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Re: You are dead wrong....again& again & again

Posted by morricab (S) on August 18, 2006 at 05:32:51

In Reply to: Re: You are dead wrong....again& again & again posted by Dan Banquer on August 17, 2006 at 18:25:05:

I guess you haven't read the papers by Otala et al. on IIM (interface intermodulation distortion) then have you? I am sure old Soundmind didn't or he would think of feedback like a black box. I expected you to know this as well.

I now have AES papers from 1978 and 1980 that show that he phenomenon is real and potentially quite stongly affecting the sound. Shall I quote their findings to you?
Here is the abstract:

"The possibility of dynamic intermodulation distortion at the amplifier-loudspeaker interface is discussed. THis distortion is produced in amplifiers using high values of negative feedback, and having moderate or high open-loop output impedance in comparison with the loudspeaker impedance. The mechanism is intermodulation between the signal and its delayed versions, generated by the loudspeaker and propagated in the feedback loop. Experimental measurements showing the probability of considerable distortion are described."

Here is the Summary:
"The analysis and the measurements show that:

at its various mechanical resonant frequencies, the loudspeaker may feed back to the amplifier much of the energy it received.

The loudspeaker may also act as a signal generator due to mechanical excitation of its voice coil by cone break-up, sustained oscillation of moving parts, delayed responses, etc.

provided that the power amplifier has an OPEN-LOOP output impedance in excess of a few ohms, and simultaneously substantial overall feedback, these backward energies are not subject to intrinsic damping at the amplifier output, but a corresponding feedback signal is generated within the amplifier, in trying to counteract the loudspeaker-generated signal and keep the output voltage constant.

The signal in the forward path of the amplifier thus consists of two components, the original input signal and the loudspeaker reaction signal, both of the same order of magnitude.

These two signals my interact in the nonlinearities of the amplifier, generating intermodulation products between the two. Loudspeaker nonlinearity is here to be considered equivalent to any open-loop nonlinearity of the amplifier, since it will effectively be situated inside the feedback loop, unless the open-loop output impedance of the amplifier is considerably lower than the specified load impedance.

this distortion, here termed interface intermodulation, IIM, will be most prominent at low frequencies wehre the loudspeaker-generated signal is at its greatest.

The susceptibility of the amplifier to IIM distortion can be measured bz using a modified difference-tone method, where on of the signals is injected to the input, and one to the output of the amplifier. Maximally, the latter signal should equal in power tthe maximum output power of the amplifier, to create a conservative worst-case fr this effect."

So, I am not making this stuff up as you can see.

Here is the abstract and conclusions from the 1980 paper:

"The previously presented theory of interface intermodulation distortion (IIM) is extended to cover the analysis of the influence of certainj power amplifier design parameters on this effect. It is shown that the amplifier distortion spectral density is altered by the loudspeaker, and that the most important single parameter increasing this effect is a high open-loop output impedance. The usual specification of the closed-loop output impedance of the amplifier has, in this respect, little relevance."

So in effect, Dan, what Soundmind said about the output impedance closed loop is absolutely wrong with regard to the back EMF. What is important is the actual impedance of the output stage and the amount of negative feedback used (ie. the impedance of the feeback loop).

now the conclusions:

"It has been demonstrated that the loudspeaker can considerably alter the nature and composition fo the internal singals of an amplifier. Porvided internal nonlinearities of any kind exist within the amplifier (my note: all amps are somewhat nonlinear so this is a give), the loudspeaker thus has a capability to change the amplifier distortion properties. The effect is strongest at the low frequencies, where the loudspeaker reactive properties are most outspoken.

The susceptibility of an amplifier to this loudspeaker effect is different for different amplifier topologies. The most important parameter in this respect is the amplifier open-loop impedance Zol. A low value of Zol prevents the loudspeaker to have any remarkable effects, but if Zol by virtue of the circuit topology is large, the loudspeaker generated reaction signal inside the amplifier may be of the same order as the original input signal.

The effect described does not normally increase the amplifier distortion considerably. Instead, it has the capability to change the amplifier spectra, and its audible character."

Now, I doubt that you have done the same kind of measurement they have, which essentially duplicates back EMF. Do you know the open-loop output impedance of your amp? How high is it without feedback? If it is high then for sure your amp could have problems with this phenomenon because I know you use copious quantities of negative feedback.

I am willing to send you these articles because there is much that I couldn't include that is interesting and also good advice for amp designers. Also, there is rather convincing experimental data in the articles that I cannot present here using Yamaha NS1000 and Acoustic Research AR3a loudspeakers and 4 different circuit topologies.

I am again offering you proof of a phenomenon that for sure affects the sound of amplifiers. In fact if the particular high feedback amp has a high Zol then it is possible that the sound of this amp would be more variable from speaker to speaker than a SET! For sure this IIM cannot sound good, which is another big problem.

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

Kimber Kable

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