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Which VA rating for a toroidal transformer recommended to nrun this?

Count Yorga

New Member
Hi I wish to use these amplifier boards for various projects. I convert old radios etc into Bluetooth speakers.


I will use a 9v toroidal transformer with a bridge rectifier which will give me just over 12v. I have no idea what VA rating I require to comfortably run this. I will be using them with either 4 or 8 ohm speakers, not sure if this is relevant, but I need a VA rating which will comfortably run either, but have no idea which VA Transformers to buy.

Can anyone help? Thanks.
 

crutschow

Well-Known Member
Most Helpful Member
For the stereo amp with dual 30W output, the power required would be 60W/65% = 92W since a Class AB amp has about 65% efficiency.
The transformer VA rating should be derated about 60% when driving a bridge rectifier/capacitor supply, due to the high RMS transformer current such a configuration generates.
So the worst-case transformer VA rating should be 92/0.6 = 154VA..
But since music does not normally continuously generate maximum power, you could likely readily get by with a transformer of 100VA or less (such as 9A @ 9Vac).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Except the claimed power is basically imaginary, from a 12V supply and 4 ohm speakers you can only get about 16W per bridged channel.

I would suggest a 50VA amplifier should be fine for a bridged stereo amplifier.
 

Count Yorga

New Member
Thanks yeah I could probably get away with 50va as I'm not pushing the amp hard at all. The integrated volume potentiometer is set at about 75 percent, then the volume is controlled by a separate pot on the input line. It's only a small bakelite radio with three way bass, mid and treble speakers. Using one channel for mid and treble, the other channel drives the bass. Signal is converted to mono after the potentiometer on the signal line then using crossover filters to separate the frequencies. Sounds really crisp and clear with just a 12v Switiching PSU but I want to use a proper PSU with a nice filtered bridge rectifier. I plan to use the two rectified 12v outputs, one to drive the amp and the other to drive a Bluetooth card. The cost of 100va vs 50va is pretty insignificant so probably go for 100va for good measure.

Once I have a design I'm happy with I plan to use it on multiple projects. I'm just experimenting at the moment. Those lovely old radios are easily available and have nice big cabinets for speakers and they always have a steel chassis too. I'm sure purists will be horrified but I like to build things which are lovely to look at but are actually functional and sound great.

Thank you for your help with this.
 

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