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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. Vasiliosn
    This is my receiver,   http://www.ebay.com.au/ctg/Sony-STR-DE597-6-1-Channel-100-Watt-Receiver-/46557150?_tab=1 
     
  2. proton007
     
    The 'volume' of sound refers to perceived loudness, and is variable in two respects. One, the frequency determines how loud it will sound at a particular dB level, that is, different frequencies at different dB can sound equally loud. It's called the frequency and loudness contour.
     
    Second, this response will vary due to natural variations, occupational effects, and many other individual reasons (listening habits, health issues, age etc).
     
    In addition, the recording itself, the dynamic range and the mastering will also play a role. At the same volume setting, some albums sound louder than others.
     
    So, as such you can determine the dB levels for impedance matching, but volume setting is mostly an exercise left to the listener.
     
    There are some tips in the main topic on page 1, but to add on, remember that our ear is also subject to fatigue and auditory masking (meaning the louder some frequencies get, they hide other frequencies from being perceived), so just keep this in mind when setting volume levels.
     
  3. Ruben123
    So... Ive bought a Voodoo-ed Samsung GS1 for driving my IEMs, BUT, no one actually knows the impedance but it should be around 8 Ohms. Does that mean it is a waste of money since my IEMs are 12-50 Ohms? (8x8=64 so none ´fits´)
     
  4. stv014
    You could measure the impedance yourself, it only requires the following as a minimum:
    - generate a test signal (sine wave) with any suitable PC software, and copy it to the GS1 in a supported file format
    - a test load, ideally a resistor, but even a headphone or IEM can be sufficient if it has a known resistive impedance at a frequency
    - a splitter to be able to measure the voltage on the load, one with 1 female and 2 male 1/8" TRS connectors should be suitable
    - something to measure the ratio of AC voltages with - a cheap digital multimeter, or even the line input of a PC sound card (the HD audio codec on the motherboard is sufficient) with software to analyze RMS levels
    Measure the voltage with and without the load (if it is an IEM or sensitive headphone, make sure it can handle the level used in the test), and then use the following formula to calculate the output impedance:
     
    Z_out = ((V_unloaded / V_load) - 1) * Z_load
     
    Depending on your IEMs, a high output impedance may or may not be a problem. Some have fairly "flat" impedance and will not sound much different, while the effects on multi-driver balanced armature IEMs can be significant. You can check if your model(s) are measured at InnerFidelity, and how much their impedance varies over the audio band.
     
  5. thepooh
    Hi ! I noticed a strange fact on my Iphone 6, while switching from a 60 ohms headphones Koss Porta Pro to a 16 ohms Sennheiser IE80, I have to raise 10 % more volume on the iem than on the headphones to get the same loudness !
     
    How could a 60 ohms/102 db headphones sound louder than a 16 ohms/125 db iem ?
     
  6. stv014
    The Sennheiser's sensitivity is probably specified in dB/V, rather than dB/mW, in which case it is not as much more sensitive as the specs would suggest. At InnerFidelity, the Porta Pro was measured to require 47 mV for 90 dB SPL, while it is 36 mV for the IE800 (2013) - not the same as your IEMs, but maybe similar enough. Additionally, if the iPhone has high enough output impedance, the voltage on the IEMs will be lower (by about 2 dB if the output impedance is 5 ohms). There might also be some random manufacturing variation, or the IEMs not having a perfect seal, or the perceived loudness could be affected by frequency response differences.
     
  7. thepooh

    Thanks mate ! I'm fixed now  [​IMG]
     
  8. snip3r77
    Nice post
     
  9. Zizou21
    Excellent post! But I'd have to say I couldn't understand it at all. I went fine through the water sprinkler explanation and then I got lost.
     
    I am interested in acquiring a portable amp for my HD558 but got concerned about the part it says it may damage my headphones with a wrong setup.
     
    Should I get a Portable Amp at all? And what one? I've been reading about those FiiO's out there. They look good.
     
  10. proton007
     
    It's fine if you could understand the first explanation...that's the jist of it, the rest is just putting 'measurements' in their proper context so that we can predict and calculate things without having to rely on feel.
    That goes for most science btw. Things are not as hard as they seem.
     
     
    The HD558 would work fairly well, but the impedance bump (250Ohms at 100Hz) would need some power. Best go for a standalone amp if you don't need to carry your headphones around.
     
  11. stv014
     
    It does not. It needs low output impedance to maintain a flat frequency response, but the power output at the resonance frequency is actually lower. For a relatively constant power output (1 dB variation over the audio band), one would need an amplifier with ~120 ohms output impedance, but that would add a 7.5 dB bass boost compared to a zero output impedance amplifier.
     
  12. proton007
     
    Thanks for the clarification!
    This would mean a near zero output impedance amp would work, portable or not.
     
  13. joegiz
    Excellent thread! Helped me get a good handle on impedance. Now if I can find an input source (good cd transport or high res streamer to match my my WA2 and HD800's) I will be all set!!
     
  14. HiFiGuy528
    My reference IEM is the Sony XBA-4 & AKG K3003 both are @ 8 Ohms.
     
    http://store.sony.com/4-driver-in-ear-headphones-zid27-XBA4/cat-27-catid-EOL-Headphones-Earbuds
     
    This is a tricky IEM for a lot of solid state amplifiers to drive effectively.  I see that most SS headphone amps if not all are rated to be used with hps between 16-600 Ohms.  Besides the negative effects of high output impedance on an amp when using this IEM (must be 1 Ohm or below), how come amps can't drive this load or not designed to drive it? 
     
  15. arnyk
     
    As is pointed out, V_loaded V_loaded and V_unloaded are divided out which has the side effect of making the recommended measurements of them with uncalibrated equipment and equipment that only reads out voltage in arbitrary units as good as anything.  Thus $5.95 DVMs and sound card inputs can be used for this measurement as unbelievable as this may seem. 
     
    I put the equation into a spread sheet so that it is easy to do a number of measurements (for example at different frequencies) and have a lot of results with very little effort.
     
    "Drives just about everything well" headphone amps such as the Topping NX1 and Fiio E11 (must be used with inexpensive separate USB power supplies that are isolated from the source) are economical solutions when the source is substandard or unknown in this regard.
     
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