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Old 5th February 2005, 05:14 PM   #1
Adam M. is offline Adam M.  United States
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Default LM1875/lm1876 single supply?

Has anyone tried national's single supply schematic for either of these chips? I'm looking to build a 2 channel amplifier for computer speakers to actually put into the speaker enclosure with the heatsinks making up the back of one of the speakers.

I'd prefer the lm1876, but I am looking for a minimal design, mostly because it makes the PCB layout, and the LM1876 needs the transistor and a couple extra components for the mute function.

Is there a minimal design out there for either of those chips? Ideally, I'd like to use p2p construction, but the circuit looks too complex to do that.

The reason I want to do single supply is save space in the enclosure, and a transformer takes up a heck of a lot more space then a wall wart (I know, its nowhere near ideal, but these are computer speakers..)

Thanks,
Adam
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Old 5th February 2005, 07:30 PM   #2
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You could always use TWO wall warts, and go for a traditional dual rail power supply....

Otherwise, I don't think there's very many people on here (judging from previous responses) that have implemented the single supply..
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Old 5th February 2005, 08:07 PM   #3
Adam M. is offline Adam M.  United States
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How would you use 2 wall warts?
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Old 5th February 2005, 08:53 PM   #4
squadra is offline squadra  Netherlands
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Default Re: LM1875/lm1876 single supply?

Quote:
Originally posted by Adam M.
The reason I want to do single supply is save space in the enclosure, and a transformer takes up a heck of a lot more space then a wall wart (I know, its nowhere near ideal, but these are computer speakers..)

Thanks,
Adam
If the wall wart has AC output, you could create dual rails.
See the attached image.
This is not ideal, but it would work.
Of course you would have to add capacitors and snubbers etc.
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Old 5th February 2005, 09:06 PM   #5
Adam M. is offline Adam M.  United States
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of course! You are saying have an outboard transformer, and just drop the bridge/filter inside. Am I understanding your suggesition correctly?
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Old 5th February 2005, 09:18 PM   #6
squadra is offline squadra  Netherlands
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Yes, you have the transformer outside, either as a wall wart or a transformer in a housing on its own.

If you are going to use the wall wart, and it has only one secondary coil, then you could use the diode layout as drawn above to get 2 rails.
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Old 5th February 2005, 09:40 PM   #7
mateo88 is offline mateo88  United States
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The single supply is very easy. Just create a virtual ground using two 2.2k resistors (or even anything close to that, really), and then use the same schematic you would for a dual supply. I did it a few weeks ago with an lm1876 and it works like a charm.

I'll actually probably use this from now on, just because I only have to use one bridge rectifier and I didn't notice any difference in sound.
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Old 5th February 2005, 11:47 PM   #8
paulb is offline paulb  Canada
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I've built a one channel amp using a single supply. For a desktop speaker set, it ran from a 16VAC wallwart. This would be better than the dual rail half-wave arrangement posted earlier because the AC is full-wave rectified.
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Old 6th February 2005, 03:05 PM   #9
Adam M. is offline Adam M.  United States
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Quote:
Originally posted by mateo88
The single supply is very easy. Just create a virtual ground using two 2.2k resistors (or even anything close to that, really), and then use the same schematic you would for a dual supply. I did it a few weeks ago with an lm1876 and it works like a charm.

I'll actually probably use this from now on, just because I only have to use one bridge rectifier and I didn't notice any difference in sound.

In that case, how do you get a true ground? The virtual ground will reference the rail voltages properly, but the virtual ground will still be 1/2 the supply voltage above true ground. Is this not a problem, and you just keep virtual ground an true ground seperate? I would think that would create a DC offset on the output.

Also, what kind of power rating would the resistors need? I don't see much current going through those resistors, but want to make sure.

Thanks,
Adam
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Old 7th February 2005, 03:12 AM   #10
mateo88 is offline mateo88  United States
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I'm not sure if I get what you're saying exactly, but there is no dc offset created by using a virtual ground. I suppose it would screw something up if you grounded the circuit via the ground pin on a power outlet, but otherwise it will be fine. Just make the virtual ground and pretend like you're building the amp with a dual supply.

I used 1/4 watt 2.2k resistors, but if you go too much lower than that you should probably use 1/2 watt to be on the safe side.

Good luck!
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