power amp

[audioguru]

Hi Daniel,
Your transistor circuit is a rectifier. For the half-cycles of signal it passes, it is an emitter-follower without any voltage gain.
A piece of wire would work much better as an audio amplifier. :lol:

What did you feed to your transistor? Was it the speaker output of an audio amplifier? Since it sounded "perfect" then this transistor must be shorted, base to emitter.

 

[Someone Electro]

Burned out transistors apear shorted too.

Just add an op-amp and it sod work beter.

His speaker is 8 Ohm so there will be much lees heating than mine wich was driving an 3,2 Ohm speaker(The speaker takes more of the voltage difrence and voltage diference makes transistors hot.)

 

[audioguru]

Hi Someone Electro,
Your 3.2 ohm speaker draws more current than an 8 ohm speaker, and the voltage output of the amp is the same for either speaker. Since there is more current in your speaker then its output power and the output power of the amp is higher. Your amp's transistors are supplying more current to your speaker so they heat more. :lol:

 

[Someone Electro]

Oh yea Ohoms law.Dumb me.

 

[audioguru]

Try shorting the output of your amp. The current will skyrocket to max and the negative feedback will sense that the output voltage is too low and tell the amp to supply more. :lol: :lol:

 

[Someone Electro]

That wod mean bye bye power transistor.

The heatsink is not that big ether.(It didnt need to be since in the PSU that heatsink was coled by a fan,in my amp it isent)

 

[audioguru]

At work, I hooked-up an amp that supplies 1200W RMS continuously at extremely low distortion to an 8 ohm speaker. The sales guy had a demo coming up so he asked me to connect two 8 ohm sub-woofers in parallel to it. The amp has a temperature controlled variable-speed fan and I knew it would be OK. It produced close to 2400W and shook the concrete floor!
I don't know if the lights were dimming in the building with each beat of the music, or if it was my eyeballs dimming with the vibration. :lol:

 

[Someone Electro]

I sure wodnt like to be in there wen that is playing!

 

[audioguru]

The sound was so clear that it didn't sound deafening at my desk near the closed showroom door. I didn't go in there.
I think distorted music sounds much louder than clear music at high levels.

 

[Nigel Goodwin]

I think distorted music sounds much louder than clear music at high levels.

Yes it does, it's why valve/tube amplifiers sound louder than considerably higher powered transistor ones.

 

[danielsmusic]

thanks guys,
i don't like op amps as they would not proguce a much current as a power transistor would.

 

[Nigel Goodwin]

thanks guys,
i don't like op amps as they would not proguce a much current as a power transistor would.

A transistor power amplifier is basically just an opamp with an output stage, usually a higher voltage one as well, as most opamps won't withstand the kind of voltages that a decent size power amp requires.

As suggested, your single transistor wasn't really anything, and wouldn't work as an audio amplifier - as also suggested, three transistors is probably the minimum you can get away with, and that's only really for very low powers.

 

[audioguru]

Opamps have plenty of voltage gain. Transistors are added to provide enough current but are not linear and therefore produce distortion. When negative feedback is added around the amplifier to reduce its voltage gain to a small usable amount, the distortion is reduced by the same amount of reduction. The result is an amplifier with as much voltage gain as you want, the ability to provide as much current as its transistors' and heatsinks' ratings, and very low distortion. A perfect amplifier if the opamp is any good. :lol:

 

[Someone Electro]

Thats exsactly what i did. An simple op-amp driving an power transistor.Worked astoundidngly well.For such an simple circuit.

I shod of aded an preamp betwen the power transistor and- the op amp

 

[audioguru]

An opamp's output current is only 10mA to 20mA but a power transistor's input current could be 160mA or more. Therefore usually driver transistors are used between them. :lol: