When an amp puts out 90 watts....

I have seen some pnm impedance measures in stereophile that show wide impedance flucuations from 4 to 16 ohms at various frequencies. Far frm the constant high impedance that an amp like that would probably do best with.

That depends on the amp. If it is OK with 4 ohms and has a 10db of feedback or more, its probably not an issue. Also, Sometimes you can get by without the feedback if the higher impedances also occur at higher frequencies. A ZOBEL network might just take care of the extra output.

I have heard from an amp designer that he has abandoned the Zobel because the Zobel network can knock down power output into the higher frequencies and diminish extension. If the speaker efficency is near 96 plus, less of an issue. Have you experienced this? Jallen

ZOBEL networks always have to be set up for a particular situation. Sometimes a choke needs to be in the network to prevent High Frequency rolloff.

Ralph, re your post about 4-16 ohms at various frequencies not being a problem if the tube amp has 10db or high NF seems correct. As you know, I posted links to Soundstage and Stereophile bench tests of the ARC VS-115 and Ref 150 tube amps. Both amps use similar amounts of NF (about 12 to 14db) and have similar output impedances of about 1 ohm off the 8 ohm tap. The output FR of both amps was about +/- 1 db over the audible FR spectrum when driving a simulated speaker load that had similar impedance fluctuations as staed above. Atkinson said the output FR might even be tighter off the 4 ohm tap where output impedance was 50 to 60 percent of the 8 ohm tap output impedance.

Bifwynne, Our amps don't use any feedback at all, yet they can work quite well with speakers that have some pretty wide impedance variation. It all has to do with the intention of the designer of the speaker. ZOBEL networks are one way you can get a zero feedback amp to work with a speaker that is expecting the amp to have a much lower 'output impedance'.