Using Stereomour as mono amp for just one speaker

[Boustrophedon]

Hi,

I'm currently putting together a mono system, with just one speaker. Originally I wanted to use a Paramount monoblock for this, but they're only available in pairs.

Is it possible to connect just one speaker to the Stereomour, and to hear the sum of the left and right channels through this ? As the outputs are transformer coupled, is it as simple as wiring the left / right speaker 4 ohm taps in series and connecting an 8 ohm speaker ?

 

[Grainger49]

I think you will want to sum the Left and Right channels before feeding both inputs. 

A pair of Y-adapters will do this.  You want one with 2 female RCAs to one male RCA that will sum the two inputs.  Then you get one that has one female (plugs into the male of the first adapter) to two males that feed the two inputs.

That is not elegant but it is effective.  I always look for a cheap answer.

 

[Boustrophedon]

Thanks Grainger - if I sum the inputs first, can I then connect the single speaker to the left / right outputs in series ? i.e., connect the (speaker 4 ohm) right +ve to left -ve with a wire, and connect an 8 ohm speaker to the left +ve / right -ve ?

 

[Grainger49]

I haven't done this myself.  Yes, I would wire both transformers for 4 Ohms for an 8 Ohm speaker.

Also, I think you described putting a jumper from one channel's negative to the other channel's positive.  This puts the transformers in series.  Then your speaker leads come from the two remaining speaker lugs. 

 

[Boustrophedon]

I haven't done this myself.  Yes, I would wire both transformers for 4 Ohms for an 8 Ohm speaker.

Also, I think you described putting a jumper from one channel's negative to the other channel's positive.  This puts the transformers in series.  Then your speaker leads come from the two remaining speaker lugs. 

Yes, that's what I was trying to describe. Thanks ! I think I'll order the kit and try it for myself...

And if anyone's wondering why I'm doing this - a large part of my collection are mono recordings, and I much prefer the sound to come from just one place. Of course, the next problem will be trying to purchase only one speaker !

 

[Grainger49]

  .  .  . 

So, maybe a center channel?  No, come to think of it, they don't have much low frequency response.

Give Clark or Molly Blumenstein a call.

 

[Boustrophedon]

Rather than use a Y-adapter, I'd like to keep the 3 switched inputs of the Stereomour so I can connect my sources directly to it.

I don't have a circuit diagram (as I haven't ordered the kit yet), but looking at the underside photo of the layout, I think the best place to sum the left / right channels might be after the source selector, but before the volume control.

Would placing a 1k resistor in series with each of the left / right channels before connecting them to the volume pots, and then connecting both volume pots together, work to sum the stereo to mono ?

 

[Grainger49]

If you intend to use multiple sources that are all mono you can sum by attaching the right and left inputs together on one "deck" of the selector switch.  You could use the other deck to switch the "ground/common" for that input to lower the noise.

If you have only one source that is mono you can do it at the one pair of RCA jacks, short right to left.  (still being cheap)

 

[Paul Joppa]

These things should work well.

Be aware the the Stereomour instructions call for one end of the output transformer to be grounded (it's more complicated if you get the impedance switch) - so you'd have to ground the other end of one of the outputs to get them in series without shorting one of the outputs.

But this series arrangement into twice the load impedance of the individual amps is what Doc B uses for his midranges. Actually, he has them wired for 1 ohms and uses fourn amps with outputs in series, to drive 32 watts into a 4-ohm midrange.

I would connect the wipers of the volume control together to get mono, as long as you never go above 3/4 of the available range. At the highest settings, the channels are shorted to each other, which may cause trouble for the source - the difference signal sees a near-zero resistance which demands excessive current from the source. A Y-cable will do this all the time, as would connecting the volume control input tabs. By using the wiper tabs at less than full volume, the about of potentiometer left unused provides a resistance between the channels. This also eliminates the poor tracking at very low volume settings, characteristic of all simple potentiometers.

Of course, the phono cartridge is going to be pretty immune to this problem as it has no active circuitry. That would effectively make the cartridge a mono cart.

 

[Boustrophedon]

Thanks Paul - I'll go with your suggestion of connecting the volume wipers together. Grounding the opposite ends of the two transformers is also noted.

I'm going to be using a Fostex FE206 full-range driver with a back-loaded horn as loudspeaker, which should give c.96 dB / W , so I think 3/4 of the volume range will be more than enough for me.

 

[Doc B]

Maybe I am confused here, but I think you need to connect the two secondaries in series and then ground just one tap. You could ground just one end, or you could ground just the tap where the two transformers meet for balanced operation. But only one tap.

 

[Boustrophedon]

Maybe I am confused here, but I think you need to connect the two secondaries in series and then ground just one tap. You could ground just one end, or you could ground just the tap where the two transformers meet for balanced operation. But only one tap.

Thanks, I meant to say I'll connect the 2 transformers in series and ground the centre where they meet. So this would mean that (say) the negative of the left channel and the positive of the right channel would be connected together and grounded. The speaker then connects to the left positive, right negative.

 

[Doc B]

Yup, that's correct.