Picture a timeline.

A fast mic pre will duplicate the timeline or come very close to the original.

A slower mic pre will copy the timeline but certain items will kind of lag behind. This can be a pleasing sort of distortion of reality. However, I sort of like the low end and the top end to be totally real in terms of where events happened in time.

Welcome to RO Claude!

I hear a lot about "fast" or "quick" preamp. Exactly what does that mean?

Ozraves says it well.

It is a lot like the way a compressor reacts when you choose fast or slow settings.

Slower preamps tend to be more mellow or softer sounding. Faster preamps tend to be the opposite and reveal much more in detail or focus. The ability to reproduce the transients as well as the original envelope of the signal is one way to determine if something is fast or slow.

Is this the same a slew rate?

Originally posted by riversedge:
Is this the same a slew rate?

Slew rate has a lot to do with it yes. But so does the way the circuit is designed. Transformers and which type are used can also make a difference.

AG, Because the reactance of the transformer causes phase(time) shift?

Originally posted by riversedge:
AG, Because the reactance of the transformer causes phase(time) shift?

Ya, that could be considered part of it. Transformers in general are not ideal in the way they process audio in freq, amplitude or time and so this affects the overall ability to accurately reproduce a signal. Mic pre designers can use this as a way to create a specific tone or tailor the tone of the circuit which is one reason why there is such a fuss over which transformers are used and for using custom made transformers.

One of my favorite weapons for tracking bass is the tube pre with direct out on my older Trace Elliot rig. I've gotta believe it was designed with low end in mind from the start. Warm & tight (heh heh heh).

Originally posted by AudioGaff:
Slower preamps tend to be more mellow or softer sounding. Faster preamps tend to be the opposite and reveal much more in detail or focus. The ability to reproduce the transients as well as the original envelope of the signal is one way to determine if something is fast or slow.

This is not strictly true IMO. Slower preamps or ones with a low slewrate spec may distort a fast transient, (such as that found on percussive signals, eg acoustic guitar) as it cannot keep up with the input signal risetime. This distortion is called Transient Intermodulation Distortion (TID) and may be revealed sonically as an emphasis of the transient rather than a softening effect. Various listening tests I have done confirms this. Transformerless preamps with a slewrate of less than 10V/uSec may suffer from this effect when asked to amplify these transients unless the signal is filtered (LPF) before entering the first stage. Remember of course slewrate and it's effects are dependent on what sort of actual level you are trying reproduce the transient at.

I think softer sounding preamps are generally transformer coupled designs and if the transformer has significant phase shift at the treble end, then the transients are distorted by time, a totally different effect to TID.

Tim.

how bout a list of "fast" preamps you'd consider, high end or budget?

Slower preamps or ones with a low slewrate spec may distort a fast transient, (such as that found on percussive signals, eg acoustic guitar) as it cannot keep up with the input signal risetime.

Ok. But preamps with poor slewrates/risetimes also tend to not be considered high quality preamps so they don't really count in my book as I try hard to avoid using them. Also, the way that one type of instrument or dynamic character reacts to a preamp is not what makes what I consider a preamp be fast or slow. It could be faster on some sources and slower on others. And although I am able to understand why a mic pre is fast or slow, and despite what a schematic and parts list can tell me, it is my experience in using them that determines how I label them. It is far more important to me as an user knowing which ones are fast or slow and which ones are not and with which sources that has the most merit to me.

Originally posted by Bloux:
how bout a list of "fast" preamps you'd consider, high end or budget?

The Audio Upgrades pre amp is among the fastest on earth. Try it on cymbals. It will amaze you.

Fast = no time based distortion.

Usually requires a very wide bandwidth and lots of headroom plus all the stuff that Audiogaff already mentioned.

Fast means all the transients make it through the preamp unaltered. Slower means the transients get filtered and you lose detail. For applications where "flavor" is desired this is good.

I like super extra amazingly clean and wide bandwidth for anything acoustic, especially if the player and the instrument have a great sound. I like color for things that naturally sound nasty like snare springs or huffing and puffing mesa rectifier or aggressive bass.

Steve

fascinating thread!

Very interesting. I'd never heard of fast or slow preamps before.

Now, I may be over generalizing here (so please correct me if this is wrong) but would this mean that the colored preamps, even the really high end ones like the Neve clones, are in the slower category and the preamps that are more transparent are the faster ones?

I know there may be more to it than and that's largely the reason I'm spending a lot of time here these days, to learn more about the technical side of all this stuff.

Dave

Now, I may be over generalizing here (so please correct me if this is wrong) but would this mean that the colored preamps, even the really high end ones like the Neve clones, are in the slower category and the preamps that are more transparent are the faster ones?

In general, I would say yes, that is correct. Again, it can be very source as well as mic dependant. Most preamps that are tube based are considered to be colored to some degree but many can be both fast and/or slow. Mic pre's using some of the newer transfomers or using custom made transformers can be colored and still sound faster than some mic pre's that don't even use transformers.

