Stereo/mono wiring kinundrum

[Krnichin]

i have one stereo amplifier. I can use it to run separate stereo speakers but i also need to run a personal monitor for myself using both channels. I know that if i simply daisy chain my left and right speakers into both inputs on my monitor, I will effectively ruin my stereo amplifier. is there a way i can wire my speaker to accept both channels without having the left and right outputs of my amp essentially hardwired together?

[AndrewT]

Hi,
you need three channels to monitor in mono AND power a pair of stereo speakers.

Since you need an extra channel, then wire the input into this extra channel to sum the stereo into a mono channel and use it for your monitoring.

Your existing amp may be able to accept a summed signal at the input but you would then have a spare channel that leaves you still short for your two channel needs.

An alternative may be to bridge your amp, if it has a switch to do that and then sum the stereo input. But you still need to acquire the stereo amp for the speaker duty.

[pinkmouse]

Or buy a self powered monitor.

[Krnichin]

I was hoping there would be a way that I could accomplish this without having to spend much. I cannot afford more equipment than what I have already, but I could spring for some resistors or something. I figured that there is a way to accomplish what I am going for only because I noticed (and this may seem out of context considering my attempted application) that my powered bass cannon in my car routes BOTH left and right signals to one speaker. the channels i wired to my woofer come directly from my front speakers, so in the same manner I thought there would be a cheap way to apply this same concept to my live sound system. I know that if I were to purchase a mono power amp i could run my existing personal monitor. I know that if i were to buy a powered monitor, I could run line outs into it or even headphone outs. If I could buy an extra non-powered monitor, then i could just sit them beside eachother and run them out of my stereo speakers no problem. however I am trying to find a way to accomplish this using solely one stereo power amp and three speakers. Thank you so much for spending your time helping me!

[pinkmouse]

Unfortunately, it is just not possible. I appreciate the problem of having no money makes your options difficult, so the cheapest thing I can think of is to build a little chip amp to power your monitor. Something like an LM3866 or even better, a LM4780 wired up in parallel should sort you out perfectly.

[kevinkr]

You didn't mention what your expectation of quality was in terms of monitor speaker nor level.

You can actually quite safely resistively sum the outputs of a tube or solid state stereo amplifier to run a mono center speaker, in the case of a solid state amp you just need to know the minimum safe load impedance of the amplifier in question, and then choose the summing resistors such that the parallel combination of the left or right speaker and the respective resistor is not less than this.

If you are using 8 ohm speakers and the amplifier has a minimum load impedance of 4 ohms, then you can safely use 8 ohm resistors to do the passive summation. (This is safe even for the worst case condition of the two channels being 180 degrees out of phase.)

Note that any speaker driven this way should probably be somewhat more efficient than the left/right speaker pair and utilize a well damped woofer alignment or the bass will be pretty flabby. In some cases you might want to use an LPAD to control levels.

Old passive quad/surround sound adaptors worked in a similar fashion and I have encountered a few that even derived a summed center channel this way.

This is not the highest quality approach, but if you are looking for something cheap and quick with modest performance requirements this will work fine.

[AndrewT]

Hi Kevin,
how do we connect the resistors?

What if the centre speaker impedance is similar to the L & R speaker impedances?

[richie00boy]

The resistors go one end on each of left and right, the other end joined to form your mixed point. Then your speaker connects from this to ground like normal.

Centre speaker impedance relative to others is not such an issue as long as the amp min load requirements are met. What is more important is matched sensitivity* between drivers.

The centre will need to be approx 9dB more efficient due to the 6dB passive summing loss and 3dB radiating area efficiency of the two left and right vs one centre (assuming all speakers otherwise identical).

[AndrewT]

Hi Ritchie,
thanks.
Have I analysed the effect on the stereo performance correctly?

If all speakers and each of the summing resistors are 8ohm and the amplifier can manage to drive 4ohm loads then the following applies.

Assume an output of 8V at right amp output, then 8V drives the right speaker.
In parallel with this is a series combination of 8r resistor then a parallel combination of 8ohm speaker // 8r+8ohm left speaker.
The voltage after the 8r resistor is about 3.2V feeding the centre speaker and then drive passes through the final route to 8r+8ohm left speaker. The voltage driving the left speaker will be about 1.6V or about -14db. If the centre speaker impedance is raised the crosstalk will be worse.

The connection of the passive centre mono has increased the crosstalk in the stereo pair to just -14db. Stereo sound stage will appear to have significantly narrowed for the sake of mono monitoring.

[richie00boy]

No Andrew, only two resistors are used. These are the summing resistors and go to the centre channel only. Crosstalk is not affected due to the amp output impedance being significantly lower than the load impedance.

If all speakers are 8 ohms and 8 ohm summing resistors are used, the worst case load when channels swing in opposite directions is

8 || (8 + 8) / 2) or Zleftorright || (Rleft + Rright) / 2)

and if just one (left) channel swings fully is

8 || (8 + (8 || 8)) or Zleft || (Rleft + (Rright || Zmono))

EDIT: also bear in mind the original poster just wanted a mono monitor channel. The fact that this method can be used to create a passive centre is just something else. So sensitivity might not matter to the original poster.