Transformer used implications to output power?

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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 :drink:
 
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.
 
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?
 
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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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?
 
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.
 
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.
That is easily solved. Use an 8ohms speaker and you will get good professional usable 50W of power and your amplifier will probably last longer driving the 8ohms speaker. Guitar amplifiers can be driven hard for longer periods than for normal music reproduction !
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?
A pair of bridged amplifier rated for 100W into 4ohms is actually two amplifiers each rated as 50W into 2ohms.
A 2ohms rating for an amplifier is a mighty onerous requirement, especially for live music PRODUCTION !!!!!

Bridging is an amplifier destroyer. Do not go into that without knowledge of what you are doing.
 
What about the 50VA trafo at 50W?
The normal recommendation for domestic duty ClassAB power amplifier/s is use one to two times the maximum total output power as your transformer VA requirement.

i.e. one 100W power amplifier works with a 100VA to 200VA transformer for domestic listening conditions.

PA duty is more onerous !!!!!!!!!!!!
Customers can be quite vociferous if the amplifier/transformer/speaker breaks.
 
AndrewT thank you for answers and advice.

I like the suggestion of using an 8 Ohm speaker, it did not occur to me, but it is so simple.

About the transformer power estimations...
For me I like the more scientific approach.
I would measure current drained at the maximum output (measured on a resistive load). This I think has to be measured for each amplifier topology. You cant generalize about this. Different topologies,classes will drain different currents, right?

Then according to the current measured I would pick a transformer (and a slow blow fuse). Just the next higher wattage (for example if I measure the total power drain of the amplifier at full load is 98 Watts, I would pick a 100VA transformer) right?

Btw the transformer (well of course depending on the build quality, and how much tought was put into it) should be a little powerful than the rating. Since when you properly build a transformer you pick a width of wire that is usually slightly thicker than calculated. Simply for the fact that rarely there is an exact width made, that you calculate. You simply pick the nearest highest width.


I read some articles about bridging amplifiers, can you elaborate more on this topic? What about the amplififer in my first post? Class D 50W+50W with a single rail supply of 24V, are there any special implications?

In the description is stated that "Maximum power: 2x50W / 8 Ohms
Mono BTL power: 100W / 4 Ohms"

Would I simply connect the speaker like this?

22222.png


Why wouldnt it be BTL 100W / 16 Ohms???
 
somorastik said:
Then according to the current measured I would pick a transformer (and a slow blow fuse). Just the next higher wattage (for example if I measure the total power drain of the amplifier at full load is 98 Watts, I would pick a 100VA transformer) right?
Wrong. A continuous DC power draw of 100W from a capacitor input PSU needs a transformer rated at around 200-300VA. This is because the current is taken in short pulses.
 
In the description is stated that "Maximum power: 2x50W / 8 Ohms
Mono BTL power: 100W / 4 Ohms"
this seems wrong to me.

If the amplifier is rated as 50W+50W into 8+8ohms, then when bridged it is indeed 100W into 16ohms. Bridging gives double the power into double the impedance, no extra power is magically created.

The 100W into 4ohms seems to be for a paralleled output, not bridged. See a pa100 chipamp and compare to a ba100 chipamp to see the differences.

Does the user manual show an arrangement for paralleling the outputs?
 
Btw can i bridge output of this (or in fact any stereo common supply) amplifier?
TDA7492 2 x 50W D Class High Power Digital Amplifi

It is an already bridged amp. 4 power stages, 2-2 in bridge, you can not bridge further. The mono option is Paralleled Bridge Tie Load.

Everything is on the datasheet.

ClassD with normal, slightly overdriven music and speaker load consumes very small average power. Absolutely no danger of overload as long as there is no short circuit. Peak power must be ensured by a big capacitor.
 
But please be very sceptic, when reading descriptions of sellers! They are misleading often. In this case they mixed datas not corresponding to each other. 50 W is the maximum of the chip, but not on 8 ohm with 24 V PSU. You need more voltage or less impedance. Use basic math, or check datasheet!
 
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For an amp to output 100W, powered by a 100VA transformer/PSU, it would need to be 100% efficient and the load would need to be non reactive (power factor =1).


If the amp is only 60% efficient. At 100W the transformer will output 160VA. 100W for the load and 60W for the amp.

Speakers are a reactive load. The Power Factor is the cosine of the phase angle between current and voltage. A power factor of 0.8 means that the PSU needs to supply 125VA to output 100W.

Then there is the choice of voltage and current. A 50V 2A transformer is not going to be able to supply 100W to a 4R.
 
Coping with reactive loads etc. is a matter for amp, not the PSU. Mismatch between supply rail voltage and load could also be regarded as a matter for the amp - it could use a DC-DC converter or bridging or an output transformer.

No, the real issue is the power factor of the typical cap input PSU. That is why, as I said, a 100W continuous DC power draw may need more than 200VA from the transformer.
 
the real issue is the power factor of the typical cap input PSU. That is why, as I said, a 100W continuous DC power draw may need more than 200VA from the transformer.

Yes, for continuous 100 W DC power a 200 VA transformer is needed. But 90 W continuous real power output from the amp can burn 2 pcs of "200 W" rated speakers. Music signal doesn't require high effective power, but very high peaks.
 
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