LIQUID MERCURY - Tap Tempo 8 Stage Phaser – Mutron Phasor II with TAPLFO

[drolo]

The phaser circuit is derived from the Mutron Phasor II, adapted to operate with 9V supply and augmented with 2 additional non-swept phasing stages. The LFO is provided by Electric Druid's TAPLFO (as well as the tap function of course). The PWM of the TAPLFO chip drives 3 VTL5C3/2 through an 2N3904 each. One could certainly use other Optocouplers or even home-rolled ones but these are still fairly cheap and have a good response to the speedier settings of the LFO.



I built it in two different versions. One with identical phase capacitors, and one with staggered capacitors as found in the Uni-Vibe phasers.

I could not really decide which one I like best so I will keep both on my pedalboard :-)

Hear for yourself:

https://www.youtube.com/watch?v=B4P5OAxIgfA

Note that I refrained from playing any Pink Floyd ...  

Here are the schematics:





The available controls are:

Multiplier – Sets the ratio for the tap tempo. Available multipliers are 1/2; 1; 3/2; 2; 3; 4
Rate – Sets the speed of the modulation manually
Symmetry – Bends the wave shape to the left or to the right
Mix – Blends between the wet and dry signal
Feedback – Sets the amount of signal that is regenerated and increases the phasing effect
Depth – Sets the depth of the modulation
Wave shape – select between the following shapes: Square, triangle, sine and random steps

All Op Amps are TL074 but you could use just about anything.

I decided to use only 4 of the 8 wave shapes available in the TAPLFO. This way I was able to use a 4 position rotary switch for the shape selection. I could not find any small PCB mounted 8 position switches that worked for me. The lumps and sweep shapes that I left out, sounded too close to the sine or triangle anyway and the ramp up and ramp down shapes can still be obtained when setting the symmetry control completely to the left or to the right.

I added a blend knob that was not in the Mutron Phasor. I borrowed the values and principle from RG's article "panning for fun". It's nice to have as you can dial in more subtle sounds when turning it CW or have a vibrato only if turned completely CCW

While the circuit works perfectly, I am still puzzled by one detail, namely the input stage. I could not really understand how it could NOT cut lots of treble with a 100n cap in the feedback loop. It actually doesn't but I wonder why ... perhaps someone can explain this. I could not really hear any difference without it but since it was in the original mutron circuit I just left it there.

I built the circuit on 2 PCB's, one for the pots and the rotary switch plus the TAPLFO chip and one for the audio circuit. This way I was able to fit it in a BB enclosure. It was the most complex PCB I ever did and when I got it from the fab, I realized I had forgot one resistor and missed to connect one of the phasing stages ....

Not too bad still, I just had to cut a trace and add 2 jumpers, and also connect the missing resistor to the corresponding points.

I have some spare PCB's in case someone would be interested to buy one set (despite the errors mentioned above). Just PM me.

[samhay]

Sounds very cool.
Did you consider using a rotarty switch to switch between the idential and staggered caps?

[drolo]

Sounds very cool.
Did you consider using a rotarty switch to switch between the idential and staggered caps?

Boy did I consider this ...
But I just could not come up with a good way to do this and still fit everything in a BB box.
I just decided I would make 2 and chose my "phaser du jour" according to my moods

[Mark Hammer]

Still need to give the demo a listen, but you were wise NOT to include switching for staggered caps.

For a few reasons, not the least of which is that it already does a lot of things and has no obligation to do everything.

But the other reason is that Univibes function by providing shallow broad dips, rather than focussed deep notches.  How many shallow broad dips can the limited bandwidth of a guitar have before the dips are largely inaudible?  Probably two.  Maaayyyyyyyybbbbeee 3, but I'm doubtful.  Certainly not 4.

So, if you implemented a switch that would change 4 cap values, and tap the output after 4 stages, that would work.  Note as well, that slow speeds on a Vibe are next to useless, as is feedback/regen.  Converting a simple 4-stager to a Vibe can be a productive mod, but converting something that has so many phasing-related features into a vibe is not so much "gilding the lily" as shrivelling it.

A tip of my hat for including some lowpass filtering on the feedback path.  Higher feedback settings do have a tendency to introduce more noise, so that was a smart move.

[Govmnt_Lacky]

Early in the demo, I detected a bit of Sample/Hold going on there. Dont know if this was planned OR a happy accident 

Sounded amazing by the way. Great work!! 

[midwayfair]

Spectacular work, David.

Just curious ... did you consider using a single LED and driving a ring of LDRs, instead of the VTL5C3/2?

