4 8 or 16 ohms.
Hi group I just purchased the Audio Research REF 750s mono amps. On the back for the speaker terminals are 3 sets of binding posts. 4 8 and 16 ohms. I will be using B&W Matrix 800 speakers. I believe they are 8 ohms but not 💯 sure. I looked in the manual but can’t find anything about ohms. Someone said to me it does not matter what ohms I connect my speakers to. He said just use the ones (ohms) that sound best. Does that seem right to do ? I don’t want to damage my speakers or amps.  BTW if it matters I will be using an Audio Research REF 6SE pre amp.Â
Look at the impedance graph in the speaker review. It's all over the map. The amp will sound different on each tap. Choose the one you like best. It won't hurt the amp or speakers. @larryi Transformers transfer maximum power when source and load impedances are equal. |
Tubes do not work harder driving 4Ω loads. The tubes drive the transformer primary impedance which remains essentially the same. Transformers are happiest driving a matching impedance. When the impedance is mis-matched, so is the power. Tubes excell driving planars which have a mostly constant resistive impedance. Roller coaster impedances like the B&W 800 Matrix are best served by transistor amps as they can supply the current necessary to generate the same power regardless of the load impedance. The B&W drop below 4Ω which can give some SS fits. As far as sounding lean on 16Ω, the speakers are 16Ω at the very bottom, dropping to minimum in the upper bass and fairly constant ≈11Ω from 1k up. 16Ω could be just the ticket - depending on how the room affects the low end. @tattooedtrackman Did you audition the AR amps before purchase? |
@tattooedtrackman If you bought the amps used, it behooves you to have a AR tech go through the amps and verify tube performance on a proper test that tests valves as run, not just go/nogo. While tubes maybe matched when new, they age and may not match with as few as 500 hours.
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Biasing often adjusts the current at a single point. I’ve never seen an AR REF750 schematic, so can’t comment on the efficacy of the bias adjustment as tubes on each OPT leg age. AR had very clever engineers so it’s likely very good. The REF 750 Service Manual is available on HiFi Engine, so it’s worth getting if you don’t have it. It details the biasing procedure. General Info for tube power amp owners: Tubes that don’t have the same Gm at various points, conduct differently and may increase distortion. Many tube amps have a single bias pot for multiple tubes and this may severely compromise performance. See Transconductance - Wikipedia & Matching by Gm or Ip? - The Amp Garage |
