1 word: OVERLOAD. This applys to MC and MM stages. Signal will be so overloaded you would have no way to verify its quality or its ability to pass the much lower voltage a cartridge has. I wouldn't risk it.
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Use pocket cd-player with adjustable volume control set to the very minimum(you may check the output level with voltmeter to make sure that you do not exceed 25mV) and adapter from headphone output to the line RCAs for the phono input. This way you can check out MM section. Otherwise to check an MC section, I'd assume it's working or I'd only recommend turntable record and cartridge.
Hessong, you can in fact use a CD player to "test" your phono section providing you are using a player with a variable output. Many players, Levinson, Sony, Denon, Cary, Audio Aero, etc. do provide you with variable outs. Some actually attenuate in the digital domain which obviously would not apply in your application.
Of course, begin testing with the volume control at it's absolute minimum output level and you should have no problems. All of the above is assuming you are "testing" only.
Sidssp, I have to assume Hessong is simply testing to assure that the phono circuits are passing signal. But, of course you know what happens when you assume. I am not in any way implying that this could be used for "listening" tests. Most CD players with variable outs will have an output level as low as 0.1v. which in no way will harm the actual phono circuit, although it will allow you to test for signal pass. But your correct, it won't sound great.
I use interstaion hiss from an FM tuner with variable output to break in phono stages, so it should work for this purpose. It should have a very rolled off, bassy quality due to the riaa EQ, but should not have any type of cyclical noise. Any 60 cycle component will be boosted, but this may be a good thing.