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Old 30th January 2008, 12:50 AM   #1
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Default 120vac straight from the wall rectified

How come you just can't take 120v straight from the wall, and rectify it, and hook it up to some big caps. Why the use of a trans former to get like 90vdc when you could build an amp with about 140vdc? with no transformer? Just a question I have wanted to ask for years.
Old 30th January 2008, 01:08 AM   #2
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I think it needs to be isolated for some reason... I'm not sure why, but I'd be quite interested to know also.
Old 30th January 2008, 01:10 AM   #3
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power from the wall is pretty dirty might not work so well.....
be interested to hear from some one that knows more or has tried...
Old 30th January 2008, 01:20 AM   #4
hermanv is offline hermanv  United States
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It all depends on whether the Neutral and Hot wires are kept straight ( and they often are not).

If you rectified the wall voltage and filtered it, the speaker lead could easily have 120VAC on it. The pre-amp output might find itself hooked to 120VAC on the power amp input and the end user might find himself dead when he grabbed the outer shell of his RCA jack.
Old 30th January 2008, 01:22 AM   #5
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Its still 60hz on the secondary side? It would be so nice just to take a dryer outlet, and have 2 separate 140vdc supplies for a killer amp. Even 120x20=2400va correct? Filter it somehow. chokes, and caps. I have a pair of speakers that have 20 drivers in each cabinet that would much appreciate a strong set of amps.
Old 30th January 2008, 01:54 AM   #6
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duplicate... sorry
Old 30th January 2008, 03:50 AM   #7
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because the mains is earthed if it is not isolated it is very easy to get electrocuted.

earth is attached to neutral or 'cold' and although it is used for safety it creates a whole lot of other dangerous situations.

eg. touch a live wire and the current can travel through you to ground and you get killed.

some old valve amps didnt use mains transformers and just straight rectified the mains. eg. the Harmony h400 guitar amp.

nowadays i'm guessing it would be law to have mains isolation on equipment wherever it is feasible.
Old 30th January 2008, 04:07 AM   #8
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he's right
Old 30th January 2008, 04:09 AM   #9
CBS240 is offline CBS240  United States
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This is a real bad idea and should be considered a NO-NO )

Unless you isolate the audio circuit from the line, you are setting yourself up to be hurt very badly or killed. It would be a tradgety to have a DIYaudio member check out due to a misguided action. BTW, a 20A breaker should trip somewhere around 18A continuous.
All the trouble I've ever been in started out as fun......
Old 30th January 2008, 04:18 AM   #10
Eva is offline Eva  Spain
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You are not proposing anything new. There are some application-specific very high power amplifiers done that way where the input signal is the one galvanically isolated. But for lower power consumer equipment it's much more advantageous and safe to have everything isolated and working on lower voltages.

Also, full-wave rectified 120V mains will produce anything between 140V and 200V DC, and half-wave will produce +/-140V to +/-200V. Both options are well above the voltage that can be reasonably handled with a class AB output stage. These voltages call for class G, H or D. Discard class G and H because a power supply is required in order to obtain the voltage taps. High voltage class D is not for beginners either, it has a tendency to explode in your face if you make any mistake

What do you expect to power with 160V peak? A 4 inch diameter voice coil (or bigger) is required to handle the resulting power, even with music. This translates into high power 15 inch and 18 inch bass drivers (there are bigger ones too...), not something everybody can have in his living room.

BTW: Discussion of direct amplifier connection to mains line is forbidden by forum rules.
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