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Advice needed

 

Todd S
Unregistered guest
Hello.

Ok here's what I want to do, though I'm not clear if it is something I should do... I recently acquired an old Motorola wall mounted stereo with a fold down phonograph. Tube Amp and all. I got the box mainly to convert the casing into a mp3 type jukebox. Was planning on gutting it and replacing all the needed hardware with computer stuff while maintaining the vintage look. For kicks when I got it home I plugged it in to see if it was still functional. It powered up and worked with out problem, sounded quite nice too. So then I thought that I might just use the existing amp to power the pc sound. When disassembling the phonograph I saw that it had two RCA (two Males) hookups. So I thought hmmm... maybe all I have to do is just go from the pc to those RCA plugs, switch the amp to phono and it's almost a done deal. I didn't have the right RCA plugs so had to do a male to male connection to test output. I also had to re-plug the power cable into the turntable and switch it to On to get audio output. I got output with lots of noise/hum. Before I get too far into this, is using the phono RCA in a bad idea for use with pc sound? The 2nd question is, any ideas what I need to do to fake the amp into thinking that the phono is on?

Thanks for any feedback you can give.

Todd
 

New member
Username: Mike_w

Post Number: 2
Registered: Nov-04
Gday Todd

Be very careful feeding pc output into a phono input on an amp. A moving magnet phono cartridge generates something in the vicinity of 5 millivolts, although your phono sounds old enough to have a ceramic cartridge which generates higher voltage output, but still nothing anywhere near what the pc will be putting out, which could be up to 5 volts. So the phono input on the amp would be designed for a line voltage of a thousandth of what it may get from the pc. The noise you are hearing is probably line noise from the pc which is always there, but at a level to low to be audible... until you feed it into the phono input of an amplifier.

Even if you could do it, you would have a problem with equalisation. Phono inputs are equalised to the RIAA standard which reduces some of the higher frequencies and increases the lower to flatten out the cartridge output. So any output from the pc, which would already be equalised to your final preferred mix, would be distorted by the RIAA equalisation.
 

Todd S
Unregistered guest
Thanks for the feedback. Decided to go with the original plan.
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