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Old 17th October 2010, 11:13 AM   #1
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Join Date: Oct 2010
Default Hello and advise

Hi
I have just bought
Class A amp
Transformer
RCA phono
Headphone Socket
The seller has said they will send 10k pot
I need a knob for the pot
I have suitable power switch

I know I will need to mount it all in a suitable box.
Will i need anything else?
Can I connect the transformer directly or will it need filter/regulator

will be used with Iphone and ATH-m50s

I'm a newbie I only know what I have picked reading here and other sites over the last week, So any other advise warnings or assembly tips welcome.
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Old 17th October 2010, 01:05 PM   #2
Jay is offline Jay  Indonesia
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Join Date: Feb 2003
Location: Jakarta
Quote:
Originally Posted by heliguy View Post
Hi
I have just bought
Class A amp
Transformer
RCA phono
Headphone Socket
The seller has said they will send 10k pot
I need a knob for the pot
I have suitable power switch

I know I will need to mount it all in a suitable box.
Will i need anything else?
Can I connect the transformer directly or will it need filter/regulator

will be used with Iphone and ATH-m50s

I'm a newbie I only know what I have picked reading here and other sites over the last week, So any other advise warnings or assembly tips welcome.
On the PCB there are diodes/bridge (1N4002) to convert AC to DC. So you can plug directly from the transformer 12-0-12.

You need cables and coaxial cable of course.
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Old 17th October 2010, 04:58 PM   #3
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Thanks Jay
Any idea where I'd get nice box to put it all in?
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Old 17th October 2010, 11:20 PM   #4
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If was to add an led to indicate power on, what would be the best place to connect it?
I'm sure I could connect it between one of the 12v and 0v off the transformer but I'm not sure if that could cause some kind of imbalance. Probably a stupid question but you don't know what you don't know.
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Old 18th October 2010, 11:22 AM   #5
Jay is offline Jay  Indonesia
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Join Date: Feb 2003
Location: Jakarta
Quote:
Originally Posted by heliguy View Post
If was to add an led to indicate power on, what would be the best place to connect it?
I'm sure I could connect it between one of the 12v and 0v off the transformer but I'm not sure if that could cause some kind of imbalance. Probably a stupid question but you don't know what you don't know.
Transformer output is AC. A led has positive and negative ends, so you need a DC. You can get the DC (+ or -) after the rectifier diode bridge (4x 1N4002). In a diode bridge, 2 diodes join together (ends with strips) into positive supply and 2 other diodes join together to form negative supply. You can get the DC either from pos to gnd, or gnd to neg (or pos to neg for twice of voltage).

But you cannot connect the LED directly to the 17Vdc supply, or it will burn. Use a resistor at either ends of the LED to reduce the voltage. The value of the resistor depends on the type of the LED and the supply voltage.

With 12Vac transformer, after rectifier the voltage will be around 17Vdc (1.4x). You need resistor around(17V-2V)/0.02A=750ohm up to 1K5. 750 ohm usually too bright. I think 1K or 1K2 will do. With blue/white LED you need twice of resistance.

As for the casing, I have no idea. For such as small amp, why not DIY from wood or biscuit can hehehe.
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Old 18th October 2010, 11:28 AM   #6
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by Jay View Post
With 12Vac transformer, after rectifier the voltage will be around 17Vdc (1.4x). You need resistor around(17V-2V)/0.02A=750ohm up to 1K5. 750 ohm usually too bright. I think 1K or 1K2 will do. With blue/white LED you need twice of resistance.
LEDs often have a maximum current limit ~20mA.
That 0.02A is the maximum. I think you will find that 20mA and even 10mA will be far too bright for an indicator.
Aim for 2 to 5mA (0.002A). You can then add a second resistor in parallel to increase the current/brightness to suit your listening room without havingto remove/damage anything.

I like using Green for OK indicators and using Red for Bad indicators and Flashing Bright Red when the operator must intervene.
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Old 18th October 2010, 01:41 PM   #7
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Are you suggesting 8K5 resistor (did I do that right) to drop the current to 0.002A

I like the idea of making a nice wooden box! ( I know more about that than the innards)
Thanks all for help so far
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Old 18th October 2010, 01:55 PM   #8
Jay is offline Jay  Indonesia
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Join Date: Feb 2003
Location: Jakarta
Quote:
Originally Posted by AndrewT View Post
LEDs often have a maximum current limit ~20mA.
That 0.02A is the maximum. I think you will find that 20mA and even 10mA will be far too bright for an indicator.
Aim for 2 to 5mA (0.002A). You can then add a second resistor in parallel to increase the current/brightness to suit your listening room without havingto remove/damage anything.
I strongly agree with Andrew. That is my experience too (tho I think that the specified maximum is usually 25mA or more). I just thinking that it may be only me who thinks that it is too bright, may be other will think that it is KEWL to have it too bright like that

So it is 7K5 to 5 times that. Parallel with 10K to make it brighter. But I think there is a certain brightness that will give a beautiful look (work in combination with the face plate color and material), so you can experiment with the resistor. I don't like red by the way.
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