Home Propeller Head Plaza

Technical and scientific discussion of amps, cables and other topics.

[ Follow Ups ] Thread: [ Display  All  Email ] [ Propeller Head Plaza ]
[ Alert Moderator ]

Re: Boy, You Opened Up a Can of Worms.... [-;

Hi Todd

There is a small group of engineers who believe (X,Y or Z insert something here), what does that have to do with what is?
From what I remember, Otala identified a problem associated with low open loop bandwidth and high negative feedback and the square / sine wave signal was a way to see it on a scope.
How is that "fraud", it may have been uncomfortable for Kenwood, Panasonic etc or been counters who saw the fix as increased parts cost, but the problem isn’t pretend.
Back then, many of the SS amps really did sound funny compared to a good tube amp, enough so the difference was initially sold as part of the benefit until too many said "yeah but". I worked at a TV store that sold Hifi gear in the early 70’s and saw this happening, kind of like how early CD’s were mixed very "Bright" in order to sound different than LP’s..

Conversely, one may not be able to envision the problem if one thinks in the normal sine wave / FFT sense.
For example, Bode criteria correctly describes the conditions needed for a stable negative feedback system case closed.
Well, all that is true for fixed level sine waves BUT music its not.
Altering the amplitude of a "pure, zero distortion sine wave" increases the bandwidth the system needs to pass that signal.
For example, a common test signal is a 5 cycle gausian amplitude envelope tone burst. While this is ONE pure frequency, it occupies about 1 / 3 of an entire octave of bandwidth.
When one is dealing with a short transient, one can have a very wide bandwidth requirement for what appears to be a simple signal.
Down load that fireworks recording I mentioned, look at one impulse wave shape, figure its period, then look at the bandwidth that occupies.
Fwiw, this is why the fundamental chart for musical instruments does not tell you what spectrum they are capable of playing music.
Those charts only tell what comes out AFTER enough time has gone by for the signal to "BE" a given frequency.
Similarly for the amplifier, what it has to deal with may be more problematic than what appears on the surface.

Back EMF shifts the current phase relative to the Voltage output, this makes it potentially "different" than driving a resistance. For the resistor load, Imax and CE voltage min are coincident, I^2 * CEV = instantaneous device dissipation.
For a fully reactive load, the current is 90 degrees from Voltage, now, the Voltage across the output in zero when the current is maximum which is a HUGE change in I^2 CEV.
The familiar "bow tie" safe area curve for an output stage is not the right shape for wildly reactive loads. SOA Protection circuits can step on the signal well before normal levels.
How reactive can a speaker be? Under some conditions, the back EMF can even be in opposition to the amp Voltage and so the driver can appear to have an impedance LOWER than the drivers Rdc.
This can push operation into the forbidden quadrants in the output stage.

A reason for that (in addition to safe area) can be seen going back to bode criteria and consider the load is potentially drawing current at greater than + - 90 degree phase angles.
I don’t design amplifiers for a living like John but I have designed a number of them including one flown on the space shuttle which HAD to be happy putting out 300W around 21 KHz with a +80 degree capacitive load.
If I were designing a hifi am now, I would go with highly degenerated stages as opposed to high negative feedback.
Best,

Tom Danley


This post is made possible by the generous support of people like you and our sponsors:
  Parts Connexion  

Follow Ups Full Thread
Follow Ups
You can not post to an archived thread.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]