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 Post subject: Phase 90 vs Small Stone
PostPosted: Sun Mar 14, 2021 8:12 pm 
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I built the both versions of the BYOC analog chorus over the past few years. I understand the first version was based on the EHX Small Clone. And the next one was based on the Boss CE-2. As I understand it, the circuits were fairly similar, but the main differences were probably the LFO shapes (sine vs triangle, maybe?) and the base delay time. I guess.

In a similar vein, does anyone know how the two phasers differ? I think both are four stage phases. So I wonder what makes them different. Anybody know?

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PostPosted: Mon Mar 15, 2021 11:40 am 
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They both use a "differential amplifier" configuration, meaning that signal is split and one half goes into the inverting input and the other into the non-inverting input, and then summed back together at the output of each phase stage.

The Small Stone uses voltage controlled amplifiers (VCA). When the LFO signal is low, the amplifier isn't doing anything, but there is a bypass cap and resistor that allow a non-inverted signal to always pass. When the LFO signal is high, the VCA increases gain and sums the inverted signal with the non-inverted signal creating the phase shifting vibrato effect.

The phase90 works exactly the same, but inverted (no pun intended). The phase90 uses standard op-amps with a set gain that never changes. The inverted signal is always amplified. The JFETs in the Phase90 circuit simply act as a variable resistance to ground at the non-inverting input. When the LFO is low, signal can pass to the non-inverting input of the op-amp and the two phases are summed. When the LFO signal is high, the JFETs "open" and bleed off the non-inverting input signal to ground allowing only inverted signal to pass, thus creating the phase shifting vibrato effect.

These phase shifting vibrato stages only create a pitch bending vibrato effect. It needs to be blended with a dry signal for a phaser effect, much in the way chorus needs its pitch bending vibrato effect to be mixed with dry signal to be chorus. Otherwise it's just vibrato. The difference with chorus is that it's vibrato effect doesn't change phase. The swooshing effect of a phaser is created when the vibrato signal goes in and out of phase with the dry signal.

You can observe the exact same sort of concept with the Univibe circuit, only instead of two different inputs and a single output, each transistor based phase stage has a single input and two different outputs. Signal goes into a bootstrapped transistor buffer. Inverting signal comes out of the emitter. Non-inverting comes out of the collector. The LDR cancels the inverting signal when the LFO is low and allows it to pass when the LFO is high.

Most optocoupler based phasers are exactly the same as the JFET based Phase90. It's just the LDR that is bleeding non-inverting signal to ground rather than a JFET. If you compare the Flying Pan to the Phase90, the phaser portions of the circuit should look pretty much identical.

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PostPosted: Tue Mar 16, 2021 10:50 pm 
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Awesome! Thank you!

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PostPosted: Wed Mar 17, 2021 7:07 am 
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Bottom line is, building them both is a must.


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