Slew Rate?

Can anyone explain what a 'slew rate' is and how it relates to a power amps performance? What to look for as a measurement? I notice Parasound publishes the slew rate of their amps, but haven't noticed many other manufacturers.

ALSO, I'm looking at purchasing a planar speaker -- I'm looking at Eminent Technology ---- and I believe someone once said or wrote that what one should look for in a power amp to power planars is a lot of "Volts" as opposed to "watts" --- any comments?

Thank You
Slew rate and risetime are related terms that refer to how quickly the amp can change states to follow the input voltage to output voltage. Think of "acceleration." This can be a good indicator of how "quick" an amplifier sounds: getting the leading edges of transients accurately reproduced, for example. It also impacts resolution and detail.

A corrolary to slew rate is "settling time" and this is just as important to how resolving the amplifier can be. As the name suggests, settling time reflects how quickly an amplifier can return to its nominal state after passing a large voltage swing. Bumps in settling time will be reflected in the trailing edges of transients and can show up as ringing and added sibilance.

Spectral and Atma-Sphere are two other manufactures that publish a slew rate for their amplifiers. Atma-Sphere quotes this as "risetime" (using the same 600 volts/microsecond measure as Spectral), whereas Spectral makes a distinction between risetime and slew rate.

However, like any other measurement it's not worth reading too much into it because it's only one aspect of an amplifier's performance envelope and there are so many aspects of sound quality that we certainly differentiate in listening but don't know how to measure.
Repeating something I have mentioned before, "settling time" tends to be the same as rise time, but really is not of interest in audio amps because musical sounds have leading edge transients, but die away gradually. This characteristic is used in the restoration of antique recordings. They are played backwards, and any transients that appear are identified as defects, and removed.
Another spec closely related to slew rate, and usely more widely published, is bandwidth. Amplifiers with wider bandwidths are faster and equate to larger valued (faster) slew rate specs. An amplifier that can reach 100kHz is faster, and would have a better slew rate spec, than one that goes only to 50kHz - the higher the number the better.

Current is the electrical property that is needed to drive speakers well, not voltage. Producing lots of current for a given power rating is part of what makes better amplifiers heavier and costlier. Current specs are rarely published, so one way to get an idea of an amp's current capacity (besides opening the amp and looking at the fuses) is to compare power ratings at different speaker impedances. An amp rated at 100W into 8ohms and 165W into 4ohms has more current capacity than an amp with 100W into 8ohms and 125W into 4 ohms. Please keep in mind though, that this only works if manufacturers aren't misleading when publishing their amp's specs. Product reviews where specs are measured are a good source of data for comparisons.

I have Magnepan planer speakers and use amps with high current capacity to drive them, with excellent results. Keep in mind that when choosing a high current amp, planer speakers need a certain amount of minimum overall power as well. I would recommend 100W as a minimum, as long as current capacity is there, but more would be better.

Hope this helps. :o)
The slew rate states the voltage increase the amp is capable of in one microsecond. This is important for very dynamic recordings (classical music has the greatest dynamics)and becomes more important the more inefficient the connected speakers are.
It is in no way related to an amplifiers bandwidth.
I've seen amps with a bandwidth of 10Hz-35kHz and a slew rate of 45V/microsec and amps that do 1Hz-175kHz and a slew rate of 23V/microsec.

Wattage can be increased by either increasing the voltage capability OR the current capability OR ideally both! The defining part for this in an amp is the powersupply, unfortunately this is also the most expensive part of any amp. The best indicator for a good powersupply is how close the 4ohm output is to the mathematical ideal of twice the 8ohm output.
I know of only one amp that achieves this: the asr emitter. It actually goes one step further and is capable of quadrupling the output into 2ohm.
For something cheaper try QUAD amps, they are designed to run electrostats.
If you like cars you see engines have two ratings torque and horsepower. Sheer horsepower effects top end spped. torque effects low end power.

