Headphone Amp - Hum - Impedance Matching?

Discussion in 'Headphones' started by Me Tarzan, Jan 14, 2019.

  1. Me Tarzan

    Me Tarzan EARTH

    Messages:
    1,130
    Just got this little guy for work to make my day's a little more enjoyable.
    https://www.amazon.com/Multi-Hybrid-Headphone-Amplifier-Audio-Preamplifier/dp/B07DLRFHXT

    • Working voltage: DC24V, 1.5A at least
    • Input sensitivity: 50mV; Gain: 32dB; Input Impedance: 100KΩ
    • Frequency response: 10Hz-60KHz (±0.25dB)
    • Audio input: 3.5mm audio jack & Stereo L/R RCA
    • Audio output: 3.5mm headphone & 6.35mm headphone & Stereo L/R RCA
    • Output Impedance: 20Ω~600Ω; Signal/Noise Ratio: >90dB
    • Dimension: W102× D102 × H37.5 mm (4.02 × 4.02 × 1.48 in)
    • Net Weight: 310g (0.68lb)
    711TmlBHS8L._SL1200_.jpg

    The sound is not great (neither are my earbuds) but there is very noticeable hum that is driving me nuts.

    The output impedance: 20Ω~600Ω. My earbuds have a Nominal Impedance of 16ohms. Is this likely the cause of the hum? I tried some cheap over ear headphones last night with phone as source and I think they had hum ass well. Strange thing is, while sitting here typing this post with music stopped, the hum drops down to an acceptable level. When I turn the music back on the hum comes back and remains even after stopping the music for about a minute or so. Source is iMac/Spotify App headphone jack out to Nobsound. It's like the hum source is being shut off from either the iMac or the Nobsound until the music starts. I need new headphones anyway. Should I buy them first or consider returning the amp? Any more tests I can do?
     

     

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  2. JoeESP9

    JoeESP9 ESL's & tubes since 83

    Messages:
    16,949
    Location:
    Philadelphia PA
    Although headphones with different impedances will sound different they don't cause hum in a device. It's likely the amplifier is defective.

    If you connect the headphones directly to the iMac do you hear any hum? If not the Nobsound is the cause of the hum.

    I have a Bravo V2 which appears to be quite similar to the Nobsound. It is noiseless with any of the headphones I've tried with it. When I use it the signal path is laptop > HiFime DIY USB DAC > Bravo V2 > headphones.

    I have also tried connecting headphones directly to the laptop. IMO the sound is not as good as using the USB DAC/V2.
     
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  3. Me Tarzan

    Me Tarzan EARTH

    Messages:
    1,130
    I tried a couple pairs or headphones, rolled a few tubes and played around with the bias adjustments and came to a combination where the hum is much better. Some tubes had more hum and others had less. Still annoying so probably defective. I guess I will put original tube back in and get unit replaced.

    ETA: It's boxed up and ready for return, not exchange. I think I would be better off putting the money into some decent IEM of some sort...
     
    Last edited: Jan 16, 2019

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