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Headphone & Amp Impedance Questions? Find the answers here!

Discussion in 'Sound Science' started by proton007, Apr 25, 2012.
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  1. obobskivich

    I think the biggest problem with discussions about audio, is the risk of over-generalizing. For example, bigger drivers can mean more bass, but by themselves do not guarantee it.

    Does Accudio have the Zoros in its database? I also remember hearing about an Audyssey app for iDevices that includes an EQ - might look into that. I'm sure both of these cost money btw. :xf_eek:
  2. radioguy1
    This is reassuring, but I still wonder if these (relatively expensive) headphones I'm about to buy might have any sound degradation or distortion because the headphones have such low impedance.  I'm about to buy Sony MDR-7520 headphones, specs say impedance is 24 Ohms.  My old home receiver is a Denon DRA-275R, specs say amplifier impedance is 8 Ohms.  That's less than the 1:8 impedance load ratio that's suggested.  Do you think they'll still sound good?
    Other specs on the headphones: closed-ear, sensitivity is 108 dB/mW, power handling is 4,000mW, frequency response is 5-80kHz, driver size 50.0mm.  Thanks!
  3. xnor
    The amplifiers output impedance is 0.1 ohm, but that's the speaker output. The headphone output on receivers is usually in the hundreds of ohms. Yamaha for example uses 470 ohm on many of their receivers.
    I doubt you'd even come close to the 1/8th rule on most receiver headphone jacks even if your headphones had an impedance of several hundred ohms.
  4. radioguy1
    Thank you very much, xnor!
  5. MrMateoHead

    That is extremely high impedance. Why, given the impacts of high output impedance on phones, would manufacturers do that? Are there any brands known for having really good headphone jacks?
  6. mikeaj
    Doesn't cost much to just put a big resistor between the speaker amp output and the headphone jack and call it a day. (Adding extra parts to implement a headphone amp most people wouldn't use would cost money.) Plug headphones in, and for many you'll get a reasonable volume. For plenty of headphones, this won't particularly alter the sound much; for others, it could be different, but different isn't necessarily worse. (well, usually it is, in these scenarios)
    That said, I think a lot of those class D chip amps these days include an integrated headphone amp in the chip, so that's kind of a freebie that can be used instead that may have decent specs for driving headphones.
  7. shaocaholica
    Hypothetically, what can happen if speaker level input is fed into a headphone amp?  Lets assume all different kinds of powered amps here.
  8. xnor
    It depends on the design of the headphone amp.
    With something like the O2 the op amps in the gain stage could "die", or you'll at least get extreme clipping. With other headphone amps you may be fine but only if you turn the volume control all the way down. Still, I wouldn't suggest hooking up a headphone amp to a speaker amp, not even using the headphone jack of the amp/receiver. We have line-outs with standardized line levels for a reason.
  9. shaocaholica
    Thanks.  So what exactly is 'line-out standardization'?  As in, what are the expected maximum levels?  How can I measure a source to see if its in spec?  I have a 'Pro' Denon cassette deck DN-790R which has its HP output tied to its line out (by design) with no volume adjustment on the HP out.  Its way too loud to drive any HP and I'm just going to hook it up using a passive limiter (Art Headtap) but I'm curious how I can measure the Denon's output to see how high it can go.  What kind of instrument can I use?  Can I use a DMM with Vrms and say a 440hz tone with the record levels at max?  Pink noise?
  10. stv014
    Use tones at a few frequencies (some DMMs are only usable in a rather narrow frequency range). Something like 50 or 60 Hz, and 400 or 1000 Hz. If you are lucky, the measured levels will not differ too much (for example, I get less than 1% variation in the range 30 to 2000 Hz with a cheap old DMM, but others might be much worse, especially some newer cheap ones with "true RMS" capability). Otherwise, if there is a large difference, you should probably trust the value at 50-60 Hz. It is also recommended to measure how much the level drops with a resistor load, and calculate the output impedance.
  11. xnor
    We have the consumer -10 dBV which is about 0.316 Vrms and pro audio (usually balanced) +4 dBu which is about 1.228 Vrms. Those are nominal levels. Professional equipment for example may have headroom up to +24 dBu so there won't be clipping up to about 12.3 Vrms.
    CD players usually output 2 Vrms for a full-scale sine wave, as do some DACs.
  12. stv014
    Perhaps related to this topic, I did some tests on how the output impedance of the amplifier affects the distortion of dynamic headphones. The headphone tested was a DT880 Pro (250 Ω), using sine sweeps at 0.68 and 0.22 Vrms voltage on the drivers at 1 kHz, and with output impedances of 10.3 Ω (Xonar Essence STX headphone output) and ~110 Ω (using 100 Ω serial resistors).
    Unfortunately, the measurements at low levels contain too much microphone and ambient noise. At the higher level, I repeated the tests twice, to be able to verify the repeatability of the results. The sweeps played were equalized so that the SPL and frequency response are matched at the low and high output impedance. The graphs below assume that 1 Vrms voltage on the driver at 1 kHz produces 100 dB SPL. This is probably not accurate, but the levels on each graph should be consistent nevertheless. Only the right channel was tested.

    These are the results I got (click to zoom):
    Frequency response: from left to right: microphone, headphone driver, amp output
    mic_freq.png     drv_freq.png     amp_freq.png
    THD vs. frequency: from left to right: microphone, headphone driver, amp output
    mic_thdf.png     drv_thdf.png     amp_thdf.png
    Driver impedance vs. frequency:

    Increasing the output impedance of the amplifier does indeed seem to increase the bass distortion of the driver, especially around its resonance frequency. This cannot be explained with the amp "working harder" (see graphs), or the headphone having to produce a higher SPL in the bass range (because it is equalized). The effect is not major, however, it is a difference of about 2 dB, but it is there. With a low impedance source that is equalized to match the frequency response of a high impedance one - which is what I did  - the distortion is lower than it is with actual high output impedance.
    Although the effect probably varies depending on the headphone model, and some are more affected than others. It would be interesting to try the same with orthodynamic headphones, to confirm if they are not only a more or less purely resistive load, but also fairly linear as well.
    Kaffeemann likes this.
  13. proton007
    Hi guys,
    After searching for a simpler way to explain these concepts to the many people who ask for advice, I've added a Hydraulic Analogy in the beginning. I think it can help form a good abstraction for the concepts in the proceeding sections.
  14. eujaee
    Thank you, proton007.  Your explanation of electrical dampening was the easiest to understand of all the ones I read.  Your article is very useful and informational, thank you for taking the time.
  15. proton007

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