50 Watt RMS MONO Audio Amp 12 volt DC

Thread Starter

teliocide

Joined Sep 26, 2013
76
Greetings

I am searching for a 50 Watt RMS MONO Audio Amp 12 volt DC
It is needed to drive an alarm system - not HiFI
The Ebay units I have found require an AC power supply

Suggestions please
 

BobTPH

Joined Jun 5, 2013
3,315
Without a voltage boost circuit, the impedance would have to be 1.4 Ohms.

That is with a 100% efficient class D amp.

Bob
 

crutschow

Joined Mar 14, 2008
27,212
A bridge output amplifier will output, at best, a 24Vpp signal into the load with a 12V supply, so the maximum theoretical RMS power into an 8 ohm load is about 35W.
The power you can achieve is determined by the impedance of your alarm load.
 

Audioguru again

Joined Oct 21, 2019
3,186
The impedance of your speaker and the power supply voltage determine how much power an amplifier can produce.
Most car speakers are 4 ohms and most car amplifiers are bridged (actually 2 amplifiers) so the output power is about 14Watts.
Many car amplifiers advertise 35W (35 whats) which is the peak power (double the real power) with lots of distortion making the number bigger.
A truly powerful car amplifier uses a circuit to boost the power supply voltage.

1) What is the impedance of your speaker?
2) Post a link to a 100W amplifier.
 

sghioto

Joined Dec 31, 2017
2,363
The most you're going to get at 12V is 30 W.
Correct with that amp.
Back in the day I remember the Federal Signal police sirens putting out 55 watts RMS into a 10 ohm resistor in the siren mode.
25 watts in the PA mode.
That was running off 13.8 volts. The amp was push pull driving a transformer.
1613585239565.png
 
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Audioguru again

Joined Oct 21, 2019
3,186
I you crank up the output level so that it sounds like a buzzer then the output per channel is 10W at the fundamental frequency plus another 10W for the distortion harmonics.
Adding the second channel you get 40W.
Increasing the supply voltage to 14.4V when a car battery is charging then you melt the IC after it produces 80W.
 

Audioguru again

Joined Oct 21, 2019
3,186
RMS power into 8 Ohms from 12V comes out to 9W by my calculation:
(12 x 0.707) ^ 2 / 8 = 72. / 8 = 9
Bob
Your calculation uses the peak voltage but the peak-to-peak voltage should be used. Then the power in 8 ohms if the losses are ignored is only 2.2W.
Most modern (for at least 30 years) car amplifiers are bridged. Bridging uses two amplifiers, one for each speaker wire with one amplifier inverting. Then the voltage across the speaker is almost doubled resulting in about 3.5 times more power than a single-ended amplifier, about 7W into 8 ohms or 14W into 4 ohms.

A car battery is dead if its voltage is only 10V. It charges at 14.4V, then the bridged amplifier power into 4 ohms is about 18W to 20W.
 

Thread Starter

teliocide

Joined Sep 26, 2013
76
I think I failed badly with my original post and people got mislead and focused on the arithmetic and therefore the apparent ignorance inherent in my post.
I do understand that 50 Watt RMS MONO Audio Amp 12 volt DC is not possible but many amplifiers sold are described as this.
Many Ebay sellers are very liberal with their interpretation of science.
There are many amplifiers based on TDA7293 but the ones I have found require an AC power supply.
If needed a DC-DC 12V TO 24V Volt Step Up Converter could be used.
I am simply trying to keep it all DC and loud enough to be an effective alarm at 20 meters and built from off-the shelf units.

Thanks everyone
 

Audioguru again

Joined Oct 21, 2019
3,186
The TDA7293 and most other amplifiers are powered with a fairly high DC voltage.
The datasheet shows a plus and minus power supply and a single positive supply that you can use.
Instead of using the AC power terminals you can connect the output of your DC to DC voltage converter to the rectifier bridge wires.
 

MisterBill2

Joined Jan 23, 2018
8,760
Moost alarm siren/speakers are driven directly. So is there a reason that you want to use an amplifier? Is the sound more than just a wailing siren sound? Certainly an H-bridge amplifier can deliver a lot of power, and if tou use a transformer output you can have a lot more than 50 watts from a 12 volt powered amplifier.
 

PeteHL

Joined Dec 17, 2014
358
Correct with that amp.
Back in the day I remember the Federal Signal police sirens putting out 55 watts RMS into a 10 ohm resistor in the siren mode.
25 watts in the PA mode.
That was running off 13.8 volts. The amp was push pull driving a transformer.
View attachment 230689
Did that use what is called a Parallel-DC, Series-AC amplifier? If so, would that be today an outdated method to achieve a high output level from an amplifier with relatively low power supply voltage?
 

Audioguru again

Joined Oct 21, 2019
3,186
A bridged amplifier is used in almost all car radios for many years. it uses two amplifiers (one inverted) for each speaker, to drive each of the two speaker wires to effectively almost double the voltage swing and produce almost 4 times the power of one amplifier driving the speaker. Also, the large output coupling capacitor is not needed to block the DC to the speaker.

I had a car that used bridged amplifiers driving 2 ohm speakers for lots of power.
 
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