I’ve seen many questions on the internet relating to preamps and if they can drive speakers. Some people may be confused about the premise of powered amplifiers and preamplifiers, so I wrote this article to illustrate what preamps and amplifiers are and the differences between them.
A preamp increases a low-level input signal to that of a line-level signal utilizing transistors. The reason it cannot drive speakers is that the amount of power a preamp pushes out in terms of watts and amps are just not strong enough when compared to a power amplifier.
We are going to take a look at what a preamp is, what it does, and what it is used for. Then we will be able to understand why we can not use it to power loudspeakers (studio monitors, PA speakers, or home entertainment speakers). We will then take a look at amplifiers, what they are, how they are used, why we need them, and what makes them able to drive loudspeakers.
Why won’t a preamp power speakers?
A preamp, we can kind of say, is a smaller version to that of an amplifier. Amplifiers are the daddy, while a preamp is the baby. Preamps input signals are always a low-level signal, usually that of a microphone. These signals are small and need to be boosted in order to be used.
They only have to be boosted a small amount in order for them to reach a line-level signal. Preamps are designed this way, and the power needed to drive our loudspeakers are just not available from that of a preamp.
By understanding these concepts we will be able to understand why preamps are not able to drive speakers (active or passive).
What is a preamp?
Take a look at my article here, where I dive deep into what preamps are and how they work.
A preamp is a specifically designed type of amplifier that is made to take a weak signal and boost it dramatically so that that signal can be used for recording, mixing, and playing.
A preamplifier and a powered amplifier (amplifier) are two different things. They are similar in how they are built and what their primary function is. However, they are used for very different things.
When thinking of a preamp you can think of an audio interface or a mixing desk.
Watch White Sea Audio explain what a preamplifier is.
What is a preamp used for?
A preamp, as we said, takes a low signal and boosts it so that that signal can be used for audible recording, mixing, and playback through an audio system or application.
To record or to set up a live sound we need to mic up whatever instrument we would like to record or play with. For that we use microphones. No matter what instrument you want to record or play with at a live event you need it to be “miced up” and we use microphones for this. There is no other way a signal can be processed from a sound wave into a signal that an electrical device can use.
The signal a microphone converts the sound waves to is a very weak signal and thus the preamp enables us to take that signal, boost it, and then push it down the chain of our gear to be used.
Essentially a preamp boosts a weak signal to line level. Let’s look at what an amplifier is and what it does now.
What is an Amplifier?
A amplifier is a device that is used to boost a line-level signal so that we are able to use that signal and push it through our speakers to get varying levels of audible sound.
An amplifier has a two-port electronic signal. It uses direct power from a power supply, and it increases the amplitude of that signal. Then that signal is applied to its input terminals and produces a proportionally greater signal at its output.
An amplifier can raise the voltage of a signal without changing the current, and it can raise the current of a signal without changing the voltage, or it can raise both. It can do this because of its external power supply that it draws an electrical current from.
What is an amplifier used for?
As we said, an amplifier boosts a line-level signal so that our speakers can use it. This is different from a preamp because a preamp boosts a low-level mic signal to a line-level signal that we can then process however we want.
In layman’s terms, it is used so we can increase or decrease the volume we hear when we listen to audio through speakers. This could be studio monitors in your home studio. Studio monitors have built-in amplifiers. It could be a set of speakers in a PA system used for live sound, and then it could also be just your ordinary home entertainment speakers used to listen to music or watch TV.
Why can a power amplifier drive speakers?
Most amplifiers use a component called a transistor that allows them to perform the function we need them for. A Transistor, in its simplest definition, is a valve for electricity.
A transistor is made up of 3 parts, namely the input, the output, and then the valve. There are jargon terms used for these 3 parts, but we do not need to go into that in this article. All you need to know is that by applying a small voltage to the valve of the transistor, we can control a larger circuit and the current there within. This is how an amplifier works and is able to drive and give you various volumes on your speakers.
Watch AKIO TV describe what an amplifier is.
How to choose the right amplifier to drive your speakers?
It’s actually not difficult to determine what amplifier you need with regards to your system. There are basically only two things you need to take into consideration when considering an amplifier. They are:
Speaker Impedance
Speaker impedance when considering passive speakers (active speakers will have power amplifiers built into them) will come in the format of a number variable. This will usually be either 4ohms or 8 ohms. Amplifiers will also be rated typically in 4ohms and 8ohms. Therefore you can simply match the ohms on both the speakers and amplifiers to get a match.
Speaker efficiency (sensitivity)
There are a few technical aspects we will not get into in this article; however, let’s take a look at something you may have seen on an amplifier with regards to speaker efficiency. You might see something like this: 97db @1w/1m, on an amplifier.
This little line of numbers tells us that the amount of power (volume) your amplifier can press out at 1 watt over 1 meter (approximately 3 feet) is 97db. If the db is lower, then the more power the amplifier would use to drive the speakers. If the number was higher, then the amplifier would use less power to drive the amplifier.
Watch Emotiva Audio explain how to select a proper amplifier.
What is the difference between a preamp and an amplifier then?
Both a preamp and amplifier utilize a transistor in order for them to function and created the boosted signal we are looking for.
So taking all the information we have acquired, the main difference between a preamp and a powered amplifier is the amount of power between the two.
We know now that a preamp boosts a low-level signal into a line-level signal that we can use for various applications down the chain of our electronic devices. Then that signal is boosted by an amplifier to drive the sound through our speakers. Those speakers can be studio monitors, PA system speakers, or your home entertainment system.
Why can a preamp not drive speakers?
The main reason why a preamp can not drive speakers is that they are built to raise low-level signals and the components that are used in a preamp are just not powerful enough to drive speakers in terms of watts and amps.
Conclusion
In conclusion, we come to understand that an amplifier uses a transistor the same way a preamp does to raise a signal. However, an amplifier is built with a higher impedance value and is built with more power than a preamp in terms of watts and amps.
Therefore a preamp is simply to weak to drive speakers.