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What Transformer to Use?

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Warlord_1011

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right I was talking in the chat section with some of the other members and i have gotten confused about what transformer rating I should get.

Right So It will be used in a 120W per channel Stereo Audio Amplifier.
It Produces 120 Watts into 8 Ohm, therefore 30.983 Volts, at 3.87 Amperes.

It requires a dual PSU at + and - 55 VDC, so before rectification thats about + - 38 VAC

so each secondary winding I need to be rated at 38 V, 3.87 Amperes, i.e 150 VA, is that right?

so In order for me to have a Dual PSU for a single channel I will need (2*38*3.87= 295 VA) around a 300 VA transformer, so for BOTH channels will i need around a 600VA transformer??
 
A class-AB amplifier is about 60% efficient when blasting at its rated power. So each 120W amplifier heats with 72W and draws over 192W from the transformer. The total power from the transformer is more than 384W if the amplifier is playing at full blast for a long time. The rectifier also uses power and gets hot so the total max power from the transformer is about 400W for a stereo amplifier.

Does anybody play an amplifier at full blast with a continuous tone? No. Music fluctuates its levels. Then you don't need a 400W transformer.
A 300W transformer might be fine.

A 120W amplifier has an output of 31VAC into 8 ohms which is 43.8V peak or 87.7V p-p. The output transistors have losses so to make 120W into 8 ohms, the supply needs to be 97.7V p-p which needs a transformer that is 36VAC for each polarity.
 
Yes and No. Note that something is wierd: 600 VA transformer and 120 * 2 watts or 240 Watts.

You could say that something is amis and it is. You need the current of 3.87 A to support the 8 ohm load: Using (I^2)*R or (3.87^2)*8. You also need voltage to support the load. If this were a custom transformer, you could get buy with 38 V * 4 @ 3.87 A; 300 VA, but that probably won't happen. 4 x 38 V would be how a torroidal transformer would be specified. You need the windings to support the current, BUT only one half can be active at anytime, therefore the core VA could be less.

I went the custom torroidal route, but didn't at the time (30 years ago) understand the significance. I went with 4 x 35 @ 3A and I had a supposed 100 W/channel amp. ( I^2)(R) is 9*8 or 72W. Interestingly enough, the guy that designed the amp (Marshal Leach - search for Leach Amp) suggested a 3 A total. In the article, he appeared to be driving electrostatc speakers, thus he didn't need the current. I eventually went with the 4 x 35V @ 3A with 9600 uf x 4 capacitors which gave me separate supplies for each amp. I also have a 500 VA Ac regulator on the input. I did use a 70 V CT (Constant Voltage Transformer) at 16 A for a while and the amp sounded really good, but the hum from the transformer was excessive. The AC regulator made a big improvement. The 4 x 35 V @ 3 A custom cost me $120, 30 years ago. The AC regulator, I picked up on Canal Street when I was visiting NY for $120. It retailed for about $1000. 40,000 uf of capacitance is pretty nasty to handle on power on, so I had to deal with that.

I would reccomend that you size the current for a 6 ohm speaker rather than 8. 8 ohms is the nominal impedeance of the speaker. The more current you can supply, the better the bass will sound.
 
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