O.T. Car Amplifier with preamp

In article , Jon writes

You won't find one, because it doesn't work.

Hi-fi loudspeakers are usually 8 ohms, and the impedance is too high for a 12V supply:

V=IxR and W = I^2 R

The maximum possible is around 18W per channel (peak output). With 4 ohm (car type) speakers you get 36W (theoretically). That's taking no account of the efficiency of hi-fi loudspeakers, which are typically very poor.

So the voltage rails in a decent small hi-fi amp are typically 30-0-30 or higher (60V. - a theoretical 450W per channel max.). Car speakers are

4 ohms or less so that you can get more power from lower voltage rails. The drawback is the need for much heavier speaker cable.

If you get a _good_quality_ car booster amp, it may well have a power supply that converts +12V to higher voltage rails, say 24-0-24 (or better). This isn't without problems, but it can help, and will let you drive more conventional speakers.

Beware though, as I've seen some truly awful designs of these things. I (stupidly) bought one from Maplins about 10 years ago that claimed to be

100W/ch MOSFET. This would've been a good thing for technical reasons. In fact it wasn't. It was a conventional class B transistor amp, with a solitary MOSFET in the power supply, capable of little more than 30W/ch at best. It's unstable, and goes into oscillation when connected to any inductive source, such as a transformer. Piece of rubbish really, and Maplins withdrew it the following year (annual catalogue).

It's a minefield, but you're probably best off sticking with 'car' audio components. for an easy life.

Regards,

Simonm.

Thanks for the information. I am having problems sourcing a car amp / preamp with 2 or more inputs at a sensible cost. An alternative might be to feed a conventional home amp with a 12v inverter. Don't much like the idea of 240v in a damp Landover though!

Oh well, I will keep looking.

Jon

In article , Jon writes

Now that one's easier!

Presumably you'll be wanting radio, tape and CD, or more than that?

A home made passive switch box will suffice if they all output around the same level. The quality will be as good as from the most expensive hi-fi (don't believe all that guff about expensive 'interconnects' - it's largely rubbish). Carry the earth straight through and use a good quality two-pole rotary switch to select L & R inputs at the same time.

If you need to control the volume put a potential divider after the switch (ganged stereo pot):

incoming signal----------+ | | | Ganged 10k | | log pot | | | Earth ------------------+------

This shows one signal connection for one of the channels. The earth goes straight through. As you look at the spindle of the pot, from the front, with the connections at the bottom, the connections are usually:

1. to earth 2.(wiper) to amp 3. signal source.

You can use a similar arrangement before each switch if you want, to match up signal levels, but you shouldn't need to really, as most things output at around -10dB which is the domestic 'line level'. I think you could get any bits you need from Maplins, but I seem to remember being told they've dropped some pots (potentiometers, AKA volume controls) from their range.

It sounds a horribly complex solution, and pretty inefficient electrically, but it will work.

Regards,

Simonm.