Hello, and welcome back to the blog! We're carrying on this week, with our focus on General Microphone Technical Aspects, by taking a look into why we need preamps, what they do, and a short look into their history to really understand where in time they came into existence properly.
We've covered preamps quite a few times on this blog, specifically looking at the way they sound rather than what their job is and how they go about doing it. So, today we'll take a look at maining the inner workings, and hopefully by the end of the blog today, you'll understand your own preamps a little more than you did before you started reading.
Let's dive in!
What Is A Preamp?
The term preamp derives from the full word, preamplifier. Split that word in 2, and we have a pretty good idea of what it is already. See, the problem with microphones is that they have a very low output voltage, which means that by themselves, if you were to try and record a microphone without anything helping the signal get louder before you listened back to it, the best you'd get is a very quiet and very noisy recording.
That's where the job of the preamp comes into full effect. Its main purpose is to amplify the signal from the microphone without adding distortion, thus making it easier to listen back to and record. The preamp in the signal chain is vital to ensure you can get a great sound from your microphone, but they're not all built the same, and there are high quality designs, and very low quality designs.
Compared to the first preamps and their technologies, we have a much easier time recording in the manner we want today. The first microphones actually didn't have anything more than a cable running from their output, which would then go straight into the recording device; only around the 1920's did we start to see the introduction of preamps being used, but even they were primitive. The first preamps were designed based on the invention of the vacuum triode valve, discovered in 1906, but unlike today, they still had to overcome a massive issue - how to find the gain needed to amplify the microphones signal. So really, the first preamps would be pretty fixed in terms of the output they allowed you to achieve, and in the design below there wasn't even an output volume control, just a fixed output stage.
This is an early 1930's preamp design which was apparently the work of the BBC.
It was only in the late 1950's where the invention of firstly the germanium transistor, and then later and more importantly the Silicon transistor, were incorporated into preamp designs giving birth to a plethora of design options. This component brought many advantages by lowering the distortion (early designs would have 10% THD which is a lot!) and allowing for more gain staging options within the design. It was also around this time that 2 of the most important companies ever to exist were founded; Fairchild in 1955, and Neve in 1961. Fairchild was the company that brought in the next wave of discovery with the invention of the Integrated Circuit, which in turn, led to the invention of the Op Amp. With Op Amp designs, there was a much simpler design that was formed by:
Input Transformer > the Op Amp (which is now a single gain stage) > The amount of gain is set by the ratio of 2 resistors > lastly to the output transformer
This simple design is what really started the boom of preamps and companies being able to put their sound into the design. The signal chain above is a basic form of one of the most beloved mic preamps on the planet, the API 312.
A vintage API 312 Preamp - that black box with the arrow is the Op Amp Integrated Circuit.
Using Preamps Yourself
So now you have the basic understanding of how they work, and the main technologies in valve and Op Amp designs, you may be wondering how you can use this knowledge to help you? Well, the designs mentioned above all have their own sonic imprint on the sound of the microphone, and so this is where the concept of matching a microphone and a preamp becomes so important.
For example, the Op Amp design is a much simpler, much cleaner sounding design with just one gainstage needed; even the transformers are really only there to help the the impedance load for the input and output but the transformers could be clean sounding too which will give you more of the sound of the microphone you're using as opposed to something such as a Valve mic preamp which inherently has both more noise (different types of valves can change this too), as well as more harmonic distortion which can be desirable and actually enhance the sound of your recordings. The same could be said for designs from Neve which leant into the sound of the transformers, giving you another type of harmonic distortion and is one of the most desirable sounds from a preamp you can get - but it does change the overall sound of the microphone by adding harmonics to the signal.
The simplest way I have found to test out my ears and figure out what I prefer with each microphone I've ever owned, is by using the cleanest way to record I can (i.e. my interface preamps which are designed to be as clean as possible), and then try out various plugins emulating each design of preamp to figure out what the best variant is for me. With plugins becoming much better at bringing the analogue sound into the digital domain, this has been the fastest route to learn what I like, and then save up to get the real thing!
Hopefully this will help you in choosing your next preamp, or upgrading your current preamps perhaps if you're only using your interface. Whatever the case, let me know if you enjoyed this installment, and what you'd like to read by writing to me at Harri@jzmic.com and until next week, stay creative!