anyone know anything about amplifier design?

[jgilbs]

[WARNING: non-engineering types may find this EXTREAMLEY boring]
ok, so im starting a new project for my x. for my apt next year i am building an mp3 player stereo reciever unit and before i tackle that project, i want to become familiar with basic amplifier design, so im going to build an amp for my car. So I plan on building a 100W amp(actually 2 50W amps with inverted signals and a common refernce - ie a "bridged" 50W amp) that will connect to a siren speaker (11 ohms, not standard 8, so this must be taken into account) so I can use the following:
1)Electronic airhorn(real harsh-souding electronic ones cops use)
2)Radio Rebroadcast for my scanner
3)PA system

so ive read up a lot on this and ive got a few ?s for anyone with some experience in this area

first(and easiest question i have) - for anyone who has heard an electronic airhorn siren tone, you know it sounds VERY harsh - much more irritating than a standard automotive or airhorn. I want it to be like this(the stock explorer horn doesnt command very much attention) - does anyone know WHY it is so harsh? I know a standard airhorn tone is 400Hz and 440Hz added together. Since this is a natural tone, it is a sine wave. Does the electronic version sound so harsh because it(possibly) is a square wave - and therefor more unnatural to our ears?

next, power supply. Can I just use an oscillator on the base of a power transistor(at 20v, 25a so the circuit is oversized to provide 300W) to pulse a 11.4v(~12v@60Hz) squarewave across the secondary of a 120v-12v torodial transformer(like use a transformer designed to step down 120v - do they work in reverse like this?) so that I have(with center tap) 60V, 0V and -60V. then use 2 4,700uF capacitors from both rails to the 0v reference to smooth out the sqaurewave(so the output voltage is about 60v peak to peak) so i now would have ~30v, 0v, ~-30v.

second, amp stage. i am going to take the input of the mic, use an op amp to provide some gain, then output that into a summing op am that will take it, the signal from the scanner and the output from the airhorn tone generator and add em all together. i am then going to use op amps with negative feedback at the input to provide inital voltage amplification. i am going to use 2, wired oppisite of each other so that I have to waves 180 degrees out of phase(youll see why later) for the supply references, should I use a resistor network across the 12v to provide 6v, 0v, -6v? or should i used the 30, 0 and -30 voltages from the output of the power supply?

next, current source. Do I use a resistor network across the 12V to create the current source? Or do I use the +/- 30V supply? Also, for the amp part, I am going to use a push-pull config for the transistors from the op amp's output to provide some additional gain. Do I bias the transistors across the 12v that the op amps signal is in reference to, or do I use the +/- 30V that I want to output from my output stage?

for output protection, can I just place a circuit breaker in series with the waveform output? or do I have to use one of the other fancy methods I've seen on other sites.

oh, and in case you were wondering why I am using a bridged configuration, heres why:
I park in a garage at an apt building. The way my spot is, it is quickest to actually exit from the enterance. So there might be times when I need to use the airhorn inside. If I were to use a 100w output, if someone was walking near when I hit the 100w airhorn tone, they could possibly suffer some hearing damage with the constructive interference from the echos off of the concrete walls in there. Also, when in traffic not responding to an emergency, if I want to tell someone to get moving when theyre stopped at a green light, I could use a lower power since I really dont need the warning power of 100w. The way I designed my console that the airhorn switch is on, it is a large montary switch with a SPDT connection. So I have two positions(up and down) I want to use UP for 100w operation, DOWN for 50w operation. I plan on doing this by using a simple SPDT relay to select between the 2 modes depending on what part of the switch was pressed.

well thanks for reading all of this, and I appreciate any help anyone can provide.

 

[briwayjones]

I know nothing about this stuff but it sounds interesting.

 

[MONMIX]

I can tell you going higher than 440 will be a good start.
A nice 1k to 2.5k would be good and irratating.

 

[collkid]

I also would like to learn about this...

 

[jgilbs]

here is what i want it to sound like:
http://www.autostyles.com/autoairhorns.htm

goto the one called MODEL 418 POWERHOUSE

i dont want to get this one cuz i want it to sound more electronic

 

[jgilbs]

ok, since a few people have expressed interest in this, im gonna start updating this thread with my progress(ill give some design notes now, and the next update when I start building the actual device).

turns out, on the inverter stage, transistors do not like to drive purely capacitive loads, so I am going to add some resistors to lessen the stress on the transistors.

i found a pretty stable current source and sink by using some transistors with feedback. this will provide a constant 3A at 34V, or any voltage that I need to get the 100W. Remember audio amps work by varying the VOLTAGE with a constant current to product output power - so current MUST remain stable. If it did not, then as the voltage varied, the current flowing would also vary per Ohm's Law (V=IR) and since Power = Current * Voltage - If they both varied, the amplification wouldn't work. So I'll have a current source wired to the positive 17, rail, a current sink at the -17v rail, both producing a stable 3A. The output stage will be a pair of voltage amplifiers that will supply the voltage to the speakers(from the current sources)