120vac straight from the wall rectified

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ben62670

Member
2006-12-27 7:58 am
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.
 
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.
 

ben62670

Member
2006-12-27 7:58 am
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.
 

guitar_joe

Member
2007-05-04 2:54 am
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.
 
:att'n: This is a real bad idea and should be considered a NO-NO:whazzat: )

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.:smash:
 
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.
 

ben62670

Member
2006-12-27 7:58 am
OK I have a 4 1/2 YO, and 2 1/2 YO daughters. I will not build an amp like this. hmmm IB subs under a floor GFI 20 amp breaker. Never mind bad idea. Unless... Nope never mind.

I wanted to power these
DSCN0923.jpg


And the sub(or 4) in the center of the pic, and some surrounds of decent proportion, and a big center.

BTW they are in my office/workshop 10x10 doesn't give them enough room to breath even with only around 500 each it hits over 126bd(rat shack meter doesn't go any higher)

My whole idea is to build a huge multi channel amp that powers the stereo pair with 6 channels, 2 sub amps, 2 surrounds, and a center, and a couple extra channels. A transformer would be like a grand to put out that kinda VA. I like my Adcom GFA-585 the most, and I have over 400 sd424, and sd554 transistors, and I would like to do a one cabinet water cooled amp for all.
 
Yeah, suppose some moron replaced the receptical you plug your amp into and wired the hot to the neutral. You plug a CD player up into a different receptical. Your amplifier ground reference would be 160V from the case of the CD player. Even if you don't touch the CD player case and get the pis$ shocked out of you, when you plug it to the amplifer, something is going to cook.


It would be unethical for this forum to promote such a thing and any post suggesting it a good idea should be removed.
 
ben62670 said:
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.

The reason is that we want to avoid getting people killed...

dave
 
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