Adding a Mono Switch

Hi guys.  I continue to struggle with adding a  mono switch to a couple of phono preamps.  I have decided not to mess with the preamp but to work at the phono-stage - less risky.   I have run across two circuits that seem to be simple enough to implement.   What do you think.  I would mount the switch at the output of the phono stage.  Advantages of one circuit over the other?  What do those resistors do in the second circuit?  I had Don Sachs build my SP14 with a mono switch but this is for a different set-up with a Cary SLP98.
Thanks in advance.
Dave

if you combine two equal signals you'll wind up with a signal twice as big. So the resistors do two things, they cut the signal down a little, so the sum is about equal to the parts,
and they also give some isolation to the signals from each other.
BTW, so how do you like your SP14 compared to the Cary??

I'm afraid the comparison will have to wait a while. The SP14 is on loan to a friend and the Cary just arrived. I'll get to listening to them side by side over the next month or so. Probably after the new year in actuality as he needs tunes for entertaining.

Sounds like you are recommending schematic #2? Also, how does one determine the size needed for those resistors?

Another option ...



Adjust the resistor value for a good match. Trimmer pots and a decade box would make that a lot easier.

Skizo, that sums to a single mono output. I want dual mono. How do i determine what is a good "match"? What is a decade box? The more I look into this, the further down the rabbit hole I go!
BTW, I see you're in Michigan, as I am. I live in Royal Oak. You?

connect the single mono output to both stereo inputs. As for matching, you can do it by ear, or use a sig gen and an o'scope, either way works, simple or complex, your choice.

WntrMute2 - It appears as though you are desirous of a mono signal from a stereo source - a phono cartridge - which plays mono discs with all the subsequent baggage of stereo noise - pops, clicks, icky stuff - from the disc. But you wish to listen thru a 2 channel system. I cannot fathom how combining both sides, which contain essentially the same signal, and playing them thru two channels, will require any attenuation via resistors. If this be the case, then the simplest solution should be sufficient. Your first solution is the correct one in your case, if I have sussed it properly. And you are right - mono recordings of the vinyl variety shoukl be listened to in true mono to appreciate the noise-cancelling effect.

I appreciate that Kentley; but almost everyone on a multitude of sites say use the resistors. Doesn't make sense to me either but that's what I did and it seems to be working well. I uses 2.2K resistors in this circuit and it worked fine.

If it works....kinda like the arguments pro and con about balanced/unbalanced circuits. The goal is to eliminate the UNCOMMON signal. Opinions vary on the implementation of this principle. In theory vs. In practice. Praxis Vobiscum. Pray to the Vinyl God. Resistance is Utile.

WntrMute2 wrote:Skizo,  that sums to a single mono output.  I want dual mono.   How do i determine what is a good "match"?  What is a decade box?  The more I look into this, the further down the rabbit hole I go!
BTW,  I see you're in Michigan,  as I am.  I live in Royal Oak.  You?

Yup ... connect the single mono of the circuit to both output jacks.

A decade box is simply a resistor network that allows you to dial in a specific resistance on the fly for testing circuit design. Don't need it that often, but a handy little devil when I do.



And I'm west state, Grand Rapids area.

There is a good sized audio club here in south east Michigan. You can check it out here:
http://www.michiganaudioclub.com/

"if you combine two equal signals you'll wind up with a signal twice as big. So the resistors do two things, they cut the signal down a little, so the sum is about equal to the parts,"

Not strictly true Kevin. If you were to "add" in the purely mathematical sense then yes but in this case if you have two 1vpp sine waves in phase and paralleled them together (assume zero source impedance) you'd still have a 1vpp sine wave. Think of the case where you parallel two 1.5v cells. What voltage do you get? Yep 1.5v.

When combining signals that differ from each other then this is where the resistors come into play. If we still use our use case from above, the 1vpp sine waves, if they are 180 degrees out of phase, they would cancel out. The result would be a zero output. Now lets go back to the cell analogy. If we were to wire the cells back to back the result would be next to zero voltage. BUT we would now have a current flowing , limited only by the internal impedance of the cells.

The purpose of the resistors is to generally limit this current and the consequential loading of the source. An overloaded source is not a happy one and can suffer increased distortion. (Your comment re isolation is true here)

Going back to a practical example, where we have load and source impedances there is of course a real world chance that combining two signals into a single source will increase the amplitude.

Hope this makes some sense.

Steve.