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Impedance Adapters/Cables | Explained & Listed

post #1 of 111
Thread Starter 

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.

Output Impedance

 

 

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.

 

Premade:

Adapter/Cable Price R-ohm Format
Etymotic ER4P -> ER4S adapter $50 75 Cabled
Hisound Golden Impedance Cable $60 70 Cabled
Ultimate Ears $50 100 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

 

Custom Resistance:

Company Price Format
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,

 

EXAMPLE:

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.

 

350x73px-LL-b858a90e_adapter.png

 

 

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?

 

Quote:
Originally Posted by stv014 View Post


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.

 

Thanks to:

stv014

Chris J

Steve Eddy

post #2 of 111
Thread Starter 

wow no1 knows any others?

post #3 of 111

There was this UE impedance adapter, not sure if it counts as a cable though. I believe it was rated at ~100 ohms.

post #4 of 111
Thread Starter 

I found it, thanks

post #5 of 111
Thread Starter 

Updated

post #6 of 111
Thread Starter 

updated again

 

EDIT: Weeee post 100


Edited by WiR3D - 3/22/12 at 12:27am
post #7 of 111
Thread Starter 

I'm contemplating stating the resistors used, it will take sum time to find out, if there is a demand for it, I will do it.

post #8 of 111
Thread Starter 

updated again

post #9 of 111
Thread Starter 

I'm amazed this isn't getting more attention, especially with the importance of impedance matching.

post #10 of 111

I'm not sure this does what you think it does?

What are you trying to do?

A true impedance matcher is a transformer.

post #11 of 111
Thread Starter 

well originally and from popular consensus, in the posts i read, it appeared that they would raise the noise floor, lower volume, and ADD to the resistance of the headphones.

 

However I am currently being educated by stv014 and it seems its not the case.

What the adapter does, is pretty much raise the noise floor and lower and the volume, BUT it adds to the AMPs output resistance not to the total resistance of the headphone.

 

So I'm going to update the main post soon, after i understand a bit more of the minor details.

 

My actual goal is to either increase the resistance of my headphones which now seems impossible,

OR decrease the output resistance of an Amp to make it pair better with low impedance cans.


Edited by WiR3D - 3/23/12 at 6:13am
post #12 of 111

Broadly speaking, if you have a good, low impedance desktop amp and you are using 32 ohm Grados or 600 ohm Beyers or anything in between, then you do not need an "impedance adaptor".

 

An impedance adaptor made up of one or two (or more) resistors will not decrease the output impedance of an amp.

If that is what you are looking for, then, unfortunately, you need a lower output impedance amp.

 

Some guys use resistive networks they call "impedance adaptors" to allow them to use very sensitive IEM with their desktop headphone amp.

Reason:

Their headphone amp has too much output (actually has too much voltage gain) for their IEM, they have a hard time controlling the volume, i.e. volume control cannot be use above 9-10 o'clock, they hear too much hiss.

 

this may be useful to you:  

http://www.head-fi.org/a/glossary-of-terms

 

and this:

http://www.head-fi.org/a/headphone-impedance

post #13 of 111
Thread Starter 

Updated

post #14 of 111
Quote:

Originally Posted by Chris J View Post

 

An impedance adaptor made up of one or two (or more) resistors will not decrease the output impedance of an amp.

 

I have already explained this, but you are wrong. If you still do not believe, here is a simple test with a source that has 100 Ohm output impedance, and a 250 Ohm headphone. The left channel was connected directly to the headphone, but its level was reduced by 20 dB. On the right channel, I used a 680 Ohm potentiometer as a serial resistor, and a 27 Ohm parallel resistor. I adjusted the potentiometer so that the levels were matched. At that setting (~220 Ohm), the source "saw" a roughly similar impedance load on both channels. I have then created a frequency response and 40 Hz distortion graph, recording the voltage from the headphone. The result is:

adapter1.png    adapter2.png

As you can see, the frequency response is nicely flattened out, and the THD is reduced by a factor of about 4. This is consistent with what would be expected from an output impedance reduction from 100 Ohm to 25 Ohm.

 

post #15 of 111
Quote:
Originally Posted by WiR3D View Post

Updated


I think you mean you can use MORE sensitive IEMs with your high gain desktop amp.

This is because the resistive network will decrease the apparent gain of the headphone amp
 

 

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