Can someone take a look at this amp schematic?

Question

I'm trying to build a guitar amp, but the thing isn't working.

http://img45.imageshack.us/img45/4600/design2ve0.png

Here is the schematic. For some reason when i trace the voltage of the output (before the transistor) it idles around 8 volts and the voltage fluctuates very little. Any clue whats going on? Thanks for looking.

Answer 1

You are asking to much of the circuit. While it should work, all that you'll get out of it is a pre-amplification of the signal from the pick up on the guitar. The 2n3904 is a small signal transistor. It was never designed to deliver a lot of power to a speaker. You need to go back and get a different circuit if you want to be able to hear what you are playing.

Your 8 volts or so at the Base is the bias voltage that the 33,000 ohm resistor is producing. Your Base current is about 0.00026 Amps, or 0.26 Milli-amps. The capacitor that you are using to couple the pick up signal to the transistor Base MIGHT have to much capacitive reactance to allow more of the signal than what it does allow, to flow through to the Base of the transistor. In any case, the voltage swing that you mention is the "AC" signal from the pick up. This is not going to make the transistor voltage change. The change is going to be at the transistor Collector. As I have said, you built a preamp, for a more powerful amplifier. You now need a power amp to  go with what you have. This circuit is NOT a power amplifier, regardless of what you believe, or were led to believe.

Answer 2

This is a low gain low power amplifier. It has a voltage gain of about 7 with no load, and cannot drive any load with a resistance lower than about 10 k ohms.

This is a very poor example of a preamp, and is not a power amplifier, and cannot drive a speaker. Nor can it drive an 8 ohm resistor.

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