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
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@ieales Yes I did buy them used. Previous owner put on 600 hrs on tubes and only had to bias them one time. I also bought a brand new set of matched (40) KT150s brand new from the seller that was bought from Audio Research.
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@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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@atmasphere Thank you very much for your feed back. It is most appreciated.
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@tattooedtrackman looking at the impedance curve measured by Stereophile on that speaker, I would seriously try the 8 Ohm taps as well. Most of the speaker's curve is well above 4 Ohms. There is a bit that is 4 ohms and slightly less in a limited range in the mid bass. The low bass is dominated by the cabinet resonance and is considerably higher. The lower the bass, the more energy. So I would try both taps and see what you think! You won't hurt anything.
FWIW dept.: Most tube amps can lose as much as an octave of low frequency response between the 8 Ohm and 4 Ohm tap. That is because on the 4 Ohm tap in a nutshell the output transformer is less efficient. It will actually run warmer too- that will be because of that inefficiency, some of the amplifier power is simply being converted to heat.
You'll find that 4 Ohm speakers are more sensitive to speaker cables too, especially on a tube amplifier. For this reason you want to keep the speaker cable as short as possible for best impact.
Amplifiers in general, tube, solid state and class D, make more distortion into lower impedances. So for transparency (since distortion obscures detail) its to your best interest to use a higher impedance speaker, all other things being equal (which they never are... but that's how amps behave FWIW).
Your speaker really isn't what I would call a 4 Ohm load though- not like a 4 Ohm Magnaplanar or the like. So you may not suffer all these ills. Try it on both taps and see which you like better. Make sure that the system has been on for at least an hour so warmup isn't affecting your impressions.
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I owned a Dyna 70 and still own a pair of Dyna MK IIIs. They had posts for 4, 8, and 16. I was always told it did not matter, but matching them to the speaker impedance almost always sounded best.
My Futtermans have only one post. The engineer says that they like 16. I am driving my 604s at 16 ohms (they can be modified to 8). The speakers at 16 have a 101 db efficiency.
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I think you should try 4 and 8 ohm taps and hear what sounds better to your ears. There is no way that the 16 ohm tap would be the right ones to use as the B&W speakers are pretty well known to have lower difficult impedances. As the other who have answered here said you won't hurt the amp or the speakers unless you turn it up real loud and get hard clipping.
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@ieales No I didn’t audition them never heard them live before but I read a lot about them. I’ve wanted them for years
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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?
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Thank you everyone for this explanation of ohms. I’ve been putting together sound systems since I was a teen but never bothered or had the need to understand this until now. One of my friends just gave me another example of this. I think it was a good one too so I will write what he wrote. Short answer try 4 and 8 ohms. The difference between 4 8 and 16 is how much electrical energy there is to drive all the speakers. 4 ohms is a heavier load than 16 ohms. A 4 ohm speaker will extract more power from your amplifier than an 8 ohm speaker , about twice as much. Think of pushing a heavy ball up a hill. The load. The woofers take the heaviest load and take the most power to move. Most box speakers like B&W are 8 ohms settings overall but dip to 4 ohms. Other types of speakers like 16 ohms. That’s why the amp has different settings. If u connect just the two 800 bottom woofers it needs 8 ohms. But if u connect the top two woofers also it is twice the load so 4 ohms is needed. The bass should be strongest with 4 ohms. But u should also try 8 ohms. With 8 ohms the highs might sound clearer u will hear more detail but the bass will be leaner. If u put it on 16 ohms it will really sound lean. It’s all subjective to the music u like and what u prefer. So try 4 and 8 ohms. You need to understand with a tube amp it works harder with 4 ohms. The tubes will have the least life in hours. It is opposite to the Krell FPB 600 u had. With the Krell design 4 ohms is easier. But it still sucks a lot of power and 16 ohms is harder on a Krell amp. Just different designs. Your speakers actually dip to 2 ohms at some frequencies. The Krell was made for the 800s and provides a very punchy bass. It will be interesting to see how the ARC amp compares. It’s not about how much wattage but the quality of watts. You can drive the 800s with a 100 - 200 watt amp. Hope this helps.
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Always use the recommendations of complete strangers over your own hearing as hey, you don't want to insult anybody.
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It does matter. I had an amp with two sets of red binding posts. The manual made no mention of the purpose so I connected black to black and red to the post nearest to black. Long afterwards someone suggested one may be 4 Ohms and the other8. Via Audiogon I verified that and I had been connected to the wrong impedance. The sound when I corrected my mistake was clearly better. It matters.
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The people advising you to use the tap that sounds best to you are correct(Don't waste your time with 16 ohms). And there's np danger with any tap as advised. As to power with your amps there will be way more than enough power with both the 4 and 8 ohm taps.
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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.
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@curiousjim Thank you Jim. I’ve heard those other words also from my wife. 😂
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What a great system you have assembled!
I’d say I was jealous, but my wife has other words for me.😁
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@larryi Thank u for another great explanation of this. I very much appreciated it. 👍
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The different taps have different source impedances. The 4 ohm tap has the lowest impedance of the amp itself in order to minimize the frequency response changes from the interaction with the low impedance of the speaker. This tap would also lower the frequency response changes of higher impedance speakers too, but those response changes are much smaller and of less consequence. The tradeoff of using the 4 ohm tap is that less power can be delivered to your speakers.
Use what sounds best. It might be the case that you will like the frequency response changes wrought by using the 16 ohm tap with your 4 ohm rated speaker. This is not usually the case, but it doesn’t hurt to give it a try.
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Holy smokes! Yer gonna crack the foundation with that system!
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@hoosierinohio Ah ok. I see that the impedance is 4 ohms. Is that what u are referring to ? So I would need to hook them up to 4. Ohms for maximum performance.
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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.
'Someone' is correct (you won't damage anything).
Enjoy your new amps!
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I believe it is 4 ohm.
The 4 ohm input impedance characteristic was chosen for two principal reasons. First, the B&W engineers felt that most good amplifiers (with which this system will, no doubt, be used) would deliver their maximum power into 4 ohms or less. The second refers to the ideal situation where the driving amplifier runs out of volts and current at the same time, thereby delivering the maximum power to the speaker load. This, according to Dibb, is more likely to occur with a 4 ohm load.
Copied from a Stereophile write up.
https://www.stereophile.com/content/bampw-matrix-800-loudspeaker-page-2
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