Impedance matching circuits

To make it a bit more clear on our end, iEMatch has at the max under 3 Ohm output impedance. The respective changes in amplitude with frequency generally pretty much parallel the impedance.

So with the iEMatch at 1/7th of the output impedance of the above case, the amplitude change with frequency due to this impedance would only be around 1/7th, or less than 0.8dB at the maximum, for this specific example. This should be at the limits of audibility and certainly should not cause large changes in sound quality.

There are some possibilities though. First, the parts used in the matching network are Vishay MELFs. These do appear to show quite a pronounced “burn-in” behaviour and often cause a bright and forward sound in the first few hours.

It may be advisable to perform a forced burn-in using the burn-in tracks iFi have made available. There is no need to keep the headphones or IEM’s connected for that. Simply select “high sensitivity” and with a headphone attached set a level for playback that does not cause the amplifier to distort, play the burn-in track(s) on repeat for 48 – 72 hours and then try to listen again.

Please see the included table.

It compares the impact of the iEMatch in high and Ultra settings and of an “impedance adapter” with 33R on the frequency response as well as the respective SPL if using a smartphone as source (approx 1V out) or a serious desktop headphone amplifier (appx. 10V out).

Headphone is Shure SE846 based on inner fidelity measurements. As a general rule 3dB is considered the smallest difference in overall loudness that can reliably be identified.

Though more recent research places this more at 1dB SPL difference. Similar research suggests that changes in frequency response need to be beyond 1dB if affecting wide ranges of frequencies and between 3-6dB if affecting narrow ranges of frequencies.

As can be seen, the 33R impedance adapter would produce gross changes in frequency response, with bass boosted 3dB and the presence region around 5kHz cut by 5dB. This will drastically change the sound character of the headphone.

In high sensitivity (which provides a comparable reduction in 1kHz SPL as the 33R adapter), iEMatch limits changes in frequency response to +0.8dB/-1.5dB which may just be at the edge of audibility, but is actually similar to the L/R differences in the measured frequency response of the SE846 by inner fidelity. Using a Smart Phone with appx. 1V output will still raise 115dB Peak SPL, enough to ensure permanent hearing damage with long exposure.

In ultra setting the iEMatch will change the frequency response by =0.3dB…-0.8dB, this will challenge the golden of ears to tell. In this case, the SPL from a smartphone at 1V SPL is reduced to 102dB, which may be a little too low for headbanging but will still produce SPL sufficient to damage your hearing with long exposure.

All the above applies to only and strictly the Shure SE846, as measured by Innerfidelity, many headphones and IEM will show much less variation in impedance and hence much less variation in frequency response.

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