old school center channel wiring

[Paducah Home Theater]

This is strictly out of curiosity due to wading through memories, but something we used to do back in the day with car audio was to make a center channel that was wired up with something like the left negative and right positive as if you were bridging the amp, but the amp also ran in stereo at the same time. Basically the center channel worked if both left and right were active but when one side dropped out the center content went away. So, wasn't really a phantom center, and it wasn't a dedicated center. We would put a variable resistor in-line to get the levels perfect. Actually worked quite swell since in a car you are never really centered. Anyway, has this been done in home applications at least at some point? Do they even still do it in car audio? Just curious.

[mustang guy]

That's cool. I've never heard of that. 

 

As long as it doesn't damage the amps, and keeps crosstalk from happening, that's would be a good solution for the 1979 truck I'm restoring. I am putting woofers in the lower rear pillars, tweeters mid way up the pillars, and a single center coax would be awesome, since the bracket and grille are already built in.

[WMcD]

I've written on this a couple of times before.  But I'll give a short run down.

 

PWK used the "bridge" center with his live stereo recording.

 

The issue is that the center speaker (any speaker or device) responds to the difference in voltage across its input terminals.  Because it was bridged across the right plus and left plus, the speaker output was Left minus Right.

 

He published an article showing that the music from the center of the stage was in random phase and therefore Left plus Right or Left minus Right did not make a difference. 

 

As valid as that may have been, things changed in the recording industry.  With mixing boards, vocals were mixed hard center (in phase in both Left and Right) and therefore the singer was perfectly identical in both side channels.  Therefore, a center channel bridge gave Left minus Right (being equal) which worked out to zero -- the singer would disappear from the center speaker.   Also, some classical recordings were made with a third, center microphone which resulted in the same problem. 

 

This is why, IMHO, PWK switched to using schemes so that the center channel speaker actually did respond to Left plus Right.  The most easy to understand is the center channel mixing box which required a third amplifier.  Basically, 5.1 Dolby did this for the center too.

 

But back then there was the goal of finding some scheme to derive a signal from the output of two amplifiers which would form a Left plus Right to feed to a center speaker.

 

The solution was to rewire connections to the output transformer of one channel, say the Right, so that polarity was inverted, to give minus Right at the output terminal, or at least where you connect at the terminals.  That really was not a problem for the right channel speaker because you could hook it up inverted -- which is to say, red speaker input to amp minus and black speaker input to amp "hot" or plus.  Where these occur at the amp output must honor the rewiring.  But essentially, this was polarity reversal and another polarity reversal, which equated to no polarity reversal as far as the right speaker output was concerned.

 

But look at what was happening to the center bridged speaker.  It was connected across Left and "minus Right" where minus Right means reversed polarity of the signal. As a result, the center speaker was responding to Left minus, minus Right.  And like in arithmetic, that minus times minus is a plus, and we get Left plus Right.  Magic.  This is in Eargle's paper in the Klipsch papers and PWK's complicated switch box.

 

To recap: John and Paul are singing into a microphone which at Abbey Road is mixed hard center on a two-track recording, which means the microphone output is recorded identically on the Left and Right tracks.  This will lead to a phantom center output from two regular stereo speaker, if you're right between the speakers and the room acoustics are balanced left and right .  But if you have a Left plus Right center speaker the image is no longer a phantom.  John and Paul are coming out of the center speaker. 

 

There is some additional math to this.  The center does too good a job because we are adding voltages in the center.  But power goes up as the square of the voltage.  The result is that we have to turn down the center a bit.

 

For the record, there was another center channel scheme from the passive Dynaco circuit. The negative sides of the left and right speakers were connected together.  Then this would be fed to the plus of a center speaker and the minus of the center would be connected to amplifier ground, or minus.  Therefore, the ground returns for the left and right would go through the center.  But, there must be cross talk between the Left and Right so that separation is compromised.  I tried a Dynaco box but it did not seem to work for me.

 

In old Hi-Fi magazines we see ads in the back for a device which "removes the singer" so you can sing along.  I'm sure this is because it extracts Left minus Right.

 

WMcD

Edited March 24, 2015 by William F. Gil McDermott

[garyrc]

For the record, there was another center channel scheme from the passive Dynaco circuit. The negative sides of the left and right speakers were connected together. Then this would be fed to the plus of a center speaker and the minus of the center would be connected to amplifier ground, or minus. Therefore, the ground returns for the left and right would go through the center. But, there must be cross talk between the Left and Right so that separation is compromised. I tried a Dynaco box but it did not seem to work for me.

