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Impedance Adapter

post #1 of 4
Thread Starter 
Hello all, I'm slowly trying to learn the science behind audio signal/components and recently was recommended a purchase I found interesting. I've been using my fairly low impedance IEMs with a portable amp (JDS C5) even though my IPC yields plenty of volume. In an effort to justify the use of the amp since I carry it around with me anyway, I was suggested to purchase an impedance adapter that will raise the impedance level of my IEMs. Could someone weigh in? Both on whether this claim will improve/affect SQ AND explain how an impedance adapter ADDS impedance. Many thanks, Joe
post #2 of 4

An impedance adapter is a series resister with the capacitive reactance load of a headphone from cable, driver, and voice coil.  For a low impedance headphone ( R < 20 Ohms ) there can be issues with amping due to poor damping.

 

You can read about damping here:

http://en.wikipedia.org/wiki/Damping_factor

 

You can learn more about headphone impedance here:

http://www.head-fi.org/t/571384/low-impedance-vs-high-impedance-huge-difference

 

Impedance Adapter Thread from 2007:

http://www.head-fi.org/t/244680/impedance-adaptor-for-headphones

 

A good read about headphones:

http://www.headphonesfordummies.com/p/headphone-101.html

 

Series Resistors:

http://physics.bu.edu/py106/notes/Circuits.html

 

In general OTL ( Output Transformerless ) amps do not function well with low impedance loads like IEMs because they do not supply enough current to the load.  At low impedance values the source of power from an amp is mostly current coupled instead of voltage coupled.  A ton of amps out there cannot handle the current load and this end up having noise issues and other problems.

 

Adding impedance is series with a headphone driver / voice coil will increase the impedance, but may have other unforeseen effects such as crosstalk and added noise via reflection.

 

Do you have a link to the particular adapter in question?

 

The output impedance of the JDS C5 is 2.2 Ohms which should not cause serious issues with an IEM, but if proves to be noisy installing a series resistance adapter could help.

 

http://www.jdslabs.com/item.php?fetchitem=70

post #3 of 4
Quote:
Originally Posted by NA Blur View Post
 

In general OTL ( Output Transformerless ) amps do not function well with low impedance loads like IEMs because they do not supply enough current to the load.  At low impedance values the source of power from an amp is mostly current coupled instead of voltage coupled.  A ton of amps out there cannot handle the current load and this end up having noise issues and other problems.

If by "a ton of amps" you mean "none", then I'll agree.  I did a quickie survey of Z in about a dozen brands of IEMs (data from headphone.com) and found most above 30 ohms, only a few under 30 ohms, which is still not a low impedance load.  There are a hand full down at 16 ohms. But it's still not a problem, because they take no power to drive.  As to high current, well, IEMs are typically highly sensitive, being shoved down your ear and all, and need next to no power to achieve eardrum-fracturing levels, so low power + high sensitivity = low current.  Of all headphone types, IEMs are the flat-out easiest to drive, OTL, or OT, no matter.

 

Take for example the Etymotic HF3, 16 ohms, 105dB@ 1mW.  That means you hit 105dB with .13 volts, and 7.8mA of current.  Anything can do that, even opamps from the 1970s can do that!  And as to noise, well the C5 specs noise at -105dBu, which, when used with the HF3s puts it at 15dB SPL, the equivalent of a really decent recording studio, and the blood rushing through your ears will be more of an issue.

Quote:
Originally Posted by NA Blur View Post
 

 

Adding impedance is series with a headphone driver / voice coil will increase the impedance, but may have other unforeseen effects such as crosstalk and added noise via reflection.

Nope, no added crosstalk by adding a series resistor.  None.  Nada.  Though why would you anyway, there's simply no need. \\

 

 And what the heck is all this "added noise via reflection"???  I've seen my reflection...it's noise free, just not nose free. 

 

 

(edit: changed "output Z" to "Z" in first 'graph)

post #4 of 4

"hiss buster" can be a divider with 2x R, another low value R to gnd in parallel with the headphone drivers as well as the series R - can get back some damping/frequency flatness

 

multi-driver iem can have significantly "lumpy" Z with frequency - with just a series R the bumps/dips show up as modified frequency response compared to lo Z amp output drive that most are designed for

 

really sensitive iem do require substantial volume turn down even with DAP which may have 0-5 - 1 Vrms output

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