• Registered: 2016-10-13
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Topic: Low impedance headphones (25 ohm) and the RME AIO Headphone output

I have a question for MC. I own the RME AIO. I'm looking at the Fostex TH900mk2 headphones. They have an impedance rating of 25 ohms. I've gathered that ideally the source impedance should be an 1/8th or less of the load impedance. So in the case of the Fostex, 3.125 ohms or less from the source output. Otherwise, frequency response can be affected.

Looking at the specs for the headphone output for the RME AIO, it's 30 ohms. This is great for 250/600 ohm headphones, but I'm expecting it to be an issue for the super low impedance loads like the Fostex.

MC, could you share your thoughts or confirm what I'm thinking above.

Thanks

2 Reply by MC

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Re: Low impedance headphones (25 ohm) and the RME AIO Headphone output

The rule with 1/8 becomes most relevant with IEMs, which often have a wild impedance swing over frequency, which then causes audible sound changes. The Fostex TH900 has a quite linear impedance versus frequency curve, mostly at 25 Ohms, up to 32 Ohms in bass, that will not cause audible changes with a 30 Ohms output. So you will have no problems with it at the AIO output.

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Matthias Carstens
RME

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  • Registered: 2016-10-13
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Re: Low impedance headphones (25 ohm) and the RME AIO Headphone output

MC wrote:

The rule with 1/8 becomes most relevant with IEMs, which often have a wild impedance swing over frequency, which then causes audible sound changes. The Fostex TH900 has a quite linear impedance versus frequency curve, mostly at 25 Ohms, up to 32 Ohms in bass, that will not cause audible changes with a 30 Ohms output. So you will have no problems with it at the AIO output.

Awesome news! Thank you, Matthias. Now I just have to figure out how to lower the noise floor of the headphones output. I have a slight hissing sound. I've read if it even the PCI bracket touches the metal on the case (has to happen unless I insulate the bracket) this can happen.

4 Reply by MC

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Re: Low impedance headphones (25 ohm) and the RME AIO Headphone output

Should be free of noise as this effect only affects line connections to other equipment. That noise is coming from external ground connections - which you not have when wearing a headphone...

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Matthias Carstens
RME

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  • Registered: 2016-10-13
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Re: Low impedance headphones (25 ohm) and the RME AIO Headphone output

MC wrote:

Should be free of noise as this effect only affects line connections to other equipment. That noise is coming from external ground connections - which you not have when wearing a headphone...

Thanks for the reply. Do you have any suggestions or ideas on how I can eliminate that noise I'm getting from the headphone connection?

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Re: Low impedance headphones (25 ohm) and the RME AIO Headphone output

You have noise with only the headphone connected? And TotalMix not sending an input signal to the phones output? That is quite unusual. I don't know if something like the tape isolation will help, but you could try.

Regards
Matthias Carstens
RME

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7 Reply by aplats (edited by aplats 2016-10-18 12:43:33)

  • Registered: 2016-10-13
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Re: Low impedance headphones (25 ohm) and the RME AIO Headphone output

MC wrote:

You have noise with only the headphone connected? And TotalMix not sending an input signal to the phones output? That is quite unusual. I don't know if something like the tape isolation will help, but you could try.

Actually I did have my monitors plugged in to the line level. So they can be causing a ground loop. I'll disconnect them when I get off work and see if it goes away. If it doesn't then I still suppose it could be coming from grounding to the case.