[drolo]

But the other reason is that Univibes function by providing shallow broad dips, rather than focussed deep notches.  How many shallow broad dips can the limited bandwidth of a guitar have before the dips are largely inaudible?  Probably two.  Maaayyyyyyyybbbbeee 3, but I'm doubtful.  Certainly not 4.

I totally agree. I did notice that it sounded more Univibey with only 4 stages. With 8 it is something different. Creating an exact Univibe was not the intent, more an experiment of what kind of sounds you can get playing around with different cap arrangements. I keep changing them all the time. Good thing I socketed them.

Note as well, that slow speeds on a Vibe are next to useless, as is feedback/regen.

I have to say that the regen is quite useful. Turned completely off, you get something closer to a univibe, but with feedback on it definitely increases the phasing effect, not quite like with a phaser but still very useful and more pronounced, especially with square and random wave shapes.

A tip of my hat for including some lowpass filtering on the feedback path.  Higher feedback settings do have a tendency to introduce more noise, so that was a smart move.

All the credit goes to Mutron on that one. I just shamelessly copied :-)

[drolo]

Early in the demo, I detected a bit of Sample/Hold going on there. Dont know if this was planned OR a happy accident 

That's done with the "random" waveshape of the TAPLFO :-)

[drolo]

Spectacular work, David.

Just curious ... did you consider using a single LED and driving a ring of LDRs, instead of the VTL5C3/2?

I did consider it for a while. The original Mutron had the 6 LDR's stuck around one LED encapsulated into a little round can. It would definitely have been great for needing less components  but I could not get it to work properly. I tried a few different LDR's but none were fast enough or did not have the right on/off resistence. If someone has a good source for fast (and consistent) LDR's I would be interested. In this case I went with the Vactrols as they are more predictable and consistent (compared to the LDR's I usually get).

[electrosonic]

Very nice. Did you worry about matching the LDRs? 

[midwayfair]

If someone has a good source for fast (and consistent) LDR's I would be interested.

Did you try the Silonex 7532 (or 7530)? They work really great with a single superbright, and they're similar to a 5C3 in switching range when using a red LED. I've been getting great results in the last couple things I made with them.

[Mark Hammer]

LDRs don't need to be matched in phasers.  All they need to do is be able to keep reacting to the light landing on them, and not sit frozen at min or max resistance.  The matching sought after for JFETs is also largely a matter of identifying components that will show a continuous change in drain/source resistance as the LFO sweeps higher and lower.  Nobody ever matches JFETs for the drain/source resistance, and nobody needs to match LDRs for min/max resistance.  Obviously there should be a reasonable range of resistance sweep so that the notch frequencies move around enough to produce an audible effect (sweeping from 300-400hz is unlikely to be described as "dramatic"), but there is no requirement for every single LDR to have the exact same range.  As long as they can all keep changing resistance over a fairly wide range, and can change resistance in response to the full range of LFO speeds, you're in business.

[mth5044]

Very nice! Thanks for the project. I don't know too much about phasers, but what would happen if you flipped one of the three vactrols to opperate two of the LDRs 180 out of phase? As midwayfair had pointed out in another thread, it's not too difficult with the TAPLFO to switch out an NPN with the PNP equivalent and a bit more rejiggering to flip the phase.

[drolo]

If someone has a good source for fast (and consistent) LDR's I would be interested.

Did you try the Silonex 7532 (or 7530)? They work really great with a single superbright, and they're similar to a 5C3 in switching range when using a red LED. I've been getting great results in the last couple things I made with them.

Thanks for the tip, Jon. I will definitely check these out.

[drolo]

Very nice! Thanks for the project. I don't know too much about phasers, but what would happen if you flipped one of the three vactrols to opperate two of the LDRs 180 out of phase? As midwayfair had pointed out in another thread, it's not too difficult with the TAPLFO to switch out an NPN with the PNP equivalent and a bit more rejiggering to flip the phase.

The Lovetone Dopplegaenger actually does this. It modulates 2 sets of phase stages out of phase. You can check on youtube for some demos.

[StephenGiles]

Super!

[drolo]

Thought I'd post a few pics of the insides




You can see from my shabby wiring why I try to PCB mount as much as possible ... 

[drolo]

Little update
I have been trying out FET's instead of BJT's to drive the Vactrols. I had good results with 2SK30A's with the drain connected to the supply and the vactrols and bias pot on the source.
Somehow the sweep seems more even (even though, since the signal is PWM, it should just act as a switch, if I understand right). Perhaps it's just psycho-acoustics and perhaps it's just driving the LED's in a different range .. go figure.

As for the "Vibed" phaser, I decided to reduce the number of stages to 4 in the end (after a lot of back and forth testing ...). More stages just sound too phasey. Now it does a really good job at imitating the real thing. Not quite the same but close enough for me.