How to Properly drive Low Impedance loads?? HELP
May 27, 2013 at 7:06 AM Thread Starter Post #1 of 12

psgarcha92

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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

"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. Current handling at Higher impedance is ok, but what about current at low impedance, for instace at lower than 10ohms?

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
 
May 27, 2013 at 7:15 AM Post #2 of 12

Chris J

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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



"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. Current handling at Higher impedance is ok, but what about current at low impedance, for instace at lower than 10ohms?




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


Have you looked at NWAVguy's O2?

IEMs are very efficient so they don't need a lot of current/power anyway.
They just need a low noise, low output impedance, low distortion, wide bandwidth amplifier.
 
May 27, 2013 at 7:32 AM Post #3 of 12

psgarcha92

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Ofcourse i have looked into the O2 too, and have read here and there about the performance issues that NwAvguy himself noted and wrote about, namely a little high distortion at low impedances.
 
I take into account what you have said, that IEMs are efficient and do not need alot of current, as i have read at places, but why is it that people at the same place say that low impedance phones need more Current swing than Voltage swing?
 
I personally have tried to understand why high efficiency phones with low impedance would not need good amount of current going through them. If they are so perfect, why do they even start to mess with an Amplifier's distortions? Why is it that when a low impedance phone is plugged into the O2, the distortion goes up (not that much ofcourse, but does go up)? 
 
Please do not get offended by what i have posted, i just want to learn and nothing else. 
 
Regards
 
May 27, 2013 at 12:23 PM Post #4 of 12

Chris J

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Quote:
Ofcourse i have looked into the O2 too, and have read here and there about the performance issues that NwAvguy himself noted and wrote about, namely a little high distortion at low impedances.
 
I take into account what you have said, that IEMs are efficient and do not need alot of current, as i have read at places, but why is it that people at the same place say that low impedance phones need more Current swing than Voltage swing?
 
I personally have tried to understand why high efficiency phones with low impedance would not need good amount of current going through them. If they are so perfect, why do they even start to mess with an Amplifier's distortions? Why is it that when a low impedance phone is plugged into the O2, the distortion goes up (not that much ofcourse, but does go up)? 
 
Please do not get offended by what i have posted, i just want to learn and nothing else. 
 
Regards

 
High efficiency 'phones do not need a lot of power, and since they don't need a lot of power they don't need much current or voltage to drive them.
 
One way to look at high efficiency vs. low efficiency headphones is, relatively speaking:
 
high efficiency headphones:  don't need much power
 
low efficiency headphones: need a little bit more power, maybe 10-40 milliWatts
 
High impedance 'phones:  need a little more voltage, a little less current
 
Low impedance 'phones:  need a little less voltage, a little more current
 
I think NWAV guy is referring to the fact that the little integrated circuits in the O2 (i.e. the little Op Amps) do not like to drive low impedance loads. Mind you, you many not be bothered by the extra distrotion the O2 gegenrates when driving IEMs, most people complain about the noise (or hisssss or ssshhh) they hear from some headphone amplifiers. 
 
May 27, 2013 at 11:42 PM Post #5 of 12

psgarcha92

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@Chris J
 
I looked at what you said about IEMs not being power hungry, and i calculated a bit
 
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.
 
Can i use an NE5532 amplifier for my 20Ohm phones?
The answer is a no, because distortion becomes relatively high for any phones lesser than 600Ohms with the NE5532.
 
Can i use maybe a OPA2132, the OPAMP of choice in the CMoy amps people make?
BurrBrown lists their Short Circuit current to be 40mA. 38.72mA is what i require and i think i might be abusing the OPA2132 with the needs of the IEM i have.
And here also, THD+N is said to be at 0.00009% in 600Ohms and 0.00008  in 2kOhms. 
 
So maybe the problem is that OPAMPs even today are not designed to run Low impedance loads.
 
Regards
 
May 28, 2013 at 7:34 AM Post #6 of 12

Chris J

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Yeah, I think most Op Amps are designed to drive high impedance loads, 600 Ohms or 1,000 or 2,000 Ohms or higher.

Have you looked at the IC headphone drivers TI makes? I'm sure they make one designed to drive 16 Ohm headphones.
Have you looked at the FiiO E12? It should be able to drive low impedance IEMs.
 
May 28, 2013 at 7:41 AM Post #7 of 12

psgarcha92

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Can u elaborate a little bit as to where i should be looking? I checked the TI website, and found ICs that were application specific, for example, Headphone amplifier ICs to be put into CD players and MP3 players. I also  found some pretty awesome OPAMPs but they were CMOS based, and i did not have an idea if they are designed for Audio or not. Can you elaborate a little more as to what i should be searching for? its a bit confusing on the TI website as the datasheets do not list what the OPAMPs are designed for.
 
I have been searching google for this too, not a lot comes up in the searches, then maybe i will have to look at stuff more closely.
 
Regards
 
May 28, 2013 at 7:52 AM Post #8 of 12

Chris J

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I'l have to do some digging later today, I can't seem to find the TI spec sheet I want right now.

I think the rating on RE272 is 1milliWatt @ 103 dB SPL.
Try calculating voltage and current for 1 milliWatt at 10 Ohms.

Also, take a look at the FiiO E12 webpage, they list a TI headphone IC as used in the E12.
 
