20W Power Supply for Stereo Amplifier

[ChrisETO]

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?

 

[crutschow]

They are non-isolated and pass (usually) the negative straight through. That's OK to reduce the positive side, but not the negative.

True.
I forgot about the common ground between the plus and minus.

 

[Nigel Goodwin]

Do you mean that he can't use two identical converters, or that it isn't possible to buy a positive and negative pair like you can with 78xx/79xx regulators? I can't find any negative ones.
I get the impression that he could use two positive ones, provided that the ground in the one for the negative rail was isolated - I'm not clear on what that means.

This solution looks more and more like a desperate bodge. If he was to use buck-converters, he might as well go the whole hog and replace the transformer as well by using an AC to 24-0-24V switched-mode-power-supply. I don't really think this is a rational choice. I think his only sensible options are 1) and 2) from post-11:

1. Exchange the transformer for a 24-0-24 one and use 2 regulator chips.
2. Check whether the amplifier could manage on a higher supply voltage and use 2 regulator chips.

Neither are sensible options - either get 2 x SM 24V mains PSU (nice and cheap), or get the correct size transformer - NOT 24-0-24, which is creating the same problem again. You don't need regulators, you need the proper transformer.

Here are the 24V SMPSU's I used (two off) to power a mixer:

https://www.aliexpress.com/item/1005005980518269.html

They do three types of 24V ones, 2A, 4A, and 6A - for a mono amp the 2A should be fine, for a stereo amp I'd be happier with the 4A.

 

[ChrisETO]

He NEEDS a proper transformer, something like a 15-0-15 or 16-0-16,

I've also looked at ChatGPT, have you any comment on its analysis?

Let’s calculate for 16-0-16 V or 17-0-17 V transformers:​

Rectified DC voltage after smoothing:​

• 16 V RMS → peak = 16 × 1.414 ≈ 22.6 V → after diode drop (~1 V) → ~21.6 V DC
• 17 V RMS → peak = 17 × 1.414 ≈ 24.0 V → after diode drop (~1 V) → ~23 V DC

Is this enough for 78xx/79xx regulators?

• Standard 78xx/79xx regulators need ~2 V to 3 V of dropout voltage.
• So, for +24 V output, 21.6 V or 23 V DC is NOT ENOUGH — you can’t get +24 V out from 21.6 V or 23 V input.
• Verdict: 16-0-16 or 17-0-17 V transformers will NOT give you regulated ±24 V DC rails with standard 78xx/79xx regulators.

 

[rjenkinsgb]

Does the PSU actually need to be regulated, or will just plenty of smoothing do??

Without regulators causing extra voltage drop, a 17-0-17 or 18-0-18 (commoner) would give something around 24V.

You do also have to allow for the transformer regulation factor. The rated voltage is at full load current; the off-load voltage will be somewhat higher.
The factor percentage should be in the transformer data sheet. Generally, the bigger the transformer, the less the off-load change. A 100VA one I have here has 7% regulation - the output is around 7% over the rated voltage off load.

 

[Nigel Goodwin]

I've also looked at ChatGPT, have you any comment on its analysis?

Let’s calculate for 16-0-16 V or 17-0-17 V transformers:​

Rectified DC voltage after smoothing:​

• 16 V RMS → peak = 16 × 1.414 ≈ 22.6 V → after diode drop (~1 V) → ~21.6 V DC
• 17 V RMS → peak = 17 × 1.414 ≈ 24.0 V → after diode drop (~1 V) → ~23 V DC

Is this enough for 78xx/79xx regulators?

• Standard 78xx/79xx regulators need ~2 V to 3 V of dropout voltage.
• So, for +24 V output, 21.6 V or 23 V DC is NOT ENOUGH — you can’t get +24 V out from 21.6 V or 23 V input.
• Verdict: 16-0-16 or 17-0-17 V transformers will NOT give you regulated ±24 V DC rails with standard 78xx/79xx regulators.

I wouldn't ever use anything like ChatGPT, but I said 15-0-15 or 16-0-16, simply on the basis that 15-0-15 is more likely to be a standard transformer, and I thought 18-0-18 was perhaps getting a little too high.

So part 1) is theoretically partly correct, but offers transformers that aren't standard values, and would need to be custom made. The output voltages will be higher than it suggests though, as the rated value is for full load, and under minimum load will be a fair bit higher.

Part 2) is only there because you asked about voltage regulators - as I've repeatedly said, you don't need regulators if you use the correct transformer.

Check almost any amplifier circuit, it's EXTREMELY rare to find a regulated power supply - one of the few exceptions being the Quad 303 power amplifier, which used a single rail with a regulated power supply, presumably for hum reduction?.

Cost wise, the cheap Chinese SMPSU boards I suggested would be far cheaper than a conventional transformer, rectifier, and capacitors - and give a regulated supply, as a free bonus.

A 50VA 15-0-15 toroid from RS is about £24, then you need a bridge rectifier, and two large expensive reservoir capacitors.

 

[ChrisETO]

Does the PSU actually need to be regulated, or will just plenty of smoothing do??

I think he is pretty keen on that, but I'll check with him.

 

[Nigel Goodwin]

I think he is pretty keen on that, but I'll check with him.

What is the amplifier design?.

 

[ChrisETO]

.............................

 

[Tony Stewart]

For a Darlington Class AB BJT Amp drop +/-2.5 V from supply of +/-24V results in 21.5V driving stereo 8||8 ohms needs 5 3/8 A pk so the 6A supply will run hot but cooler than the 4A SMPS yet able to drive 115W /2 .

Automotive stereo amplifiers get 22W per channel using differential power amps rather than single ended using 14.2V.