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Old 28th November 2005, 02:33 AM   #1
Pato is offline Pato  Chile
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Default Need some help

Hi folks.
This is my first post here.
The case is I need some help.
As I was young, (yes,believe me, I was young some time) I built several amps and electric guitars, some good ans some bad sounding. But due to lack of time and other interests I lost the way at the begining of IC's and now I am building a chipamp with 2 LM3886's in stereo.
The fact is that I don't know if it needs a preamp. I am going to use it as a standalone for my turntable, tape etc.
Your help will be welcome.
Greetings from Chile.

Oscar.
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Old 28th November 2005, 02:42 AM   #2
Stocker is offline Stocker  United States
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You will want some way to attenuate the volume if the sources do not all have volume level controls. This could be a potentiometer on the amplifier itself. Also a preamp can have proper equalization for the record player, if it is not built into your turntable.
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Old 28th November 2005, 05:45 AM   #3
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Hi Oscar and welcome

Since tape decks generally have a much lower output than components like CD players, it will most likely need some gain, and I'm assuming your turntable is an older one which would almost certainly require a phono preamp (which goes before the main preamp, though you may build a preamp which includes a phono stage). There are now available turntables with line level outputs, however I beleive that the market segment they are aimed at is people wanting to record (to their pc) their old records, and I'd doubt that they are very high quality.

So I think that yes you will need to build a preamp

You can probably get away with just a pot for the tape and components other than the turntable, but you may not have high enough input levels to get maximum power out of the gainclone I don't know off the top of my head what it's minimum input level is for full output.

Tony.
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Old 28th November 2005, 05:59 AM   #4
Nordic is offline Nordic  South Africa
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Quote:
Normally the gain is set between 20 and 200; for a 40W, 8Ù audio amplifier this results in a sensitivity of 894 mV and 89 mV, respectively. Although higher gain amplifiers provide greater output power and dynamic headroom capabilities, there are certain shortcomings that go along with the so called “gain”. The input referred noise floor is increased and hence the SNR is worse.
this is from the LM3875 datasheet, I'd assume the 3886 is similar
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Old 2nd December 2005, 03:06 AM   #5
Pato is offline Pato  Chile
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Thanks for your answers.
I understand that my turntable needs a preamp due to the magnetic cartridge, but the other periferials will work without it?
Of course they will need some equalization, so the final sound level will be the same level to all of them.
By the way, what is a buffer for?

Thanks a lot.

Oscar.
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Old 4th December 2005, 09:24 PM   #6
Stocker is offline Stocker  United States
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a signal source that can't push the current you need.

Say you have a whopping 2V output from your CD player... into a 10Kohm load. The current is pretty small. now give it a 600ohm input impedance Pass-designed amplifier... The CDP tries to give the current but can't and the voltage output is going to drop also. Insert an op-amp set up as a buffer that has a friendly 10K input impedance but also can push some current and the CDP is happy to swing its 2V again. Buffered.
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