Piezo horn, or is it a speaker ?

I have a single piezo horn in my system that I need to replace with 3 horns ( swim event starting horn ).

The existing horn spec is :

Impedance 1000 ohm @ 1 kHz
Voltage Nominal 15 V
Voltage Peak 35 V
Frequence Response 3 kHz - 30 kHz
Sensitivity 1W,1m 92 dB

and I am driving it with the attached circuit.

The tone and volume is perfect, but I need to replace this with 3 horns that I can position along the start line of the pool.

I have 3 other horns, but connecting these and testing gives a much lower volume - hardly audiable. The tone is correct, but the volume is way too low.

The only specs that the supplier can give me on the new horns is 75W.

I measured resistance over the terminals.

Old horn : no circuit over the terminals.
New horn : 4 ohms.

So what am I working with here ?

Is the old horn a piezo and the new horn a speaker ?

Any guidance on how to get the new horns working would be really appreciated.


Well, obviously 4 ohms isn't 1000 ohms and open circuit at DC, so yes, the new one is clearly not a piezo.

How about you show us a picture of them all? {Remember: perfectly focused and resolution no greater than 1024 by 768.}

I would have preferred if you had kept this in the previous thread.

And - just how did you wire them up? All in parallel I gather?

Apparently that H-bridge driver can't handle 2 ohms - which is not surprising.

The 4 ohm horns need to be connected in series to make 8 ohms - if the H-bridge can't handle them, you will need the LM386 after all.

Hi Paul

Thank You for your feedback.

Both horns look visually almost identical to the center pic on this page : Pro Sono & Lumiere

They are connected in parrallel ( same wire from H-bridge pin 3 with 150 ohm resistor to the + terminal on each speaker, and H-bridge pin 7 with 150 ohm resistor connected to the - terminals ).

I also thought about the possibility of the 3 speakers / horns being a problem, and disconnected the last 2, so only 1 speaker remained connected - no change in volume.

My supply voltage on the H-bridge pin 8 is 12VDC.

You say : Connected in series : so like this :

H-bridge pin 3 > 150 ohm resistor > horn 1 + pin.
Horn 1 - > horn 2 +
Horn 2 - > horn 3 +
Horn 3 - > H-bridge pin 7.

This should be no different to the above test I did by connecting only 1 of the speakers.

I also tried using only 1 output from the H-bridge to the speaker positive pin, and the negative pin direct to ground. Same volume.

I think I understand this now.

The piezo ( open circuit over terminals ) is a high impedance device, and only needs high voltage / low current to work.

The speaker ( 4 ohm closed circuit over terminals ) is low impedance ( 4 ohm ) and requires high current.

The H-bridge is capable of a 1 Amp output current up to 36 V. I am using 12V.

So while the piezo worked nice and loud on the 12V ( low current ), the speaker can only draw a maximum of 1A from the H-bridge.

The speaker is rated 75W, so at 12V it should draw a maximum of 6.25A.

But limited to 1A by the H-bridge, the speaker will be operating at 1 / 6.25 = 16% of its potential volume.

Does it sound like I am on the right path here ?

Solution would therefore be to use the H-bridge for the single piezo at the finish line, and to user a different component to drive the 3 speakers at the start line.

On second thoughts, the 3 speakers X 6.25A each = 18.75A current required ? Sounds like a dangerously high number to me. Maybe I need to look at replacing the 3 speakers with 3 piezo horns and keep the H-bridge.

Maybe I need to look at replacing the 3 speakers with 3 piezo horns and keep the H-bridge.

Well, that would make sense.

You are getting muddled!

Your piezo horns are rated for 12V or so, but not 75W. You can indeed parallel 2 or 3 with impunity.

Your other voice coil horns are rated for 75W, but not (limited to) 12V. Noting that we are going to be talking about AC RMS voltages and currents, not DC, then to deliver 75W to a 4 ohm device, Watts = Voltage2/R so V2 = 75 x 4 = 300, V=17 and current would be 4.3 amps.

If more practically, you delivered 12V peak to peak to one such speaker, you would be delivering about 4V RMS, 1 Amp and 4 watts. Put 2 in series and you can drive them with one half of your H-bridge using an output capacitor.


Thank You very much for the replies.

I do not understand the RMS calculations that you quoted, so that is on my ' to do ' list to research and grasp.

I do think that it would be easiest of I change the speakers to piezo horns. The circuitry is already in place and I just feel more comfortable working with the lower current.

Regards Dave