What is meant by a "fast" pre amp

I have heard a pre amp described as "fast", what does that description mean?
I have "heard" should read I have "read" a pre amp as described as fast, what does fast mean?
It means that its response to incoming signals has a fast rise time. This requires wide bandwidthe--flat response at least to 60KHz and preferably 150KHz or more.

If you were to look at a test report of an amp or preamp that shows both frequency response curves and square wave response (several Stereophile tests do), you'd find that there is a correlation between wide bandwidth and how square the square wave is--in other words, how immediate the response to incoming signal is. This is speed, and it's what is meant by a fast component. Fast components have sharper transients and generally more space between the notes and therefore more clarity.
I think it is an overused adjective that has very little substinance.
how square the square wave is
What about the overshoot? Does that mean it's too fast?
To me fast is the equipments ability to track the musical signal.
Usually if the equipment sounds fast it's not because if it is fast it will sound like misic.
What about the overshoot? Does that mean it's too fast?
Hi Bob,

It could reflect a number of things, including a frequency response rise at high frequencies; less than ideal "compensation" for stray capacitances that may be present in the circuit or in individual devices; the amount of time required for signals to propagate through a feedback loop, if present; phase shifts that are not proportional to frequency; etc.

It's more complicated than one might imagine. One of my college textbooks has a 70 page chapter on "compensation" of wideband amplifiers, referring to techniques that may be necessary to achieve clean edges and flat tops in response to square wave or stepped input waveforms. It is filled with complex equations that I wouldn't wish on anyone!

But I would say that the answer to your question can be thought of as being "yes," in the sense that wider bandwidth/faster risetime, while potentially having the advantages that Johnnyb53 cited, can make the factors I listed above more critical.

Best regards,
-- Al
Thanks Al -- always appreciate your comments.
Brf, Is the technical definition what you wanted? The last thing you want to do is buy equipment based on specs or some long elaborate explanation. The tweak industry is full of this kind of talk with space age materials and methods that only a NASA engineer would know.
The real question is; Does it sound like music?
Buying on specs only leads to disappointment.
............Are you serious?? The guy asked for audio help, not english lessons.
Sorry, I thought someone else made that comment.
Pace, rythym, & drive equals fast.
I would add to Phd's well chosen words, dynamics, both micro and macro.
Is this the same as "slew rate" ?
I pride myself on owning a very fast sounding system. Just the other day I was listening to a track on one of my favorite CDs, as soon as the song started, it was over. C'mon folks that is very fast, left me alot of time to do other things. This is not funny but more than likely true. How about amps that weigh 50 lbs or more. By virtue of their sheer weight they are dragging the sound down to a slow pace.

Honestly, good engineering and a short signal path can produce a fast sounding product.
I am pretty ignorant regarding the technical aspects of an amp, but, surely if it sounded faster, that is, if the sounds don't take long for the attack and the decay then a CD with a song length read-out of, say, five minutes exactly would play shorter than five minutes. OK, so this obviously isn't the case, so I am assuming it refers to the length of time for the amp to take the sound/s to a particular sound volume level......does this make sense????......Wait a minute, that doesn't make sense neither - the song would still finish earlier!
I am guessing it has more to do with what Phd refers to as "sounding" faster than actually "being" faster.......the more I think about it the more confused I get. Of course the whole speed thing is totally determined by the RPM of the record or CD. Could "faster" simply be semantic quibbling that could just as easily be referred to as "crisper"?
Sorry if I have confused you even more Brf.
Tom -- While I think that the term "crisper" can correctly be used to describe what is meant by "fast," Johnnyb53 is also correct when he says that "fast components have sharper transients and generally more space between the notes and therefore more clarity." Think of it this way:

Regardless of how "fast" or "slow" the preamp or amp is, the amount of time between the very beginning of one note and the very beginning of a subsequent note will not change. However, the amount of time between the very beginning of one note and the very end of that same note (or another note) CAN change, if the "risetime" and "falltime" (or, correspondingly, the bandwidth) of the component is sufficiently slow to attenuate the high frequency content of the signal.

It is well established that the ear tends to "latch on" to the leading edge of transient sounds, and give them disproportionate emphasis relative to what follows during some subsequent number of milliseconds. If the speed of that edge is degraded, the subjective result may be described as lack of crispness, dullness, poor definition, reduced clarity, etc. Similar subjective effects can occur at the trailing edge, if the output signal doesn't stop as quickly or as cleanly as the signal from the source.

The total duration of the track or recording, though, will not be affected significantly by any of this. The amount of time from the start of the first note to the end of the last note will only vary over an amount corresponding to the falltime degradation of the very last note on the recording. I think that will be clear if you consider the second paragraph in this post. That variation will be measured in microseconds (millionths of a second), or perhaps tens of microseconds -- in other words, totally insignificant.

6550c -- Slew rate and risetime/falltime are loosely related, but not quite the same thing. Slew rate is defined as the maximum rate at which the output signal can change, usually measured in volts per microsecond. Risetime is defined as the amount of time required for a signal to change from the 10% point to the 90% point of a positive-going step change in voltage. Falltime is similar, except that it applies to a negative-going step change in voltage, and is measured from the 90% point to the 10% point. Risetime and falltime inversely correlate with bandwidth.

