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what is a "fast " Preamp

Discussion in 'Preamps / Channel Strips' started by claude, Feb 11, 2004.

  1. claude

    claude Guest

    Hello and thanks in advance for all answers. I hear a lot about "fast" or "quick" preamp. Exactly what does that mean? Thanks again.
  2. ozraves

    ozraves Active Member

    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.
  3. AudioGaff

    AudioGaff Well-Known Member

    Welcome to RO Claude!

    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.
  4. jonyoung

    jonyoung Well-Known Member

    Is this the same a slew rate?
  5. AudioGaff

    AudioGaff Well-Known Member

    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.
  6. jonyoung

    jonyoung Well-Known Member

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

    AudioGaff Well-Known Member

    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.
  8. jonyoung

    jonyoung Well-Known Member

    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).
  9. Tim Farrant

    Tim Farrant Active Member

    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.

  10. Bobby Loux

    Bobby Loux Active Member

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

    AudioGaff Well-Known Member

    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.
  12. e-cue

    e-cue Active Member

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

    sdevino Active Member

    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.

  14. Paladyne

    Paladyne Guest

    fascinating thread!
  15. doctorfish

    doctorfish Guest

    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.

  16. AudioGaff

    AudioGaff Well-Known Member

    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.
  17. doctorfish

    doctorfish Guest

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

  18. Tim Farrant

    Tim Farrant Active Member

    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).

  19. realdynamix

    realdynamix Well-Known Member

    :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?

  20. e-cue

    e-cue Active Member

    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.

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