Amplifier Phase margin - what is a good value?

[MasterApex]

I was listening to webinar last night that talks about good amplifier design and behavior
It mentions to pay attention to phase margin value which is rarely disclosed by manufacturer but is important to be able to low impedance without affecting tonal balance due to oscillation at supersonic frequency that would affect the sonic tonal balance.

Is this value measurable ?

 

[DVDdoug]

I've never heard of the term "phase margin", or I've forgotten...

After a guick-Google it seems to be a characteristic of a "raw" open-loop op-amp (or gain stage). Op-amps are never used open-loop unless the op-amp not being used as an amplifier. It would be important to the amp/preamp design engineer but if the final amplifier is designed properly it should be of no consequence to the listener/user.

Any normal-slight phase shift in the final design is also inaudible.



...I do understand the consequences of phase-shift in a bad design with feedback. Negative feedback (corrective feedback) is a good thing with all kinds of good-effects but if the phase gets shifted/inverted you get positive feedback (a very bad thing) and you get oscillation similar to what happens when you have a PA system and feedback from the speakers gets into the the microphone.

Many years ago I built a preamp that turned-onto an RF oscillator and I fried a power amp! Early op-amps needed a " frequency compensation" circuit when used at low gain to prevent oscillation, and that's terminology that I do remember! I don't think this is even an issue with any modern op-amps, they all seem to be internally compensated so they are stable at any gain..

 

[egellings]

I use 60 degrees or so.

 

[DonH56]

Phase margin refers primarily to the phase shift of the applied feedback signal. If the negative feedback phase is 180 degrees, i.e. opposite polarity, at any frequency where the amplifier has gain (i.e. is above unity-gain), then applying it to the negative input means it will be inverted and add to the input, causing the output signal to grow without bound. At least until it clips or something breaks. The amplifier turns into an oscillator.

The phase margin is the magnitude of the phase at the amplifier's unity-gain point. Typically 45 degrees is a minimum goal, with 60 degrees desirable. A similar metric is gain margin; the gain when the phase reaches 180 degrees (you want it to be less than 1 or less than 0 dB).

This is usually a design criteria and not in a consumer product's datasheet. Any designer will simulate and measure the phase margin along with a plethora of other parameters not appearing in the consumer data sheet.

https://en.wikipedia.org/wiki/Bode_plot#Gain_margin_and_phase_margin

 

[egellings]

Sounds good to me. My amplifiers (home brew) have all been well behaved, even with somewhat capacitive loads.