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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.

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    Interesting. Try passing the output through a capacitor to remove DC. – Johannes Nov 17 '15 at 9:36
  • You have a level difference here I think. You should try a line level input. – Marc W Nov 17 '15 at 14:52
  • I think you actually want just a TS (single ring) 3.5mm plug on the microphone side. Frankly, plugging into a 3.5mm mic/line input jack on a sound card is usually terrible for quality. You really need an audio interface. The hum could just be from the computer's power supply getting into the sound card ground connection. – Todd Wilcox Nov 17 '15 at 15:51
  • Will try the capacitor. What should I consider when selecting the value?I don't have a line level input. I am not too concerned with the quality. The recording computer doesn't have any issues with quality or the sound card ground connection. As a test I can record music the headphone jack of my macbook with more than suitable quality for my purposes. Also, I mentioned the hum is no longer present if I remove the 150 Ohm resistor from the device that I am recording. With resistor, hum. No resistor, no hum. However I can't simply permanently modify the device generating the sound. – Dennis Nov 17 '15 at 17:11
  • The 60Hz hum isn’t the mains en.wikipedia.org/wiki/Mains_hum ? – Simon White Feb 6 '16 at 20:51
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There are a few things that are causing problems for you here. Mainly that your source is designed to drive a set of headphones and your load (sound card) is designed to accept an electret microphone at its input.

I haven't been able to get the headphone output to work using simple headphones through a 1/4" to 3.5mm adapter

First confirm that your source is working properly. Depending on what it is, you might be able to plug it into a line in on a different USB interface. That'll work fine for iPhones/iPods and things like that.

You don't want to plug a headphone amp into a microphone input on a sound card like that. They are not designed for each other. The headphone amp could clip the input of the sound card and you will have noise issues because neither device is seeing the impedance it expects.

The sound card is providing DC phantom power for a microphone which you probably don't want to connect to your source.

The 150 ohm resistor in the headphone amp is there to make the performance more consistent with various impedance loads (headphones).

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