Are you looking for a signal to tell you the gain is set? Do you want a buzzer that says "stop turning that knob? The simplest and least expensive way to set your gains is with a piezo tweeter.
Setting the amplifier gain matches the input of the amplifier to the output of the deck. We know the amplifier gain is too high when the amplifier output distorts.
It is difficult for your ears to discern clean sound vs distorted sounds. Discerning distortion is even more difficult at high volumes. You will need a tool to help set your amplifier gains.
There are expensive tools that help with this process. They use a blinking light to tell you when the amplifier output distorts. These tools are great if you set gains on amplifiers day in and day out.
But do you need an expensive tool? What if you could use an inexpensive speaker that buzzes when the amplifier distorts? The answer is a piezo.
What is a Piezo?
A piezo is a crystal that gets excited when a voltage passes through it. Engineers figured out how to convert these crystals into speakers. Piezo tweeters were common in the 80s and 90s in full range car stereo boxes. Those piezo tweeters loaded into a horn assembly to play loud. Companies like Kicker and MTX would pair these with woofers for a full range car audio box.
One of the properties of a piezo tweeter is that it is capacitative. That means the circuit resistance increases as the frequency decreases. That means a piezo tweeter blocks bass frequencies without a crossover circuit.
When an amplifier begins to distort or clip, the amplifier produces harmonic frequencies. A harmonic frequency is a multiplication of the target frequency. An example of a clipped signal is the "crunch" of an electric guitar's over-drive circuit. While that is great for rock and metal music, it is terrible for your subwoofer and your amplifier.
So if we're playing a 40hz "natural" tone, the harmonics will be 80hz, 160hz, 320hz, 640hz, and so on. These harmonics will play through the piezo and sound like an audible "buzz". The piezo will play the buzz but not the main tone.
And they're cheap when you buy them in bulk from Amazon!
Youtuber Taylor Fade of Everyday Audios compares the piezo tweeter to the SMD AMM-1. You can hear the piezo buzz when Taylor pushes the amplifier to distortion. The light on the AMM-1 also turns on when the amplifier distorts. Give it a watch:
Here are the steps to setting amplifier gain with a piezo tweeter
- Unplug the speaker
- Plug in the piezo at the amplifier speaker leads
- Turn down the gain
- Play a 40hz tone
- turn the deck to 90% volume
- turn up the gain until the piezo clicks and back it off one notch.
- Tell your friends not to touch the gain knob. Ever.
Final Thoughts
Setting your gain is a necessary chore to ensure you are sending a clean signal to your subwoofer. One should make sure the gain is set in a way that will not damage speakers. If you have an amplifier that is more powerful than your speakers rating, you can use a digital multi-meter to set the gains.