May 28, 2013 at 9:46 AM Post #9 of 12

psgarcha92

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I found something
 
http://www.ti.com/product/tpa6111a2
 
Its rated for loads >=8Ohms.
THD+N, when driving a 16-
color]
 loads, the THD+N performance is 0.005% at 1 kHz, and less than 0.5% across the audio band of 20 Hz to 20 kHz.
 
Would the distortion be audible??
 
Also, you mentioned E12, MontBlanc,
It uses a buffer, LME49600, and the OPAMP is an OPA1611, again designed for headphones above 600Ohms.
Amplifier design states specs for Headphones with 32Ohm impedance.
 
As to  what i understand till now, not being able to provide current to low impedance loads is not as great a problem, as reaching higher THD+N levels while driving the loads is.
 
When looking at Power Amplifiers, the problem seems to be higher THD+N (TPA6111A2 has 0.005% driving 16Ohms compared to OPA1611 having 0.000015% for a 2kOhm load). 
 
This is what i have been able to find out looking at the datasheets.
 
 
For what u asked, the current comes out to be 10mA.
Regards 
 
EDITED: Typos
 
May 28, 2013 at 12:15 PM Post #10 of 12

Chris J

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Quote:
I found something
 
http://www.ti.com/product/tpa6111a2
 
Its rated for loads >=8Ohms.
THD+N, when driving a 16-
color]
 loads, the THD+N performance is 0.005% at 1 kHz, and less than 0.5% across the audio band of 20 Hz to 20 kHz.
 
Would the distortion be audible??
 
Also, you mentioned E12, MontBlanc,
It uses a buffer, LME49600, and the OPAMP is an OPA1611, again designed for headphones above 600Ohms.
Amplifier design states specs for Headphones with 32Ohm impedance.
 
As to  what i understand till now, not being able to provide current to low impedance loads is not as great a problem, as reaching higher THD+N levels while driving the loads is.
 
When looking at Power Amplifiers, the problem seems to be higher THD+N (TPA6111A2 has 0.005% driving 16Ohms compared to OPA1611 having 0.000015% for a 2kOhm load). 
 
This is what i have been able to find out looking at the datasheets.
 
 
For what u asked, the current comes out to be 10mA.
Regards 
 
EDITED: Typos

Hey Thanks!
Those are good examples of the part numbers I was looking for!
So a 10 Ohm load pulls 10 mA at 1 milliWatt and (more calculations) 100 mV at 1 mW.
No wonder you need a very low noise amp to drive IEMs! They don't need much voltage!
 
The TPA6111A2 spec sounds rather vague: 0.5% THD across 20 Hz - 20kHz.
But if you take a look at the spec sheet, the distortion rises at high frequncies, but highest distrion levels are below 100 Hz.
Also, distortion remains fairly low below 30 mW into a 32 Ohm load.
So you probably wouldn't hear the distortion.
You could actually use two TPA6111A2 to create a balanced driver for your IEMs
 
The LME49600 actually looks like a lower distortion amp, too bad the spec sheet doesn't show distortion from 20 Hz to 20 kHz.
It does show THD = 0.0005% at 20 kHz at 3 Vrms into a 32 Ohm load.
There is probably more distortion at lower frequencies bu the human ear is more tolerant of distortion below 100 Hz.
But it has more than enough current to drive almost any headphone.
 
Anyway, I agree, the problem is low enough distortion into a low impedance load across the audio bandwidth.
 
There is another problem, the amp has to be a low enough noise amp to drive IEMs.
I have a pair of Shure SE210, I have only one amp that I can use for these IEMS:  my iBasso D12 which runs off a small internal battery.
Any other amp I have tried either has too much hiss or you can hear a faint bzzzzz from the power supply.
So a battery power supply is nice for an IEM amp.
 
May 28, 2013 at 1:34 PM Post #11 of 12

psgarcha92

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The 0.5% THD+N of the TPA6111A2 is what concerns me too...
 
What other options are there? Where can one get enough current and low distortions??
 
Also, high gain produces hiss most of the time....sometimes its just the source. My Mini^3 is really very very quiet when it comes to hiss. My Fuze actually used to hiss more than my Mini^3 does. And the power supply buzz or hum you hear...i bet these are grounding issues....
 
Regards
 
May 28, 2013 at 5:03 PM Post #12 of 12

Chris J

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Quote:
The 0.5% THD+N of the TPA6111A2 is what concerns me too...
 
What other options are there? Where can one get enough current and low distortions??
 
Also, high gain produces hiss most of the time....sometimes its just the source. My Mini^3 is really very very quiet when it comes to hiss. My Fuze actually used to hiss more than my Mini^3 does. And the power supply buzz or hum you hear...i bet these are grounding issues....
 
Regards

 
Try the LME49600. It doesn't look too bad.
 
Grounding issues: could be:  when I use the iBasso D12 the battery is fuly charged, therefore there is no ground connection via the iBasso D12, ground is via the source only. The iBasso D12 is both a DAC and a headphone amp. I don't know whether it uses a pulse transformer for isolation in the SPDIF input.
OTOH, I have a Matrix M Stage which has a fairly large toriodal power supply transformer in it, you can hear a soft bzzzz when I use IEMs with it. but the Matrix M Stage was not really designed to drive IEMs.
 
Obvioulsy, you don't need voltage gain to drive IEMs, unity gain with attenutation would be all you need, as long as your amp can provide enough current and low enough distortion when driving 10 Ohm loads, which may have a complex impedance WRT frequency.
 

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