Magnavox 9303: Switching speaker impedance

[Moonwatcher]

Tube newbie here: So, depending on what I'm listening to, I use both 8 ohm Wharfedale W60s and 4 ohm Smaller Advents with a Magnavox 9303 modified (not by me) with Edcor transformers that have 4, 8 and 16 (I think) ohm taps. Currently the 8 ohm taps are attached to the binding posts, because I am using the Wharfedales.

I tried connecting both the Smaller Advents and Wharfedales in series, to take advantage of the former's deep bass and the latter's rich midrange, but I wasn't impressed, at least not at first.

I'd like to be able to easily switch between pairs of speakers without having to go under the hood to change transformer binding taps, but I'm unclear whether the 4 ohm or 8 ohm tap would be best to use for both pairs. There's a lot of confusing information out there.

I guess I could try to fashion some kind of elementary switching mechanism, but that might be above my skill level. That said, if there's such a gizmo available from somewhere like Parts Express, please don't hesitate to let me know.

Any thoughts would be much appreciated!

 

[whoaru99]

The most straight forward solution is likely to modify to be able to use the the 4 ohm and the 8 ohm tap.

Alternately, you could connect either pair to the existing tap. It will change the operating point of the amp to put the 4 ohm speakers on 8 ohm tap but it's not going to damage anything AFAIK.

Some years back I think I showed a connection scheme using 4 pole Speak On connectors that "automatically" connected the speakers to the correct tap based on what terminals you used in the connector installed on the speakers. But, that takes modification too.

For example, a four pole Speak On connector has 1+, 1-, 2+, and 2- terminals. On the amp socket if you connected common to, say, 2-, you could connect the 16 ohm tap to 2+, the 8 ohm tap to 1- and the 4 ohm tap to 1+. Then, on the speaker side, for 8 ohms, you'd install a Speak On with the speaker - connected to 2- and the speaker + connected to 1- terminal.

Then, with your 4 ohm speakers, you could install the Speak On connectors using 2- to the speaker - and 1+ to the speaker +.

In either case, then, when you connect the Speak On plug to the Speak On jack, the impedance is "automatically" matched.

 

[Moonwatcher]

That makes sense. I'm sure that would be pretty straightforward. I guess I wondered if there might be any advantage to running the 4 ohm speakers on the 8 ohm tap or vice-versa. Thanks!

 

[whoaru99]

No inherent advantage unless the operating points are miscalculated/designed.

That said, some people find that "mismatched" connections sound better to them in certain cases. It's one of those things you just have to try.

If you had the impedance curves of the speakers you might get some idea if the curves deviated substantially from the "nominal" rating. If so, then maybe going by the curve might yield better results than going by the nominal.

 

[Moonwatcher]

Thanks, Whoaru99. I think the Smaller Advents are somewhere under 4, like maybe 3.4 or something. Researching the impedance curves would at least be educational.

 

[FlaCharlie]

Instead of connecting the Wharfedales and the Smaller Advents in series, you could find another pair of 4 ohm speakers and run them in series with the SAs. Maybe even another set of SAs? Then you could switch between systems and stick with the 8 ohm tap.

 

[gadget73]

not really had wonderful results running speakers in series that don't match. The variable impedance makes things go a bit goofy. Easier to just bring the other taps out so you can just connect to what is appropriate.

 

[Moonwatcher]

Great advice. Forgive my ignorance, but what if I put a 4 ohm resistor in series with the 4 ohm Smaller Advents? It's not that easy where I live to find Smaller Advents on the curb or Salvation Army shelf. I tried for years!

 

[gadget73]

you'd be throwing away half the power but the amp wouldn't care. You'd get less volume and significantly reduced damping so the bass might get a bit flabby sounding.

 

[Moonwatcher]

Thanks, and since the point is to be able to use both the Wharfedales and Smaller Advents, it seems like making both the 4 and 8 ohm taps easily accessible must be the way to go.

 

[opnly bafld]

Thanks, and since the point is to be able to use both the Wharfedales and Smaller Advents, it seems like making both the 4 and 8 ohm taps easily accessible must be the way to go.

The best way to go is to use both taps.
I assume the Wharfedales are more sensitive, so if you used just the 4 ohm taps the Wharfs will get less power but amp distortion should be lower.
The Advents then will get max power.

 

[gadget73]

if you're talking about running them both at the same time, connecting one to the 4 and one to the 8 will load down the output stage considerably. It would be the same as putting a 4 ohm load on the 8 ohm tap. To do this with an 8 and a 4 ohm pair you really need a 2 and a 4 ohm tap, which basically nothing has.