[mlundy57]

Mike,

I do not believe your subjective conclusions are necessarily due to the differences in the power output between the Rotel and ACA.

The mu follower output stage in the ACA has a relatively high DC impedance, but it is an active component with an AC negative resistance. It contributes to raising the apparent output impedance of the amplifier. Mu followers are popular in the tube world but typically in preamps. Pass' use is both unusual and creative which is what is wonderful about him!

Pass in his Amp Camp article stated that the damping factor of this amplifier is about 3. So output impedance assuming a vanishingly low speaker cable impedance (in addition to the wire in your crossovers), is going to be 8 ohms/3 or 2.66 ohms on the best day (i.e. short thick/beefy speaker cables). Various fellas have already confirmed Pass' output impedance measurements as true. Of course a higher output impedance and/or low damping factor all things being equal will contribute to a 'looser bass.' Usually, these designs are more compatible with woofer drivers that have very low Qts. But that doesn't mean it sounds bad as you have heard.

It's likely that the Odwatt unit has a similarly low damping factor.

Just my $.02.

Enjoy!

Anand.

Anand,

I'm not sure if you think I am saying the ACAs sounded bad or if you are agreeing with me that they do not sound bad, in fact they sound pretty darn good. They just don't play as low as the Rotel does.

I think I follow your explanation. However, my background is social sciences, health sciences and military. My engineering background and understanding is sketchy at best.  I don't understand all your technical points (mu follower, damping factor  ) but if I get the gist of what you are saying the reason the Rotel plays lower than either the ACA or the OddWatt is not due to the power difference between them but rather due to differences in how the amplifier circuits are designed and executed.

Mike

[poseidonsvoice]

Exactly. And yes, my impression was that you thought the ACA sounds good. I was merely illustrating why the Rotel may have better bass from a "technical standpoint."

Apologies. Sometimes I sound like a professor, and I don't mean to, but a lot of audio boils down to science.  And explaining it is important. Over the years, I have found that there is hard data, calculations and measurements that can explain why an amp (for example) sounds the way it sounds, not just cause it "does." And that is something I continue to explore...

My background isn't engineering either 

Best,
Anand.

[WC]

Mike,

Have you used the ACA amp with your X-otica MTMs. I built the ACA amps and am interested in the X-Otica MTM over an H or W frame 12" servos. Seemed like you had some impedance matching issues when you used the ACA amps.