Quote:
opamp circuit uses default 100k to gnd for the bias resistor,10k in the feedback loop,and 1k to gnd.and 47ohm in output
hiss is still present,mild though,with the casing on using a opa2132pa.i can check it up in college using a oscilloscope though.no change in opamp temperature(does not get warm or any such thing) .
is this because i am using iem's?will it go away when used with higher impedance headphones?i used the standard gain of 11(tangent's schematic),so i have to lower this?have no headphones as of now,as my sennheiser hd202 distort very badly.
A couple things to look at.
It looks like the 47ohm resistors are IN the feedback loop. These resistors do occasionally help to improve stability & reduce noise, but when the op amp is driving a load they usually cause more trouble than they are worth. They make more sense in "pure" gain stages that are not driving the load (like a gain stage with a buffer after it). I suspect that these resistors are causing the distortion you are hearing. I would suggest reducing their value to less than 10ohms, or just jumpering them with a bit of wire. The noise may go up slightly after this, but the distortion you described with the HD202 should hopefully go down.
I'l get to really killing the noise in a minute but since were already talking about output resistors.
The alternative to output resistors IN the feedback loop is output resistors OUT of the feedback loop. They do help with noise in very sensitive low impedance headphones, but they bring their own compromises too. With single driver headphones I often think the change is for the better with many headphones which makes the compromise an improvement to me, but with multi-driver headphones (IEMs) its just kind of sucky. Multi-driver headphones really need to be driven from an amp with as close to a 0-ohm output impedance as possible. Now I don't like the idea of using output resistors as a bandage to cover up the fact that the amp hiding behind them is all hissy, and it sounds like you actually need a low output impedance for the IEMs so just file this info away until later. Maybe experiment with it a bit and see if you like the changes.
So to really kill the hiss I would start my search with the gain of 11. Voltage gain of 11 is WAY HIGH for most headphones & sources. The real problem isn't so much gain, as the junk that lots of gain usually brings along for the ride. The problem is that ANY noise inherent to the circuit is multiplied by the gain - random junk IN the op amp its self, resistor noise, RF pickup, and the like. By reducing the gain we reduce how much this random junk is amplified and usually take a few steps towards a nice quiet amp. I'd try voltage gain of 3. 3 is still a little high for most systems*, but it offers you plenty of options for stable op amps that would not fly with less. When dealing with gain in hi-fi systems - less is more.
One of the compromises of the Cmoy is that it can be difficult to find an op amp that can drive a sensitive low impedance headphone with an adequately quiet/low output noise without resorting to output resistors. There are some op amps that can truly "do it all" but they are not really easy to use. Is it really a Cmoy when you have to add a dozen parts not on the original schematic to make the amp run stably?
*personal definition of too much gain:
If you have the pot on the quiet side of the middle (about 25-30db of voltage overhead) at your preferred listening level with the most hungry headphones you have you have too much gain. This ideal leaves you plenty of extra pot for quiet recordings and gets even cheap pots into their best matched range but in practice it hard to actually achieve with headphones. It is also worth note that people freak out about spinning the knob up so high and equate spinning the knob that much that with lack of power (which is silly) so I would only apply it loosely. If you can get the pot 1/3 of the way up you are better off than most.