I am about to build this one-is it possible to modify it to be an 8-stage phaser for deeper whoosh whoosh-have any of you tried this?
Unless it is something like some of the basic phase-shifter designs where you literally alter the critical resistor by sweeping a dual pot with your foot or something, you can usually add on as many allpass stages to a phaser design as you want, provided you know where to stick them, have the parts, and can put up with the extra noise you acquire each time another stage is added.
In principle, the EPFM 4-stager is the same as the MXR 10 stager. They both use op-amp based stages with an LDR as the control element. Where they differ is that the MXR unit has 6 variable allpass stages and 4 fixed ones, providing 10 stages of phase shift in all. Even though the fixed stages may seem like they contribute nothing without sweeping, they do. Bear in mind that where the notches occur depends on how many cumulative degrees of phase shift there are at that particular frequency. In each of those op-amp stages, the number of degrees of phase shift goes up (to a maximumof 90) starting from a frequency set by the value of the .01 cap and the LDR to ground.
The depth of the notches created has little to do with the number of stages. The apparent depth of the *effect* however, increases as you increase the number of stages, and also the resonance/regeneration.
The EPFM design has a phaser/vibrato switch that cancels the dry signal to yield a nice wobbly pitch shift. As JC Maillet pointed out recently, in a vibrato situation, increasing the number of phase-shift stages increases the amount of pitch deviation per amount of sweep. In other words, for the same setting of the LFO width, 8 stages will get you more pitch deviation/wobble than 4 stages, and 4 will get you more than 2.
Mike Irwin has built some phase shifters with as many as 24 stages and demonstrated them to me (I have a sound clip somewhere). Once you get up into the 16-stage+ zone, the differences between phasers and flangers start to diminish although they never disappear.
Just note that if you add more stages, it WILL contribute a bit more noise, although there are ways to tame that. Also note that you will want to keep track of where your regeneration receive and send points are located. If you have 8 stages, ideally you want the regeneration signal to be tapped after the last stage, and fed back to the second stage. Adding 4 more stages onto the last stage won't do that, unless you change where you tap the regen signal.
One of the things that hasn't come up much, and maybe needs some experimentation reports, is what happens to Univibe cloning when you move up from 4 stages. The staggered cap values that permit stock phasers to sound more like a Univibe (staggered values distribute degrees of phase shift differently,and yield a more watery, less focussed sound), are generally observed in 4-stage environments. Although I'm familiar with a 6 stage design (appearing in an old National Semiconduictor audio circuits book I believe) that used progressively smaller cap values over 6 stages, I've never seen anything that aimed for the staggered values arrangement in an 8-stage context. Of course you have no obligation to do that, but that doesn't make me any less curious. At the very least, switching 7 caps (you can probably keep one constant) is a BIG deal, and probably to be avoided. On the other hand, if you build in a switch that lets you tap from either 4 or 8 stages, you could easily build in cap switching from 3 components.
I mentioned in another thread that I had added two extra phase shift stages to my Anderton phase shifter.
The extra stages were wired up on perfboard.
The result is a deeper effect (obviously). I think it is worth it.
I socketed the phase shift caps in the first 4 stages so I could compare the stock values of 0.01 uF to the univibe values. The difference is clear and I never went back to the stock values. It does lend a univibe feel to it.
8 stages should sound cool. Driving 8 optos takes quite a bit of current (and buying 8 optos eats a lot of $). My 6-stage shifter eats around +/- 15-20 mA. That's close to 40mA total! A power supply is definitely in order, as batteries probably won't last too long.
I love my phase shifter, even though the sweep isn't as wide as I'd like and I'm not crazy about the triangle LFO. I tried to mod it to use a sine LFO, but I don't think the one I tried had enough current drive for all the optos. It is still on the to do list.
It DOES sound great paired with Anderton's Retro-Stereo circuit that Mark mention's in another thread. I think that is one Guitar Player article that should have made it into Anderton's DIY Projects for Guitarist.
