Ross Phaser - build report

[RickL]

I used the layout available here: http://www.tonepad.com/project.asp?id=25 and after correcting a few mistakes I made by trying to mod it before I got the basic project working first it worked fine.

I think I prefer the sound of the Small Stone to this one but this has lots of possibilities for mods that I'm going to take full advantage of.

I used spdt centre-off switches to choose phase cap values of 0.001, 0.0032 and 0.0057 µF. I think I might make the biggest one bigger (maybe a 0.01 µF in parallel with the 0.001 µF) to get a little more contrast between the choices.

I also did the phasefilter mod suggested by Mark Hammer on each of the stages (a switch to change the 27k resistor from the inverting input of the opamp to ground). The most dramatic effect is with two of the stages in filter mode and two in phase mode but all the combinations are usable.

A mod I always do on phasers (also suggested by Mark) is a switch to disconnect the 27k mixing resistor from the straight signal to the mixing stage. This gives vibrato when all the phasefilter switches are in the phase position and intensifies the effect when some of the switches are in the filter position.

I replaced the 15k feedback resistor on IC1b with a 15k resistor in series with a 500k pot and added a 100k volume pot in place of the 150k output resistor. I also added switchable clipping diodes in the feedback loop. This gives anything from a slight volume boost with the diodes switched out and the 500k turned up a bit to heavily fuzzed phasing with the diodes in and the 500k cranked. I also tried clipping diodes from the output of the same opamp to ground but they didn't work. Unless the 500k was cranked and the volume turned up no sound came through and even then the sound was heavily gated like something was misbiased.

I also tried Ge clipping diodes in the regeneration path, from the wiper of the Regen pot to ground, but I couldn't hear any difference in sound. I guess, as Marked thought, there isn't enough signal to reach the threshold of even germanium diodes.

I tried switching the phased part of the signal to the non-inverting input of the mixing section with mixed (!) results. I had to add a 15k resistor from pin 5 to Vb to get any sound at all and even with the 27k mixing resistor increased by about 40k (with a trim pot) I couldn't get the signal as loud as into the inverting input. I'll fiddle with it a bit more but I think it may not be worth the bother.

Still to try: Marks suggestion of filtering in the regeneration loop (switch to choose between 1 µF cap and a couple of smaller values), smaller cap(s) in the LFO for faster (ring mod territory) speeds, more phasing stages (I'll start with four more, selectable two at a time, then add more if I like the sound) and switches to choose where the regeneration goes to. I'd like to try reversing the phase of the LFO to send to the second set of four stages but I think it may be like the Small Stone and not work.

I should have more to report in about a week. Thanks Francisco for a great layout.

[moosapotamus]

I also did the phasefilter mod suggested by Mark Hammer on each of the stages (a switch to change the 27k resistor from the inverting input of the opamp to ground).

I thought the phasefilter mod involved switching the capacitors to ground. Does it sound similar with the resistors going to ground instead?

I also added switchable clipping diodes in the feedback loop. This gives anything from a slight volume boost with the diodes switched out and the 500k turned up a bit to heavily fuzzed phasing with the diodes in and the 500k cranked.

Cool!

I tried switching the phased part of the signal to the non-inverting input of the mixing section with mixed (!) results. I had to add a 15k resistor from pin 5 to Vb to get any sound at all and even with the 27k mixing resistor increased by about 40k (with a trim pot) I couldn't get the signal as loud as into the inverting input. I'll fiddle with it a bit more but I think it may not be worth the bother.

So, you kept the straight signal connected to the inverting input? Did you try connecting both the straight and phased signals (the 27K/27K junction) to the non-inverting input of the mixer? How about trying a dedicated buffer (before the mixer) for the phased signal, configured as a switchable inverter?

Still to try: Marks suggestion of filtering in the regeneration loop (switch to choose between 1 µF cap and a couple of smaller values), smaller cap(s) in the LFO for faster (ring mod territory) speeds, more phasing stages (I'll start with four more, selectable two at a time, then add more if I like the sound) and switches to choose where the regeneration goes to. I'd like to try reversing the phase of the LFO to send to the second set of four stages but I think it may be like the Small Stone and not work.

I should have more to report in about a week. Thanks Francisco for a great layout.

Thanks for the nice build report, Rick. Great stuff! I'm looking forward to hearing how those other mods work out, too.

Thanks
~ Charlie

[RickL]

Brain freeze  :oops: , of course it's the caps not the resistors.

I got the mod that changed the polarity of the phased signal working. I kept the dry signal going to the inverting input and switched the phased signal so it either went to the inverting input through the original 27k resistor or to the non-inverting input through a trim pot set to 20k. Remember that it only works if you connect pin 5 to Vb through a resistor (I used a 15k). This gave pretty much the same volume in both positions. I got the idea from Craig Anderton's Phaser in Electronic Projects for Musicians. It gives a little different sound in the two positions and should sound cool when I add some more stages. I'll make the extra stages switchable either in series or in parallel, in phase or out of phase.

I did the regeneration cap mod but to get any audible change I had to use choices of 0.001µ, 0.01µ and 1µ. The smaller cap values lowered the apparent amount of regeneration, I guess since only higher frequencies were being fed back. It's done now so I'll leave it, but I don't think I'd bother to do it on another build.

I did the speed range cap mod. It's the 3.3µF cap. I switched in a 0.22µF cap that gives a range of medium fast to ring mod speeds. In fact the LFO gets so fast that you can't hear it in the last 1/8th of a turn or so of the speed pot. There is enough overlap in the speeds that it might make sense to raise the value of the 3.3µf cap to 4.7 or 6.8µF to get some super slow rates.

More in 4 or 5 days.