My brother gave me a cheap phono preamp by Pyle
The only active device is a F4558 dual op amp. Aside from that, a couple voltage dividers and a few filtering capacitors and an "on" LED. The unit operates off of a 9v battery or a 12v (marked) wall wort that puts out about 19v at open circuit. 18.8v at input with amp on. Then it runs through a resistor and a diode to provide 12v to the device.
He says it doesn't work, but it appears to work to me, other than I think it is just a garbage device.
I have 3 pictures of the scope attached. All 3 are 2mv +/-. They are 100hz, 1khz, 10khz.
I can't understand how you can have an audio amplifier and not have a negative rail voltage - but that may just because I don't know what I am doing.
The 100hz seems like an acceptable output - 2mv in, 750mv out. Wave is slightly offset of center
The 1khz, the gain shrinks considerably and the output is completely positive - 2mv in, 200mv out. Wave is completely above zero
The 10khz I had to change the scale to 50mv/div, but it is has a considerable ?dc offset? - 2mv in, 50mv out, 80mv offset.
10khz is high for audio, but I am just trying to understand more than anything.
While technically the same tone from the speaker, this output would only push the speaker and not pull it - correct?
OP amp is 12v+ and ground for input. Audio goes in the non-inverting input.
I realized that I had the input probe set at 10x so that has the input wave scaled wrong, but the signal generator was 2mv
How did I get more than a 2mv negative wave on the output without a negative rail?