"Bridging" Stereomour II

[Jamier]

I want to bridge a pair of Stereomour IIs creating 7 watt monoblocks.
This would give me enough power to drive my 92 db efficient MTMs
in a small, semi near-field listening area. Has anyone done this with
success? (Yeah, I know I should have gotten off the dime and got some
Paramounts, but now they are all gone. The upside is that I'll have 7
Watts of 2A3.) Maybe, if there is enough interest in this topic, an
official Bottlehead guidline will be created.

 

[Doc B]

It's relatively easy, depending upon the impedance of your speakers. Assuming they are 8 ohm speakers, the first step is to adjust the transformers to 4 ohms output impedance.

Next jumper the negative (black) binding post of the right channel to the positive (red) binding post of the left channel. That will wire the two output transformer secondaries in series as a single 8 ohm secondary. Next connect the two outboard binding posts to your speaker in the usual way, i.e., right red to speaker pos, left black to speaker neg.

Now get an RCA Y adapter. Configure whatever type of adapter you use to split the signal cable from one channel out of your source into two cables, so that it can feed both left and right input RCAs of one of the input pairs on the Stereomour.

You now have two single ended amps strapped in parallel as one monoblock. This is more trick than a traditional parallel single ended amp in that the output tubes are completely isolated from each other and one won't hog current from the other.

 

[Paul Joppa]

You must also disconnect the safety ground of one of the black binding posts (otherwise that channel's output will be shorted).

If you disconnect the left black post from ground, then the output will be balanced; if you disconnect the right black post from ground it will be unbalanced. You can re-use the chosen wire to make the jumper between channels.

If you have the impedance switch, it must be in the "UNBAL" position.

Set the balance control to the middle so that both tubes get the same signal.

 

[Jamier]

When the amps are bridged, will they still be able to drive a
4 ohm load?

 

[Paul Joppa]

Yes indeed. Stereomour and Seductor both use the same transformer, and can be set for 2, 4, 8, or 16 ohms. When series-bridged as described by Doc B, they will optimally drive 4, 8, 16, or 32 ohm speakers. The Kaiju (still in development) will have the same ability.

The SEX amp uses the same output transformer but at a higher impedance level, so it can be set for 4, 8, 16, or 32 ohms. It can be series-bridged to drive 8, 16, 32, or 64 ohm loads with 4 watts per monoblock.

 

[drewh1]

I think it is a great idea. Just FYI. I drive a pair of 91db speakers very nicely with a stereomour. Plenty of volume. Might want to build one amp and try it first.

 

[Jamier]

That's good advice ! I ordered 1 today and I'll see how it goes.I
do have some concern about driving this particular speaker with bridged
Stereomours as the impedance curve dips to about 3.5 ohms above
5K Hz. Paul says their 4 ohm good but what about below that?Any thoughts?

 

[Doc B]

3.5 ohm speakers on a 4 ohm tap is essentially the same as a 4 ohm speaker on a 4 ohm tap. Go check a bunch of 4 ohm speakers and you will see they are quite often not exactly 4 ohms. Since the lower impedance is above 5kHz in this particular case the 1/2 ohm difference is pretty much a non-issue.