 

Wasn't there a Dynaco type circuit in which the negatives were connected to each terminal of a speaker behind the listener,  that recovered out of phase sounds, and another circuit (in phase) for the front center?  I think I remember an engineer back then saying that some amplifiers wouldn't "like" this, and would freak out.  Or did I dream it?

Edited March 24, 2015 by garyrc

[agile1966]

Thanks for the post William. Excellent read!

[JJkizak]

I'm wondering if these problems still exist because I just hook my center speaker to the center channel out jacks and  set the AVR to PLCIIx music and everything sounds fine.

JJK

[WMcD]

Here you go Gary. Sleep well.

[garyrc]

Here you go Gary. Sleep well.

 

Thanks!

 

Furlongs?  Did Speakerlab move from Seattle to Imperial Britain when I wasn't looking?  While I was asleep?  I think with 6 furlongs I could wire the Cow Palace.

 

I actually had something like this in the '70s, but with only one rear channel, dead center behind the listeners.  With some records it was very, very good, but with others there was (out of phase?) rumbling, and I kept wondering if someone was rattling the windows behind me.

Edited March 24, 2015 by garyrc

[babadono]

Didn't Dynaco make a "quadaptor" or some such? I think I read a couple of our fine members here raving about it.

[babadono]

PWK used a center Belle between KHorns did he not? I know I have seen how he hooked it up, perhaps in a 'Dope from Hope' article.

[babadono]

yep, its 'Dope from Hope' Volume14  #4. Wish I knew how to put a link to it here. I'm a dumb-dumb.

[WMcD]

Gee, I seem to be always referring to my old posts.

 

Here is the link.

 

https://community.klipsch.com/index.php?/topic/48332-bridged-center-loudspeaker-dope-from-hope/?hl=%2Bmini-box+%2Bmixer

 

People seem to like my technical explanations.  So I'll tell you about it.

 

We would like an output which is Left plus Right for the center channel amplifier.  It is not as easy as just hooking a wire or Y connector to both the left and right output of the pre-amp or the left and right outputs of the power amp.

 

If we did that with the pre-amp out it might work to create a center channel but now a connection would be made so that the left and right power amp inputs would be getting a signal from the other side, and as a result, we'd have three speakers giving left plus right, or just mono.  Not good.

 

We have to understand something out the nature of the output circuits of the pre-amp.  These have a relatively low output impedance and I'll say it is 1000 ohms.  Thus means that if you don't put too much of an electrical load on it, the voltage output will be what it supposed to be. That is to say, what the music is doing.

 

The Y of resistors do not present too much of an electrical connection. Therefore the pre-amp outs keep on doing what they should.  This is so even though there is a little signal leaking through from the two high resistances in the Y connection.  The output circuit of the pre-amp is strong enough to overcome this leakage.

 

But we have the junction point of the Y.  Here the voltage from the left and right channels add, to form left and right.  So we have, essentially a mono signal there.  The input to center channel amp does not load this down.  The concept of "not loading it down" is to say that the pre-amp has a high impedance input.  With high impedance (or resistance) it does not load the junction very much.  Yet it does respond to the signal at the junction.

 

BTW, the voltages at a pre-amp output is probably about 1 volt or 2 at maximum levels of music.

 

There is a second circuit shown which hooks up to the speaker output of the left and right amplifiers.  It is basically the same circuit.  Here the power amplifier outputs have a very much lower output impedance because it is meant to drive 8 ohm speakers. It can and does put out a lot of current.  The resistors in the Y connection can not possibly influence the outputs. The voltage of the music there can be pretty high.  But consider that horn speakers fed with 2.828 volts is very, very loud.  In any case, you can knock it down with the potentiometer. 

 

I dare say that in some cases, the voltage level from the line out is higher than the voltage output of the power amp.  Of course the power amplifier is putting out much more current because of the 8 ohm (or so) load.

 

WMcD

[babadono]

Resistive adding of signals will result in a fraction of the left channel in the right and vice versa. Whether you can hear it is debatable and of course depends upon how much mixing or 'bleed over' there is. The output impedance of the driving circuit, the input impedance of the input and the mixing resistors in the box all come into play. This would be a perfect job for a summing amplifier.

