[juswyq]

Hi guys, I've built a few cmoys before. Now I'm looking to get a cmoy with the lowest possible noise to power my sensitive IEMs. Any ideas on the measures I get the lowest possible noise floor?

My ideas: Use the optional R5 (which type of resistor should I use for the best sound quality?)
Use a low gain
Remove the potentiometer (Rely solely on the volume control of the source)
 
How about positioning of the wires and such? And I heard about a ground plane but I'm not sure how to implement it - any tips?

Thanks in advance

 

[Avro_Arrow]

Yes, use R5 to raise the impedance seen by the op amp.
You will need a little extra gain to compensate for R5.
Keep the parallel value of the gain and feedback resistors
to a value of 300 to 500. If you etch your own board, use
a ground plain design. A volume control of 10k will not
add much noise. A gain of five should be low enough to
keep the hiss down but high enough for good stability
and compensation for R5. An R5 value of 51 ohms should
be enough. Keep R5 as close to the op amp as possible.
R5 also isolates the op amp from cable capacitance.
You can use the same resistor type for all resistors.

 

[jcx]

an amp is simply the wrong approach for sensitive iem
 
http://www.head-fi.org/forum/thread/553094/continued-sidetrack-discussion-from-tiniest-portable-amp-i-can-build-nikongod-microtransformer-based-impedance-step-down-box 

 

[juswyq]

thank you for the very informative posts. 
 
I intend to build a high quality cmoy for some cans as well, so having information to build a low noise cmoy would be very helpful.
 
for the transformers:
I understand that it's meant to make your low-impedance IEMs appear as high-impedance so that the source can drive them better. however, can the cheap transformers cause any distortion or add in any unwanted colourations to the sound?
 
a separate question: imods have a dc voltage offset if you don't use capacitors. would running them through this transformer set up also get rid of the dc voltage offset? since transformers don't work with dc circuits. that would eliminate the need for an output capacitor, by far my greatest problem with the imod (i don't like running my audio signal through a capacitor - was looking for a way to remove it from the cmoy but it seemed like the potential cost of ruining my headphones far outweighed the sound quality gain)
 
I apologise if I'm making any mistakes in my electronics theory, I'm not EE trained so I'm seeking enlightenment. Tried to search for answers to my questions but I couldn't find any.

 

[Avro_Arrow]

The windings of the transformer would appear as a very low impedance to DC.
This could cause problems for your iMod. On the other hand, the transformer
will not pass DC.
Transformers will color your sound. Whether or not that is a bad thing
is up to your ears.
 
Quote:

thank you for the very informative posts. 
 
I intend to build a high quality cmoy for some cans as well, so having information to build a low noise cmoy would be very helpful.
 
for the transformers:
I understand that it's meant to make your low-impedance IEMs appear as high-impedance so that the source can drive them better. however, can the cheap transformers cause any distortion or add in any unwanted colourations to the sound?
 
a separate question: imods have a dc voltage offset if you don't use capacitors. would running them through this transformer set up also get rid of the dc voltage offset? since transformers don't work with dc circuits. that would eliminate the need for an output capacitor, by far my greatest problem with the imod (i don't like running my audio signal through a capacitor - was looking for a way to remove it from the cmoy but it seemed like the potential cost of ruining my headphones far outweighed the sound quality gain)
 
I apologise if I'm making any mistakes in my electronics theory, I'm not EE trained so I'm seeking enlightenment. Tried to search for answers to my questions but I couldn't find any.

 

 

[nikongod]

RE distortion - 
EVERYTHING distorts. At best you can pick where & how. Transformers do have some inherent distortions, but at low levels they are pretty small. The key point for IEMs is that a transformer adds basically zero noise to the signal, which is hardly true for active circuits. 
 
Compared to a Cmoy (some of the newer ultra-high current op amps designed for low impedance loads dont suffer as badly) you have to account for "overloading" the op amp. 
 
RE DC offset:
The transformer will block DC but I'm not sure how well the little guys in that thread deal with DC current. I'm also not sure that the line out on the ipod will deal well with sourcing output current. If its any consolation you could probably get away with a 10-22uf cap
 
Something I have been thinking about for a while is an active voltage shifter to replace the output caps in the ipod. 

 

[limpidglitch]

IEMs rarely do need any voltage amplification, even with the .5V output of an iPod, so transformers, as Ari and jcx suggests, or an active buffer with good current performance should fit the bill.
 
NwAvGuy just reviewed a Cmoy designed to these goals, based on a JRC4556 and dual 9V power supply. Maybe some of the more tech savvy folks here can find ways to further improve on the design to drive IEMs, but already it is looking very promising.

 

[nikongod]

I built a bridged/balanced "JRC4556 cmoy" for my Grado RS-1 headphones. Its a fun little amp. I built it as a joke about "balanced amps are inherently better" (I honestly wanted it to suck) but it turned out better than I hoped. Grumbles.
 
As a tangent, does anyone know what the DC offset of the ipod line out is before the coupling caps? 

 

[juswyq]

my diymod measures 1.5v.
 
great information in this thread, thanks
 
by the way, i recall that some people modded their er4p into er4s by adding a 75ohm resistor to each channel.
 
wouldn't this increase the impedance felt by the amplifier too? is it the same effect as using a transformer? or am I missing out something big in the theory department, haha. seems to me like a resistor would have much less distortion and colouration than a transformer, though I might (again) be wrong.
 
on a separate note, i was about to build a mini3 when i stumbled upon nwawguy's blog. it seems like he disproves many 'myths' about headphone amp design, and refutes a lot of amb's claims regarding his amps, especially the mini3. can anyone well versed in electronic design comment on how credible his findings are?

 

[jcx]

series R can change the headphone frequency response if the headphone impedance varies
 
single driver headphones may have a hump in their impedance curve at low frequency which givies a small bass boost with a hi output resistance amp (or with a normal amp and added series resistor)
 
by making a resistor divider you can reduce the output level and have a smaller equivalent added series resistance  - see the "hiss buster" thread
 
  http://www.head-fi.org/forum/thread/198828/the-hissbuster-for-sensitive-headphones
 
some multidriver iem have very "lumpy" impedance curves and series R would be expected to give audible frequency response coloring
 
but a transformer is the most "efficient", literally and in terms of giving lower noise and can be wound for exremely low added series R 
 
 
 
 
 
 
 
 
 
 
 

 

[nikongod]

Thanks for the info on the DiyImod voltage offset. Im going to think about this one for a while. I have some ideas though 

 
Regarding the ER4 P-S resistor. Yes, adding a resistor does reduce the load on the amp in a kind-of-sort-of similar way to the transformer BUT the sensitivity of the ER4S is much lower than the P so the amp has to swing more voltage. It has to swing more voltage into the transformer too, so I cant say for sure which is the easier load to drive. Id like to guess its the transformer as 100ohms is still in a kind of tricky range for a lot of op amps and >1Kohms is pretty easy but the voltage swing could do the opamp in.
 
Something nice about the transformer is that it has applications with the multi driver IEMs where a resistor cable definitely does not. Resistor cables like the Ety P-S cable interact very poorly with the crossover networks in the multi-driver IEM's and generally mess up the sound. The transformer has a low output impedance and knocks hiss down a bunch.