Low impedance phones, Please suggest amp design?

[noobman92]

I own a pair of Low Impedance IEMs, the RE272s, rated at 20Ohms at 1KHz.
I recently read somewhere that Headphone impedance is different at different frequencies, which i could infer from manufacturers rating Headphone impedance at one particular frequency. Well, how much the impedance varied with frequency, i did not have an idea of that.

Quoting NwAvguy at NwAvGuy: Headphone Impedance Explained

"YOUR IMPEDANCE MAY VARY: Virtually all passive headphones (without their own powered electronics) have a varying impedance that changes with frequency. As described above, they don't behave like a simple resistance when driven with typical audio signals. The gold colored graph below shows the impedance, in ohms, of the Ultimate Ears SuperFi 5 Pro in ear monitors. You can see they’re only at their rated 21 ohm impedance below about 200 hz. The impedance rises to nearly 90 ohms at about 1200 hz and drops to below 10 ohms at 11,000 hz"

This scares me a bit. I have the 272s at 20Ohms, and I imagine their impedance varying from 10ohms to 30-40ohms maybe.

Can i have an amplifier designed for the current needs of my IEMs? Would it actually help me in getting better sound? i was looking at
http://www.ti.com/lit/an/sboa031/sboa031.pdf

But they list the nominal impedance as 40-300Ohms in this design. What else should i look at?

The aim of the amplifier is going to be able to drive the IEMs effortlessly, as in, being able to have all the current the IEMs can ever require.

Regards

[Cornelis Spronk]

Loudspeakers and earphones have a mechanical diaphram that is made to swing back and forth by an electromagnet coil. The diaphram and coil is a mass that is allowed to move against a restoring force of a springlike material.

If you put a mass on a spring it will vibrate best at its natural frequency. Also, at its natural frequency, the coil produces the most back emf. This back emf will hold back any current the amplifier attempts to push into the headphone.

There is nothing you can do about this. The headphone designer has worked hard to keep this effect at a minimum by controlling a sort of friction on the springlike motion (also called damping).

The only thing you need to be concerned about is that any amplifier will give enough current at the lowest impedance. Then at the higher impedance, the current the amp will provide is more than enough. The higher impedance (or changing) impedance need not scare you.

Feeding a lower impedance into a higher one is not a problem. Feeding a higher impedance source into a lower impedance will result in current limiting, which may be a problem.

Total frequency response and distortion are generally much more important to concern yourself about.

Also, a peak in the response curve is more annoying than a dip of similar size.

Most opamps, being very small, do not give enough current for a 30 ohm earphone.

You are looking for a suggestion.

Look around in this forum for headphone amps with larger discrete output devices. Many of them are excellent. They will give plenty of current for any headphones.

Better yet. Build two different ones, and report on your results. Learning is life.

[noobman92]

Originally Posted by Cornelis Spronk

The only thing you need to be concerned about is that any amplifier will give enough current at the lowest impedance. Then at the higher impedance, the current the amp will provide is more than enough. The higher impedance (or changing) impedance need not scare you.

That is exactly what I was talking about in the post, its the Current that the IEMs get at Low impedance

Originally Posted by Cornelis Spronk

Total frequency response and distortion are generally much more important to concern yourself about.

Does it mean that i should not really be worried about the IEM getting enough current or not? Also, To keep Distortions at the lowest, an amplifier should be able to provide all the current required by the IEM easily right? as in, the max current requirements of the IEM should fall under moderate operating range of the amplifier? Did i understand you correctly?

Originally Posted by Cornelis Spronk

Most opamps, being very small, do not give enough current for a 30 ohm earphone.

You are looking for a suggestion.

Look around in this forum for headphone amps with larger discrete output devices. Many of them are excellent. They will give plenty of current for any headphones.

Better yet. Build two different ones, and report on your results. Learning is life.

I should be looking into Discrete amplifiers? or specifically amplifiers with discrete output stages? i have never handled a Discrete amps before, can you please provide a link maybe if its not too much to ask for?

Regards

[Avro Arrow]

Something like this would drive your IEM quite well.

[Redshift187]

Try to find a The Wire PCB, they are fantastic for the cost of the build and capable of powering pretty much any earphone/headphone.

[noobman92]

@Avro Arrow,

I cant seem to find enough info on that particular amplifier.

Also, can you explain why it can do what i am looking for?

Also, i need to ask and don't know if i should be starting a new thread for this,
Why exactly cannot OPAMPs drive low impedance phones properly? In all the datasheets of OPAMPs, i have never read the nominal impedance of the speaker to be less than 30 ohms. Why is it that way? I know for sure that low impedance phones mess up an amplifier's graphs, as in Distortion levels, TDH+N and others. Why is it that way? Is it because they demand so much current? i calculated a little bit here.

My RE272s are rated at 20Ohms. I assume that the lowest impedance they get at is 10Ohms. Max rated input is 30mW, Rated input is 10mW. So, i assume i want the volume at 15mW.

SO
15mW= R x I^2 (assuming purely resistive load, tell me if i am wrong here)
15mW=10x I^2
I^2 = 15mW/10
I^2 = 0.0015
I = 38.72mA

38mA is the typical output current that a NE5532 can provide.

Why then, do OPAMPs have problems driving low impedance loads??

Regards

[bimo]

Originally Posted by noobman92

@Avro Arrow,

38mA is the typical output current that a NE5532 can provide.

Why then, do OPAMPs have problems driving low impedance loads??

Regards

Distortion is relative high if NE5532 drive impedance below 600 Ohm. If you can accept the distortion you can use NE5532 for driving headphone.

[noobman92]

@Do you know of any OPAMPs which have relatively low distortion at Low Impedance? What is the lowest impedance OPAMPs are ok in driving with typical distortion in them?

Regards

[bimo]

Originally Posted by noobman92

@Do you know of any OPAMPs which have relatively low distortion at Low Impedance? What is the lowest impedance OPAMPs are ok in driving with typical distortion in them?

Regards

You can chip amp that design for headphone amplifier. Many chip amp is capable to use split power supply and work exactly like an OP-AMP.

[noobman92]

By chip amps you mean Power Amplifier ICs right? like the LM386 types??
Can i get very less distortion levels from these ICs? are Chip Amps generally used for Headphones? Also, unlike OPAMPs, are Chip Amps designed to work for Low Impedance loads????

Regards