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Old 26th June 2011, 04:53 AM   #1
karmik is offline karmik  Italy
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Default Single 12v and +-12v needed, What options?

Hi, i'm currently working on a very space constrained project and i'm in need of a single 12v supply line for a power amp (a Arjen Helder ta2020 module from ebay) and a +-12v for a preamp.

The whole thing is tightly constrained inside a small speaker cabinet (this is gonna be a set of active speakers for a desktop) so at the moment i picked an external laptop power supply (24v) and used the schematic of Rod Elliot's Split DC Power supply to get the relevant +-12v for the pre.

From the same power input, i need to get straight 12v for the power amp but i can't tap the +12v from the dual as it would create a very strong current imbalance and that's not really advisable as i understand.

At the moment i'm thinking of using a LM7812 but i don't know how much current will flow through the regulator.

How much would the ta2020 absorb in a standard setting? Speakers are 5w rating and they are 8ohm, so the *sound* power the amp will put out is around 12w.

Is the 7812 a valid solution or i'm risking setting something on fire?

here's a rough schematic of how the whole thing is going to work

Click the image to open in full size.

Thanks for your input

Last edited by karmik; 26th June 2011 at 04:55 AM.
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Old 26th June 2011, 08:38 AM   #2
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That circuit will DEFINITELY NOT WORK

As drawn the output of the 7812 will be +24V (or close to it). The 0V of the 7812 will be floating as it is not tied to a HARD GROUND.

Best bet would be to use 2 x 7812s one for pre +12V and one for power +12V. Then build a simple invertor using the TL497 (simple applications are shown in the datasheet.
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Old 26th June 2011, 08:50 AM   #3
karmik is offline karmik  Italy
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As it's already obvious, i'm a newbie but i don't understand why the output of the regulator should be 24v, it was a very quick sketch but what i meant is :
  • Feed 24v to the regulator, with the + on the input of the regulator
  • Ground to the ground of the regulator
  • As i understand, the output pin of the regulator should have +12v?

The capacitors are mentioned in the datasheet of the regulator as useful for eliminating transients and they are connected between each pin and ground, as instructed in the datasheet (in one of the demo circuit illustrations).

If it's not too much bother could you please tell me what's wrong in the above? Thank you very much for your input.
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Old 26th June 2011, 08:58 AM   #4
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The +/- 12V circuit you have drawn only works with very low and equal currents being drawn from both sides + & -.

If you connect the 0V of the regulator to the 0V of the +/- circuit, regulator will try to use the -ve supply as its return path.

The +/- will no longer be +/- 12V and the 7812 will kick out 12V above whatever it is seeing on its 0V terminal.

Under no-load it would appear OK but as soon as you try to draw any current the voltages will all go to s**t.

Also you have the problem that the amps 0V is not at the same potential as the pre-amps 0V.
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Old 26th June 2011, 09:06 AM   #5
karmik is offline karmik  Italy
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I see. Well i have a DC power jack coming from the laptop brick.

I was planning on running 1 ground and 1+ from the jack for the +/- board and 1 ground and 1+ for the lm7812 board.

Shouldn't the 0V of the LM7812 refer to the 0V of the laptop brick that way?

Also, the +/v part will use an opamp to fix the current imbalance, and it's meant for a pre so current should be enough (i'm following Rod Elliot's recommendations and the target preamp is P88 from his website).

Thank you very much again for your support

PS: Well i'm really open to suggestions, i just need a dual power supply for the pre and a single-ended one for the power amp. Any solution that can fit on a 60x60mm (maybe a little more) breadboard and that i can possibly build myself is welcome.

Last edited by karmik; 26th June 2011 at 09:09 AM.
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Old 26th June 2011, 03:41 PM   #6
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Use CT transformer 12-0-12 and use 7812/7912 regulators for the rails
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Old 26th June 2011, 03:47 PM   #7
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How would that solve the problem? I'd still have a dual power supply with one rail draining much more current than the other. As far as i know that is really bad?
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Old 26th June 2011, 03:49 PM   #8
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Quote:
Originally Posted by karmik View Post
How would that solve the problem? I'd still have a dual power supply with one rail draining much more current than the other. As far as i know that is really bad?
Its not a problem at all.
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Old 26th June 2011, 03:50 PM   #9
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Best bet is to build your own SMPS. Not a 240V one as they are very difficult to build, but one that runs from your 24V plug-in.

I'll draw up an idea and post it in half an hour or so.

Just to give you a rough idea.

Use a basic NE555 running in Astable (Oscillator) mode to drive a MOS-FET.

The Drain Load on the MOS-FET is a simple ferrite transformer.

The secondaries of the ferrite can be constructed to give you your +/- 12V and +12V secondaries.

If the load is constant you shouldn't need any regulators except the 15V reg for the NE555.

With a bit of practice low voltage SMPS designs are easy to build.
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Last edited by Andy5112405; 26th June 2011 at 04:04 PM.
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Old 26th June 2011, 04:11 PM   #10
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Quote:
Originally Posted by karmik View Post
How much would the ta2020 absorb in a standard setting? Speakers are 5w rating and they are 8ohm, so the *sound* power the amp will put out is around 12w.
t
According to the data sheet for13.5 V supply into 8 ohm, maximum power will be 10 W with 10%! THDN or 7 W with 0.1% THDN.

Regulate the 24 V laptop brick down to 13.5 V for the power amp. You can use an LM317, LM7815, etc. for this. LM338 would be better (5 A instead of 1.5 A capacity). You will need a heat sink for the regulator.

Run the pre from your virtual split supply as planned.

AC-couple the pre output to the power amp input using an MKP or MKT capacitor, 1 to 10 uF, depending on the input impedance of the amp. The pre output, power amp input, or both, may even already be AC coupled....
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