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Old 13th July 2016, 08:39 AM   #1
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Default Transformer used implications to output power?

Hello,

Recently I became interested in power amplifiers with over 100W. This implies a good supply.
My question may be basic, dumb even, but I would like to ask.

If I have an amplifier class B lets say 100W/4 Ohm output power. The recommended transformer for this amplifier is 120VA(secondary:2x30VAC)

What happens if I use a 100VA transformer with the exact secondary voltage?
What happens if I use a 50VA transformer?
What happens if the amplifier is Class D? Something like: Free shipping 1pc IRS2092 CLASS D Audio Power Amplifier AMP Kit 200W MONO Assembled Board-in Amplifier from Consumer Electronics on Aliexpress.com | Alibaba Group

Btw can i bridge output of this (or in fact any stereo common supply) amplifier?
TDA7492 2 x 50W D Class High Power Digital Amplifier Board AMP Board+ Radiator-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group

Cheers and thank you for answers and understanding
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Old 13th July 2016, 08:58 AM   #2
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Downrating the transformer but maintaining the same secondary offload voltage just means you'll get poorer and poorer regulation (aka voltage sag) under load. So assuming you just hit your 100W power target with a 120VA trafo, the 100VA will under-deliver by a small margin. The 50VA will under-deliver by a substantial margin. A classD amp is more efficient so you can get away with a smaller transformer.
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Old 13th July 2016, 09:01 AM   #3
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What does poorer regulation imply (voltage sag)? Will the output start to distort? Is the transformer at risk of destruction?
I dont mind if a 50VA transformer will underdeliver, but what will happen at peak signal input? Will the output be <50W???
Btw what about bridging the output of the amplifier I mentioned?
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Old 13th July 2016, 09:10 AM   #4
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Poorer regulation means higher output impedance of the power supply. So the more current that's taken the lower the output voltage.
Yes the output will be distorted at 100W with the 100VA trafo if you just manage an undistorted 100W with the 120VA trafo.
I've not followed your link so not going to answer about bridging.

No, there's no major concern about destroying the trafo unless you're running at high average powers for long periods, in such cases the transformer will overheat. Smaller transformers overheat faster.
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Last edited by abraxalito; 13th July 2016 at 09:13 AM.
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Old 13th July 2016, 10:49 AM   #5
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Note that running a 100W amp on a 120VA transformer assumes that the amp will be used for real music only, so a low duty cycle with occasional peaks. If you want continuous high power then a 100W Class AB amp may need a 300VA transformer.
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Old 13th July 2016, 11:02 AM   #6
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I want to use the amplifiers mentioned for instrument (guitar/bass) amplifiers.

You know the problem I try to solve is: I have a transformer of 100W and an amplifier of 100W/4 Ohm. Is the transformer going to get damaged? Is the signal going to clip/distort? When exactly?

Or another common situation. I have a transformer of unknown power (I can estimate by the core size), maybe with multiple winings, I dont know the exact rating of each winding (sometimes I can measure the wire width to estimate the winding current). I can measure the AC voltages of course. And I want to use this transformer for an amplifier.

btw what about bridging the amplifier I mentioned in the first post? Or what about bridging amplifiers in common?
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Old 13th July 2016, 12:50 PM   #7
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If you run continuous loud music through a 100W Class B amp fed from a 100VA transformer then eventually the transformer will overheat and fail. Until then, you will get significant droop and clipping.

If you want reliable operation then use a 250VA transformer, and add a little series resistance to get whatever droop you require.

You can run a 100W 4ohm amp in bridged mode with another 100W 4ohm amp, to get 200W into an 8ohm speaker.
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Old 13th July 2016, 12:57 PM   #8
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And what if I run lets say a 100W amp (2x30VAC trafo) with a 50VA (2x30VAC trafo) at less then 50W (not to overheat the transformer)

btw a thermal protection switch (inside some trafos) should prevent from damage from long term overheating?
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Old 13th July 2016, 01:51 PM   #9
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Yes, but be aware that this internal safety switch is not always resettable. It is a fire prevention device, not a transformer protector. The transformer is expendable; your house is not.
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Old 14th July 2016, 11:38 AM   #10
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What about the 50VA trafo at 50W?
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