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What is a 'fast' amp?

Tone_Terrific

Silver Supporting Member
Messages
34,420
Assuming a tube amp for this discussion.
Are we talking transient response?
If so, is it measured while observing a square wave?
At what power level and failure to meet a spec does it start to feel 'slow' to a player?
Which components contribute?
Are we sure fast/slow amps are not fast/slow speakers?
Where is this latency sourced?
Do players confuse speed of attack with brightness or crispness i.e. they identify a peak on the waveform edge as fast, even if the transient is actually slower than a dull amp?
Squarewave response drops at lower freqs as it stresses the amp, so can we have a fast amp at high freqs that is a slow amp down low?
Let's get those questions out of the way:banana
 

FattMetheny

Active Member
Messages
85
Assuming a tube amp for this discussion.
Are we talking transient response?
If so, is it measured while observing a square wave?
At what power level and failure to meet a spec does it start to feel 'slow' to a player?
Which components contribute?
Are we sure fast/slow amps are not fast/slow speakers?
Where is this latency sourced?
Do players confuse speed of attack with brightness or crispness i.e. they identify a peak on the waveform edge as fast, even if the transient is actually slower than a dull amp?
Squarewave response drops at lower freqs as it stresses the amp, so can we have a fast amp at high freqs that is a slow amp down low?
Let's get those questions out of the way:banana
Amps always have felt "fast" to me. My amps all have solid state rectifiers though, not much sag to be had. I consider myself to be an extremely fast picker (not a tapper).

In terms of "speed of attack", I think the amp's response time is far quicker than the conglomerate of pedal lag, so many here experience. I use rackmount effects to avoid that.

I don't play low frequency square waves on any of my guitars. Occasionally I will on my guitar synth though. "SAW" waves are more fun.

Analog tube circuits can be modulated in the GHz, nothing inherently slow about them.

I guess what I'm saying is, do you have an example of a "slow amp" you'd like to share with us?
 

sanhozay

klon free since 2009
Gold Supporting Member
Messages
11,962
i have no technical means to be descriptive, or any assortment of exact reasons why. but for me, a fast amp is immediate response from digits to sound projection from the amp. the embodiment of that characteristic is a vox ac15/30 style amp. there's probably many factors in the equation, but i've never been able to, or interested in how it occurs. all i know is it doesn't happen in any fenders i've played, least of all the blackface circuits.
 

Tone_Terrific

Silver Supporting Member
Messages
34,420
^ You see, assuming that you are sensing something, what is that you are sensing? A BF Fender is pretty straight forward and so is a Vox.
Whassup with that? I don't mean PS sag or tube saturation.
 

Promit

Member
Messages
2,481
Assuming that when players say "fast" they are actually talking about reproducing transients properly and not making up terminology for something else entirely --

How fast an amp is will depend heavily on the structure of the power supply and power amp. Usually high gain chugging puts the most strain on an amplifier's power supply to swing to a much higher current output quickly. In many amps, the B+ supply voltage will drop and then come back on high power transients. This 'sag' effect is most obvious with tube rectifiers but there are a variety of other factors. Amplifiers that use a power resistor for final DC filtering rather than a choke will sag considerably under high demand. If the power transformer is running close to saturation (undersized), it may not be able to respond fast enough to deliver the power being demanded downstream. Mesa Dual Rectos and their ilk are famous for this type of behavior. I suspect the OT's ability to swing between frequencies is also involved but I'm not sure.

Not sure of what impacts the preamp's ability to reproduce transients.
 

drbob1

Gold Supporting Member
Messages
29,047
To my ears, a fast amp has a hard attack on the note. Some folks refer to it as the notes "flying off the fretboard". It is, I believe, a consequence of the power supply, the output configuration, not so much the preamp. The fastest, or hardest amps I have in my collection are AC30 inspired and Trainwreck style amps-very simple preamps, very stout power filtering, little negative feedback, power tubes biased in the very low class AB so they're conducting most of the time-that means when called on to reproduce a transient they're able to react right away. It can be done with a high gain preamp, witness the Dumble inspired amps which also tend to have really solid transients, at least clean.

The drawback to a fast amp is that there's nowhere left to hide from a technique point of view-if your picking isn't consistent or you fluff notes, people WILL hear it. So, I think many of us are a little happier with amps that are more forgiving!
 

Belmont

Member
Messages
3,465
you want fast transients? try a Hiwatt DR-103 on about 3 or 4, before the sweetspot.
 

WillTM

Member
Messages
65
I usually associate an amp being "fast" with the type of rectifier it uses. Tube rectifiers add "sag" to the amps sound, and that usually makes the amp feel "slower" but also warmer. Diode rectifiers have no sag and shave a much more immediate sound
 

drbob1

Gold Supporting Member
Messages
29,047
I used to think this ^ too, until I realized that a DC30 with single GZ34 was one of the fastest amps I own and it's tube rectified (obviously). It's more about how the power supply is built-you can compensate for sag from a tube rectifier with enough filtering BUT that means a huge PT to be able to drive the caps. Which is the reason that Fenders tend not to be fast even if they have lots of output tubes: not big enough iron and not enough capacitance!
 

stratovarius

Silver Supporting Member
Messages
9,141
I learned something here today. I used to think "fast amp" referred to the length of time between a glowing review and an emporium listing.
 

Blue Strat

Member
Messages
30,218
I learned something here today. I used to think "fast amp" referred to the length of time between a glowing review and an emporium listing.
ROFL! Excellent!

What is a 'fast' amp?
I guess you'd have to ask the person who said this. I've never heard anyone with a technical background use that description.

Probably transient response as you mentioned. Leading edge of a square wave would be the typical measurement if our assumption is correct, but I can't imagine anyone who knows about transient measurements using the term in question...so...
 

Tone_Terrific

Silver Supporting Member
Messages
34,420
ROFL! Excellent!

What is a 'fast' amp?
I guess you'd have to ask the person who said this. I've never heard anyone with a technical background use that description.


Probably transient response as you mentioned. Leading edge of a square wave would be the typical measurement if our assumption is correct, but I can't imagine anyone who knows about transient measurements using the term in question...so...
The term certainly gets used but what does is mean, technically? That is the question.
Is it a design feature that is engineered into an amp or do amps just pop out of manufacturing and get a rep based on somebody's subjective evaluation?
I have played rigs that have latency and not liked it beyond a certain point, but that seems not to be what others refer to.
I do not feel BF Fenders to be slow or forgiving, whatever it means. I'm talking clean, here.
 

mad dog

Silver Supporting Member
Messages
11,008
It's a feel thing to me. Any fixed bias amp not a 1x15 which has a SS rectifier feels that way. As opposed to the tube rectified amps (same with 15" speakers) that are more my usual fare.
MD
 

capnjim

Senior Member
Messages
1,228
What a strange thread. I've owned dozens of amps and have never had an amp faster or slower. You hit a note, you hear it right away.
TGP is getting weirder and weirder every day.
 






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