Question about the Rob Squire DIY passive monitor controller

[Carl]

My first post here, so first up, g'day all!
(Well, actually my second attempt at my first post. My actual first post languishes in solitude elsewhere on the forum...)

Now, been thinking about building this handy dandy looking device, saw it in AT mag a few years back, my question relates to how the stereo to mono switching works.

My understanding is that just joining the left and right wires of a stereo send (i.e. tying L hot to R hot and L cold to R cold) to create mono was a big no no, but this is what Robs design does.

Am i missing something here? Not dissing the design, but would like to understand how this works, or more precisely why it's ok in this scenario.

FYI, I have a basic electronics understanding, can wield a mean soldering iron and have built many bits and pieces etc. but really don't have an in depth knowledge of the hows and whys of those pesky electrons, so please feel free to illuminate me!

Ok, cheers
Carl

[Sammas]

My understanding is that just joining the left and right wires of a stereo send (i.e. tying L hot to R hot and L cold to R cold) to create mono was a big no no, but this is what Robs design does.

It doesn't if you look at the overall design though.

That being:

Input selector -> Stereo volume control -> mono/phase -> output selector.

The left and right hot and cold all run through independent 4.7k resistors in the volume control. It is a simple design, but not as simple as just tying left and right straight together.

I am no expert either, but the resistor network in the stereo volume control looks quite similar to a passive summing network that you see in passive summing circuits. I would take a stab and say that Rob knows what he is doing. He is the expert after all.

[rob]

Ha! Yeah, I know what I'm doing .... some days anyhow.

yes. .. it is a summing network.

[Carl]

Cheers, thanks for the replies!

Hmmm, yeah i figured it might have something to do with those resistors...

I'm presuming the pot (variable resistor) is serving the same function as the resistor that ties across the Hot/Cold legs at the output of a simple passive summing device, but i can't say i totally get it.

However, as electronics isn't my main gig i'm happy to differ to those that know; time to just get on with it and build it!

Anyways, thanks again, and i have no doubt Rob Squire knows what he's doing.

Most days....

Ciao

Carl

[Sammas]

The pot is just wired as a variable resistor between the hot and cold of each output.
Hot and cold being 180 degrees out of phase with each other if the connections are balanced.

Pot full anti-clockwise = no resistance across the pot = hot added to cold to result in silence.