Re: Bose Acoustic Wave

In article <dcid4g$djk$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Jem Raid <jemraid@xxxxxxxxxxxxxxxxxxxxx> wrote:
> I don't know the technicalities of circuits, but that did surprise me
> when I first saw the design, I do know that power supplies do need
> smothing caps, but not how to work out the value.

For a full wave rectified PSU, use approx 1000µF per 0.25 amp. So with
preferred values:-

125mA 470
250 1000
500 2200
1A 4700
2A 6800
4A 10,000
8A 22,000

The working voltage should be 1.4 times the off load voltage or twice the
nominal RMS - both approx, so just go for the available rating which
exceeds both of these.

However, you only need smoothing for the maximum power actually drawn,
rather than the output of the transformer which seems pretty potent. ;-)


> I thought the
> circuit, even with my limited knowledge was a joke at first. Building
> one, and you can lash one up in minutes, makes you realise that it's
> far from it, exept for the laughter at the fact that it does work and
> after running in will sound great :-)

> Apparently, within the design of the chip (LM3875) there is a ripple
> rejection circuit which takes care of the problem.

Probably for ripple from the car alternator, but then a car has a pretty
large smoothing capacitor in the form of a battery. And you've said there
is audible hum.

I'd also put 0.1µF ceramic disc capacitors across each leg of the bridge
rectifier to cut down RFI. Ie, 4 per rectifier.

> You would be very welcome, if you live near to the West Midlands to pop
> along and have a listen Dave.

Thanks very much, but it's a fair trip. ;-)

--
*The statement below is true.

Dave Plowman dave@xxxxxxxxxxxxxxx London SW
To e-mail, change noise into sound.
.

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