Make sure to wire the regeneration as Mark mentioned. That means that lug 3 of the resonance pot will now go to the output of the last phase stage and not to pad "S" as shown in the schem.
Good luck.
J.
:D Thank you kindly for the generous amount of info, fellas. My best wishes go out to you for the new year!
I've posted earlier on this subject and had no response. Anybody figure what could be my problem?
http://diystompboxes.com/sboxforum/viewtopic.php?t=17118&highlight=
I'll have to check where the regeneration is going again, just to make sure.
Regarding phasers,non optical,one of the problems is fets matching...i think one could build a phaser by using 2 cd4007s ,having then 6 stages,because each has 3 pairs of mosfets...i haven´t tried it,but at least sounds possibe...
happy new year
Maneco
You haven't lived until you've heard a 24-stager. Some 20 or more years ago, there was a 6-stager in Electronics Today International, based on the MOSFets in a CD4049 as control elements. Mike Irwin did some redesign to tame the noise and stuck 4 of them end to end. Listening to that thing sweep white noise is awe-inspiring.
Go check out some of the phase shifters that Larry Spence has posted over at modezero.com. Just make sure to have a bowl handy to catch the drool. :)
Larry has plenty of soundfiles, but more importantly he has pictures of the insides of a lot of pedals, and pretty decent photos I must say. The Moogerfooger 12-stage phaser is a cute one to see. Half a dozen LM13600's (or were they 13700's?) packed in tightly into the corner of the board with SIP-style resistor networks to produce a very compact board for what it does.
That's http://modezero.com
Be sure to click on gallery when you get there.
:P Cool!!!!!
Mark,
is that th eti phaser posted in CAG?
i build it some time ago,but found it noisy,could you please direct me in the right direction for noise reducing it? i remember it got worse when i tried to add feedback to the circuit...that's why the originall design didn´t have that control...
regarding reducing noise,i remember an ibanez phaser with a couple of caps and resistors around the fets,maybe that's the ticket to improvfing SNR...
thanks
All the best
Maneco
I didn't check, but I suspect its the same one. If the layout calls for single op-amps, that's it.
Here's the body of a note Mike sent me some time back: "The ETI phaser is pretty noisy - probably one of the reasons there
is no regen control is that the noise will become even WORSE!
The problem is that the FET's in the CD4049 chip distort badly
when more than about 50 mV p-p is applied to them - so the signal
level must be kept low (and is attenuated before being fed to the
allpass stages), which makes the noise more apparent when the
signal is later restored to it's original level. Some thing I tried that
works well (and that has been mentioned on this forum at least
twice in the past 6 months) is the method used on most (maybe
all?) of the Ibanez phasers - 470K in series with each gate input
(where the LFO voltage normally goes) and a series 470K and
0.01 uF between the gate output and gate input. This helps reduce the distortion, allowing a larger signal (perhaps 150 - 200 mV p-p) to be applied to the allpass stages, and reducing the noise accordingly. I think some of the Maxon, Korg and MXR phasers which use FET's also use this technique."
Another trick you'll see used is to stick a HF-limiting cap in the feedback loop of one or more of the all-pass (phase shift) stages. Realistically, I don't think you'll need much more than 10khz bandwidth from the phase-shift path anyways, so if the all-pass stages use 100k feedback resistors (between input and output pins of the op-amp), stick a 120-150pf cap in parallel with that every couple of stages to attenuate any accumulated noise above 8-10khz. It's no panacea, but between being able to run a hotter input signal, and trimming back on hiss, you should get a better S/N ratio.
....I just built the Small Stone from JD's site (with lots of quality help from JD) with the metal can CA3094's-there's lots of swoosh!...can't wait to build the other one I need for stereo operation!....then I may finish the Neovibe soon!....great projects-great people!.......
Mark,
thanks for your reply...
your postings are small essays of music electronics,i'll start collecting them...
you're my hero.
keep the good work
thanks again
Maneco