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Old 16th April 2012, 10:37 PM   #1
3n2323 is offline 3n2323  United States
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Default stereo to mono input

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hi all,

i found the above diagram from the forum, when searching for a way of feeding the stereo singal from a CD player to a simple lm386 mono amp.

two newbie questions,

1) if LR hot are connected together directly, without the 1K's that is, what possible damages might occur to the CD player, how or why?

2) what do the two 1K resistors do, and how possilble damages to the CD player are avoided with them?

thank you for helping!

Last edited by 3n2323; 17th April 2012 at 01:14 AM. Reason: accuracy
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Old 17th April 2012, 12:24 PM   #2
AndrewT is offline AndrewT  Scotland
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use an inverting opamp to couple together a pair of inputs.

The inverting opamp acts as a mixer/adder/sumer(summer?)


Vout = V1in + V2in if the three resistors are equal value.

Two signals of 300mVac, that are exactly in phase, will output 600mVac after the opamp.

The schematic shown combines the signals but also partially cancels the stereo. While the mixing cable is connected you cannot get a stereo signal from the Source.
Missing out the 1k resistors should not damage the Source amplifiers. But I cannot give a guarantee.
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Old 17th April 2012, 12:40 PM   #3
wwenze is offline wwenze  Singapore
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It won't (usually, hopefully) damage the source because the source usually has its own output impedance, can be any 2-digit to 4-digit ohm value. But each channel will have more distortion as they try to cope with the lower load.

An ideal amp will have it's output pin behave like zero ohm to ground.
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Old 17th April 2012, 06:44 PM   #4
3n2323 is offline 3n2323  United States
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Join Date: Apr 2012
Quote:
Originally Posted by AndrewT View Post
use an inverting opamp to couple together a pair of inputs.

The inverting opamp acts as a mixer/adder/sumer(summer?)


Vout = V1in + V2in if the three resistors are equal value.

Two signals of 300mVac, that are exactly in phase, will output 600mVac after the opamp.

The schematic shown combines the signals but also partially cancels the stereo. While the mixing cable is connected you cannot get a stereo signal from the Source.
Missing out the 1k resistors should not damage the Source amplifiers. But I cannot give a guarantee.
thank you Andrew!
now it is clear that the above schematic is not ideal, i wonder why people do it this way, i've such scheme at many places.
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Old 17th April 2012, 06:45 PM   #5
3n2323 is offline 3n2323  United States
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Join Date: Apr 2012
Quote:
Originally Posted by wwenze View Post
It won't (usually, hopefully) damage the source because the source usually has its own output impedance, can be any 2-digit to 4-digit ohm value. But each channel will have more distortion as they try to cope with the lower load.

An ideal amp will have it's output pin behave like zero ohm to ground.
thank you wwenze for your reply!
alright there's no need worring about the source, distortion bothers me now. guess the best way is as Andrew said, use an opamp.
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