[WMcD]

Here is the circuit I mentioned, i.e. with the center wired as the ground return for the flanking speakers. Of course it is from that Dynaco guy, David Hafler.

You can go to the USPTO site for a full copy of the patent.

WMcD

[garyrc]

PWK used a center Belle between KHorns did he not? I know I have seen how he hooked it up, perhaps in a 'Dope from Hope' article.

 

I believe he first used a Heresey between the flanking Klipschorns, then a Cornwall, and finally a Belle.  He said that the perceived distortion of the whole system tends be similar to the distortion of the speaker with the highest distortion, so the Belle was used because it had less distortion than the Heresey or the Cornwall.  I believe he was talking about lower midrange & bass distortion only, and that it was of the frequency modulation type.

[whatever55]

 

For the record, there was another center channel scheme from the passive Dynaco circuit. The negative sides of the left and right speakers were connected together. Then this would be fed to the plus of a center speaker and the minus of the center would be connected to amplifier ground, or minus. Therefore, the ground returns for the left and right would go through the center. But, there must be cross talk between the Left and Right so that separation is compromised. I tried a Dynaco box but it did not seem to work for me.

 

Wasn't there a Dynaco type circuit in which the negatives were connected to each terminal of a speaker behind the listener,  that recovered out of phase sounds, and another circuit (in phase) for the front center?  I think I remember an engineer back then saying that some amplifiers wouldn't "like" this, and would freak out.  Or did I dream it?

 

 

No dream... Here it is  I hope  The only special requirement was that the amp HAS to have common grounds between channels and that can be checked with an ohm meter

Edited March 28, 2015 by whatever55

[mustang guy]

 

 

For the record, there was another center channel scheme from the passive Dynaco circuit. The negative sides of the left and right speakers were connected together. Then this would be fed to the plus of a center speaker and the minus of the center would be connected to amplifier ground, or minus. Therefore, the ground returns for the left and right would go through the center. But, there must be cross talk between the Left and Right so that separation is compromised. I tried a Dynaco box but it did not seem to work for me.

 

Wasn't there a Dynaco type circuit in which the negatives were connected to each terminal of a speaker behind the listener,  that recovered out of phase sounds, and another circuit (in phase) for the front center?  I think I remember an engineer back then saying that some amplifiers wouldn't "like" this, and would freak out.  Or did I dream it?

 

 

No dream... Here it is  I hope  The only special requirement was that the amp HAS to have common grounds between channels and that can be checked with an ohm meter

 

That's good info, and there is no reason this couldn't be a single front speaker and dual rears as in an old pickup truck. I don't understand the ground, however. Every chassis has a common ground doesn't it? I always thought the negative wire coming from an amp used this common ground. I could really use some help understanding how to identify this with an ohmeter. Simply see if there is a closed circuit between the negative terminals? If they are closed, then you disconnect the negative terminals of both stereo speakers and connect them together? This is very odd and I am having some trouble wrapping my pea brain around it.

[whatever55]

 

 

 

For the record, there was another center channel scheme from the passive Dynaco circuit. The negative sides of the left and right speakers were connected together. Then this would be fed to the plus of a center speaker and the minus of the center would be connected to amplifier ground, or minus. Therefore, the ground returns for the left and right would go through the center. But, there must be cross talk between the Left and Right so that separation is compromised. I tried a Dynaco box but it did not seem to work for me.

 

Wasn't there a Dynaco type circuit in which the negatives were connected to each terminal of a speaker behind the listener,  that recovered out of phase sounds, and another circuit (in phase) for the front center?  I think I remember an engineer back then saying that some amplifiers wouldn't "like" this, and would freak out.  Or did I dream it?

 

 

No dream... Here it is  I hope  The only special requirement was that the amp HAS to have common grounds between channels and that can be checked with an ohm meter

 

That's good info, and there is no reason this couldn't be a single front speaker and dual rears as in an old pickup truck. I don't understand the ground, however. Every chassis has a common ground doesn't it? I always thought the negative wire coming from an amp used this common ground. I could really use some help understanding how to identify this with an ohmeter. Simply see if there is a closed circuit between the negative terminals? If they are closed, then you disconnect the negative terminals of both stereo speakers and connect them together? This is very odd and I am having some trouble wrapping my pea brain around it.

 

 

No problem.  Yes most amps have a common ground but there are some that do share a ground and would cause a problem. You can check this by checking for continuity between the negative posts on your amp with it tuened off and no connection to the speakers. ( Just to be safe)