Bob Cordell's Power amplifier book

[Bob Cordell]

Originally Posted by JPV

Jan, where did you order it? I did it via amazone uk and i have to wait until december.


To Bob Cordell

In your book, do you analyze the influence of a reactive load on crossover distortion.
I am puzzeled by the fact that crossover distortion is happening when the current goes through zero in the output transitors. Due to the phase of the input impedance of the load, the voltage can be high at that moment and will then have a glich. Could it then happen that again due to the phase of the voltage input, pressure ouput transfer function of the system, the output pressure could be high and experience at that moment a glich which makes it louder than what would be expected from a zero crossing signal.

The same can be asked for the optimum bias current if the load is reactive.

If this is true, custom phase equalization of the system becomes a very important topic at least for diy.

JPV

Hi JPV,

In connection with reactive load, I believe that on occasion I have seen some evidence of increased THD with reactive loads in some amplifier reviews in Stereophile. Those reactive loads were the simulated loudspeaker load usually used to load the amplifier in the frequency response plots.

I'm wondering if anyone else here has seen that effect, either in measurement or simulation.

Cheers,
Bob

[JPV]

Originally Posted by Bob Cordell

Hi JPV,

In connection with reactive load, I believe that on occasion I have seen some evidence of increased THD with reactive loads in some amplifier reviews in Stereophile. Those reactive loads were the simulated loudspeaker load usually used to load the amplifier in the frequency response plots.

I'm wondering if anyone else here has seen that effect, either in measurement or simulation.

Cheers,
Bob

What would be interesting is to measure the crossover distortion in the pressure wave taking then into account the phase of the system. Of course it is impossible to separate this from the distortion generated by the speaker itself.
An interesting experiment is to simulate with spice a closed box and a vented box ( distortion free), connect them to the simulated amplifier with crossover distortion). In this way the influence of the phase shift on the level of crossover distortion in the pressure wave can be measured. These two systems have radically different phase shifts.

I am waiting for your book to start finally to experiment in depth with spice.
Leach has some good spice models for different speaker systems, it is rather straightforward but good to have on hand

JPV

[jcx]

didn't Cherry look at "interface distortion" - and conclude that there really wasn't anything surprising there?

basically with common power amp's distortion dominated by output Q behavior with current load - even if reactive loads draw more current at different V the distortion isn’t substantially different than what would be with resistive load drawing the same current at audio frequencies

so sizing output, driver, ect. for the peak reactive current should suffice - including allowing for the higher thermal load on the outputs driving reactive loads

[JPV]

Originally Posted by jcx

didn't Cherry look at "interface distortion" - and conclude that there really wasn't anything surprising there?

basically with common power amp's distortion dominated by output Q behavior with current load - even if reactive loads draw more current at different V the distortion isn’t substantially different than what would be with resistive load drawing the same current at audio frequencies

so sizing output, driver, ect. for the peak reactive current should suffice - including allowing for the higher thermal load on the outputs driving reactive loads

After quick review of Cherry's paper it seems to me that he is addressing the interface intermodulation problem. This is different. Some have stated that the load beeing an emf generator, it is a kind of active circuit and could inject signals in the output that can intermodulate with the signal of the input on any internal nonlinearity.
Cherry has shown that the distortion generated by intermodulation of two signals injected at the input is alway greater than the intermodulation of the same signals one injected at the input and the other at the output.
The back emf source is not an independent thevenin source but a dependent one and is totally equivalent to an RLC circuit representing the load. Therefore there is not a special intermodulation phenomenon that would be worse than the one excited by two input signals.

But injecting a signal at the input and another at the output we could drive the amplifier in all the operating points ( Vce, Ic) that an arbitrary signal with any reactive load could reach and it is a valuable test method providing you go through all the realistic points
He mention ( en passant ) that these operating points outside the resistive locus could generate parasitic oscillations, triggering of protection circuits or accentuation of crossover distortion and this is my point.

If the accentuation is even more on the pressure loudspeaker output signal for phase reasons, then crossover can be more audible.

This is only hypothetical


JPV

[Bob Cordell]

Originally Posted by JPV

What would be interesting is to measure the crossover distortion in the pressure wave taking then into account the phase of the system. Of course it is impossible to separate this from the distortion generated by the speaker itself.
An interesting experiment is to simulate with spice a closed box and a vented box ( distortion free), connect them to the simulated amplifier with crossover distortion). In this way the influence of the phase shift on the level of crossover distortion in the pressure wave can be measured. These two systems have radically different phase shifts.

I am waiting for your book to start finally to experiment in depth with spice.
Leach has some good spice models for different speaker systems, it is rather straightforward but good to have on hand

JPV

Hi JPV,

Simulating an amplifier's distortion with simulated loudspeaker loads is a good idea. I show a simulation model of a simple closed-box woofer in my book, but I don't go as far as a vented one, which would be interesting.

Indeed, one could put together a simulation model of a complete three-way speaker system with a fairly complex crossover.

In the real (measurement) world, one can synthesize a loudspeaker load using a second (back-feed) amplifier and active filter techniques. I say a bit about this in the book.

There is also another approach that I mention in the book. It rests on the observation that the phase angle of a load just corresponds to the voltage and current excursions being out of phase. If a second amplifier is used to back-feed an amplifier under test through a "load" resistor, and the back-feed amplifier is fed with a slightly different frequency, a beat frequency will result that will cause the output voltage of the amplifier under test and the current to go through different phase relationships. In a sense, you can think of the back-feed amplifier as emulating the counter-EMF of a loudspeaker.

Looking at the THD residual in the output voltage of the amplifier under test as a function of time could be very revealing.

Cheers,
Bob

[orjan]

Hi,

I also orderd my book at amazon.uk and delivery is planned early dec. Reading JPV's question brought up things I been thinking about.

It's often said that measuring thd with resistive load gives the best results.

If a the load is inductive at higher frequenses, the loading is less where the feedback starts to decrese, and can give less distorsion. I saw this first in an JAES engineering report by Paul Klipsch (JAES july/aug 78 p547).

If this is true then it would be interesting to know if crossover distortion is less with such a load.

Then for crossover distortion with reactive load. If the crossover glitch is offseted would there be a masking effect? The only listening test I read is Geedes and Lee (http://www.gedlee.com/downloads/Distortion_AES_I.pdf) and I'm not sure if they covered that case. But they say that nonlinearities at lower signal-levels are more audible and if the glitch is moved does that change masking? Even if the size of the glich itself is the same.

/örjan

[apolon34]

Received the amazon shipment notification. It should be there (in France) on october, the 11th.

[osscar]

got my today fro bookdepsitory !!!

[richie00boy]

Mine was delivered today, had a very quick flick through and looks good.

[Bob Cordell]

Originally Posted by spind

Copyright 2011??? Did I miss a few months somewhere?

Steve.

Hi Steve,

Your eagle-eyed observation made me curious, so I checked. It turns out that it is customary in the publishing industry to use the following year's date for anything published after the end of August - sort of like with the automobile industry.

Cheers,
Bob