are needed when in phasefilter mode, given that the basic EH/Ross design does not permit one to manually "tune" the range of the circuit.

Originally, the phase-filter mod consisted of using a DPDT toggle to redirect the other end of the cap tied to pins 5+7 and 10+12 so that it either went to ground (lowpass filter is created) OR to the 27k input resistor (creating an allpass section).  That arrangement works just fine if one uses the same 3300pf range-setting caps in both allpass (phaser) and lowpass (phasefilter) modes.  My sense is that in lowpass mode, somewhat smaller-value caps are called for, perhaps 2700 or even 2200pf.  The stock caps tend to mute the sound a little too much when using only the wet signal (and if you implement a phasefilter mod, you really MUST also have a vibe/wet-only mod as well, since the phasefilter sound is simply luscious when used without dry signal).  So, I'm thinking that the "ideal" phasefilter mod does the following:
a) the 3300pf caps tied to the 27k input resistor remain there but the end of the cap tied to pins 5+7 or 10+12 is lifted,
b) a 2700pf or perhaps 2200pf cap is connected to ground for each OTA/filter section,
c) the pin 5+7 pad and pin 10+12 pad is left "uncommitted",
d) a DPDT toggle is used to select between the free end of the 3300pf tied to the input OR the free end of the 2700/2200pf cap tied to ground.

In this way when one selects phasefilter mode, you simultaneously select a slightly higher cutoff frequency range for the lowpass sections and allow more mids through.  You should still hear the cyclical muting of the guitar sound, only not quite as mute.  Incidentally, set this baby for phasefilter (with no dry signal mixed in) with slow sweep, with a fuzz in front of it and a chorus after it, and you get some delicious synth-like "pads" that swell gradually.

[Fp-www.Tonepad.com]

Mark,

I just want to say I really enjoy reading your posts related to phasers. Sorry I don't have anything constructive to add.

How about some sound samples or photos? would that be possible?

Fp

[Mark Hammer]

I'd be more than happy to post some soundfiles...of a LOT of things.  Just having a little difficulty finding my Cool Edit installation CD at the moment.

Oh, and thanks.   I enjoy using your layouts just as much!

[jmasciswannabe]

Thanks for the suggestions on the phase filter mod! I will definitely have to implement the update. I am using the ross phaser layout from tonepad along with a add-on board.....and I got another just add on board but haven't gotten around to adding it on. But now I will definitely have to! One question. Is there a way to increase output volume on the ross circuit? Typically I want a little volume boost when engaging the phaser.

[Fp-www.Tonepad.com]

you can increase the 15k resistor between pins 6 and 7 of IC1.

Fp

[Shepherd]

One of the other things I seem to get is a kind of "bounce" at the sweep turnaround point when in 8-stage mode.  This does not happen in 4-stage mode, so I'm wondering if anyone else has experienced it. 

Are you referring to a little extra "animation" in part of the 8 stage sweep?  I'm pretty sure I usually dial it out with the speed and depth controls.

Another thanks to Tonepad.  I simply adore my Ropez.

[jmasciswannabe]

Thanks francisco!! I have been wanting to do that forever.... :icon_redface:sadly it is the reason why it's not on my gigging pedal board.

[Fp-www.Tonepad.com]

I'm glad to help.

[jmasciswannabe]

Does anywone experience input distorton on this circuit? I am getting distortion in the sweep when using heavy attacks. I am using humbuckers but don't get distortion when bypassed. Should I maybe replace the 4558. Or would this be a problem in one of the 13600s. I am out of town and away from my audio probe or I would check it. By the way, the volume increase mod works and I have adjusted the values for the phase filter mod and am very happy with it!

[Mark Hammer]

Just for the heck of it, try lifting one end of both D1 and D2.  These are supposed to provide some protection against excessive audio input.  Of course, if they provide clipping because your input signal too often exceeds their forward voltage, then I guess you're not doing yourself any sonic favours.  Alternatively, maybe the "cure" is to use diodes with a higher forward voltage ratings, or two Si diodes in series to replace D1 and D2.  Let us know if it makes an audible difference.

Glad to hear the component swap improved the phase-filter sound for you.

[Shepherd]

IIRC I used tl082's. 

Never noticed distortion, and I've slammed it with a humbucker equipped bass.

[jmasciswannabe]

I am wondering if I don't have a component problem or something. I tried the diode lift as well as the two in series and noticed no audible difference. I also swapped out the 4558 for another 4558 and still had the distortion problem. Any other suggestions on why it would distort?

As for that resistor between the lfo and OTA stages i am using a 2.2k resistor in series with a 25k pot. Only half of the rotation is useful though....gotta bump it down to fifteen or so.

[jakenold]

I'd be more than happy to post some soundfiles...of a LOT of things.  Just having a little difficulty finding my Cool Edit installation CD at the moment.

Oh, and thanks.   I enjoy using your layouts just as much!

As one of the "benchwarmers" on this forum, who is absolutely dazzled with the information you and plenty of others make available, I'm very curious to hear your experiments.

Audacity is a freeware recording program, which shouldn't have that steep of a learning curve, if you are just experienced with another piece of software like Cool Edit. It can be downloaded here: http://audacity.sourceforge.net/

Kind regards, Jake