amplifier's "slam-factor"

Pbb, I'm with you brother! Here we go again, Slam by whose standard. One mans slam is another mans flab! An amp is suppose to reproduce the source---exactly. If it doesn't, then it is a coloration. The problem is, who has ever heard a source "Perfectly" reproduced? I guess if we can come up with a definitive definition of slam then we can go from there.

Stehno: J. Peter Moncrieff of IAR showed why one cdp sounds different from the next. He did this many, many years ago to specifically demonstrate that some "golden eared reviewers" really were hearing differences and not imagining things. This is one of the reasons that i like Peter i.e. he not only gives you aural impressions of the gear but also takes the time to actually measure and explain why gear has the sonic personality that it does.

By playing the same identical note on one player and measuring / charting the response and repeating the same procedure on another player, it became quite visible on a waveform that one player had better transient response ( faster rise and fall times, which are what gives us the "attack and decay" characteristics that you mention ), greater dynamic range ( higher positive and negative peaks), lower noise floor i.e. "increased inter-transient silence" ( reduced ringing for greater silence between notes / increased separation of notes), etc...

As you can see, all of these factors could contribute to a very different presentation in terms of "slam", timing, pace, etc... even when using identical source material. I find it both amusing and appalling that a magazine with the stature of TAS could not understand / know why such things occur.

Bear: Your comments intrigue me regarding power supply sag. Does this take place in designs that are fully regulated ? Do you know of any designs that regulate the output section ?

Bob Crump: While i can agree with your comments in theory, who is to say that all music / speaker systems can get by with less than 30 - 40 watts in Class A ? While you did specifically state 89 dB speakers, who is to say how big of a room one has or how loud that they like to listen to specific types of music ? What if the speakers being used were of a low impedance design ? That in itself would change the operational area of the amp in terms of switching from A to AB. Sean
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Sean, thanks for validating my previous statement and for adding some of the detail that I overlooked. It is my understanding that Moncrief really knows his stuff about audio.

Two other questions, Sean:

1. You're probably really kicking yourself for not purchasing that Primare P30, aren't you? heh, heh.

2. Why not enter your system into virtual systems here on AG?

Aball, the damping factor is calculated by dividing the resistance of the speaker by the output impedance of the amplifier: DF= Z(load)/Z(out). For an amp to have a damping factor of 100 into an 8 ohm load its output impedance would have to be .08. Since a speakers load/resistance changes with frequency (most anyway) the damping factor of an amplifier varies with frequency too. Above 100 is pretty good. When the numbers start getting really high they kind of get trival/splitting hairs. The difference between a damping factor of 175 and 1750 is a 0.5% change in control over the speaker. Some claim those really high damping factor start becoming marketing hype--particularly if other aspects of the design may have been neglected. Concern rises when an amp has an output impedance of say 2 ohms and into an 8 ohm load we now have a DF of 2, worst case (and it does happen with those SET's) things can actually go into the negative zone. (The equation is simplified: to account for the speaker cable, its resistance is added to the output impedance of the amp and then that sum is divided into the aformentioned equation.)

(Current drive amps are the only time one tries to match the output impedance of the amp directly to the speaker for a DF of 0: this in only done in active systems and rarely to my knowledge. Any other time the amplifier aspires to an ouput impedence of 0)

Slew rates matter: its an important spec. However, their worth has to be interpreted in the context of power and load resistance too. A less powerful amp doesn't need as high a slew rate as a bigger one. Into an 8 ohm load an 11V/us is acceptable for a 32 watt amp; for a 1,000 watt amp 64V/us is appropriate. It does depend on the bandwidth of the amp. And if slew rates get too high it can cause new problems.

This article briefly attempts to correlate some of the electrical/measurable aspects of amplifiers performance to subjective impressions.

http://sound.westhost.com/amp-sound.htm

An output impedance of 2 into an 8 ohm load gives a DF of 4, not 2 as incorrectly stated. (This is ignoring the change caused by the cable) Although if you look at tube amp specs you'll often see ones claiming DF's of <2. I believe the Cary 300SEI (from mid '90's) actually went up to a 3.8 ohm output impedance.