I'm not particularly expert in this area, but I believe that with respect to audio components slew rate tends to be primarily relevant to "large signal" amplitude changes, meaning those that approach full scale (full power in the case of a power amplifier), and the idea is that the slew rate must be faster than the maximum expectable rate of change of the input signal, or distortion will result. While risetime and falltime can be relevant in the context of both small signal conditions (which typically comprise most of the music) and large signal conditions, and can affect the signal by limiting its bandwidth (i.e., by rolling off very high frequencies), as opposed to introducing distortion.

It is common for the full-power bandwidth of a power amplifier to be different than the small signal bandwidth, because different design parameters come into play under the two sets of conditions.

Rrog -- I doubt that there are many Audiogon members, or other audiophiles who are at least semi-serious, who "buy based on specs." However, that does not negate the value of having as good an understanding of specs as possible, for at least two reasons:

1)Ruling out potential component acquisitions that would be mismatches to other components in the system (gain mismatches, impedance mismatches, level mismatches etc.), or that would be mismatches to the listener's requirements (peak volume capability, deep bass capability, etc.).

2)Providing clues as to particular things that should be listened for when auditioning components.

-- Al

That was a hell of an answer. You are always one to dig deep when it comes to figuring this stuff out - good to know you are around.
Thomastrouble, you knew my last post in reference to fast was tongue & cheek and was written to be amusing, not to be taken seriously. Right?

Yes, I got that - I was just referring to the "sounding" bit.

As for me, I am just a listener and am happy to let brainy people like Almarg (who obviously loves the technical challenges) do the dirty work for me.

Your quip about heavy amps got me thinking about something - I have a couple of CD players, both highly rated - a Bryston BCD 1 and a Naim CDX2. One weighs about four times the weight of the other (rough guess). The Naim feels like a block of lead - I wonder why. They both sound just as good, the only difference I have found is the shipping cost to get them to me.
Thomastrouble, I too agree with you that Almarg post was both intelligently written & clear. He most definately is an asset to this audio community. I'm more in your camp and will leave the technical description to folks like him.

The Naim CD is a block of lead, no doubt contributed to its heavy case. I have experimented with Naim gear in the past and found out it is designed to generally excel with other Naim components and with its unique connections, makes it almost impossible to mix and match with other brands. If I could give you any advice I would tell you to purchase as new as possible because this technolgy has been progressively improving as time goes on.

Agreed, Al's post was lucid & well-written for most people to understand this technical subject.
Just a small addendum - as far as specs are concerned, the higher the slew rate the better. Slew rate can also be thought of as the rate at which the amp can charge a capacitor i.e. besides the units of Volts/microsecond, slew rate has equivalent units of Amperes/Farad. Thus, slew rate can give you an idea of the robustness of the ampilifier's power supply. For difficult to drive speakers, amps with higher slew rates generally do better as their power supplies can provide more current.
(current is not the only requirement for hard to drive speakers, one also needs wattage which provides the max possible output voltage swing. So, one needs both watts & current for hard to drive speakers & that's what makes these amps very large chassis units).
Higher slew rate amps generally cost more as more robust power supplies are not cheap in parts or labour.

As alluded by Al, slew rate & bandwidth are inter-related: if you view slew rate in its dual definition as the ability to charge a capacitor, that current is related to gain of the amplifier stage & in turn, the gain of the amplifier stage & the total capacitance in that amplifier stage defines the bandwidth. So, higher slew rates, higher current generally yield wider bandwidth (or "faster") amps.
Interesting perspective, Bombaywalla, which I hadn't thought of or seen stated before. And it sounds right.

I (current) = C (capacitance) x dv/dt (change in volts per unit time)


dv/dt = slew rate = I/C = amperes/farad.


Best regards,
-- Al
I have been told that a fast preamp is one with good phase purity.
Supratek preamps sound very fast.
I also owned a Metaxas Marquis SS preamp which was fast.
These are the two best sounding preamps I have heard.

Once you get used to the speed there is no going back.
'good phase purity' comes from wide bandwidth/high slew rate.

In order to get to 20KHz without phase shift, the upper cutoff of the circuit needs to be 10X higher, IOW 200KHz. YOu can use that as a good rule of thumb for seeing a 'fast' preamp in its specs.

Mind you, 'fast' is not the same thing as 'bright', in fact brightness is often caused by phase shift issues due to poor bandwidth. Brightness is also caused by very minimal distortion of the 5th, 7th and 9th harmonics.
All of this can be summed up in one word, MUSIC.

A preamp could have all of the best specs in the world, but if it doesn't sound like music, you have nothing.

If you think your equipment sounds fast then it probably isn't.
Agreed- 'fast' by itself would strike me as being a coloration. 'Fast' with a host of other aspects (detailed, transparent, **relaxed**, wide bandwidth), no worries.