Single 12v and +-12v needed, What options?

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karmik

karmik

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Joined 2008
didgt.info
! said:
Since you are considering an external power supply, your transformer need not be a toroid type, magnetic field is mitigated by the distance of the external PSU. It should be much easier to find a 12VAC transformer good for 2+ amps at low cost. One common source that comes to mind is outdoor garden lighting transformers, though they are usually higher current capacity than you would need for TA2020.

Use two of them with secondaries in series, where you join the secondaries is your center tap ground. Even though the two may be slightly different voltages it will work if you put regulators on positive and negative rails. It's just one idea, not necessarily the best one. You could also use two separate transformers, a tiny center tapped for the preamp and a larger for the TA2020, then you have no issue at all about power rail noise getting to the preamp, providing you use proper star grounding as with any amp.

How hard do you plan to push this amp? I powered a sonic T-amp with TA2024 (15W(?) version) at about 13.something volts and a moderate sized heatsink never gets excessively hot using LM317 to regulate. While I'm thinking about it, I threw it together quickly out of spare parts I had lying around and cannibalized part of an ancient computer router PCB and case to put it in, so it looks frankenstein ugly inside, but it gives you an idea how close I was able to put an E-core transformer without hearing any sound change, though just for the heck of it I put i a plain steel partition anyway.

I've used the amp in my garage over the past 5 years or so in (up to) 95F summer temperatures and it hasn't failed yet, though I usually only run it at about half volume so I don't annoy the neighbors.

An externally hosted image should be here but it was not working when we last tested it.


Hmm, the link isn't working for me, here it is:
http://img16.imageshack.us/img16/7563/sv300539.jpg
Click to expand...

I'm sitting 30 cms from where the speakers (with the amp inside) will be. They will be on the table where i work every day. Next to a window that is open from spring to fall. And i have neighbors too :) So no, i won't push it i think.

If i go the DIY psu way, i will use two transformers and two separate circuits. My only worry is: won't the final cable carrying 3 voltages from the psu to the amps be a very big antenna picking up noise? The preamp is opamp-based so it shouldn't be a problem (i believe i read somewhere they have very good power supply noise rejection)


discrete said:
Output of brick: 0 V and 24 V
Output of LM317 regulator: 0 V and 13.5 V

Feed to pre-amp: 0 V and 24 V from brick, with a virtual earth at 12 V to bias the pre amp
Feed to power amp: 0 V and 13.5 V from LM317

Signal from pre-amp to power amp: 0 V to 0 V and signal out to signal in via a capacitor.

So no problem with 0 V....

Possible problems that may, or may not, manifest:
- Power on thump due to DC offset, but power amp should have a mute delay.
- Ground loops, depending on layout and how solid the virtual earth on the pre-amp is


Try it, worse case you will learn something :)

PS. The LM317 should be mounted on a heat sink or metal sheet exposed to air from outside the wooden box. It will shut itself down before setting fire though.
Click to expand...

I'm even more confused now. How would i achieve that? Would i have to create a virtual earth with an opamp like in this article?
 
karmik

karmik

Member
Joined 2008
didgt.info
Mooly said:
Not so :)

The output from the DC/DC convertor is fully floating and isolated so you connect zero to wherever you want... in this case the 0 volts of the main PSU.
That gives 0-24vdc and -12/0/12 vdc.
Click to expand...

That's what i thought!

Ok so, i worked a bit on the schematic and i think much of the incomprehension came from my poor skills rather than actual problems, i apologize for that.

To draw this schematic i copied information from:


So here is it, what did i get wrong this time?

Thanks for your feedback!

psub.png
 
karmik

karmik

Member
Joined 2008
didgt.info
discrete said:
It is the same idea as the ESP article you quoted in the beginning.
The wiki article you linked has a good link as well:

Virtual Ground Circuits
Click to expand...

I believe my second schematic is correct according to Rod Elliot's site?

Mooly said:
Like this. Now your virtual earth point can be connected to ground giving a true -/+ supply.
Click to expand...

Wait. The dc/dc converter you linked has an input voltage of +12v and can output a +/-12v? But then i don't need 24v anymore.

I only used 24v in the beginning because rod elliot's split supply used 24v to obtain +/-12v. My laptop brick has selectable output, i can very well just buy that dc/dc converter and use +12v straight from the power brick!

I could then have the poweramp fed directly from the dc jack (maybe with *minor* regulation, requiring no heatsink), and use the dc/dc converter to have a *real* dual power supply!

Is this correct? am i understanding this correctly?
 
A

Andy5112405

Banned
Joined 2006
karmik said:
That's what i thought!

Ok so, i worked a bit on the schematic and i think much of the incomprehension came from my poor skills rather than actual problems, i apologize for that.

To draw this schematic i copied information from:


So here is it, what did i get wrong this time?

Thanks for your feedback!

psub.png
Click to expand...

Just don't connect Virtual 0V to 0V because they are different DC potentials.
 
M

Minion

Member
Joined 2006
If you can get a 12v AC transformer you can use a Voltage doubler and regulators to get your +/- 12v DC then use a seperate rectifier and regulator for your 12v DC for the power amp ......
The Preamp won"t use very much current so a voltage doubler would be suitable and there isn"t a problem with it sucking more from one rail .....


Cheers
 
Mooly

Mooly

Administrator
Joined 2007
Paid Member
karmik said:
I'll fix the schematic as soon as possible, but i think i'll go with Mooly's solution. Would it be a good idea to regulate the 2 rails after the dc/dc converter as well? or would that be overkill?

Best regards,

GC
Click to expand...

You have many options... you could go with a 15/0/15 convertor and use 78and 79L12 regs. The 3 volts is sufficient headroom for these to work well.

You could just use a simple R/C filter feeding the opamp rails.

Or just use it "as is" with suitable caps on each rail.

Or again with a 15 volt reg you could use a resistor/zener/cap arrangement which in many cases for low currents is a very good option... this "shunt" arrangement is the one I use in my integrated amp.
 
S

sarathssca

Member
Joined 2014
Mooly said:
You have many options... you could go with a 15/0/15 convertor and use 78and 79L12 regs. The 3 volts is sufficient headroom for these to work well.

You could just use a simple R/C filter feeding the opamp rails.

Or just use it "as is" with suitable caps on each rail.

Or again with a 15 volt reg you could use a resistor/zener/cap arrangement which in many cases for low currents is a very good option... this "shunt" arrangement is the one I use in my integrated amp.
Click to expand...

Do we need to filter the split rail that we are achieving from the DC/DC converter, as both this and the main PSU are switching power supplies?
and switching power suppliers are known to be noisy especially for audio pre amplifiers.
Do we need to look for additional high bandwidth PSSR LDO after DC/DC rail splitter or a pi CLC filter or any other filtering ?

thanks and regards
S Sarath
 
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