[xmokshax]

i have a soft hiss in my CMoy when listening to my iPod through very efficient headphones (25ohm IEMs). the hiss goes away abruptly when i lower the iPod volume to zero, so i assume it must be the source of the noise. i lowered the gain from 11 to 5 by replacing the R3s, and i think it improved the situation, but the hiss is still louder than i'd like.

my question is this: will adding a 10kohm at R5 possibly help this problem?

can i add said resistor to one channel and test the amp without killing it, or is it unwise to run the amp with this kind of imbalance?

 

[MisterX]

Yes, adding R5s will help but I dunno that 10 ohms is going to be enough.
(what phones and what op-amp are you using?)
And yes you can add the resistor to one channel at a time if you wish
(running the amp like that it far from ideal but it is not the end of the world either)

 

[xmokshax]

thanks for the quick response! i'm using UM1s and the "stock" OPA2132PA.

i actually posted my original question AFTER soldering in the 10k R5, but before testing it - i thought i'd better check before risking killing the amp.

 

[Joshatdot]

Quote:

Originally Posted by xmokshax /img/forum/go_quote.gif i have a soft hiss in my CMoy when listening to my iPod through very efficient headphones (25ohm IEMs). the hiss goes away abruptly when i lower the iPod volume to zero, so i assume it must be the source of the noise. i lowered the gain from 11 to 5 by replacing the R3s, and i think it improved the situation, but the hiss is still louder than i'd like. my question is this: will adding a 10kohm at R5 possibly help this problem? can i add said resistor to one channel and test the amp without killing it, or is it unwise to run the amp with this kind of imbalance?

Quote:

Originally Posted by xmokshax /img/forum/go_quote.gif thanks for the quick response! i'm using UM1s and the "stock" OPA2132PA. i actually posted my original question AFTER soldering in the 10k R5, but before testing it - i thought i'd better check before risking killing the amp.

UHH...you don't need to use 10,000 ohms (10k ohm), just a 50 ohm or 100 ohm will do fine.

 

[xmokshax]

ahh wow, misread the troubleshooting page... ok, i have some 47.5 ohm sitting here, i'll throw one of those in. thanks for saving me.

 

[MisterX]

See what I get for assuming 10K was a typo?

 

[xmokshax]

Quote:

Originally Posted by MisterX /img/forum/go_quote.gif See what I get for assuming 10K was a typo?

ha... no, i'm a raw newbie, so i don't really yet have that good a feel for the circuit and preciesly what everything does. i got so used to using resistors in the kohm range for the rest of the circuit that i think i just assumed the potential R5s would be in the same range. big mistake.

 

[xmokshax]

hmm... so the 47.5ohm R5 doesn't seem to have done a whole lot... the hiss is still comparable in the two channels. any other ideas? i'm not sure how much lower i want to go on the gain... has anyone else had this type of hiss problem when using a basic CMoy with low-impedance phones such as IEMs?

 

[majkel]

How do you use your iPod? It is recommended to set its volume near maximum, otherwise you will amplify hissing coming out of your iPod.

 

[Joshatdot]

Quote:

Originally Posted by xmokshax /img/forum/go_quote.gif hmm... so the 47.5ohm R5 doesn't seem to have done a whole lot... the hiss is still comparable in the two channels. any other ideas? i'm not sure how much lower i want to go on the gain... has anyone else had this type of hiss problem when using a basic CMoy with low-impedance phones such as IEMs?

do you get hiss from any other source? PCDP, Computer, other DAP devices?

it could be your iPod is causing the hiss.

 

[xmokshax]

i haven't tried other sources yet... i had pretty much identified my iPod as the source of the hiss already, because it goes away when the volume is all the way down, then starts immediately as soon as the volume is turned to "1". i think the iPod's volume control must have an integrated switch, such that its internal amplifier is disabled when the volume is turned to "0". interestingly, the hiss seems pretty much constant in volume up to about halfway on the iPod's volume control, then intensifies steadily from there to the max volume setting. at max volume, it really is quite unbearable.

in response to what you said, majkel, once i have the pot integrated later this evening, i'll try what you suggested - turning the iPod volume all the way up and controlling the volume via the amp. i'm not sure this will be effective since the hiss seems to intensify near the iPod's max volume, but it's certainly wort a try.

 

[tangent]

If the noise is coming from the source, adding R5 shouldn't do much at all. Its purpose is in quieting noises generated by the amplifier itself.

The proper solution is to lower the amp's gain. With an iPod, I'd use a gain of 2 at most. They're quite loud, for a portable.

 

[xmokshax]

ok, i'll lower the gain again. is it a pretty good bet that the OPA2132PA will remain stable at a gain of two in a reasonably well-constructed circuit? i know the chip is unity gain stable under ideal conditions, but will small perturbations in the circuit affect that stability significantly?

 

[ericj]

Yeah, the 2132 is extremely stable.

 

[firefox360]

Also why not use the line-out from the iPod? Allows for better audio quality, and may eliminate the hiss that you're getting from the headphone out of the iPod.