Similarly,amps have a power output and a current output. This is why when playing a tube amp and solid state amp of similar power the tube amp appears to have more power. Because it generaly puts out more current.
For some reason planar speakers seem to require more current to move thier large diaphrams. The ET is reknown for it's inefficiency. I have found the conrad johnson to be very good in this area. Early Krell and Levinson(company not the man) where also excellent.
The only way that an amplifier can deliver current is by applying voltage to the load. You cannot deliver amperes without volts, so it is not an either/or proposition. However, an amp with a "weak" power supply might be designed to deliver a high voltage that permits signal peaks without clipping, but which, when applied to the load, will result in current that the amp cannot sustain very long. This is why "music power" ratings can be so much higher than continuous rms power ratings. A "weak" power supply produces high rail voltages when the signal is small, but these voltages drop greatly when a large signal persists, because the transformer and rectifier are too small to keep the capacitors charged up. Larger capacitors can help here. "Weak" power supplies actually do make sense because they are economical, and to tell the truth, music signals are characterized by peak voltage far in excess of that which corresponds to the rms voltage. It can't hurt to have a power supply that is capable of pumping out the amps without strain, but unless the audio signal is loud, compressed and peak-limited (pop music perhaps) this capability will not come into play.
The slew rate of an amp will tell you how fast the amp is capable of reproducing an output signal with respect to the input signal. The output signal of an amplifier is similar to an enlarged xerox image of the input signal's magnitude. But the input signal also has a time factor - that is, it has a voltage rise is over a period of time. If the amplifier can produce the enlarged image just as fast as the signal enters the amp, then the slew rate is infinite. If there's a time lag, then that's the slew rate. The higher it is, the quicker it generates the output signal.

The relationship to performance is that the slew rate is related to the amp's POWER bandwidth - not signal bandwidth, as most amps are capable of exceeding 20kHz. The power bandwidth is defined as the highest frequency the amp can reproduce at one-half it's rated power. So the higher the power, the higher the power bandwith and the higher the slew rate needed to accommodate it.

Don't look to slew rates as a measure of how an amp will sound because no one spec will be the defining factor. Slew rate is not a stand-alone indicator of quality; however if an input signal is so quick that the amp can't keep up with it, then there will be transient distortion which may be audible. Two amps with the same power rating but different slew rates may sound different due to slew rate - but more importantly, the one with the lower slew rate will probably have other shortcomings that account for the lower slew rate.
Talk about confusing !!!!

"Frequency response" on an amp is typically rated at 1 watt of output whereas "power bandwidth" is rated at full rated power. An amp that may have a frequency response of 1 MHz at 1 watt may only have a "power bandwidth" at full rated power of 20 KHz ( extreme case for demo purposes ). This will obviously vary with how linear the amplifier is at various output levels. It is therefore possible to have both a high slew rate yet still be limited in bandwidth. In most cases though, designers that strive for high slew rates also strive for wide power bandwidths too.

Other than that, slew rate tells how much voltage the device can swing in a given period of time. The more voltage that it can swing in a shorter period of time, the more likely that it is to remain truer ( more linear ) to the input signal without adding its' own distortions to the signal. This takes into account that the device is stable in design as changing the load or impedance that it sees could very well change the slew rate.

As i've stated before, high levels of speed and stability are what make for good performance but one without the other makes for a less consistent, less desirable product. As either performance attribute ( speed or stability ) is reduced, the more that "system synergy" comes into play. That's because you have to start balancing out the flaws of one component with the strengths of another & vice-versa. Problem here is that when you chance one factor in the equation by "upgrading" a component, you now have different "strengths & weaknesses" that need to be balanced out.

As to what is an acceptable level of slew rate for an amplifier, most of the acknowledge "audio experts" ( like John Curl & Nelson Pass ) teach that 40 V/uS is more than acceptable. I would say that this is a good starting point, but is nothing to sneeze at in itself.

Others like Douglas Self teach that, so long as the circuit is capable of providing more signal than it is ever called upon to deliver during normal conditions, it is "plenty". As such, Self proclaims that preamps with a slew rate of 2V/uS are "plenty fast enough". His reasoning? 2 volts is typically more than enough to drive most amps to full output, so the preamp ( during normal operation ) will probably never be called upon to deliver more than that amount of output at any given time. Very simplistic logic that really only works when everything is perfectly optimized / best case scenarios.

Having said that, some amps are MUCH slower than others. For instance, some of the older Quad amps slewed at 5 V/uS, which is very slow. Some B&K amps slew at appr 14 V/uS, which is still pretty slow. If you look for common sonic signatures between both of these brands, you'll find that neither is known for "slam" ( the ability to deliver high quantities of power very rapidly ) or "high frequency articulation". The latter has to do with the fact that the higher the frequency one tries to reproduce, the "faster" the amp has to be to accurately reproduce the signal.

Now what happens when you connect an amp to a highly reactive load that is very low in impedance? If the amp is unstable and / or lacking in current capacity, the slew rate is reduced. Why is that? Simple. The amp can only climb in voltage as fast as it can deliver current. If the amp can't load into the speaker due to a lack of current or stability factors into the equation with a "tough speaker load", the amount of voltage that the amp can deliver in a given period of time ( slew rate ) is reduced accordingly. This is a fact that most EE's never think about and is commonly overlooked. That's because these spec's are based on tests conducted into non-reactive "dummy" loads, not actual speakers.

