Plugging subs directly into 120V

Inductor128 · 5th October 2008, 12:07 AM

Hello everybody. I've been reading this forum whenever it came up in a Google search, and it seems to always have the most knowledgeable members.

I am planning to build a class D amp for a sub I am building, and I want it to be as cheap and powerful as possible. Therefore, I am planning on foregoing the power transformer and simply rectifying the wall voltage and using 120V for the supply rails. I will have a lower voltage power supply of course to power the IR IC's.

I haven't tried doing a class D project before so I will probably be on here a lot as my design progresses.

I recently ordered four Power Acoustik 10" 800 watt RMS subs from amazon.com:
http://www.amazon.com/Power-Acoustik...3090994&sr=8-2
I think they are an incredible deal, especially due to the low shipping ($9) but it worked, they did ship them for that price. The resonant frequency of the speaker is a little high, but when $160 gets you 3200 watts RMS of speakers shipped to your door, I can't complain. Oddly, they say they have been on amazon.com for several years, but I can't find reviews of this speaker anywhere on the internet.

I figure if I make an amp that runs on 120V, I should have speakers I can plug directly into the wall. There are a few YouTube videos of people doing that, but they are deliberately trying to blow their speakers. I figure, however, that if I wire up these speakers as an 8 ohm total load, 120V RMS should put out 1800W RMS. These speakers can handle 3200W RMS total, and therefore should take it no problem.

I kind of feel like there will be more voltage there though. With a full bridge layout, does the speaker just see a max of the supply voltage in alternating current form, or is it twice the max? A full bridge is constantly reversing the supply rails to the speaker. If I use a 120V supply rail, can the speaker potentially see 240V RMS? I guess I don't really understand how a half bridge works; to me it seems like kind of a stupid idea.

So my questions are: Did I do my calculations correct? Can you apply Ohm's Law to alternating current like that, or is it an integration problem? Also, is designing a Class D amplifier with no power transformer a retarded idea, or an awesome idea? And, with a full bridge layout, is the speaker really seeing 120V RMS, or 240V?

Thanks for your help.

μH

theAnonymous1 · 5th October 2008, 02:02 AM

Quote:

Originally posted by Inductor128
I am planning on foregoing the power transformer and simply rectifying the wall voltage and using 120V for the supply rails.

This is potentially lethal and a "forbidden" subject on this forum. This thread will probably be closed.

DigitalJunkie · 5th October 2008, 02:24 AM

From one Portlander to another, DON'T!

Atleast use an isolation transformer to isolate you from the mains wiring. What you propose is *very* dangerous.

Pano · 5th October 2008, 04:24 AM

I've done this with tube amps (220V mains). NOT A GOOD IDEA.

All I killed was my CD player, but it could have been me or my kid. Stay away.

lumanauw · 5th October 2008, 05:00 AM

Hi, Inductor128,

This is a very dangerous and lethal practice. Please don't even try it.

This thread will be closed now.

5th October 2008, 12:07 AM	#1
Inductor128 diyAudio Member Join Date: Oct 2008	Plugging subs directly into 120V Hello everybody. I've been reading this forum whenever it came up in a Google search, and it seems to always have the most knowledgeable members. I am planning to build a class D amp for a sub I am building, and I want it to be as cheap and powerful as possible. Therefore, I am planning on foregoing the power transformer and simply rectifying the wall voltage and using 120V for the supply rails. I will have a lower voltage power supply of course to power the IR IC's. I haven't tried doing a class D project before so I will probably be on here a lot as my design progresses. I recently ordered four Power Acoustik 10" 800 watt RMS subs from amazon.com: http://www.amazon.com/Power-Acoustik...3090994&sr=8-2 I think they are an incredible deal, especially due to the low shipping ($9) but it worked, they did ship them for that price. The resonant frequency of the speaker is a little high, but when $160 gets you 3200 watts RMS of speakers shipped to your door, I can't complain. Oddly, they say they have been on amazon.com for several years, but I can't find reviews of this speaker anywhere on the internet. I figure if I make an amp that runs on 120V, I should have speakers I can plug directly into the wall. There are a few YouTube videos of people doing that, but they are deliberately trying to blow their speakers. I figure, however, that if I wire up these speakers as an 8 ohm total load, 120V RMS should put out 1800W RMS. These speakers can handle 3200W RMS total, and therefore should take it no problem. I kind of feel like there will be more voltage there though. With a full bridge layout, does the speaker just see a max of the supply voltage in alternating current form, or is it twice the max? A full bridge is constantly reversing the supply rails to the speaker. If I use a 120V supply rail, can the speaker potentially see 240V RMS? I guess I don't really understand how a half bridge works; to me it seems like kind of a stupid idea. So my questions are: Did I do my calculations correct? Can you apply Ohm's Law to alternating current like that, or is it an integration problem? Also, is designing a Class D amplifier with no power transformer a retarded idea, or an awesome idea? And, with a full bridge layout, is the speaker really seeing 120V RMS, or 240V? Thanks for your help. μH

5th October 2008, 04:24 AM	#4
Pano diyAudio Moderator Join Date: Oct 2004 Location: North Carolina Blog Entries: 4	I've done this with tube amps (220V mains). NOT A GOOD IDEA. All I killed was my CD player, but it could have been me or my kid. Stay away. __________________ Take the Speaker Voltage Test!

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5th October 2008, 02:24 AM	#3
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	From one Portlander to another, DON'T! Atleast use an isolation transformer to isolate you from the mains wiring. What you propose is very dangerous.

5th October 2008, 05:00 AM	#5
lumanauw diyAudio Moderator Emeritus Join Date: Oct 2002 Location: Bandung	Hi, Inductor128, This is a very dangerous and lethal practice. Please don't even try it. This thread will be closed now.