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If it has a claimed damping factor of 10, and if that is accurate in fact, then the amp almost certainly uses NFB in the output stage at least. In that case, impedance per se will be less of a problem. But do bear in mind that it is likely the estimate of damping factor was based on driving a speaker presenting an 8-ohm nominal impedance. Since your speakers have a nominal impedance of 4 ohms or less, you can cut the number in half at least, i.e., the damping factor when driving your speakers will be 5 or less. Even this is marginal. Moreover, there is still the general question whether 50W, even if delivered to the speaker, is sufficient for demanding musical passages or room size. But again I apologize for being negative and by all means go ahead and try it.
Follow Ups:
Hi,
Yes the amp uses about 8db of negative feedback. Damping factor is simply the inverse of the output impedance I believe, therefore, it is independent of the load attached to it. However, it can vary with frequency so I think it is usually specified at something like 1Khz or 100Hz. I will try them first without the autoformers, if i like what I hear (I have already heard them with "easier" speakers so I know they are damn good) then I will probably reduce the feedback and see if that still gives enough power. If not then I will try the autoformers as I would like to run with little to no feedback (definitely possible with the autoformers and why the work so well with zero feedback Atamspheres).
Direct from the web: "Technically, the damping factor is the specified load impedance (in use, that'd be the speaker system) divided by the amplifier's output impedance."Damping factor is not independent of the load impedance. The load impedance = the speaker resistance at any given frequency. Thus since all speakers present a reactive load, damping factor is not independent of frequency, either. Most manufacturers will stipulate damping factor into an 8-ohm load, and few bother to tell you at what frequency, but 1kHz is a good guess. Anyway, the advertised damping factor of your amplifiers is likely to be incorrect for driving your Apogee ribbons, which have an impedance way lower than 8 ohms.
Lest I confuse you or someone else, when I wrote "the advertised damping factor of your amplifiers is likely to be incorrect for driving your Apogee ribbons", what I meant to convey is that if your amps have an advertised DF of 10, which is probably based on an 8-ohm speaker load, then they will actually function with a DF of 5 when driving your Apogees (as I stated above), assuming the Apogees have a real impedance of 4 ohms, but we know the impedance of the Apogee goes even lower than 4 ohms, further lowering the actual DF achieved in practise.I am not a big devote' of high damping factor (i.e., super low output impedance) amplifiers per se; my experience tells me that there is such a thing as "critical damping factor" which was defined and described long ago by Norman Crowhurst and others. However, when the ratio of the amplifier output Zo to the speaker Zi approaches one or certainly when it goes above one, which will happen if you turn off all your NFB, things do turn sour. Zeros are a must in that case.
Hi Lew,
Got those Silvaweld OTLs at home now. First I have been listening to them on my Acoustat 1+1 and the results thus far are very positive. I am going to check the frequency response is a couple of days to see how the speaker impedance is interacting with the amps but it doesn't sound really out of whack.Next up will be the Apogees. Here I expect less problems with frequency variations but perhaps more with ultimate power on tap. I still expect some beautiful sound though :). Really fine amps.
Hi Lew,
You are correct, my apologies. You are also correct about the change in damping factor with the lower impedance. Assumming a damping factor of 10 then into an 8 ohm load (at 1Khz or so) then the output impedance would be about 0.8 ohm. Therefore the damping factor into 4 ohms would indeed be 5. As to the Apogees I own, they do indeed measure quite flat in impedance from 3.2-4.6 ohms most resistive. There won't be much variation then in the frequency response. Besides 0.8 ohm is no worse than most tube amps on the market. Again, thanks for the correction, I should have verified what I was saying.
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