Anything as " fast" as SPECTRAL gear?

02-06-15: Bifwynne
So Al, what does that mean in the context of ARC ref amps. As I mentioned above, the slew rate for my amp (150 wpc) is a paltry 13 volts/microsecond and the rise time is a sluggish 2 microseconds. The top of the line Ref 750 has a slew rate of 20 volts/microsecond and a rise time of 1.5 microseconds.

Bifwynne, doing the math for your ARC Ref 150 tube amp -
if the amp is outputting its full/rated 150W into 8 Ohms that would mean a RMS voltage of 34.6Vrms of a sinusoidal waveform & a peak of approx 49Vp of the same sine waveform. I.E. the peak-peak voltage of the sine wave would be 98Vpp. If amp traversed the 98Vpp such that the slew rate of the amp is 13V/us (as spec'd) then the maximum frequency that the amp could handle with this slew rate would be 66KHz. So, it appears that the freq BW of your amp is 20Hz (or 5Hz?) - 66KHz.
Compared to the (fictitious?) example Almarg gave where the amp was a 200W unit & had a slew rate of atleast 45V/us such that it could handle a signal as high as 200KHz.
So, the ARC Ref 150 has quite a bit lower slew rate compared the Almarg's example power amp & accordingly lower frequency bandwidth.
This also means that since the ARC Ref 150 bandwidth is just 3X (rather than 8X or 10X) the music bandwidth (of 20KHz) one can expect to hear the amp impart its own phase shift onto the higher frequencies of the music. This can manifest itself in a few ways - the highs could sound rolled-off or they could sound warmer or there could be less sparkle/shimmer compared to an amp of higher bandwidth. Nothing wrong with this sort of attribute of a power amp - many like it & many others don't. Something keep in mind.
Just some additional info, FWIW.

But are Spectral amps freaky-fast like Jimmy Johns delivery? That is what I want to know.

Really though, all interesting information here.

Is Jimmy Johns really that fast or are they just setting themselves up for failure by advertising to be?

Just wondering. have never experienced.

Bombaywalla, thanks for the additional thoughts. However, while I agree with your math, I don't completely agree with the conclusions you reached.

As I'm sure you recognize, my hypothetical example involving 200 watts at 200 kHz, as well as the figures you cited in connection with Bruce's Ref150 of 150 watts at 66 kHz, are not remotely within the realm of real world operating conditions for a home audio system. Or at least a home audio system that is not in the process of self-destructing due to some unimaginable defect-induced oscillation :-)

A key point to keep in mind is that under realistic operating conditions bandwidth and risetime should not be and in general will not be limited by slew rate, as I indicated in my long post dated 2-6-15 (see more on that below). And as I indicated in my previous response to Bruce:

... a basic point in my analysis was to show that even relative to VERY conservatively drawn thresholds the speed-related parameters of the Spectral amps are **in themselves** extreme overkill. While at the same time citing the possibility that the overkill might provide indirect benefits in the design.

Consider "VERY conservatively drawn thresholds" to mean parameters that are so extreme as to be well beyond the bounds of real world operating conditions.

As you may realize but others may not, slew rate should be fast enough to avoid any possibility of the amplifier's output slewing under any realistic operating conditions. While bandwidth (and risetime) limitations mean that frequency components above a certain frequency are attenuated or filtered out, slewing results in essentially all frequencies that represent sonically meaningful information being filtered out and/or distorted. In a sense it can therefore be considered analogous to what happens in hard clipping, although of course that relates to amplitude limitations rather than speed limitations.

I note, btw, that the ARC Ref 150 has a specified 3 db bandwidth of 120 kHz (at 1 watt, with an 80 kHz "power bandwidth" also being specified), and JA's measurements indicate a bandwidth of approximately 100 kHz (for an 8 ohm load connected to the 8 ohm tap), with excellent reproduction of a 10 kHz square wave (under small signal conditions, as noted under the graph).

So while I don't think we can completely rule out the possibility of audible phase shifts occurring in the upper treble region under some circumstances, given the bandwidth of the Ref150, I would not expect that to be as much of an issue as your conclusion would seem to indicate. And to put that into context, I'll point out that more than a few other very highly regarded amps, including some tube amps costing vastly more than the ca. $13K Ref150, have bandwidths which are considerably more limited.

Best regards,
-- Al

Map, I know this is way off the subject, but yes, JJ's is pretty darn fast for delivery, they only deliver if their typical drive time from store to your house is 5 minutes or less(barring non-standard events such as car accidents, weather etc.)Anyway, back to the good stuff....