I'm OK with digital electronics, fully capable of popping a 2n2222 and a suitable base resistor onto a GPIO pin but analogue is still quite a mystery to me, hence I'd appreciate some design advice with a much beefier output stage to drive a 5w 8ohm speaker. I already have a 3W 4ohm speaker driven from 12v through a TIP102 and it makes a reasonable din, but two things concern me: a) putting too much current through the speaker coil. I think I may already be doing this - I checked a "speaker power" calculator site and it looks like I'm shoving 4.5W through it, given that there is no base resistor on the TIP102 (my signal source is already limited to 3mA) b) being too cautious and limiting the drive so it's not as loud as it could be...it's for an alarm that I'd like to hear throughout my house.

Basic parameters: The signal is out of a UM3561 which is controlled by a Wemos D1. The spec sheet shows the max output current is 3mA and my toy 'scope shows it to be a square wave 0v - 1v with a varying duty cycle (digital FM? - great name for a radio station...). I have easy access to 3.3v 5v and 12v. My parts bin has a few TIP102s and several logic level MOSFETs and a bucket full of small-signal BJTs)

I have some notion that I need to bias the TIP102 to be in its best operating region somewhere around the midpoint (i.e. 6v and "adding" the signal to it so it fluctuates between e.g. 5.5 to 6.5 volts. Am I talking nonsense and/or is there a better way?

There's also a slight hitch: The UM3561 has a low side MOSFET switch whose gate is tied to a GPIO pin so that I can power it on and off under program control. BUT if I connect the output direct to the base of the TIP102 the Vdd find its way through to ground and thus the alarm is always on, albeit at much reduced volume. I put a 1uF cap in series to fix the problem but I don't know what the optimum value should be...very low pF values stop any sound at all and higher uF lead to unacceptably long delays as the cap charges...

So, what I'm after is circuit that will get the most sound out of the speaker without popping it and using the parts I already have, i.e. I don't wan't the complication of an op-amp etc - I'm not after fidelity or low distortion I want a loud noise to scare burglars.

Here's the broad outline: and the one final simple question: How do I improve my output stage to get the most racket out of a 5W 8ohm speaker?

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Thanks in advance for any advice


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