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Music Manic
active member

Joined: 20/12/02
Posts: 1885
Loc: London UK

Headphone Amp Impedance mismatch?
#932048 - 02/08/11 07:30 PM

Hi guys,

I've just realised that my headphone amp has a specification of <40 Ohms on its output load.

I've been using the Sony MDR 7509s for years and have been satisfied. I always wondered why it is so loud at a low level, because the Sony headphones are rated at 24 Ohms. Would this cause distortion at higher level. Are the cans driving the amp too hard?

If so, which headphone amp provides better impedance to such low impedance headphones?

Thanks

Thanks

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James Perrett

Joined: 10/09/01
Posts: 9645
Loc: The wilds of Hampshire

Re: Headphone Amp Impedance mismatch? new

[Re: Music Manic]
#932155 - 03/08/11 09:40 AM

This all sounds a little confusing at the moment.

Tell us exactly what the specification says and you'll get a better response.

If it says that the output impedance is <40 ohms then that doesn't mean that it won't work with low impedance headphones. If it says that the recommended headphone impedance >40 ohms then it means that there is a chance that you could damage the output stage with low impedance headphones.

Headphone amps sometimes have a relatively high output impedance compared to amps designed to drive speakers as it means that they can be used with a wide range of headphone impedances without a big change in volume.

James.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

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Music Manic
active member

Joined: 20/12/02
Posts: 1885
Loc: London UK

Re: Headphone Amp Impedance mismatch? new

[Re: James Perrett]
#932276 - 03/08/11 03:50 PM

Hi James

Max output power 100mW into 40W
>100mW for loads >40W

THD <0.02% 20Hz-20kHz unweighted
<0.007% 20Hz-20kHz "A" weighted
S/N ratio >80dB unweighted
>86dB "A" weighted

Power requirement 12V AC 500mA (via mains
adaptor supplied)

Dimensions (mm) 110 x 110 x 220 (W x H x D)
width and height include feet

All measurements are made at 3.5mW into 40W

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Folderol

Joined: 15/11/08
Posts: 2542
Loc: Rochester, UK

Re: Headphone Amp Impedance mismatch? new

[Re: Music Manic]
#932320 - 03/08/11 07:59 PM

Quote Music Manic:

Hi James

Max output power 100mW into 40W
>100mW for loads >40W

THD <0.02% 20Hz-20kHz unweighted
<0.007% 20Hz-20kHz "A" weighted
S/N ratio >80dB unweighted
>86dB "A" weighted

Power requirement 12V AC 500mA (via mains
adaptor supplied)

Dimensions (mm) 110 x 110 x 220 (W x H x D)
width and height include feet

All measurements are made at 3.5mW into 40W

This still doesn't look right. You sure it's not:

Max output power 100mW into 40R
<100mW for loads >40R

THD <0.02% 20Hz-20kHz unweighted
<0.007% 20Hz-20kHz "A" weighted
S/N ratio >80dB unweighted
>86dB "A" weighted

Power requirement 12V AC 500mA (via mains
adaptor supplied)

Dimensions (mm) 110 x 110 x 220 (W x H x D)
width and height include feet

All measurements are made at 3.5mW into 40R

At 24R (ohms) you will get nearly twice as much output but are likely to be severely stressing the amp unless you keep the volume well down.

If the amp is good enough to be worth it (don't know the model myself) you could try fitting 2:1 or even 3:1 step-down transformers between the outputs and the phones.

--------------------
It wasn't me!
(Well, actually, it probably was)

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Music Manic
active member

Joined: 20/12/02
Posts: 1885
Loc: London UK

Re: Headphone Amp Impedance mismatch? new

[Re: Music Manic]
#932322 - 03/08/11 08:03 PM

Yes Sorry it is the Omega impedance symbol.
It didn't transfer when I copied and pasted it.

I did have to use it at low level to get any use.
I am just having a resistor built for it.

Thanks

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James Perrett

Joined: 10/09/01
Posts: 9645
Loc: The wilds of Hampshire

Re: Headphone Amp Impedance mismatch? new

[Re: Music Manic]
#932368 - 04/08/11 09:31 AM

The 40 ohm impedance mentioned may just mean that there's a 40 ohm series resistor already built into the box. If it works at the moment with 24 ohm headphones then keep the volume sensible and don't worry. You will only damage it if it doesn't have a series resistor and you run at excessive levels.

James.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

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