Ultrapath - Advantages?

[batsong]

What are the sonic advantages of Ultrapath?

[Arnulf]

It introduces PSU noise to cathode, awesome sonic advantage if your stage has too much PSRR to begin with.

[dsavitsk]

Arguably it allows you to keep the final PS cap out of the signal current loop. Some people think it works well, others don't.

ETF Presentation

[kevinkr]

Originally Posted by Arnulf

It introduces PSU noise to cathode, awesome sonic advantage if your stage has too much PSRR to begin with.

However the addition of a capacitor across the cathode bias resistor in conjunction with an ultrapath cap produces an interesting benefit if you get the ratio right - you get greatly improved psrr. That cathode bypass cap should be ~ (mu +1) x C_ultrapath although it usually is not too critical in my experience. This technique appears to have originated in Western Electric amplifiers sometime in the 1920s.

I've used the technique successfully in the output stage of a cathode biased (!) 2A3 amp I designed a few years back, it reduced 120Hz buzz on the output of the amplifier in question by more than 6dB.

[batsong]

Thanks for all of the replies.
What I was looking at was using a capacitor in conjunction with the bypass cap on a 2A3.
I had removed the bypass cap because I felt it has resulted in more bass and treble, though it also gave the bass a little extra 'flabbiness'.
I'm guessing that if I put the bypass cap back in with an Ultrapath, I would lose this. I wonder if switching to a film capacitor for the bypass would help. Although, with this amp, even with 105db speakers the hum is not bad.

[Sch3mat1c]

Considering a triode is "0%" PSRR, anything you can do to help it is a great idea.

*That includes using pentodes instead.

Tim