Preamp and impedance question
paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?Yes it will the bass will be tighter/controlled with the 4ohm than the 8ohm tap, if the speakers are 4-5ohms Go the $500 extra you can't go backwards once it’s done, and the amp will have far better resale value down the road. Cheers George |
The output impedance of tube amplifier with a transformer is a function of the (output impedance of the amplifier section multiplied by the turns ratio) ^2 (squared). The 4 ohm tap has less total turns, so it has lower output impedance (higher damping factor). On the other side, changing the tap, changing the turns ratio, reflects a different load back to the amplifier, and coupled with the transformer, that can have some impacts on the flatness of the frequency response at the ends of the audio spectrum and the phase. All depends on the overall design. In terms of selling the amplifier, "clones", especially something that is a production clone, are of limited resale value, so keep that in mind when looking at what you spend. The transformer is a huge part of the implementation. You are building something "like" a Luxman mq-300, but certainly not a Luxman mq-300. You can ask Spatial audio if they can supply an impedance graph/chart. There really is no excuse for not being able to supply one. From there, you could make a decision at least on this speaker. They claim 8 ohm, and there is no reason to doubt that. Rather large mid-bass, so the impedance could dip, but 8 ohms is still likely best for overall performance. Different speakers, of course, will behave differently. |
@roberttdid You bring up a good point. I know the X5 use a 12” Eminence driver. I look up their web site, and found the model and curve below. It seems it dips down to 7 ohm around 200Hz. So, only 1ohm lower than the nominal 8ohm. In this case, I guess having only an 8ohm tap is ok? And for the power subwoofer on the speaker, I guess the output impedance of the amp is irrelevant, right? https://www.eminence.com/speakers/speaker-detail/?model=KL3012CX_8 |
If it only dips to 7ohm, then it will be just fine but the speaker is only part of the equation. The crossover will have an influence too. I don't know what they speaker does at the frequencies for the subwoofer. There will be some influence from the circuit for the mid. I expect there is some loading at low frequencies, but would be highly unlikely to be much less than 8 ohms. |
