120v powersupply


2008-01-11 7:14 pm
I'm new to building amplifiers so this will seem like a obvious question to most. I've got a 1000x2 factory amp that uses a 90v supply from the transformer to the rails. That's not too far from the 115v from the wall outlet. So of coarse I'm wondering if a person could rectify and condition it to be used as a supply, assuming the transistors were up to the job?
It isn't a issue of 'can' you do it, and yes line voltage can be rectified and filtered, but this is a very dangerous thing to do for this purpose. This bad idea comes around here and there on the forum, and the answer is always DON'T DO IT!!! If the polarity gets reversed between line and nuetral, then you could end up with 160VDC between the amplifier ground reference and the case(earth ground) of the source device, reciever, CD player, ect. This can present a leathal situation. Don't trust your life to some idiot who wired up the building, they won't be responsible.:dead:
IF you were to do something like that,and assuming that the power transistors,etc. in the amplifier could handle the voltage,you MIGHT be able to use a 1:1 (or 1:2ct'd?) mains isolation (or step-up?) transformer.The voltage would be the same,but you have the safety of a supply thats isolated from the mains. But remember,it's still a high voltage,so you need to be careful in that respect.

I've actually considered a +/-120V supply,using a large (surplus) isolation transformer..

The isolation from the line is the important bit,regardless of voltage.
The first rule of Fight Club is, you do not talk about Fight Club.
The second rule of Fight Club is, you DO NOT talk about Fight Club.

Discussion of non-isolated circuit like this is not allowed here... other than to request people to, for their own sake, and any children and loved ones who may come in contact with these killers, not to do it.
Note the drawbacks (before the thread is closed):

- You would have to add "mains grade" galvanic isolation to signal inputs, requiring either safety certified signal transformers or expensive linear optocouplers.

- You would have to ensure "mains grade" isolation between all live amplifier circuits and heatsinks and the metal case. This may call for 5mm or 8mm spacing between any live part and any earthed surface. Connecting the case to safety earth would be mandatory.

- You would need speaker wiring and connectors with "mains grade" insulation. Forget about banana plugs and binding posts.

- Speakers should have "mains grade" insulation too. The drivers should have to be recessed and/or hidden with the help of non-conductive horns or waveguides and/or a non-conductive safety grill.

These requirements limit practical applications to powered band-pass subwoofers with hidden driver and little more...

If you consider toroids rated at several KVA as too heavy and bulky (like me), SMPS is the alternative.

Finally, note that if you ever wanted to do some experimentation or reparing on any system intended for direct mains powering, you would have to buy and use 1:1 isolation transformers anyway because the *really* dangerous part is to work on a live circuit where every mistake can potentially kill you. The big transformers are required, no matter if you go one way or another. Furthermore, mains isolation is a must in order to be able to connect oscilloscopes and other grounded measuring equipment to the circuit.