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Old 26th June 2003, 09:57 PM   #1
lgreen is offline lgreen  United States
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Default Transformerless Amp - Possible?

This might be the stupidest question of all time, but here goes. Suppose you are building a high power SS amp, using around +/- 60 volts. Could you omit the transformer completely? Just put the raw 120 volts AC into a 600V, 35 A full wave bridge and you have a super powerful power supply with about 60 volts plus and minus?

Now I'm not saying this design would be great, it might be a bad idea from a noise and isolation perspective, but that's not my question. My question is whether you could build such a thing and have it work.

Has anyone tried this or even thought of it?
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Old 26th June 2003, 10:08 PM   #2
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One of my all time favorite threads
Still need help with my amp design
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Old 26th June 2003, 11:01 PM   #3
PRR is offline PRR  United States
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> using around +/- 60 volts

Actually you would get 168VDC. In 8Ω this gives over 400 Watts RMS sine power, almost 900 Watts in 4Ω. Pretty big amp.

The biggest drawback is that the speaker terminals are "live". Touching them is about like sticking your finger in a toaster or a light socket. With a bridge rectifier, neither side of the DC is common with either side of the AC, so even if you think you know which side of the AC is "Neutral" you still have high and possibly lethal voltages on the speaker terminals all the time.

Also you can't connect directly to a standard preamp without finding huge voltages across your shield and a short on the AC line. You have to bring in your audio with a transformer or other high voltage isolation.

It is an old-old idea. Most 5-tube kitchen radios and many tube TV sets were "hot chassis". Even with a half-wave rectifier, touching the chassis was dangerous about 50% of the time. These units always had closed plastic cases and interlocked cords so they could not be powered-up with the back off. And they got away with it because they had no external audio connections. Antenna was internal or could be transformer-coupled. No Line In or Out. Mostly no Speaker Out, though with cheap tube sets you could float the output transformer secondary winding if you tested it for hi-pot.
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Old 27th June 2003, 01:16 AM   #4
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thus isolation transformers...LOL....got zapped a few times...thats what made me so crazy..hahaha
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Old 27th June 2003, 01:55 AM   #5
karma is offline karma  Canada
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o ya joe i can do better than that. try working on a tube amp
when its running and grazing the 300v cap. than reaching for the
volume on the preamp. not a good thing. in one arm out the other
hurt like hell. i was lucky and stupid

i got some parts if you want em next time your in hamilton there free just e-mail me
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Old 27th June 2003, 07:39 AM   #6
djk is offline djk
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The Amazing Subwoofer from Bob Carver and Sunfire is a transformerless design.

An opto-isolator is used on the audio input.
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Old 27th June 2003, 10:18 PM   #7
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i read that other thread a little. the audiobahn alum12x only handles rated power in the smallest box, and probably isn't 96dB 1w1m as listed... i don't really trust audiobahn. the woofer did have good excursion qualities, but there was a lead problem where the leads would contact the metal basket leaving arcing marks... eventually the subs would just die... some love these subs others hate em.
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Old 27th June 2003, 10:57 PM   #8
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cool Karma....I`m just down the road in Caledonia

Cheers!!The DIRT®
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