0
\$\begingroup\$

I am trying to build a very simple amplifier type device to power a 4ohm 150 watt piezo tweeter. I am getting my signal from an mp3 player and then want to use a voltage source, 9-20 volts, to amplify the signal to as much as possible and as simply as possible. The goal is 120db. I have talked to a few people and all give me different ways of doing it. The simplest way I was told is using a basic transistor amp circuit and it would work.

enter image description here

Can I use this very simple circuit and just replace the small transistor with a larger mosfet and apply my 9-20 volts? Will anything back feed into my mp3 player and destroy the output?

I don't really care about quality sound since it will just be constant tones coming out. I just need them to be amplified as much as possible and very simple.
Thanks

\$\endgroup\$
1
\$\begingroup\$

No, that won't work. The piezo is basically just an oversized capacitor and for a common emitter to work effectively it needs a certain amount of dc collector current . The piezo will totally block the dc.

I'd be tempted to use an audio output transformer or some kind of push pull amplifier.

\$\endgroup\$
0
\$\begingroup\$

A single transistor with a current gain of say 300 can be used to either impedance from input (10k?) to output (300) or amplify voltage but not both the product of change in V/R gain depends on the current gain of the transistor.

120dB gain means 6 decades or 1 million.

So you do the math, and look at existing solutions instead of trying to design a "square wheel".

If the transistor in your circuit had a current gain of 500 it would offer a 1/R gain of 100k/300=333 and a voltage gain in the range of 30 dB depending on device. close but no cigar

\$\endgroup\$
0
\$\begingroup\$

Your application is very similar to the outside siren used on intruder/burglar alarm systems, in the UK its called a bell-box. In these a piezo horn is driven by a step up transformer to give about 90V pk-pk. They can easily achieve 110dB at 3m, so with a larger piezo horn you should be able to meet your target of 120dB. In a siren the piezo horn and step-up transformer have to be designed to work together to get the highest sound output without damaging the piezo, which are easily damaged if overdriven. The transformer is usually driven by a darlington transistor such as a TIP121.

Although I have shown a transformer with separate primary and secondary windings in practice it is an auto-transformer with a common primary winding.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ Another person was showing me a design similar to this. Seems pretty simple. Could I use my music players line out for signal input somehow? \$\endgroup\$ – Doug B. Oct 7 '16 at 21:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.