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Old 26th June 2011, 05:14 PM   #11
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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?
Not a problem.

If transformer is rated at full current of power amp then the reduced current on the -ve pre-amp leg is easy meat for it.
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Old 26th June 2011, 06:17 PM   #12
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These are a good option if space is tight and current requirements modest,
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Or a 15 volts one that allows you to add further regulation if you want,
Your Search Results | CPC
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Old 26th June 2011, 07:23 PM   #13
karmik is offline karmik  Italy
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I appreciate everybody's feedback immensely. Thanks to andy5112405 for the SMPS idea, i will definitely steal it sooner rather than later but running my own switching supply is a bit past my possibilities at the moment.

Quote:
Originally Posted by discrete View Post
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....
This is what i meant to do from the start. My original question was if stepping down from 24v to over 10v lower would mean having HAZARDOUS amounts of heat in a wooden cabinet. I take your answer as a no? :P

i will proceed with this way then, if the heat issue is confirmed as non-existant. I will use the 5A version with a sizable heatsink and see from there. I'll try to sketch something up soon and maybe i can abuse everybody's kindness again

EDIT: Also, it seems clipping won't be a problem given the speakers will be Fostex FE83, which are rated for 5W. So hopefully i will never hit the 7w treshold ^^

Last edited by karmik; 26th June 2011 at 07:25 PM.
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Old 26th June 2011, 07:27 PM   #14
karmik is offline karmik  Italy
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Quote:
Originally Posted by Mooly View Post
These are a good option if space is tight and current requirements modest,
Your Search Results | CPC

Or a 15 volts one that allows you to add further regulation if you want,
Your Search Results | CPC
The input voltage appears to be too low (~5v for the first, ~10v for the second) as i'm dealing with 24v. But i'll save this for future reference, thanks!
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Old 26th June 2011, 07:33 PM   #15
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You could either regulate the voltage before the DC/DC convertor or just add a series 12 volt zener diode in the feed to it to drop the unwanted volts. For a preamp I imagine your total current requirement is low.
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Old 26th June 2011, 08:14 PM   #16
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You still have the problem of 0V.
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Old 26th June 2011, 08:23 PM   #17
karmik is offline karmik  Italy
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I don't understand how it works sadly :/.

I can't have two circuits from the same power supply? How do PC power supplies work then? with dozes of different voltages? Among all the solutions suggested here (of which i'm thankful for), the only one that i canget my head around to is discrete's one.

I don't have the expertise to run my own SMPS from scratch and the trial and error would take months :/ and i can't run a transformer inside a 20x20cmx30 cabinet together with damping material, a speaker cone and 4 circuit boards.

Isn't there a way to supply the regulator a real 0V?

Last edited by karmik; 26th June 2011 at 08:33 PM.
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Old 26th June 2011, 08:39 PM   #18
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I honestly didn't foresee this many complications in something i chose exactly because it was a quicker solution (using a laptop brick instead of building a supply from scratch).

How much do you think it would cost to build a supply of my own? At this point i'm open to suggestions, provided the cost is not prohibitive. Last time i checked, transformers were very expensive (30-40euros for a toroidal of a few tens of VAs).

The choice of the laptop brick was inspired by
  1. External Power Supply (for space reasons)
  2. Lots of power on the cheap (120w for 30-35 euros)
  3. Reduced complexity.

But as it's turning out now, there is loads of added complexity, and i'm actually not saving that much room anymore. So i might as well consider other options.

I can't spend more than 50-60euros for the whole supply, from AC Mains to final supply lines (+12 for the power amp, +/-12 for the pre).

I'm very appreciative of all the hassle you are all going through to help with this problem If it's of any help, i'll be sure to post a detailed build log once i'm through (not sure if that's welcome around here?)

Last edited by karmik; 26th June 2011 at 08:45 PM.
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Old 26th June 2011, 09:14 PM   #19
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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

Last edited by !; 26th June 2011 at 09:24 PM.
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Old 27th June 2011, 02:36 AM   #20
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Quote:
Originally Posted by Andy5112405 View Post
You still have the problem of 0V.

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.
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