Phase 90 dominant pole compensation

[Aentons]

While doing some phaser research, I was reading thru the Electrosmash Phase 90 Analysis article ( https://www.electrosmash.com/mxr-phase90 ) and noticed the following statement in the "Output Mixer Stage" section:

"Later circuit revisions (Block Logo), included a capacitor C11 from base to collector. It is a Miller Capacitor which shapes the high-frequency response (roll-off the highs) providing a dominant pole compensation preventing oscillation."


Is that the same thing as the stray filter cap across the zener in the Tractor Beam project? Is it for noise, to roll off the highs, or a little of both?

I have 90's block logo and it has a nice sounding high frequency roll off that I don't really hear in other phasers. Is that the trick?

[TGP39]

I admittingly don't understand the whole concept, but the Miller capacitor helps prevent high frequency oscillation and stabilizes the phase shift in the feedback network.   
This explains it better than I can.
https://www.globalspec.com/reference/22133/203279/dominant-pole-compensation

I hope this helps. 
Steve. 

[madbean]

Honestly, I didn't know that was what it was called. But, you've seen this sort of thing before. Example - fuzzes that use a cap from Q2 collector to base to roll-off some high frequency and reduce noise at the highest fuzz setting. Big Muffs do it too! In fact, if you ever want to make a Big Muff much bassier you make those caps a lot larger in value to bring the thunder. All bass, no treble.

It's not at all the same thing from the Tractor Beam which is more about voltage stabilization (although I think Peter did describe it for noise reduction...I can't remember exactly).

[Aentons]

Ok, I think I see... so it sounds like the same thing as the compensation cap for the lm308 in the Rat.

Any idea why that cap wouldn't be included in the Tractor Beam? Is it because he used an good opamp for the mixer stage?

[reddesert]

The Tractor Beam schematic is kind of spaghetti ... but the relevant parts are decipherable.

It uses the zener as a sort of poor man's voltage regulator to provide a Vbias. The large electro cap across the zener should stabilize Vbias. The intent is possibly to reduce noise in Vbias from either power supply noise or LFO ticking.

The TB output mixer stage is different from the Phase 90 because it uses a mix pot, some mixing resistors, and a Level trimpot to adjust the gain of a typical op-amp amplifier. It probably would work with any "modern" op-amp, not just an OPA2134. By "modern" I mean anything from a 741/1458 onward. These op-amps all have tiny internal compensating capacitors to decrease gain at high frequencies (well above the audio range). The LM308 was designed before semiconductor fab could squeeze these little capacitors onto a chip, so it needs an external compensating cap, which serves a function sort of like that cap across the base and collector of the transistor.

Whether this feedback cap rolls off the audible highs of the P90 would depend on the value, and it's something one might have to experiment with to adjust to taste.

The Miller effect refers to the phenomenon that a capacitor in the feedback loop effectively has its capacitance increased by the gain of the amplifier: https://en.wikipedia.org/wiki/Miller_effect