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Old 20th August 2007, 06:42 PM   #1
ericww is offline ericww  United States
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Default LM3875 powered by 28-30v wallwart?

I've got a completed rev.A LM3875 chipamp.com kit and all motivation to complete this project is basically gone, so I'm looking for an easy way out. My idea is to power each channel of the amp using a 28vDC wallwart that outputs a couple of amps. Would this work? I'm asking this because most people use the rectifier board with bare transformers. My basic electronics knowledge says that 30v coming out of a rectified bare transformer is the same as 30v coming out of a wallwart.

Thoughts? Flames?
Thanks in advance for all help,
Eric
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Old 20th August 2007, 07:44 PM   #2
paulb is offline paulb  Canada
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You probably need a +/- supply for the amp, meaning 3 wires: +, ground, and -. Your wallwart only supplies 1 supply. You could possibly use 2 wallwarts if their outputs aren't connected to AC ground (which most wallwarts don't have anyway). If you don't know how to hook them up this way, better stay with the plan as you were given.
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Old 20th August 2007, 08:30 PM   #3
gootee is offline gootee  United States
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Default Re: LM3875 powered by 28-30v wallwart?

Quote:
Originally posted by ericww
I've got a completed rev.A LM3875 chipamp.com kit and all motivation to complete this project is basically gone, so I'm looking for an easy way out. My idea is to power each channel of the amp using a 28vDC wallwart that outputs a couple of amps. Would this work? I'm asking this because most people use the rectifier board with bare transformers. My basic electronics knowledge says that 30v coming out of a rectified bare transformer is the same as 30v coming out of a wallwart.

Thoughts? Flames?
Thanks in advance for all help,
Eric
Hi Eric,

Have you found a 28v AC/DC wall-adapter that will push 2 Amps? (I've seen a lot of 24v, but not many 28v. Just curious.)

It "should" work OK. If you need +28v and -28v, just connect the more-negative output of one of them to the more-positive output of the other one, and call that "ground".

You could also use a couple of the "desktop" type of AC/DC adapters. Those might be available with more voltage and current options than the wall-pluggable adapters.

And there are also lots of little "open frame power supply" products, which are often available in the 60 Watt range for around $30, brand new (you'd need two). However, 24v supplies are a lot more common than 28v.

Here's a 30v 2A supply for about $34: 418-CFM60S300 , at http://www.mouser.com . It's a switchmode supply, with a 2x4 inch footprint, and 1.2 inches tall. I think that jameco.com also has a wide selection of cheap open-frame or PCB-only power supplies.

Your best bet might be to look for an older lab-type dual-output linear supply, on ebay.com, probably in the industrial test and measurement section.

Good luck.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html
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Old 21st August 2007, 12:14 AM   #4
ericww is offline ericww  United States
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Thanks for all the suggestions so far, but I fear I may have wasted your time. Powering this seems to require a bit more effort than my motivation allows (I'm sure I have the technical competence to do it, I just don't want to). I'm considering just selling this stuff and buying a T-amp, since that seems to be the very easy way out. The speakers are those cheap Insignias from BestBuy, so I really don't need something awesome.

Talk me out of it
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Old 21st August 2007, 01:41 AM   #5
BWRX is offline BWRX  United States
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With a proper power supply the chip amp will be capable of more output power than a T-amp. I have both types of amps and enjoy the sound of both.
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Old 21st August 2007, 02:02 AM   #6
amc184 is offline amc184  New Zealand
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The DC from an average wall wart is probably pretty rubbish anyway, with heaps of noise and not enough filtering. You're much better off making your own, with some decent rectifiers, and good sized filter caps.
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Old 21st August 2007, 07:47 AM   #7
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The quality of a gainclone is VERY highly dependent on the quality of the power supply. If you really want to take the easy way out, gets a couple sealed acid batteries.

The voltage you use will determine the impedance of the speaker that works best with it, If you have low impedance speakers you want lower voltage, if you have higher impedance speakers a higher voltage works,

One of the best sounding chip amps i've heard was a BGT Rev 1 with a power supply voltage higher than the max, but driving 16 ohm speakers (it didn't do as well into lower impedance speakers)

dave
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Old 21st August 2007, 09:07 PM   #8
CarlosT is offline CarlosT  United States
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No sense messing with this anymore...just send me all your stuff to:

Carlos Tres
PO Box 722
Drexel Hill, PA 19026
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Old 21st August 2007, 11:25 PM   #9
john65b is offline john65b  United States
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I once tried using two 19.5VDC 3.5A Laptop Power Supplies (one supply + connected to other supply - to form the 0V centertap) for a Gainclone PS...as I had a ton of these supplies - I used a few of these laptop PS with a potted LM317/LM350 to dial in just about any Voltage I needed from 3VDC to 18VDC @ 1A - 3.5A...

Anyway, I figured I would try using two for a simple dual rail, low power regulated power supply to test out a simple point to point Gainclone circuit...sounded horrible. I really don't know why as I had the current and the low voltage should not have been a problem.

Don't know how to explain it...just sounded real bad.

Just ended up making a conventional toroid/rectifier/smoothing caps and P-P circuit sounded just fine.
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Old 21st August 2007, 11:39 PM   #10
BWRX is offline BWRX  United States
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You need to be careful with those laptop type switching supplies. The minus or negative output is often tied to earth for safety reasons. If you try to make split rails with two of those supplies without disconnecting the negative output of the supply used for the negative rail from earth there will be problems!
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