20W Power Supply for Stereo Amplifier

[ChrisOfBristol]

An acquaintance M is good at the practical side of building electronic circuits, but maybe not so good on the theory, so a friend used to help him with that. The friend wasn't able to help with this circuit, and M went ahead with building it anyway.

It is a two rail power supply for a 20W stereo power amplifier. Toroidal transformer, full-wave rectifiers and capacitors. He intended to use Zener diodes with resistors to stabilise the output voltage, but didn't know how to decide on their resistance.

He asked me. It looked a bit crude to me and I was concerned that there would be a lot of power dissipated across the Zeners. It's worse than I thought. Since the transformer is 30-0-30, the output of the rectifier and capacitors will be the peak voltage of 41V and since the output should be 24V, there will be a 17V drop which will mean about 14W dissipated per rail. Not sensible.

I would replace the transformer with something more appropriate (24-0-24?), but since he has paid over £100 for the transformer, I suspect he would be reluctant to do so.

Instead of the Zeners, he could use regulator chips like LM317/LM337, I think that the chip controls the voltage without dumping it as heat, so the 17V drop wouldn't be a problem, but I'm not sure, am I correct?

 

[Nigel Goodwin]

An acquaintance M is good at the practical side of building electronic circuits, but maybe not so good on the theory, so a friend used to help him with that. The friend wasn't able to help with this circuit, and M went ahead with building it anyway.

It is a two rail power supply for a 20W stereo power amplifier. Toroidal transformer, full-wave rectifiers and capacitors. He intended to use Zener diodes with resistors to stabilise the output voltage, but didn't know how to decide on their resistance.

He asked me. It looked a bit crude to me and I was concerned that there would be a lot of power dissipated across the Zeners. It's worse than I thought. Since the transformer is 30-0-30, the output of the rectifier and capacitors will be the peak voltage of 41V and since the output should be 24V, there will be a 17V drop which will mean about 14W dissipated per rail. Not sensible.

I would replace the transformer with something more appropriate (24-0-24?), but since he has paid over £100 for the transformer, I suspect he would be reluctant to do so.

Instead of the Zeners, he could use regulator chips like LM317/LM337, I think that the chip controls the voltage without dumping it as heat, so the 17V drop wouldn't be a problem, but I'm not sure, am I correct?

No you're not, it just moves the heat to the chips instead of the resistors - and you would need MUCH bigger regulators than those, and huge heatsinks.

He NEEDS a proper transformer, something like a 15-0-15 or 16-0-16, whatever he can find round there.

The existing transformer is MASSIVELY too high, you're going to waste loads of energy as heat, and the regulators are likely to be more complicated than the amplifier is.

To be fair, we've probably all done it at some time - I've still got a transformer that a friend and I bought back in the early 70's - we went to the nearest city, which had an electronics shop, and they hadn't got the transformer we wanted, so we got the closest 'next one up' they had. We made various failed attempts with it over the years (it was for a disco/PA amplifier), but it's still sat here on a shelf - I 'think' it's 45-0-45?.

 

[ChrisOfBristol]

What if he used 78xx/79xx chips, since they are fixed voltage, would that make any difference?
What is the advantage of chips over Zeners if they both just dump the excess voltage as heat?

 

[rjenkinsgb]

Linear regulators dissipate power in proportional to load current through them, * voltage drop across them.
No load, no heat.

A resistor + zener regulator has to have somewhat more than the maximum possible load current passing through the resistor all the time.

That current is then split between the zener and the actual load. All in the zener at zero load, mostly in the load when that's maxed out.

Fine for a few milliamps, but ludicrous for high currents.