10 ohm Load resistors on the speaker output termnals of a SEX amp

[Todd R]

Hi Gang,
I have built a couple of SEX amps over the years and I have in my hands another one that belongs to a friend.
She bought it built, didn't build it.

It didn't sound right to her with Grado phones, so she wanted me to check it out. It sounds fine here with my Senn HD-650's apart from low volume.
It has the 10 ohm loading resistors on the speaker output terminals, and removing them brought back the volume I was missing. Not sure if removing them would make her Grado's sound any better, but it's worth a try.

Anyhow....
I seem to recall that when I built my last amp, those loading resistors were no longer needed because of some change to the circuit.
Could someone refresh my memory about what was changed to allow removal of the resistors?
Thanks

 

[Paul Joppa]

It wasn't a change to the circuit, it was a change to the theoretical model of the circuit. Originally, before we had built one, the theory said we'd need those resistors, After we built it and listened to it, it became apparent they were not actually necessary. After some experimentation and measurements, I realized the theory was too simplified - a more refined theory agreed with the real-world results (both subjective and objective) that the resistors are not needed.

As I just posted in another thread, if the SEX amp is not loud enough with low-impedance phones, the 120 ohm resistor at the headphone jack can be reduced or replaced with wire. In some cases, even sensitive phones will benefit from the better damping, unless they are so sensitive that the noise floor becomes a problem. A couple of clip-leads to short the resistor makes an easy experiment to determine if this is the case.

 

[Todd R]

Thanks Paul, it's all coming back to me now