Wiring Mono speaker

[tdukex]

I have a decent ($200) center channel speaker I want to use as a mono reference for my mixes. It has one + and one - terminal. I'm going to use a speaker switch box to switch between various sets of speakers, so I will have L+ L- and R+ R- terminals coming out of the switchbox. Is there a correct way to hook this up?

Thanks for any replys.

 

[tdukex]

Barefoot, where are you?

 

[barefoot]

Here I am. (I got a message that my PM Inbox was full, so I looked around and found your thread.)

If I understand you correctly, you're looking for a way to produce a mono signal by somehow wiring the L and R amplifier outputs to a single speaker.

Well, there's really no way of doing this simply with speaker switches. The only way it could work is to add a power resistor network like the top circuit in the attached picture. Switch position B sends the signal normally to the L and R speakers. Position A sums the L and R via the 2 Ohm resistors. The 16 Ohm resistor plus the speaker impedance act as a buffer so the L and R sum equally. There are two very major problems with this circuit. 1. The speaker output is greatly attenuated due to the resistor network. 2. The speaker impedance is not flat with frequency. So the resistor network and the speaker act as a frequency dependent voltage divider causing significant frequency response deviations.

Something similar to the bottom circuit would work. It uses a 4PDT switch to sum the L and R on the preamp side as well as switch the speakers. In position B the L and R channels are active. In position A the preamp L and R channels are summed via the 1K resistors and sent to the left amplifier. Also the L amp channel is switch to the center speaker and the R speaker is switched out. The reason this works is because the input impedance of the amplifier will be fairly flat over the entire frequency range. The poles switching the speaker connections, of course, need to be capable of handling the current. You can probably come up with a more elegant circuit, but this gives you the basic idea.

Hope this helps!

Thomas

http://barefootsound.com

 

[TexRoadkill]

Barefoot - Why can't you just bridge the L+,R+ and L-, R-? That is how some PA amps work for bridging mono. Does it depend on the amp?

 

[barefoot]

Tex,

Most of the bridging schemes I've encountered reverse the polarity of one of the channels in the preamp stage, then have you connect the speaker to the L+ and R+ outputs. Without the polarity reversal you just get the L-R difference signal. I guess if all the amplifier outputs were floating you could connect L- to R+ then connect the speaker between L+ and R-. But, typically both of the negative outputs are reference to the same ground. If you try this with most amps you'll fry your output stage for sure.

Thomas

http://barefootsound.com

 

[jrosenstein]

barefoot, would something like the following work as a basic mono switch? i don't want to switch to a single center speaker, just to have the option for both speakers to output the sum of the left and right channels.

also, what does the 16R resistor accomplish in the post-amplifier summing circuit that you don't recommend? doesn't that just attenuate the signal?

with amplifier input impedances as high as they are, would the signal loss due to the 1K resistors even be noticeable? (if it is, it's ok, because my amp goes to eleven)

 

[barefoot]

jr,

Yeah, that mono switch should work fine. Most amps you see these days have input impedances of something like 20k Ohms. So the 1k resistor attenuates the signal by about 0.5dB... not a big deal. The main detritus effects of the resistors are added noise (several mechanisms having to with the nature of resistors as well as how amplifiers react to looking back into fairly high impedances) and minor frequency response deviations due to the amplifier's slightly complex input impedance. The 1k value is a compromise and chosen to present a reasonable 2k Ohm load to the previous output stage. If you happen to have a very beefy preamp stage driving this circuit, you might be able to get away with 500 Ohm or even 330 Ohm resistors.

The 16 Ohm resistor in the first circuit serves 2 purposes. If you think of each amplifier channel's output as a virtual ground, then the other output sees a load of 2 Ohms plus another 2 Ohm load in parallel with the speaker leg. If the speaker leg has a relatively high impedance like 16+ Ohms, then each channel sees approximately a 4 Ohm load - which is fairly low, but still reasonable. If you removed the 16 Ohm resistor and used a 4 Ohm speaker, then the impedance seen by each channel could easily dip down below 3 Ohms. The 16 Ohm resistor also helps ameliorate, but hardly eliminate, the large response deviations due to the complex speaker load. I'm sure there's a much more optimal set of resistor values for this circuit, but this is basically what you need... and in any case it's not very pretty.

Thomas,

http://barefootsound.com

 

[barefoot]

Oh yeah, I forgot to mention that the low frequency box tuning of a speaker directly depends on the circuit impedance. So, however optimal you make the resistor network, it's still going to mess up the bass alignment.

Thomas

http://barefootsound.com

 

[jrosenstein]

very cool. nobody could ever accuse you of giving incomplete answers.

thanks so much.

 

[barefoot]

No problem.

I should say I do really like tdukex's idea of using a center speaker for mono. Using a stereo pair in mono will create a very nasty acoustic comb filter. And you really need to be listening dead on center to avoid it's effects. For close in nearfields, just the space between your ears (or should I say the thickness of your head?) can make it near impossible to hear a flat summed response. Mono from a single speaker is really the best way to go.

Thomas

http://barefootsound.com

 

[tdukex]

Thanks for the info, Barefoot. This, of course, will be more difficult than I thought.

PS. I did try to PM you several times.