"Fast" And "Slow" Preamps


I'm ashamed to admit that I did not really know what these phrases meant until quite recently. One of our engineers, Brad Smith, had to fill me in. Thanks buddy! Anyways, this is part of the lingo that you need to be aware of as an engineer.


The easiest way to describe fast and slow is to picture a sound wave in terms of a timeline. It goes up and down repeatedly. Now picture a device that is to capture that sound wave. How well does it capture the true waveform? And how quickly does it do that? How big is the lag? That is what is meant by fast and slow. 



Slew rate is defined as the change in voltage per unit of time. So in terms of a sound wave, it represents how quickly a pre-amp can change voltage with the change in the incoming waveform. I know that sounds a little jargon-y, so have a look at the picture to the side. The red line represents a square waveform. Square waves are typical in synthesizers. Now in a perfect world, a preamp would follow the wave instantaneously, however that's not the reality of physics. (That would be akin to teleportation). So the green line represents the ground truth of how fast the transient moves within the preamp. Hence the word fast and slow


Transformers in mic pre's also have a reactance time. For the same reason, the length of time of time it takes for the transformer to respond adds to the speed of the preamp. Once again, fast and slow



There are a few general characteristics of slow and fast preamps. Generally speaking, fast preamps have a sharper and much more detailed toned, providing a bit more punch. For instance, an SSL preamp is known to be quite fast, which is why they are punchy and more appropriate for a punchier, rockier sound. By contrast, slow preamps tend to have a warmer and more coloured sound to them. The most obvious instance would be the Neve 1073 preamp, which has a fat, warm and coloured sound to it. It does an unbelievable job on softer generals such as folk, singer songwriter, jazz, etc.

Now having said that, these aren't hard and fast rules. For instance, a preamp with a slower slew rate may distort when receive a source that has an extremely fast transient, such as a snare drum. The preamp can't keep up with the rise of the waveform, therefore it does whats called, Transient Intermodulation Distortion (TID). This can actually end up sounding more punchy, rather than rounding off the transient. 


You can start to hear the speed of the preamps you use by A/B'ing the sources, reading about their transient speeds, and getting comfortable. Then you can think about what you're recording, what genre, what instrument and how you want it to sound. This can help you choose the correct preamp that will most benefit your sound.

Well, I hope I've done a decent job explaining this topic. It's something I'm still getting comfortable with, so I thought I would share it to others who may not be aware of it either.

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Cheers y'all!

Brad Simons
Velveteen Audio