20watt amplifier

[shreya]

Could somebody provide a 20watt amplifier that works with 12v (Only transistor based not chip)

[Bigun]

power into a speaker is usually quoted rms and is calculated from P(rms) = (voltage * voltage) / (speaker impedance * 2)

So, if you have 12V and I assume that means +/- 6V peak to peak, less say 1V for drop across a power device (optimistic) then you have +/- 5V.

To get 20W rms you need a load impedance of less than 1 Ohm.

Seems like a tough call with most speakers - looks like you will need to increase the voltage.

[tinitus]

Jean Hiraga's Monster is 8watt, on 12V

you could use one of those car switch mode
converts 12V into higher voltage

41hz sells classD kits for 12V

[john_ellis]

Two ways to do this, assuming you want to use standard 8 ohm speakers.
1. Use an old-fashioned transformer output stage. You will almost certainly need to wind a suitable transformer but the core size would be around 29 (EI100) or bigger depending on the frequency response you wanted
There are some companies that might wind the transformer(s) for you but they won't be cheap.
2. Use an inverter
I've built two ferrite-core based inverters one using a saturating core (eugh!) and one using a low power saturating core driving a bigger non-sat transformer (better) - these aren't as flashy as a MOSFET inverter but they are simple (sort of) and fairly foolproof.
Both produced +/- 35V from 12 V but you maybe only need +/-25V. Both have worked reliably for several years. Both need filter chokes in the audio leads, so though it is in principle simple there are four coils to wind!.
John