Safe way to run 120VAC full wave bridge rectified to get -85 -0- 85VDC transformerless for power amp?

Thread Starter

777funk

Joined Apr 13, 2010
19
Is there a safe way to run 120AC through a full wave brige rectifier to get -85 0 85 VDC? This would be to power a class D mosfet power amp The signal in and out (speaker) would be referenced to the 0 in the center of the two filter caps as ground.
 

Tonyr1084

Joined Sep 24, 2015
6,269
Full wave rectified you would only get 170VDC filtered. Without filters you would have pulsing 120 VDC (less the voltage drop across the diodes).

What amperage do you want? Why do you want to rectify mains? Do you understand how dangerous that can be?
 

Thread Starter

777funk

Joined Apr 13, 2010
19
I think I have this (chinese version of this off of ebay). Except my switching supply is -65v-0-65v and made for a 500 watt class D dual MOSFET controlled by a chip power amp. The problem is, I've bought two of these switch mode supplies and they work for a few startups and seem to run fine, but start the amp a 4th or maybe 5th time and as soon as the supply is plugged in, bang. A resistor and 2 MOSFET transistors fail violently. I'm looking to keep this little power amp small and light. The SMPS was great for this if it only worked.
 

Thread Starter

777funk

Joined Apr 13, 2010
19
Full wave rectified you would only get 170VDC filtered. Without filters you would have pulsing 120 VDC (less the voltage drop across the diodes).

What amperage do you want? Why do you want to rectify mains? Do you understand how dangerous that can be?
That's what I was figuring (170 hopefully split into a positive negative 85VDC) and at 3-5 amps. I have a big toroid that would be in the ballpark but I'm wanting to keep it small and light. I guess SMPS is my only option perhaps? The actual chip amp board (500w class D $15 on ebay) is about the size of an old flip phone which is great!
 

Papabravo

Joined Feb 24, 2006
16,830
That's what I was figuring (170 hopefully split into a positive negative 85VDC) and at 3-5 amps. I have a big toroid that would be in the ballpark but I'm wanting to keep it small and light. I guess SMPS is my only option perhaps? The actual chip amp board (500w class D $15 on ebay) is about the size of an old flip phone which is great!
Just how exactly would you propose to do that?
 

MisterBill2

Joined Jan 23, 2018
9,304
An ISOLATED switch-mode supply can get away with a much smaller and lighter transformer and thatcould work fairly well, plus have excellent regulation. And using a stack of 28 volt modules might let you build it out of modules commonly available. That could also then provide the lower voltages for the driver stages.
 

MisterBill2

Joined Jan 23, 2018
9,304
You don't see the contradiction in your statement? Isolated and transformer?
The transformer i a current technology switchmode supply is MUCH smaller and lighter than a mains frequency transformer for a similar power supply. I can hold a 12 volt 100 amp switcher easily with one hand, while the 60 hz transformer for a traditional 100 amp supply is a whole lot heavier. It seems that the frequency that a transformer operates at has a very large effect on the size and weight. AND, switcher supplies are lighter anyway.
 

shortbus

Joined Sep 30, 2009
8,817
The transformer i a current technology switchmode supply is MUCH smaller and lighter than a mains frequency transformer for a similar power supply. I can hold a 12 volt 100 amp switcher easily with one hand, while the 60 hz transformer for a traditional 100 amp supply is a whole lot heavier.
Missed again. just the use of the word "transformer"(which you did use) infers isolation. Other than an "auto- transformer" like a variac.
 

Audioguru again

Joined Oct 21, 2019
3,544
Please post the ebay ad for the cheap and small "500W" class-D amplifier so we can see how soon it will blow up, the same way your ebay SMPS blew up.
Many ebay amplifiers are rated in Whats, not Watts.
 

Audioguru again

Joined Oct 21, 2019
3,544
They say it is HiFi then say its 500W distortion is 10% that ain't hifi.
This 500W amplifier actually produces 250W into 8 ohms or 340W into 4 ohms at low distortion.
I wonder how soon the brushes in the fan motor will wear out.
 

Thread Starter

777funk

Joined Apr 13, 2010
19
They say it is HiFi then say its 500W distortion is 10% that ain't hifi.
This 500W amplifier actually produces 250W into 8 ohms or 340W into 4 ohms at low distortion.
I wonder how soon the brushes in the fan motor will wear out.
There's a guy doing some testing on youtube with the amp. I'm sure the fan is pretty cheesy but the amp itself seems to do what it advertises. I think he gets 450w of clean unclipped signal if I remember correctly. Maybe 2 ohms... maybe 4 can't remember. One thing is for sure on the topic of distortion... class D distortion isn't easy on the ears. But even at 350W of clean power into 4 ohms, that's pretty good for $15.

The problem at this point is the PSU. I have a big toroid that will work, I was just hoping to find a way that thing could be small and light.
 

Papabravo

Joined Feb 24, 2006
16,830
There's a guy doing some testing on youtube with the amp. I'm sure the fan is pretty cheesy but the amp itself seems to do what it advertises. I think he gets 450w of clean unclipped signal if I remember correctly. Maybe 2 ohms... maybe 4 can't remember. One thing is for sure on the topic of distortion... class D distortion isn't easy on the ears. But even at 350W of clean power into 4 ohms, that's pretty good for $15.

The problem at this point is the PSU. I have a big toroid that will work, I was just hoping to find a way that thing could be small and light.
Anybody can make a claim, and most people have no idea what the claim even means. Actual measurements with real data speak VOLUMES.
BTW "2 or 4 ohms ... can't remember" is part of a claim that's as useless as a screen door on a submarine.
 

Thread Starter

777funk

Joined Apr 13, 2010
19
Anybody can make a claim, and most people have no idea what the claim even means. Actual measurements with real data speak VOLUMES.
BTW "2 or 4 ohms ... can't remember" is part of a claim that's as useless as a screen door on a submarine.
He uses a scope, meters, and a signal generator and has a credible setup. RMS Power isn't a difficult calculation. Look it up if you doubt, it's public domain on youtube.
 

Papabravo

Joined Feb 24, 2006
16,830
He uses a scope, meters, and a signal generator and has a credible setup. RMS Power isn't a difficult calculation. Look it up if you doubt, it's public domain on youtube.
People make videos all the time with fancy equipment and dubious claims. I'm not a video detective, but then again I don't need to be. You can believe whatever you want,
 
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