Audio Systems: hiss, headphone output, output stage

Question
QUESTION: Hi - I have an Onkyo home a/v receiver model TX-SR303. So far its been a great component and I have no complaints. But, when you put headphones in the jack, on the phones you hear white noise when you turn the volume low or the CD is paused. This is not a hum, this is white noise/hiss, kind of like the hiss you'd hear between songs when listening to cassettes. The hiss does not change when adjusting the volume, its just buried when you turn the volume up from 0 and the music starts to come in. So if you're listening to a cd with the volume very low such as position 1, 2 or 3, you still hear the hiss along with the music. The problem isnt the headphones, I tried them on another component, and also tried other headphones in the Onkyo, so the problem is the Onkyo. It happens no matter which input you're on: CD, tuner, etc..
Thanks.

ANSWER: Do you hear the hiss noise on loudspeakers? Or only on headphones?

IF: Only on headphones it is a noisy transistor or IC that feeds the headphone output.

IF:  It is present on both headphone and speaker it is a transistor or amplifier module between the output of the volume/level control and the final stage of amplification before power output stage.

Of course, it can be a noisy capacitor but that is very rare; usually it will be an active device like a transistor either a discrete one or one inside an IC.  

Onkyo are, as a general rule, very quiet with a very high S/N ratio.  Very good products.



---------- FOLLOW-UP ----------

QUESTION: I just found out this hiss is also coming from the "A" set of speakers, for some reason there is no hiss from the "B" set. I never noticed it from the "A" set because you have to put your ear right up to the speaker.

Answer
But if your speakers are connected to the A terminals only you would not hear anything on the B terminals unless you switched the wires over to B.

Thus, I am assuming the hiss is in both speakers and headphones.  The reason it is more audible in the headphones is because they drive to full power with only about 20 milliwatts whereas speakers require much, much more power.

Answer is as before: the amplification device after the volume control, be it a transistor or IC, is noisy and will probably need to be replaced.

Only an Onkyo repair facility would have the schematic diagrams and be able to identify the part that must be replaced.  I looked on the Internet for free schematic but did not find; they are usually $10 to $30 from the mfr or private companies.

BTW: Here is a bulletin for hiss correction on an old model indicating the same sort of thing your set has but I don't know if this bulletins correction will apply to your set because it is a much newer model.  

http://www.schematicsforfree.com/archive/file/Audio/Products/Receivers/Onkyo%20-

I recommend you take your receiver to an authorized Onkyo service shop.

Best wishes.