The mini “motherphaser”


I built a “phasey” vibe. The idea was to create a cheap vibe easily build with OTA and classic op-amp circuitry. The result wasn’t great. More phaser than Univibe (even if the goal wasn’t to reproduce a particular univibe sound, I wanted something in the spirit of the vibe effect). After serveral modifications and tweaking , the sound remained unattractive and dull. I guess the transistors , photocells and bulb has something to do with that.


Instead of taking appart the whole circuit I had the idea of making a smaller version of the “motherphaser” . This one will have only 4 phase stages , no ramp effect , just the usual layout of a phaser plus few additions.


The modification weren’t hard. The vibe and the phaser share the same topology only 4 caps have to be changed. I won’t go deeper in the detail because , after all, it’s a standard 4 stages phaser made of OTAs . However, I will expose the additional circuit that I came up with.


The VR bias op-amp is here just to give a nice low output impedance bias source. You don’t need to build a specific one for the LFO , used the one presents in your circuit (if there’s one). I found out that Ota based circuits work better with a buffered bias source.

The shape circuit is a variable inverter gain op amp which can go from triangle to a square. It desn’t do this by itself actually. The op-amp goes to saturation (obvisouly the saturationwill depend on the amplitude of the incoming signal) and therefore give a square wave. Although it isn’t on the circuit right now I thought it could be nice to show it. This part isn’t necessary if a traditional phaser sound is required but for more experimenting and crazy stuff it will do. Note that the pot could be replaced with a switch with different values (as in the chopper tremolo)

the second op-amp is a summing op-amp. The trick here, borrowed from the moog phaser, is to introduce a DC voltage in the operation. It’s the role of the “Sweep” pot.  .  It offsets the output and therefore modify the frequency where the phaser is applied. Sorry, I haven’t more technical words and unfortunately I haven’t an oscilloscope either to show the result. It is an efficient way to modifiy the sound. Although it is also not essential it’s a nice feature to have on a phaser. Close to the positive rail it gives a deeper and stronger effect whereas close to the negative rail it creates a subtler effect.

For a stronger effect I recommend to use a Rail-to Rail op amp , the waveform will be bigger and less limited by the output of standard op-amp. For more details , go to the “dalek light bulb” post where there’s a nice video of this understand the “rail-to-rail” thing.

A low current op-amp can also be used if the circuit is battery powered .

The “depth” is acting as a voltage divider , the usual “depth” control. bringing a portion or the total LFO signal into the Iabc pins of the OTAs. I’m not really happy with this kind of “depth” control because it doesn’t make a “balance” between dry and wet signal. If not at max the LFO is mixed with a fixed DC voltage (in this case, VCC/2 or “bias” or “vr”) , it reduces the LFO impact but also creates a filter with the DC voltage . Therefore, there’s no ‘dry’ signal in this operation . The ratio on the ouput is the same , 50%  “dry” and 50% “wet” . Nothing change that. The depth is only making the “wet” signal weaker . The other drawback is that at min the “depth” gives a filtered sound and not a “only dry” sound that the user would except.

I tried several ways to create a real “depth” (mix between dry and wet) but it appears that this topology is weaker and oddly enough offers less possibilities. An interesting way of doing it would be to put a voltage controlled resistor after the LFO (an other OTA wired as a VCR) . There will be no fixed DC voltage only a full or attenuated LFO . However, the circuit will be more complex and will need more space , will draw more current ect….After all , it’s a phaser that you want , so let’s build a phaser!

In this regard I think the “depth” control can be removed totally . It may be the reason of the simplicity of the PHASE90 or the small stone that I understand now.

The “feedback” pot , is a feedback circuit , nothing special in here. It’s set to be at the limit of the oscillation.

The 4 “phase” caps chosen were 2n2 which give a kind of phaser/wah wah sound.  Interesting sound ! But I changed them for 5n6 , more traditional sound.

I also added a final gain stage to put a volume control on the panel. It’s a commun complain to read and hear volume drop or volume boost with modulation effects . The pedal has a volume boost built in and a volume pot for the output volume. (Severe boost can be created ! ).

As always the wirring job was a pain in the ass , really ! I hate doing it it’s not fun at all but the result is nice and exploitable in a pedalboard (whereas the “motherphaser” is a big monster which offers nice other things though) . The look of the pedal is an accident , I don’t know what happened but it turned out to all rusty. A nice “mad max” touch …

thanks for reading

the old “vibe” circuit =

the post presenting the “motherphaser”=


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