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calx44 16th December 2012 07:54 PM

power supply
I am ready to put the electronics in my amplifier project. I have pretty much finished the cabinet, or at least as much as I can without the circuitry in place.

I am sorted in terms of pre amp and power amp, but I am not sure what to do with a power supply. My power amp will use the lm3886 chip which requires about 24V, if I remember correctly. The preamp circuit requires +-15v and can be seen here; 100W Guitar Amplifier (Mk II)
There is a power supply featured on this page, would something like this do the trick?

I would also like to run some LED strip lights off it, which use roughly 12v, as well if possible to backlight parts of the cabinet.

Any schematics or a point in the direction of some would be great, any advice much appreciated. Thanks.

AndrewT 16th December 2012 08:06 PM

The chipamp needs dual polarity supply.
Not a single polarity 24Vdc.
Unless you modify the chipamp schematic to work with a single polarity supply.

GeorgK 17th December 2012 11:27 AM

The LM3886 also won't deliver 100 Watts...
it is rated at 86W into 4 Ohm loads.
using a 8 ohm speaker gives you about 40.

this requires a dual 28V supply, I would get a 2x20 V 80VA toroidal transformer, 6A Bridge rectifier, Filter caps (2x4700/40V should do).
the supply on the page is ok so far, but do not use the suggested 2x25V transformer, this will result in overvoltage for the LM3886!
If the +- 15V for the preamp are not reached because of the lower voltage, you can change R18/19 to 560R.

Do not forget adeqate cooling for the LM!

calx44 17th December 2012 08:56 PM


Thanks, I am not worried about it producing 100Watts, that was just the page that happened top have the preamp on it. So it is possible to get this working then?

GeorgK 17th December 2012 10:20 PM

Just saw that the 3886 can handle more than +/- 28 Volts, sorry - so go ahead, the 2 x 25V transformer should work. But I would recommend not to use a speaker below 8 Ohms then.
you are going to use a standard circuit round the 3886 then?
should be no big deal as far as i see.
If you are not familiar with power supplies and line voltage take care! have more than a second look every time. again, do not forget cooling! Attaching tle 3886 to the chassis may not be sufficient. (Heat transfer paste + isolation material anyway!!!)

calx44 21st December 2012 08:00 PM

sorry for the delay,

ok, I have a celestion speaker that is rated at 50 watt and i belive is 4 ohm, how could i alter the supply to make this sufficient? Or would it just be easier to use a less powerful chip?

Yes, I was planning on just doing a standard circuit as this is a school project and needs to be kept to a deadline.


GeorgK 21st December 2012 09:35 PM

In this case, just choose the operating voltage. Mathematically, 50 Watts @ 4 Ohms means ca 14V RMS. (U=sqrt(PxR)) Your power supply has to deliver slightly above 2 x 20 Volts then. No transformer reaches exact values at changing loads, so 2 x 15...18VAC transformer voltage will work. After rectifying and filtering this results in the required 20...24 Volts DC.
Why do you think the celestion has 4 Ohms? Is there a sticker on it or have you tried to measure with a multimeter? If you read about 6 Ohms then you have an 8 Ohm speaker.

calx44 22nd December 2012 04:39 PM

oh ok, thats not too bad then :) Will that still be ok for the Preamp supply?
I'm pretty sure it is a 4 ohm as I remember there being a sticker on it, however It is currently at school so I am relying on my memory which is not always that trustworthy...

Minion 22nd December 2012 05:33 PM


Originally Posted by calx44 (
oh ok, thats not too bad then :) Will that still be ok for the Preamp supply?

If your power amp runs off of say +/- 24v DC you can use a couple regulators to get the +/-15v DC for the preamp ......

When building solid state guitar amps I usually tend to design the preamp to run of off a single supply so I can run it off of the positive DC power amp supply , it saves from having to build a separate regulated supply for the preamp ......


GeorgK 22nd December 2012 10:56 PM


Originally Posted by Minion (
If your power amp runs off of say +/- 24v DC you can use a couple regulators to get the +/-15v DC for the preamp ......


The preamp has its own Z-Diode regulation if it's the same as in the schematics. If the +/-15V cannot be reached make the two resistors smaller as I described before.

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