I've made small amplifier for hand-made electric guitar - it is just two transistor cascades - and I've found one curious effect when increasing amplification. Some annoying noise (just typical electric interference from nearby devices) appears if I raise amplification on the last cascade, but not if on first.
And I don't understand why is it so.
Now here are details:
The amplifier is necessary because my hand-made pick-up has comparatively small coils (in comparison with normal electric guitar).
Here you see the amplification of each transistor is determined by relation between resistor in collector and in emitter.
E.g. with values shown, first transistor amplifies
1100/39 = 28 times, while second only by
1100/220 = 5 times.
If I increase amplification by changing 220 Ohm resistor to 68 Ohm - i.e. roughly 3 times - I get the noise described above - it occurs when the hand is close to or touches strings.
However there is no such noise if I raise amplification on the first transistor instead. I.e. I changed 39r to 4r7 and got louder but still clean sound.
Well... How does it happen?
Below is the picture of the whole assembly for better idea - power is from two batteries, to exclude noise from power supply...