What makes one preamp faster than another?

Thanks for your reply yotonic. It seems like "faster" and "less color" are interchangeable words in your explanation. That's something I can understand. I was thinking it had to do with millisecond time factors in the signal going through the circuitry, but I'm admittedly under educated when it comes to gear.

Hey Frank, yes I would say that to you if we were just having a beer, but Tony or somebody might jump all over me here and indicate they aren't mutually exclusive! Lol. Maybe I should say that there isn't necessarily "less color" with a faster preamp but it comes at you harder and more defined like a digital photo versus film.

Yeah I own some Jim Williams gear and he is the King of high speed.

I really dig fast preamps on stuff where that kind of raw, realism, really makes the music move you, especially talented acoustic artists etc. But everybody uses a different pen to communicate their vision. 

I'm guessing as a songwriter Frank that you are trying to educate yourself on gear to improve the quality of your demos or completed productions. I would add that the single most effective way to do that is through the quality of your instruments. You can make the greatest advancements in the quality of the final product by not cutting corners on the instruments, players, and microphones.

I know players who are so good that they can add more detail, eq, realism etc through their attack and control of the instrument than any preamp can add. I am always stunned when I use top session players at how huge the gap is between them and my regular session guys. Every medium sized city supports one or two guys who scratch out a living as session players. In my experience they aren't on the same level as guys in New York, LA, and Nashville who are "First Call" or whatever buzz term you want to use. You pay a little bit more but "most times" get a lot more back in return. And these guys are playing on so many peoples songs that they will have ideas to fix your writing where it's amateurish. I don't spend much money on gear anymore, only instruments and musicians. Every top session player has a world class home setup, and for live sessions studio time is cheap.

Every electronic component made has a speed limit. To obtain a fast design, every component must be evaluated for it's speed limitations. Some resistors do better at mhz's than others. Caps are dependent of film types and voltage ratings, higher voltage caps are faster.

Transistors have a FT rating that sets their operational bandwidth. Opamps have open loop gain vs slew rate.

Topology also sets your speed limits. Transformer designs are limited to under 20v/us slew rate due to mostly rise time limitations of the transformer windings, even if the following amplifier is very fast. Transformer-less designs have no such limitations. Voltage feedback topologies are limited in speed and have a -6db/octave roll-offs past their operational bandwidth. Current feedback designs do not, they maintain full operational bandwidth at all gain settings without the roll-offs seen in voltage feedback designs. They are also much faster as there are no feedback/bandwidth limiting elements, feedback is steering currents that don't have the roll-offs or speed limits of traditional voltage feedback.

Some of my High Speed mic preamp designs here do 7000V/us slew rate, far beyond any other designs. Their operational bandwidth is 400 mhz. There are zero degrees of phase shift, 20~20k hz, also unique to the industry. CCIF twin tone IMD is .00015% as measured on the Audio precision analyzer. Noise is -133 db EIN at a 50 ohm source impedance. Tony C. has one of my 500 modules with that design, maybe he can comment on how it sounds.

Actually transformer-less tubes are the fastest.

There are a few fast RF triodes that can be used for audio, not that quiet however. You might get about a 50V/us slew rate but the input transformer will not get there, the best one is the Jensen JT-16 at 150/600 ohms.

Common audio preamp tubes like the 12AX7A are limited to a 12v/us slew rate. Not too bad but far away from a quality current feedback amplifier running at a 7000V/us slew rate. The difference is like watching a sprinter run 12 mph while a spacecraft re-entering the atmosphere passes by at 5000 mph.

Echoing what a lot of these other posts are saying (but living in a lower-end world myself): I'm seeing two types of speed/slew rates in my studio.

My Focusrite interface's preamps, when paired with an SDC, feel very immediate. The spike of the transient coming off a snare drum feels like the spike of the sound coming off the drum in the room (minus the physical punch from the moving air). Sharp. Sometimes too sharp. An acoustic guitar can sound very "live" - almost in a "warts and all" way. I can almost pick out the time differences and inconsistencies between every string of a strum.

My Chameleon Labs 7602 Xmods have transformers on the input and output. My Presonus Eurekas have transformers only on the input. The same SDC setup on them feels almost like the snare has a little limiting on it. The transient doesn't spike as sharply. I'm sure it's a little of the transformers saturating, too, but it's just not as immediate (most of the time, in a good way...but that's a different topic). An acoustic guitar sounds a little more together and, when strummed, like the individual strings are standing out less. Almost covering up the inconsistency from one string to the next that I was noticing in the transformer-less preamp.

A metaphor that translates the different preamp speeds looks like this to me (though I'm no golden-eared expert or technical genius - I'm just a low-end dude trying to make some cool recordings):

Imagine a sports car and a box van driving at 60mph, not being allowed to slow down, trying to follow a curving line on the road as exactly as possible. The line has some long curves that are smooth and some curves that turn fairly tight and are difficult to manage. The sports car will be able to turn a little more sharply and, because of its better handling, more accurately follow the line. The box van will have to round some of the sharper corners in order to miss as little of the actual line overall (if it tried to turn as fast as the other, it would tip over).

You're following me. The line is the sound wave in the air. The sports car is a faster preamp and the box van is the slower preamp. The path the vehicle actually travels is the sound wave after it comes out of the preamp. Both are giving you a somewhat accurately represented sound afterwards but one of them is more like the original sound and one of them is a bit more sluggish in its handling of the original sound's details.

Slew you!

Super true.

There are definitely upsides and downsides to each end of the spectrum. That level of detail can be unusable.

To add yet another metaphor to the pile: Reducing the amount of detail in a photograph of a person's face makes their skin look a lot better. 

Current feedback amps don't use band limiting feedback caps. Bandwidth is set by the feedback resistor values. They can be more stable at higher bandwidths than comparable voltage feedback designs. The extra detail is useful for beautiful sources, warts will be exposed to all.

I throw mine out the window. First to hit the ground is fastest.