[Wishmasterx]

Hi
I have portable headphone amplifier based on this scheme:
http://www.eleccircuit.com/wp-content/uploads/2011/05/increase-output-current-of-the-op-amp-ic-in-85mA-size.jpg
only my is:
Master - OPA128SM
4x Slave - OPA627AU biased 5mA to Class A over resistors
How much are minimal safe and stable values for output resistors (R2,R3,R4 on picture)? Original 47ohm resistors I replace with 10ohm and sound is much better. I need low output impedance of headamp too for IEM.

 

[goodvibes]

Sounds like a bad idea to me for a few reasons. I'd already be checking my stability and give up on driving low impedances.

 

[Wishmasterx]

What You mean bad idea? Going with resistors below 10ohm?
My headphones are Triple.fi 10 (32ohm) and SRH-840 (44ohm). With 47ohm resistors was sound little muddy and highs was not bright. With 10ohm resistors sound excellent. Very clear, punchy bass, bright highs with overal more energy.
Amplifier is this:
http://g02.s.alicdn.com/kf/HTB1caq6FVXXXXXlXXXXq6xXFXXXn/220677691/HTB1caq6FVXXXXXlXXXXq6xXFXXXn.jpg

 

[HotIce]

That's WeiLiang E10 Class A amp, right?
So, either WeiLiang is BS-ing about the class A, or that's not the schematic.

Picking up the feedback after those 10 Ohm resistor will make the feedback phase to lag upon capacitive load.
True that C1 will roll off gain on high frequencies, but that seems like a bad idea.
Also, as is, that schematic has unity gain.

 

[Wishmasterx]

Here is really schema of this amp:
http://www.imagehosting.cz/?v=e10headamp.jpg
Opamps was not biased, biasing is my modification.
I make simulation - with 47ohm output resistors is output impedance about 12,5ohm. Now is output impedance 2,9ohm.

 

[HotIce]

Heh, totally different schematic. Much better, but those 3.3K resistors do not make it class A
This is closer to the O2, which I am not in love with either, because I do not like the idea of the signal going through another 10 BJT per OP-amp, in order to get current buffering.
With a diamond buffer ALA LME49600, within the feedback loop, you add only two BJTs in the signal path, and not waste 4 parallel OP-amps to achieve the same current throughput.

 

[Wishmasterx]

I see now on O2 schema, it is realy closer to my amp. In O2 are output resistors only 1ohm.