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Old 5th June 2007, 08:49 AM   #1
PaulBNZ is offline PaulBNZ  New Zealand
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Default Help!Building first amp......

Hi everyone!
I am really interested in building an amp for an MP3 player or other source of input. This is purely because I would like to listen to my music on it and think perhaps building one would be more interesting than just going out to buy one.
As a bit of a background I have some experience in simple DIY electronics although the audio side of things are a bit of a mystery to me.
I was looking at something stereo, with a power output of not more than 20-25W.(something like this:http://sound.westhost.com/project72.htm)
I have a pair of LM1875 chips that I was thinking of using for the amp itself.
I just have a few questions at the moment based on the little bit of research I have done:
1)Do I need a pre-amp if I am using something like an MP3 Player or Walkman?
2)Should I worry about damaging the input device, as I hear they are designed to function with the impedance of the headphones?
3)I realise the schematic above is for ~25V but would I be able to use components that would function with the 25V supply but for the moment also function with 12V. This is because I already have a few 12V DC supplies for my model planes. I understand the power out will be a lot less however.
Thanks in advance for putting up with my beginner q's!
Cheers
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Old 6th June 2007, 01:13 PM   #2
Nordic is offline Nordic  South Africa
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You will severely restric headroom off a 12V supply...ultimately meaning more distortion. suggested supply voltage is 16 to 60 V

If you can get yourself to buy a bigger transformer, I would suggest you aim for LM3875 chip... much more robust, and will probably serve you much longer... you can't buy an amp in the shop for anything close to the money it costs to build (likely a couple a times more than the cost).

http://myweb.tiscali.co.uk/nuukspot/...gainclone.html
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Old 6th June 2007, 01:57 PM   #3
teemuk is offline teemuk  Finland
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1) If you don't need tone controls, a volume control or other specific features that preamplifiers usually have then you don't.
2) If you keep the DC level in input at zero and input impedance is higher than 32 ohms there should be no risk of damage. Sources that have too high signal amplitude might cause some clipping, though.
3) The circuit in your schematic example is running from +-25V dual supply. There is no good excuse why you should try to tweak it to run from 12V single supply. If you do it, it will only give you mediocre performance. Try a different circuit instead - like some of those based on TDA2005 chip.

Can the model plane supplies deliver enough current for an amplifier?
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Old 6th June 2007, 03:02 PM   #4
Nordic is offline Nordic  South Africa
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http://myweb.tiscali.co.uk/nuukspot/...gainclone.html

P.S. I would not worry about the output level from the ipod... its pretty low, and the current is limited by input resistor...

Look for a kit maybe... should be able to find one quite easily..
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Old 6th June 2007, 03:14 PM   #5
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No need to buy new chips and build a gainclone, the LM1875 will be fine for what you want. It's also a simpler circuit. Running it on 12V should be fine as well.
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Old 6th June 2007, 08:22 PM   #6
PaulBNZ is offline PaulBNZ  New Zealand
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Thanks guys for your posts.
The model plane power supplies that I have can put out about 5A. From memory one of them has some extra tappings, but for the live of me I can't remember what they are. I'll have to measure them.

Also, at a lower voltage it is best to run lower impedance drivers correct?i.e 4ohm instead of 8ohm?

Finally, hypothetically, if I decided to power the amp with around 12v, would I have to change any of the component values (resistors etc.) to suit?
Cheers
Paul
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Old 6th June 2007, 08:38 PM   #7
Nordic is offline Nordic  South Africa
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Yep lower voltage provides more power with lower impendance loads...thats why you normaly get low empendance car speakers.
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Old 7th June 2007, 07:40 PM   #8
PaulBNZ is offline PaulBNZ  New Zealand
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Quote:
Originally posted by Nordic
Yep lower voltage provides more power with lower impendance loads...thats why you normaly get low empendance car speakers.

Thanks.
Would the component values need to be changed in order to make the amp function on 12v?
Cheers
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Old 7th June 2007, 08:10 PM   #9
Nordic is offline Nordic  South Africa
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Not realy, but I would reduce R5...

With gain of 22 you should at least have power levels equal to the input times the gain I would imagine... else your chip would at high levels try to exceed the rail voltages... not posible...at leas not like this...

Look up in the datasheet what the lowest stable gain is for the chip and aim around there... maybe a little higher for stability...
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Old 8th June 2007, 06:56 AM   #10
PaulBNZ is offline PaulBNZ  New Zealand
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At the risk of sounding like an idiot, could you explain this?

Quote:
With gain of 22 you should at least have power levels equal to the input times the gain I would imagine... else your chip would at high levels try to exceed the rail voltages... not posible...at leas not like this...
Thanks
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