Stereomour II 4 or 8 ohm for KEF LS50

[ompv62]

To my complete shock, my Stereomour II, which I have been using with 94db single drivers, is able to drive 2-way 85db KEF LS50 monitors to reasonable levels with surprisingly good dynamics and great bass in a small 11x15 dedicated listening room. I'm contemplating switching the Stereomour from 8ohm to 4ohm based on:

Quoting Stereophile:

Somewhat optimistically specified at 8 ohms, the LS50's impedance (fig.1, solid trace) drops to 4 ohms at 200Hz and to 5.4 ohms at the top of the audioband. The electrical phase angle is generally mild, but the combination of 5.3 ohms and –41° at 135Hz, a frequency where music often has high energy, will make the speaker work at its best with a good, 4 ohm–rated amplifier.

Would you go 4 ohm based on the above? I do not understand the technical aspects of the phase angle, etc.

If I decide that I want more power, which would be better with these speakers, one Kaiju or two Seductor strapped as mono? I know 2 Kaijus is probably the best but I don't want to spend that amount at this time.

Thanks in advance for your thoughts.

 

[Paul Joppa]

If the phase angle is small, that means the impedance is mostly resistive - which is a good thing for SETs with no feedback. But the magnitude of the (mostly resistive) impedance will still have an effect. With low resistance, the current demand is higher and the damping factor is worse. But this also depends on the transformer - the output tube sees the speaker impedance modified by the OPT ratio. I use a fairly conservative design, which means a greater tolerance for speaker impedance dips. I don't have a problem with your speakers on the 8 ohm tap - it certainly won't hurt anything.

So, while I strongly encourage you to try it both ways, I must say either is acceptable. If you use the 4ohm wiring, you will have a bit less peak power, but better bass damping. Your ears should be the test, not theory!

For more power, to my ears zero-feedback triodes are still the best. Remember that Stereomour can be bridged to make a 7 watt amp - so you could get to Kaiju-level power that way. On the other hand, a Kaiju can be bridged to make a 16 watt monoblock, so if 7-8 watts is not enough you could add a second Kaiju at a later date.

I hope that helps - sorry it's not a very clear recommendation!

 

[ompv62]

Paul,

Thanks for the reply. Pardon my ignorance, I was reading here:

http://bottlehead.com/smf/index.php?topic=8600.0


How is volume control handled in 2 bridged Stereomours?

Thanks!

 

[Natural Sound]

How is volume control handled in 2 bridged Stereomours?

You will use two volume controls instead of one. One for left speaker volume and one for right speaker volume. Alternatively you could could use a passive pre-amp upstream from the Stereomours. In this setup you would simply turn the volume all the way up on the two Stereomours and use the submissive to control the volume.

http://bottlehead.com/?product=submissive-volumesource-control-kit

 

[Jamier]

You could cross over to a sub with a steep filter (24 db/oct) and relieve your KEFs from that frequency.The Stereomour II sounds so complete at low volume levels that I don't feel the urge to play loud. A couple years ago I read a review by ebag4 about how complete the Stereomour sounds at low volume levels. He is so right. This really improves the WAF if thats an issue for you like it is for me. I don't know what it is about this amp, but it does sound really good at low volume levels.I really feel that with this amp we are not required to use full range singles.Good 2 ways are well within it's range.

Jamie

 

[Jamier]

Just so you know. If you feel you need to, bridging is as easy as Doc and PJ describe. I have bridged one SII to make sure I could do it. It is a pretty impressive increase in power with no sonic penalties ( from what I could hear with one channel). For my persnal needs 2 bridged SIIs would pretty much handle it all.

 

[ompv62]

After spending a week with the Stereomour II on the 4 ohm taps with the LS50's, it is indeed sufficient power in a small room.  I'm amazed by the Stereomour's abilities to drive these speakers! I've ordered the SR and MQ upgrades!

 

[Jamier]

You will love those!

Jamie

 

[Jamier]

Ohan,

I think you should install the SR board before the Mourquiet. The SR install gets a liitle tight at times and requires all the room you can get. The MQ sits up off the chassis and might create some access issues.

Jamie

 

[ompv62]

Thanks for the tip James! I was planning on doing the MQ first so you saved me...