Opamp for Iem Amplifier

[noobman92]

Hey Guys
i read somewhere that iems like more current gain than voltage gain, and my current CMoy amp, based on an OPA2132PA won't be very good for these iems.
i have a pair of HiFiman RE ZEROs and RE0s, and am looking forward to building another Tiny CMoy just for iems. the max voltage i can get is 3.7x2, using 2 very small LiPo/Li-ion batteries in series.and the gain i would be using would be 1 or 2, what opamp should i be going for? peple on head fi suggested AD8397 and the OPA690. the AD8397 is not unity gain stable, and the OPA690 is not a dual opamp. any suggestions u guys have?

I know u guys would be asking why i need an amplifier for these iems, the reasons are as follows:
i) better bass response, or more bass quantity
ii) the soundstage widens on both phones when using an amplifier
iii) my setup would be using an LOD, and without an amp, its not possible to get it to work.

please suggest what opamps to use,
Thanks

[Mooly]

Whats an iems ? and whats an LOD ?

[noobman92]

Iems stand for in ear monitors, these phones are inserted into the ears, google it, u'd understand what i am talking about.
An LOD stands for a line out dock.its used to bypass the internal amplifier of the player and carry the signal out to be amplified.

[Mooly]

Well I did highlight "iems" as typed your first post and Bing brought up this,
Iems - Bing

hence me asking

As to the opamp... the supply voltage you have available in conjunction with the impedance of the phones is a limiting factor. You can parallel opamps using small value output resistor to help equalize and share currents. These can be incorporated in the feedback loop too if desired.

Low supply voltage is the biggest problem... small DC/DC convertors are cheap giving -/+12 volts from 5 volts at 90%+ efficiencys. Or get more efficient phones...

[noobman92]

What about the opamps i mentioned above? Also,when these opamps are rated 3v to 24v, aren't they going to work at something like 3.7 volts?also, would these DC/DC convertors have a bad effect on sound quality?i just can't get more iems.what opamps can i use? Through hole and SMD, both would work.

[Mooly]

I'm not familiar with the AD8397. The OPA690! without using one for what you intend again I wouldn't like to comment. It has high input bias currents and really needs very low feedback network impedances to work correctly... its an extremely fast wide bandwidth device more suited to video than audio imo.

What you need to determine is how much voltage swing you actually need across your phones for the max output you want. If the impedance of the phones is around 50 ohms (just guessing) then no normal opamp will "officially" drive anything like that unless the output is padded by a series resistor. And to do that you need more voltage to swing... higher rails.

The CMOY has a huge following... but as you can see it has severe limitations.

DC/DC convertors used correctly should be no problem.
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[noobman92]

Hey,
my phones are 16ohms.but i don't understand, the standard opamps won't be good for my phones because they have low impedance?
How do i calculate the voltage swing?my pair is the RE-ZEROs, 16ohms, 103dB/1mw.
If the OPA690 is employed in the CMoy, would it work fine?
Also, if the OPA690 is for video, what other choice of opamps do i have?

[Mooly]

Impedance varies with frequency so "16 ohms" could well be the DC resistance of the coil. The impedance of an "8 ohm" speaker unit could be as low as 3 to as high as 30 ohms over 20 to 20khz.

The CMOY gets away with it because although the load (headphones) are far lower than the opamp can drive, in actual use the output needed is actually quite small and so the opamp can "cope" and manage to put the required voltage across the low load impedance.

I actually did some real world tests on this a while back for a headphone amp I made (results for my phones in post #2)

http://www.diyaudio.com/forums/headp...phone-amp.html

As you can see, the actual swing needed on the Sony phones isn't great.

Your phones are 103db for 1 milliwatt.

Watts is V squared/R

so

V is square root of Watts times ohms

That gives sqrt of 0.001*16

Which is 0.126 volts rms for 103db.

0.126 vrms is 0.178 volts peak or 0.356 volts peak to peak.

Would the OPA690 work... I'm sure it could be made to work just fine but I wouldn't just drop it into the CMOY circuit "as is". You would need to test and optimise using a scope and generator at the very least.

[jaycee]

16 ohms is too low for just an opamp to drive alone, and youd probably want at least +/-5V to get any decent volume out of them