I created this thread because impedances can be very confusing to non-DIYers. an impedance adapter in the general actually refers to an impedance INCREASING adapter, and is often used incorrectly. Here is a good article explaining why impedance matching is important.
Impedance Increasing adapters
Generally referred to when referring to an impedance adapter, but often used incorrectly, myself included. It essentially increases the output impedance of the source or amp.
Used to allow you to use more sensitive IEMs with your desktop amps. It lowers volume and raises the noise floor, essentially making your IEM's less sensitive.
|Etymotic ER4P -> ER4S adapter||$50||75||Cabled|
|Hisound Golden Impedance Cable||$60||70||Cabled|
|APureSound Mini to Mini||$45||75||Adapter|
|APureSound Mini to Mini||$45||120||Adapter|
|APureSound Mini to 1/4"||$55||75||Adapter|
|APureSound Mini to 1/4"||$55||120||Adapter|
|APureSound||$60||Mini to Mini Adapter|
|APureSound||$70||Mini to 1/4" Adapter|
|Ebay: AW Audio Accessory||$22||Mini to 1/4" Adapter|
|Ebay: AW Audio Accessory||$40||Mini L shaped Adapter|
|Ebay: AW Audio Accessory||$18||Mini to Mini Adapter|
|Ebay: WeMakeAmp Audio Workshop||$12||Mini to 1/4" Adapter|
|Ebay: WeMakeAmp Audio Workshop||$11||Mini to Mini Adapter|
I would like to update this list, so please add suggestions.
(Faux) Impedance Decreasing Adapters
What these would do is decrease the output impedance of a source or amp and allow it to pair better with certain low impedance headphones, which are affected quite a bit by bad impedance matching.
Note: not impedance transformers, since they can degrade sound quality, it is not a problem in very high end Amps such as those from Woo audio.
Note: The output impedance is not changed in any way, it appears lowers from the headphones perspective, and the load impedance appears higher from the amps perspective,
What you need is an adapter that essentially has the following configuration(for each channel,) the downside is it wastes power, but using the ASUS Xonar essence STX as an example it will lower the output impedance of the soundcard from 10 ohms to about 4 ohms. And still have a maximum SPL of above 110db.
Using a 24 ohm resister instead of the 20 ohm will result in a slightly higher LOAD impedance.
This is relatively simple to make just as are the impedance increasing adapters above, I have yet to find any for sale, I will be making my own for experimental purposes.
How do I build one and which resistors do I use?
Let's say we have an adapter with a 24 Ohm serial, and a 4.7 Ohm parallel resistor, the headphone impedance is 25 Ohm, and the card has an output impedance of 10.5 Ohm. This means that:
- the amplifier sees a load of 10.5 + 24 + 1/(1/4.7+1/25) = 38.46 Ohm (i.e. the headphone+adapter combination is ~28 Ohm)
- the headphone sees an output impedance of 1/(1/4.7+1/(10.5+24)) = 4.14 Ohm
- the damping factor is increased from 2.38 to 6.04
- maximum voltage on the headphone is (with 7 Vrms output from the TPA6120): 7 / (1/4.7+1/25) / 38.46 = 0.72 Vrms
- this is enough for a maximum SPL of 90+20*log10(0.72/0.064) = 111 dB (more power?)
- the maximum current (with sine output) is 0.72 * (1/4.7+1/25) = 0.182 Arms
- power dissipation on the serial resistor is 24 * 0.182^2 = 0.795 W (at least 1 W resistor is needed)
- check here for some idea on how the TPA6120 is likely to perform with a 28 Ohm load
- the worst case power dissipation on the TPA6120, with +/- 12V supply voltage and 6 V square wave output, is 6^2 / 38.46 = 0.936 W per channel. This may be of concern if the chip is not well cooled, although in practice with realistic music signals and listening volumes the power is likely to be less
Hopefully the power supply on the STX (particularly the negative rail) is good enough to actually output enough current at the maximum voltage with this load, this cannot be verified from the available information.
By the way, this headphone+adapter combination is only about twice as efficient overall as the Hifiman HE-6.
Disclaimer: I take no responsibility for any damage to your amp or source, it is your duty to ensure you are using the correct components, and to double check and tripple check.