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Old 16th July 2007, 08:37 AM   #1
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Default Transformer for LM3886

Hi guys,

I am not really good at the electrical calculations and conversation.

can someone let me know if I can use this transformer
http://sg.farnell.com/jsp/search/pro...sp?sku=9532455
for the lm3886 amp from chipamp.com

Many thanks in advance.

Rgds,
Marcus
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Old 16th July 2007, 08:48 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
this 225VA 18-0 18-0Vac transformer will be suitable for a pair of chipamps powering speakers 4ohm and above.
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Old 16th July 2007, 02:47 PM   #3
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for speakers from 4ohms and above meaning it will be able to drive anything from 4ohms to 8ohms?

I am planning to do up this amp to drive speakers and also another output driving a pair of 600 ohms headphones, will there be any problem.
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Old 16th July 2007, 03:24 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
most solid state amps can drive any resistance above the minimum. So I meant 4ohms to infinity.
It is VERY unusual for SS amps to have an upper limit for load resistance and most will operate safely with the output open circuit, so your 600ohm headphones are no problem.

But you will need an attenuator to drive the headphones.
They will only need a few mW of input to sound VERY loud.
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Old 17th July 2007, 03:32 AM   #5
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will it helps to have a output resistors of abt 220 ohms on the headout
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Old 17th July 2007, 07:18 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
adding a resistor in series with the headphones will cut volume slightly.
But, you need an attenuator.
You should be looking at -20db to -40db to be effective.
A little experimentation will give you some answers.
If you want to start with 220R in series with the phone output and try adding 22r across the phone output. This will give you approximately -20db. See how loud they sound when your volume is set to normal level and how the control feels over a range that you might listen to.
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Old 17th July 2007, 07:44 AM   #7
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Hi andrew,

add 22r across mean across the left and right channel?? or in series
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Old 17th July 2007, 09:47 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi M,
no you need two attenuators, each separately connected to the respective output.

the 220r is in series with the output. The 22r goes from output to return (audio ground). The headphone output is across the 22r.

If the power amp output is about 20Vac maximum, then the 220r will see a maximum dissipation of about 1W8, use three 600mW resistors in parallel (680//680//680=227r).
But, I would use 1k0 in parallel, each will be asked to dissipate 400mW maximum. You can use just one 1k0 to try the volume settings. If this is too quiet then add a second 1k0 in parallel to increase the headphone output. You should not need to add a third but that option or even four, is available depending on the sensitivity of your phones.
So go and buy 10 off 1k0 600mW metal film and 2 off 22r metal film.
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Old 17th July 2007, 01:42 PM   #9
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i dont quite get it is there any graphical presentation?

I am not quite an electronic guy and dun quite understand your description..
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Old 17th July 2007, 02:31 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
-34db attenuator for 600ohm headphones.

add extra 1k0 resistors in parallel to increase the volume at the headphones.

Or,

add extra resistors in parallel to 22r to reduce volume.
Attached Images
File Type: jpg headphone attenuator.jpg (10.7 KB, 440 views)
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