As such, one is ALWAYS better off with an amp that is as fast as possible. That's because there will always be conditions presented to it that reduce effectively it can operate. This has to do with the dynamic nature of music and the reactive load that most speakers present during those dynamic changes in amplitude.

Obviously, selecting an amplifier with a high rail voltage ( more dynamic headroom ) with gobs of current capacity ( less potential for clipping & better overall driver "control" ) that is of a stable design ( remains consistent in performance regardless of the load impedance it is connected to ) and can respond quickly to signal changes ( high slew rate ) over a wide bandwidth ( frequency response ) at full power ( power bandwidth ) while maintaining high levels of linearity ( various distortions i.e. THD, IMD, TIM, etc.. ) without the need for a lot of error correction ( negative feedback ) is what we are looking for.

As one can see, there are MANY spec's that tie into making a "good performing" and "good sounding" amplifier. Lose any portion of the above stated "requirements" of good performance and you compromise the overall level of reproduction and versatility ( more variation from speaker to speaker ) of the component. The end result is that the engineer / designer that developed the product has to take all of this into consideration when sitting at the drawing board AND select parts that will more than hold up under duress. Marginal designs using marginal parts are easy and less costly to produce, hence the fact that many products perform quite similarly ( poorly ) overall. Sean

PS... Slew Rate and Rise Time are somewhat the same spec's. Given the limited voltage output required for a preamp, the spec of "rise time" is more appropriate here whereas "slew rate" would apply more directly to a power amp. That's because the preamp is voltage limited in real world operating conditions to less than 2 volts, so we want to know how long it takes ( duration ) it can deliver a specific amount of voltage. On the other hand, speakers require high levels of voltage, so slew rate tells us how much ( quantity ) voltage the amp can deliver in a given time period. In effect, slew rate tells us how much voltage the device can swing in a given period of time and rise time tells us how long it takes to deliver a specified amount of voltage. Even though they are kind of the same thing expressed in different manners, good manufacturers actually provide both spec's with the specific parameters for each test result involved. If a manufacturer provides both specs, you should look for lower rise times and higher slew rates for better performance.
Ok I have done some more reading and have some more questions. Do you have some values that you can apply to your reccomendations ? If, as so many of the knowledgable people have stated, the manufactures specifacations are
dubious at best how can one get it right if they can not listen to a great many pieces ? Who or what can we trust ?
Slew rate and frequency response are both important because one addresses transient response and the other addresses steady-state response. No slew-rate spec is going to tell you what the amp will sound like. There are several reasons:

1) Transient response can vary as a function of frequency, and usually does - this is controlled by the power delivery topology of the amplifier

2) Fast slew-rates can be underdamped or overdamped, and if you are lucky, critically damped. This means that the transient overshoots when the amp is underdamped causing a higher voltage than that in the original waveform. If the amp is underdamped, then the transient does not reach the voltage that is in the recording.

There is no commonly practiced spec that captures the above phenomena.

Steve N.
Empirical Audio
While those are good points Steve aka Audioengr, how many "ultra-fast" components have you ever seen that suffer from severe over or under-shoot? This is not to say that all "fast" components are properly designed / stable into every load possible, just that they typically have a little more thought and care put into their design and implimentation.

Saki: Some manufacturers are reliable in terms of the spec's that they print. If you can't get spec's from them directly, you'll have to rely on third party testing like Stereophile, Soundstage, etc....

As far as listing specifics that i personally consider to be "acceptable levels of performance", that would open up yet another massive can of worms without accomplishing much. That's because people would end up becoming "spec readers" without really knowing what they were looking at. This is how we ended up with SS amps that had .00X amounts of distortion back in the late 1970's / 1980's. Even though the amps measured well on paper, they sounded as sterile and lifeless as could be. This was primarily due to the use of gobs of negative feedback, which was a spec that most manufacturers never provided although some did openly discuss.

As mentioned in another thread, spec's can be manipulated to make a product look "good" to the consumer and unless the consumer knows how the spec's are derived and what they really mean, they'll never know the difference. As such, providing a "seal of approval" by listing numerical values for each of the above mentioned spec's would probably only end up creating more confusion in the long run. Sean
Overshoot on transients has, in times past, been deliberately introduced so as to compensate for the slow rise time of loudspeaker drivers. I don't know if anyone does this anymore. A good idea in theory, but probably difficult to execute without problems.