So the setup that I have is that I have a device that traditionally outputs sound from a speaker but also includes a single headphone output via 1/4" jack that, when plugged in, turns off the speaker. I have been able to use a simple 1/4" to 3.5mm adapter to plug that output into the microphone input of a Syba USB sound card. The problem is that in this configuration I get a significantly loud hum which spectrum analysis of the recorded sound reveals to be 60Hz and harmonics. This hum is not audible from the speaker.
Circuit diagram of the device shows that the only difference between its native speaker output and the headphone output is that when outputting to the headphone output the signal passes through a 150 Ohm resistor. A quick test using a jumper wire to short across the 150 Ohm resistor reveals that sound recordings without it no longer have the hum. My problem is I cannot simply leave it at that, as I cannot modify anything inside the device. So, how can I achieve the same affect by only affecting things after the headphone output?
I thought to try using an op-amp (LM471CN) configured as a simple voltage follower on the signal to provide the microphone with a fresh signal with no impedance. On the oscilloscope this seems to work, until I plug the output end into the microphone input. It turns out the microphone input applies a 4.5V DC voltage (bias voltage to power a microphone I believe) and this seems to be messing everything up. Trying to record audio confirms that nothing really gets recorded. I am using Syba USB sound card with the C-media chipset and the cables I am using are three-ringed.
One more thing, I don't believe it is a matter of simply lowering the output because I wasn't getting clipping of the audio signal before, just the noise hum. And also, strangely enough, I haven't been able to get the headphone output to work using simple headphones through a 1/4" to 3.5mm adapter (the white apple headphones kind the old ones before they added volume controls and mic.).
Any input or suggestions would be greatly appreciated as I'm running out of ideas. Thanks.