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Old 27th June 2011, 05:23 AM   #21
karmik is offline karmik  Italy
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Quote:
Originally Posted by ! View Post
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.

Click the image to open in full size.

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


Quote:
Originally Posted by discrete View Post
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.
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?
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Old 27th June 2011, 05:54 AM   #22
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Andy5112405 View Post
You still have the problem of 0V.
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.
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Old 27th June 2011, 06:16 AM   #23
karmik is offline karmik  Italy
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Quote:
Originally Posted by Mooly View Post
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.
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!

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Old 27th June 2011, 06:24 AM   #24
Mooly is offline Mooly  United Kingdom
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Your "virtual" 0 volt line is at +12 volts relative to the "Gnd" points. Each part of the circuit is fine looked at in isolation but will not work together as you want.
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Old 27th June 2011, 06:31 AM   #25
Mooly is offline Mooly  United Kingdom
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Like this. Now your virtual earth point can be connected to ground giving a true -/+ supply.
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File Type: png psub.png (77.8 KB, 62 views)
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Old 27th June 2011, 06:36 AM   #26
karmik is offline karmik  Italy
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Cool, and this is using one of those IC you linked? I will look more into that, but those you linked couldn't take 24v input.

But i'm pretty sure there is something like that around. It will also save up some room, sweet!
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Old 27th June 2011, 06:38 AM   #27
Mooly is offline Mooly  United Kingdom
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The zener drops 12 volts thus giving 12 volts across the convertor. The input to these devices isn't critical as long as it's within the recommended range.
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Old 27th June 2011, 06:42 AM   #28
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Quote:
Originally Posted by karmik View Post
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?

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
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Old 27th June 2011, 06:55 AM   #29
karmik is offline karmik  Italy
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Quote:
Originally Posted by discrete View Post
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
I believe my second schematic is correct according to Rod Elliot's site?

Quote:
Originally Posted by Mooly View Post
Like this. Now your virtual earth point can be connected to ground giving a true -/+ supply.
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?
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Old 27th June 2011, 06:59 AM   #30
Mooly is offline Mooly  United Kingdom
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Yes it just needs a single 12 volts input to give the -12 /0/ +12 output...

I have to go... but it really is that easy.
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