impedance question

[Randall]

I just restored a 1961 Made in Germany Bang and Olufsen 15 watt/side stereo HiFi amp that I am running on a quad of Mullard ECL85s. The amp has a 4 ohm and a 16 ohm switch. If I want to run a set of 8 ohm speakers, which is the better position? I'm thinking 4 ohm. I can find no drawings on this amp whatsoever.

[Leo_Gnardo]

The amp has a 4 ohm and a 16 ohm switch. If I want to run a set of 8 ohm speakers, which is the better position? I'm thinking 4 ohm.

Yes I'd pick the 4 ohms tap. Also, you've looked at the OT's and no 8 ohm tap? Odd that they would have 16 & 4 but no 8.

[Randall]

Can't really tell from looking at the OT, but on the back panel under the speaker connections it is labelled 4 or 16 ohm. Maybe they intended it to be run with a pair of 8 ohm speakers on each side wired in either series or parallel?

[Leo_Gnardo]

Maybe they intended it to be run with a pair of 8 ohm speakers on each side wired in either series or parallel?

A little puzzling because Europe seems to prefer 16 ohms as a standard speaker impedance while 8 ohms is most popular in the USA. Well whatever - it may not be perfect but I have a feeling your output tubes would be happier driving 8 ohms speakers from the 4 tap. Might have a peek at those OT's & see if there's an un-assigned output tap, might be the 8 ohms you're looking for. And - maybe there's nothing.

[J M Fahey]

Yes I'd pick the 4 ohms tap. Also, you've looked at the OT's and no 8 ohm tap? Odd that they would have 16 & 4 but no 8.

Makes sense from the winder's point of view: it has 2 equal (in fact might even be bifilar wound) 4 ohms windings, which in series give 16 ohms.

[Chuck H]

+++

There is clearly a goal from the maker that doesn't suit an 8 ohm load. Perhaps the nature of available speakers at the time of manufacture.?. As an aside...

Marshall 4X12 cabs are either 4 ohms OR 16 ohms. Never 8 ohms. This could be a similar situation. A foresight on the part of B & O.

[bob p]

i'm thinking that both B&O and Marshall could be using 4 and 16 ohm taps for different reasons. in the B&O case, it amounts to OT winding efficiency. in the Marshall case, it could also be winding efficiency. But the Marshall case has an additional factor that might play in -- a quad of 16 ohm drivers. It's pretty easy to put four 16 ohm drivers in a cab and get either 16 ohms (series/parallel) or 4 ohms (parallel). kinda hard to get 8 ohms out of four 16 ohm drivers.

[Steve Conner]

16 ohm speakers were common in Europe. Try both taps and see which you prefer the sound of.

[kleuck]

Using the 16 Ohms tap is the more secure way.

[David Schwab]

it has 2 equal (in fact might even be bifilar wound) 4 ohms windings, which in series give 16 ohms.

Wouldn't that be 8 ohms?

[Chuck H]

Good question. I don't know the specifics, but would like see it run down again. I do believe it's because, WRT transformers windings, the series connection is squared.

[loudthud]

Calculate the voltage for 10W at 4 ohms, 10W at 8 ohms and 10W at 16 ohms. It should be clear to you.

E= (P*R)^.5

[J M Fahey]

Yes, that's the point.
"4/8/16" ohms is nominal, in fact we are putting windings in series or parallel, and what counts is turns.
2 x "4 ohms" windings in parallel are still 4 ohms , and many OT are made that way (Bassman among others which has 3 x "2 ohms" windings made out of relatively thinner wire) while if in series: 2X the turns, 4X the impedance.

Many very high quality Hi Fi transformers, often wound on "C" cores, have a dozen (no kidding) independent equal secondaries, or split in 2 groups, say, 6 with 14 turns, six with 20 turns, and come with a connection table which lets them have any secondary impedance between 2 and 32 ohms and a wide range of primary ones, to adapt to many different tubes and amps.

[jazbo8]

Like this one for Marshall.