Need help in designing my PSU

[adonix]

Hi everyone,

I need your help regarding my amp's PSU.
I'm intending to build a 6*30 Wrms at 8 ohm amplifier. The supply voltage is +-28v. I have done the following to compute the transformer size:
1- Compute the peak current for each channel: Ipeak = sqrt(2*Po/RL) = sqrt(2*30/8)=2.74 A
2- Since i have 6 channels, the supply should be able to deliver a peak current of 6* 2.74 A = 16.5 A!

This means that my PSu should deliver a peak power of 920 W!!

Am i doing the right calculation? Should i design my PSU basing on peak power or RMS power which is 56 (V) * 2 (A) * 6 (channels) = 672 W?

This is important since i'm hesitating between building a linear PSU or an SMPS. I'm rather tempted to build an AC/DC SMPS, but i need to know the right power before designing the circuit.

Thank you for your help

 

[Michael Bean]

Before giving advice, how are you planning to use this 6 channel amp, home theater or something?

Mike

 

[adonix]

Hi Mike,

This amp will be mainly used as a home theater set. However, if in some occasion i might need it in a party were it will operate at full power for few hours.

Adonix

 

[abraxalito]

For it to run at full power for a few hours continuously then I believe the thermal constraints will be the most onerous. You can start by assuming a worst case crest factor for your music (peak to average power ratio) - if you went for 10dB you'd have a fairly conservative design I reckon even with today's highly compressed offerings.

Then your average power output will be 18W for the max power of 180W. You then need to work out how efficient your amps are over the operating range. This isn't exactly a trivial calculation though so some approximation might be in order. Once you have the overall efficiency then you can start choosing transformers to be able to deliver the required power without overheating. Bearing in mind that most of the power goes into heating the amplifier heatsinks you can then size these accordingly.

 

[Michael Bean]

I'll assume you're using class A/B amps, which will give about 60-70% efficiency. So even if you draw max power from all 6 amps all at once (which isn't likely), that would be a worse case of about 300 watts from the power supply. So a 300VA transformer should work just fine, and if you want to be extra conservative, a 400VA trans would provide plenty of headroom. For that power level into 8ohms something like 2 X 3300uF filter caps for each amp should be OK, but 3900uF would be better. If you want use use just one pair of filter caps for all 6 channels, about 22000uF for each supply rail should work OK. You don't really need to worry too much about peak power from the supply, the filter caps will cover that. So I'd say that a 22-24volt X 2 @ 300-400VA transformer with a total of about 22000uF per rail should be about right. Understand that not knowing all the specifics about your amps, these are just rough "scratch paper" calculations so YMMV.

Mike

 

[abraxalito]

... that would be a worse case of about 300 watts from the power supply. So a 300VA transformer should work just fine, and if you want to be extra conservative, a 400VA trans would provide plenty of headroom.

VA is only directly translatable into W if the power factor is unity. With rectifier - capacitor power supplies, its not unity. A (very) rough rule of thumb is that the AC peak current will be 5X the DC output current. Seeing as the heating effect is I2R and the AC current is on a (rough, depends on ripple voltage) duty cycle of 20%, this gives a heating effect greater than expected. Given the party requirements I'd go for the 400VA as a minimum myself, though I'm a fairly conservative designer

 

[adonix]

Thank you guys for your reply.

Will the same requirements apply if an SMPS supply is used instead of a linear one?

 

[nuryev]

Hi, Abraxalito, I am considering building a stereo amplifer (BPA150 or BPA300) using LM3886.
Would the toroidal transformer seen here be adequate?
18+18V 400VA Toroid Transformer DIY HiFi Audio Amp 4218 - eBay (item 370319697684 end time Sep-04-11 17:21:35 PDT)

 

[abraxalito]

Both BPA150 and BPA300 are bridged designs? If so then this would be fine for one BPA300 (monoblock) or two BPA150 as a stereo pair. Its not going to allow you to swing the full 300W into 4R though as its a bit below the necessary voltage. I reckon you'll get around 250W into 4R from it.