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Old 30th January 2013, 06:22 PM   #2621
Renron is offline Renron  United States
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Glowing Load Resisters is more romantic than candlelight. I'd leave 'em in the circuit.
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Old 31st January 2013, 12:51 PM   #2622
darbost is offline darbost  Indonesia
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Hi all,

I've salas hotrodded dcb1 coming on the way with Antec transformer (2 x 15V, 50VA). I'm planning to set it to 600mA, but I'm not sure whether it's safe or not.
Any advice is highly appreciated.


Thanks.
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Old 31st January 2013, 04:21 PM   #2623
stajo is online now stajo  Sweden
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Originally Posted by darbost View Post
Hi all,

I've salas hotrodded dcb1 coming on the way with Antec transformer (2 x 15V, 50VA). I'm planning to set it to 600mA, but I'm not sure whether it's safe or not.
Any advice is highly appreciated.


Thanks.
Here is how you count: you have a 50 VA trafo that divides into two secondarys. That is 25 VA per secondary. You need each secondary to provide power to 0,6 A. Divide 25 VA with voltage 15 and you get the allowed current per rail 1,67A. You seem to be on the safe side if Antek (poor guys, factory just went up in smokes) holds specs, which they usually do.

Thing with audio equipment is that you dont want to max the trafos since that, even if they take the power they start to vibrate and generate physical hum.

I think your good tho.


GL
Staffan
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Old 31st January 2013, 07:37 PM   #2624
Renron is offline Renron  United States
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Darbost,
what size are your heatsinks?
Sounds like fun. any Pics?
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Old 31st January 2013, 08:12 PM   #2625
stajo is online now stajo  Sweden
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Originally Posted by Renron View Post
Darbost,
what size are your heatsinks?
Sounds like fun. any Pics?
11 DD
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Old 31st January 2013, 09:53 PM   #2626
stajo is online now stajo  Sweden
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Originally Posted by stajo View Post
Here is how you count: you have a 50 VA trafo that divides into two secondarys. That is 25 VA per secondary. You need each secondary to provide power to 0,6 A. Divide 25 VA with voltage 15 and you get the allowed current per rail 1,67A. You seem to be on the safe side if Antek (poor guys, factory just went up in smokes) holds specs, which they usually do.

Thing with audio equipment is that you dont want to max the trafos since that, even if they take the power they start to vibrate and generate physical hum.

I think your good tho.


GL
Staffan
I will for any circumstance update on my own post. If you are designing for class A as this and most preamps are, you dont have to take head up for any temporary current draw. Some class B amps, especially in the buyers segment is designed for class B but cheaps on the tranny and relies on the caps in the PSU rail. Rotel and Denon is good examples as they built good amps in their own concepts. Rotel relied more on big trannys, Denon calculated a lot on good caps. Both succeded but Denon won since it was cheaper.

In class A I wouldnt worry about caps. Keep them as low as possible and as good quality as possible. If your tranny keeps up and ripple is decent, dont go for larger caps.
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Old 1st February 2013, 12:34 AM   #2627
Renron is offline Renron  United States
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Originally Posted by stajo View Post
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Old 1st February 2013, 09:12 AM   #2628
AndrewT is offline AndrewT  Scotland
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Originally Posted by stajo View Post
.....................Divide 25 VA with voltage 15 and you get the allowed current per rail 1,67A..............
You will severely overheat your transformer doing as you suggest.

1.) the 1.67Aac is the AC current that is allowed to feed a resistor load.
2.) the power output when fed to a Capacitor input filter becomes Idc * Vac * Sqrt(2) This MUST be less than the VA of the transformer.
3.) all transformer manufacturers de-rate the transformer output to take account of the additional heating effect of the pulsing current when feeding a capacitor input filter. This reduces the available power output to well below that predicted in 2.) The usual de-rating factor is around 0.7

The result of all of these is that Idc ~ = Iac/2 when taking continuous DC current from a capacitor input filter.
For the example you gave the maximum continuous DC current will be ~840mAdc

Drawing 600mAdc from the 50VA 15+15Vac transformer will be using ~70% of maximum continuous capability.

I have posted similar information in many previous posts, but Members like you refuse to read any of my unhelpful posts.
Is this another of those unhelpful posts?
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Last edited by AndrewT; 1st February 2013 at 09:16 AM.
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Old 1st February 2013, 10:07 AM   #2629
stajo is online now stajo  Sweden
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Quote:
Originally Posted by AndrewT View Post
You will severely overheat your transformer doing as you suggest.

1.) the 1.67Aac is the AC current that is allowed to feed a resistor load.
2.) the power output when fed to a Capacitor input filter becomes Idc * Vac * Sqrt(2) This MUST be less than the VA of the transformer.
3.) all transformer manufacturers de-rate the transformer output to take account of the additional heating effect of the pulsing current when feeding a capacitor input filter. This reduces the available power output to well below that predicted in 2.) The usual de-rating factor is around 0.7

The result of all of these is that Idc ~ = Iac/2 when taking continuous DC current from a capacitor input filter.
For the example you gave the maximum continuous DC current will be ~840mAdc

Drawing 600mAdc from the 50VA 15+15Vac transformer will be using ~70% of maximum continuous capability.

I have posted similar information in many previous posts, but Members like you refuse to read any of my unhelpful posts.
Is this another of those unhelpful posts?
Sorry about that. I forgot about multiplying Sqrt(2) for VDC. I was writing on the phone on the sub and just tried to be helpful. He is still in the spec and it wont blow in his face. It might get lukewarm and give some hum. No need to blow your top Andrew.

Staffan
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Old 1st February 2013, 10:10 AM   #2630
AndrewT is offline AndrewT  Scotland
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I forgot about multiplying Sqrt(2) for VDC.
and worse, you did not mention the manufacturer's Derating for Capacitor Input Filter. Were you even aware that it exists and must be incorporated into any modeling?

When Members post rubbish and the Moderators take no action, then what is one to do?

I have taken a step back for the last few weeks and I am seeing Rubbish and Lies being posted on this Forum and no one seems to take any heed !!!!!
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Last edited by AndrewT; 1st February 2013 at 10:14 AM.
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