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Amp noise with only one headphone?

post #1 of 7
Thread Starter 

First, let me just say I was unsure which part of the forums to post this in, but this part seemed a safe bet. So let me describe my conundrum:

I have a Kenwood KR-V7070 receiver and I'm using the headphone out with a few of my headphones. I tried it with my T50rp IIs, MDR-MA900s, ATH-M35s, HA-RX900and even some low-end in ears (V-Moda Bassfreq Metal). 

Out of all these various headphones, only the RX900 suffers from a low volume static/hiss noise at even the lowest volume setting on the receiver (if it matters, the noise is equal volume at all volume settings) and additionally, the RX900 doesn't suffer from any sort of noise when using it with my Schiit Magni at normal and even moderately high volumes, nor with my Sansa Clip+ and computer.

 

Is there anyone who knows what might be up here? I'm unsure of the receiver specs, but I'll post all the headphone info I think might be useful below:

Audio Technica ATH-M35: 65 ohms 100 db/mw

 

Fostex T50rp II: 50 ohms 98 db/mw

 

JVC HA-RX900: 64 ohms 105 db/mw


Sony MDR-MA900: 12 ohms 104 db/mw

 

 

Thanks for anyone who can help me understand this my problem! c:

post #2 of 7

The HA-RX900 has the highest efficiency out of those headphones. Also, your receiver could very well have a rather high output impedance (possibly much higher than the impedance of any of your headphones), which means that the power output into the HA-RX900 is the highest as well, unlike with a low impedance source. Therefore, the HA-RX900 has the highest noise SPL when driven by the receiver, which was probably designed only for driving speakers, and the headphone output is simply connected to the speaker output through some large serial resistors.

post #3 of 7

Indeed high sensitivity means that you're more likely to be able to hear the noise produced by the amp. If you had highly efficient speakers you could probably hear the hiss as well, maybe you already can if you put your ear next to the speaker.

 

 

Why doesn't the hiss change with the volume control? Think of it this way:

Input -> volume control -> amplification (produces the noise you hear) -> output. The volume control doesn't attenuate the output, so since the noise is generated in a later stage it doesn't matter what you set the volume control to.

post #4 of 7
Thread Starter 

I see, I see. But would 1 db/mw difference really bring out the noise? I can't hear ANY noise with my MA900s, which have similar sensitivity but lower impedance. But this is pretty informative none the less, thanks.

post #5 of 7
Thread Starter 

I see, I see. This may be a dumb question, but does that mean the closer the headphone's impedance is to the output impedance (if it's higher than the headphone's impedance) the more power it'll receive? 

post #6 of 7

You have to understand two things:

- higher output impedance will reduce the voltage into the load (reduces efficiency)

- if you have a fixed output impedance > 0 then the amount of voltage the load receives also depends on the impedance of the load itself

 

 

Example: the amp (before any output resistors) outputs 1 V

 

output impedance = 0

load = 64 ohm => load gets 1 V

 

output impedance = 100 ohm

load = 64 ohm => load gets only 0.4 V

load = 12 ohm => load gets only 0.1 V

 

Without touching the volume control the 12 ohm headphone will receive 12 dB lower voltage. This difference gets worse with higher output impedance.


Edited by xnor - 10/5/13 at 5:26pm
post #7 of 7
Thread Starter 

I think I understand that, thanks for taking the time to break it down for me. :)

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