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Finding power for my project?

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spitso

Member
Hi their

I am building the 60W(per ch=120W) stereo audio amplifier based on the lm4780 ic for a project of mine. Voltage supply can be anywhere from 20V to 84V according to datasheet.

The biggest problem ive found with my project is providing the power for it any ideas?
I've been looking at a 160VA toroidal (two 25V windings in series for 50V) on ebay for around $55AUS. think you can find a less expensive alternative?

thank-you
 

BrownOut

Banned
You can try to find some busted stereo amps and salvage the transformers. To be a good hobbiest, you should be a good schrounger.
 

spitso

Member
Ive been ringing up dumps/tips and they dont let anyone schrounge around anymore. which would have been pay dirt for me :(
 

BrownOut

Banned
Where do you live? I can sometimes find them at salvage sales aka "yard sales" aka "garage sales." Also try charities, they often get busted stuff and are happy for someone to take them away.
 

microtexan

New Member
Amp Power supply

You can try to find some busted stereo amps and salvage the transformers. To be a good hobbiest, you should be a good schrounger.

To be a good hobbiest, you (should) must be a good schrounger. :D
 

crutschow

Well-Known Member
Most Helpful Member
Is that the same as a scrounger?
 

audioguru

Well-Known Member
Most Helpful Member
When the LM4780 has a plus and minus 35V supply, its output power at clipping per channel into 8 ohms is about 60W. If both channels have a little less than 60W then the heat is 136W which will overheat the amplifier and cause it to shutdown.

If both channels have 60W continuously then the transformer must supply 236VA.

But you will not operate the amplifier continuously at full blast so the transformer and heatsinks can be smaller.
 

spitso

Member
When the LM4780 has a plus and minus 35V supply, its output power at clipping per channel into 8 ohms is about 60W. If both channels have a little less than 60W then the heat is 136W which will overheat the amplifier and cause it to shutdown.

If both channels have 60W continuously then the transformer must supply 236VA.

But you will not operate the amplifier continuously at full blast so the transformer and heatsinks can be smaller.

I dont 100% understand?
If i put secondary of 25v 160VA toroidal in series i will obtain 50v which can output max of 3.2amps.
If i need 60w of power per channel then 60w/50v=1.2amps x 2channels=2.4 which means i still have 0.8amps to work with. Or is it more complicated than that?
 

audioguru

Well-Known Member
Most Helpful Member
50VAC makes a rectified and filtered 68.7VDC. You need a positive and a negative supply.


The power into the speakers is 120W and the power heating the amplifier is 116W so the total is 236W.
A 50V transformer must supply 236VA/50V= 4.72A RMS for as long as the output power is at 60W per channel.
 

stevez

Active Member
Not so long ago I purchased a 25.2 vac, 2 amp transformer from Radio Shack for about $10 US. Two of these might be sufficient - connect secondaries in series - the common point being more or less neutral or ground/earth.

Scroungers/do-it-yourselfers can often make do with a little more or less than good design might require. You might be well satisified with a $20 investment in a power supply that will run your amp at a fair rate of power - maybe not the full 60 watts continuous.

What I do not know - is the 2 amp rating for that transformer continuous. I also don't know the secondary impedance so I can't predict how far down the voltage will drop on momentary current peaks. I wouldn't hesitate to give it a try though.

Machine tools often use power for controls at 24 vac - not sure if they do that where you live though. That can be a possible source for transformers too.
 

audioguru

Well-Known Member
Most Helpful Member
The current rating of a power transformer is continuous current and the voltage when it has its rated current must be at the spec'd voltage.
If you overload the transformer with a power surge then its voltage will drop lower than its rated voltage.
 

BrownOut

Banned
Ok, now I'm confused. First you said that if both channels have a little less than 60w, the heat is 136w, which will over head the amp. Then you said if the total power is 120w, which I interpret as each channel having 60w, then the heating is 116w. How did the power delivered go up and the heating go down????
 

audioguru

Well-Known Member
Most Helpful Member
Ok, now I'm confused. First you said that if both channels have a little less than 60w, the heat is 136w, which will over head the amp. Then you said if the total power is 120w, which I interpret as each channel having 60w, then the heating is 116w. How did the power delivered go up and the heating go down????
Look at its datasheet.
The amplifier produces the most amount of heat when its output power is about half (30W) of its max possible undistorted power (60W).
With a 30W output and a 70V total supply its heat dissipation is 68W per channel.
With a 60W output and a 70V total supply its heat dissipation is 58W per channel.

But it doesn't matter because music is never played at full blast continuously by normal people. The peak sounds are at full power but they occur only occasionally.
 

stevez

Active Member
The current rating of a power transformer is continuous current and the voltage when it has its rated current must be at the spec'd voltage.
If you overload the transformer with a power surge then its voltage will drop lower than its rated voltage.

In my amateur radio reference material there is some mention of various ratings for power transformers as they relate to transmitters. One of the ratings was titled "intermittent service" and the explanation said it would deliver rated current just fine - but only for short periods - heating of the transformer was the issue. It also mentioned what I understood to be truly continuous service - 100% of the time at full load. As you already mentioned, audio amplifiers, like some transmitters, do not demand full power on a continuous basis. What I don't know is if there is currently a rating system such as I have described. I also don't know what kind of service one can expect from a Radio Shack transformer. I am guessing that it may not be heavy duty but it might be sufficient to power the proposed amplifier given all of the factors being discussed.

Good discussion - not intending to argue but rather share what I've come to understand.
 

spitso

Member
i kinda understand now. So correct me if im wrong but the 160va toroidal will be ok, because the amp wont be used at full power contiously, correct?
 

audioguru

Well-Known Member
Most Helpful Member
Your transformer will be 50% overloaded when the amplifier plays 60W + 60W. You might notice or you might not notice.
 
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