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Speaker output to Line IN

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prprog

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A music piano keyboard (yamaha) I have provide a headphone out that sound terrible (it hum at high volumes) if we connect to a guitar amplifier. Using the attach simple circuit I was able to connect to the amp. It sound better, but it still have a hum sound. It is more of a digital/clock sound , high pitch. What change to the circuit is necessary to eliminate the hum noise.

Thanks,
PRPROG
 

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  • Speaker to Line Out.bmp
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Car stereo installers have a speaker to line level adapter that uses a transformer. That provides isolation. Somewhere on this forum, somebody had the same problem ans was pleased with the result.
 
Car stereo installers have a speaker to line level adapter that uses a transformer. That provides isolation. Somewhere on this forum, somebody had the same problem ans was pleased with the result.

I peruse old threads related to speaker to line conversion and audio transformer ....learn a lot....but I don't find the one that answer my question. Can you please indicate which one is it?

Thanks,
prprog
 
search the internet (also ebay) for audio isolation transformer.
I use a small metal box with audio jacks on each end. (transformer inside)
Price range is 25 to $200. Depends on what name is on the box.
There are some home made designs on the net.
 
I have a hard time searching the forums and here https://www.parts-express.com/boss-b65n-speaker-level-to-line-level-output-converter--265-4040 is cheap one that does everything needed.

Most car stereo install stores stock something similar. It's not possible to make it at that price.

Some of the reasons this is difficult is:

1) The output of the device is "bridged". This is a way of getting increased power from a lower voltage and typically done in car systems. Two out of phase signals are fed to speaker. Using ground and plus may also work with the simple divider you have.

2) Unintentional ground loops. Small resistances, like 2.2 ohms in the negative terminals help.

3) Then there "are times" where it's unsafe: When there is no output and the chassis is connected to one side of the AC in.

4) Tube or valve systems require a minimum load in all cases.

The transformer by itself solves all but #4. It's usually a transformer + attenuator. Electronically, for an active system, you need a differential amplifier so signals common to both inputs are cancelled.
 
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