Hi David,
Weighty questions indeed.....!
1. Will a gated(i.e. pulse) response during measurement "present" the phase angle to the amplifier? I am guessing "no", and believe my initial experiment was flawed. Perhaps an MLS would work?
Possibly. The issue is how long it takes for an impulse to create a phase shift at the output, and then whether or not this phase shift will destabilise the amp via the feedback system. Destabilisation will be transient only, and normally results in well damped ringing. It's quite likely not all pulses will precipitate this ringing; it will depend on pulse length, periodicity and slew rate.
On a zero feedback amp phase shift at the load should not destabilise the amp. It should be unconditionally stable. However, the amp may not have the slew rate to handle very fast pulses, and this will show as intermodulation on music, but you really only need about 5V/uS for a power amp to avoid this problem. Tube amps are good in this respect, but even with fb fairly resistant to oscillation under phase shift at the load. Because of the hysteresis issues in transformers, conventional tube amps are usually limited to 15dB or so. OTLs will use up to 50dB. SS amplifiers can be anywhere from 50dB to 80dB. The more the feedback, the more difficult it is to ensure stability.
SS amps with global fb are a separate category entirely. Signal propagation through the amp is finite, and at some very high frequency, usually >500KHz, this delay approaches half the periodicity. This situation means that negative feedback is transmuted into positive feedback, causing uncontrolled oscillation, almost always destructive of tweeters and output stages. To solve this, we apply bandwidth limiting, so that the voltage gain of the amp is below unity by this very high pole frequency. Any capacitive phase shift at the load will compound this internal phase change lowering pole frequency and taking the amp outside the Bode/Nyquist criteria and leading to oscillation.
2. Given the 1801's impedance curve, do you think flattening the upper or lower phase bumps will have the most significant impact for the speaker/amplifier interaction? I do recall reading in a Valve amp book that a dampening factor of @15 will be fine, BUT there was NO discussion of phase angle/amplifier interaction. What do you think?
The problem with a peaky impedance curve is the underlying vectorial addition of a DC resistance and an AC impedance. Only the first order crossover gives relatively flat impedance. The arctangent of the ratio of resistance and impedance, IIRC, is the phase shift. (I may be wrong here, but the point is that the existence of an impedance guarantees that current and voltage are NOT in step.) This has repercussions for the safe operating area of the output stage, of course, but the phase shift will radically alter the Bode/Nyquist plot, inevitably leading to instability.
Damping factor need not be much more than about 25 in my experience. A low DF can add a little reverb, decay by another name, and this is one striking feature of a tube amp over SS. A very high DF seems to make SS amps sound a bit too tight, almost cold. DF is of limited use, however, as it is by nature variable as it's a ratio of impedance. Sadly (and inappropriately) it's only ever quoted at 1KHz into a purely resistive load!
A final point is that as impedance rises, current falls, and with it, sensitivity, since the speaker is driven by BiL - magnetic forces created solely by current, not voltage. To the extent that impedance peaks alter sensitivity of the speaker across the spectrum, this is a bad thing. The only ways I know to fix this problem are to use a shunt crossover network, or a 1st order crossover.
3. Do you believe the considerable phase angle of the 1801 would precipitate better results with your 55 or 100 wpc amps? Also, if the impedance were flat, which amplifier should perform better?
The AKSAs (and the new Lifeforce) use phase lead networks which compensate capacitive loads very effectively. This technique is much touted by John Linsley Hood. The AKSA/Lifeforce amps will tolerate the Quad ESLs - they are extremely stable, and pretty well drive any speaker. I don't believe reducing the phase angle of the 1801 would make much difference to the sonics - it will sound very good regardless, though the 55W would sound marginally better because the output stage is not as robust. Tube amps, particularly zero fb amps, are more like current than voltage sources, and thus the peaks and dips in the impedance curve of speakers - and notably horns - do not much faze them. Of course the geometry of a horn speaker is a band pass by another name, and this does contribute a peakiness all its own, but that's another story......
David, I'm not good at software, so can't match my comments with nice graphs, but since most good speakers use third order crossovers with two or three drivers I have set up the AKSA/Lifeforce to cope well with these types. This is a tricky load at the best of times, so if the amp is stable AND very linear, it should sound very good regardless of load.
I hope this is helpful, and apologise for glossing over some of your deeper questions. In closing, I have never yet improved the linearity of an amplifier without noticing an improvement in sonics. Linearity is still a worthy goal.
Cheers,
Hugh