Thanks Audiogaff. I figured there'd be something more to it.

Dave

It must be mentioned that just because an amplifier (be it preamp or power amp) has a wide frequency response does not mean it has a fast Slew Rate. For example, a good ic opamp like the OP275 can operate out to 200Khz in a well designed circuit, but only has a slew rate of 20V per microsecond (V/uS). The BE40 discrete amplifier we use also operates out to 200kHz no problem, but has a slew rate of 140V/uS.

I do not know what the slew rate of a Neve 1073 module would be, but it will probably never generate much in the way of TID because the signals going in are filtered (by the action of the coupling transformers) so the electronics are never asked to reproduce transients they cannot.

Bad sounding circuits are those with a wide frequency response but poor slew rate, as you are asking the electronics to reproduce something it cannot. With the elevated analog levels we now use to feed A to D converters to get FSD, slew rate should be considered an important specification. A circuit with a slew rate of say 5V/uS may be fine if the peak analog level is only +10dBu (2.45 volts), but may struggle to deliver accurate audio at +20dBu (7.75V).

Tim

:confused: I always thought "slew rate" was the speed of the ability of a power amplifier to restore the speaker to it's idle state after the onset of a transient, or it would still be in motion when the next one is applied. IMO, "rise time" makes more sense for what is being discussed here.

With the use of condensor mics, does this FAST/SLOW pre-amp speed apply to the built in mic pre as well?

--Rick

If you really want to see this in action, I highly recommend you rent or buy a Groove Tubes ViPRE. It has a selectable "rise time" which you should hear the syblince, attack, and clarity vary when you toggle around.

Originally posted by Rick Hammang:
:confused: I always thought "slew rate" was the speed of the ability of a power amplifier to restore the speaker to it's idle state after the onset of a transient, or it would still be in motion when the next one is applied.

This would be called "settling time" and also applies to electronic circuits.

IMO, "rise time" makes more sense for what is being discussed here.

With a stepped input signal (square wave), Rise Time is the time a circuit will take to reach 90% of the stepped input voltage. Slew Rate measures the "rate of change" to that waveform, or, the speed at which the circuit will respond to that waveform.

With the use of condensor mics, does this FAST/SLOW pre-amp speed apply to the built in mic pre as well?

Yes, most certainly, but these circuits are not asked to deliver +20dBu. In a condensor microphone, the preamp is more of a high to low impedance converter with a very small voltage output, so it does not necessarily require a fast slew rate. You need to remember that slew rate effects are related to the actual signal level you are trying to reproduce.

Cheers
Tim
http://www.buzzaudio.com

sorry double post...

:) Thanks Tim, I picked it up with some specs on vintage gear manuals. Slew Rate and damping factor appeared to have a relationship... :d: . It appears there is more to it than just slew rate (physics and impedances), in regards to amps and speakers. Settling time, a term I just discovered. You know, the years when specs were the main battle ground in choosing high performance gear.

Thanks for the clarification! :tu:

--Rick

Wow. This is fascinating. Ok, so dumb question time: I have a Presonus M80 which is using the standard NE5534AP opamps. According to the Texas Instruments website, they have a slew-rate of 13V/us.

In a recent TapeOp article (and on a webpage I've seen) they talk about changing the opamps to something like the Burr-Brown OPA604 which has a slew rate of 25V/us or the Burr-Brown OP627AP which has a slew rate of 55V/us. Would this account for the reported "increased clarity"?

:cool:

Even dumber question... which would be better: An op-amp with a 20V/us slew rate and a 110MHz bandwidth or one with a 135V/us slew rate and an 80MHz bandwidth?

Hi B3Groover,

To answer your first question, the 5534 is a bipolar input device, the OPA series you mention are FET input and this would (IMHO) account for a change in sound. There are so many factors to take into account it's difficult to say 100% the better slew rate provides the better clarity, it would depend on the circuit and what preceeds the 5534. But, higher speed is definitely advantageous for accurate audio reproduction.

To answer you second question, there are devices out there that are very high speed, but are not suitable for audio due to other parameters, such as harmonic distortion, noise and non-linearity. These devices are intended for other types of analog circuits like instrumentation, video etc... Whilst it may be possible to use them for audio, sonically they may be bad, so you would need to take all these parameters into account when choosing a device. If the 135V/uS device has suitable parameters for the application, then it would be a better choice I suppose.

Oh and your questions are not dumb. I've done quite a lot of tests in this area to determine what sonically the effect of slew rate has on audio gear, it's most interesting !!

Tim
http://www.buzzaudio.com

Tim,

Thanks for the reply. I'm going to assume the OPA637 has good audio characteristics because the webpage I saw talking about chaging op-amps used the OPA627 and the 637 is the same as that one, only with a faster slew rate.

I'm going to order some (there's a guy on ebay selling them cheap!) and try it out.