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Thread: 3 Channel Stereo ?

  1. #1
    Steve Gonzales
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    3 Channel Stereo ?

    I have a friend who is interested in hooking up a third speaker (4660 JBL) into his stereo system. I told him that I remembered a technique for this very thing that was done in the old days. I don't remember what it is called or how to do it. All I remember is that the third speaker is powered by taking the + from one channel and the - from another. Does this sound familiar to anyone? Please let me know if you can help. Thank you, Steve G

  2. #2
    Senior Member andywin's Avatar
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    Quote Originally Posted by Steve Gonzales
    I have a friend who is interested in hooking up a third speaker (4660 JBL) into his stereo system. I told him that I remembered a technique for this very thing that was done in the old days. I don't remember what it is called or how to do it. All I remember is that the third speaker is powered by taking the + from one channel and the - from another. Does this sound familiar to anyone? Please let me know if you can help. Thank you, Steve G
    Its called a Haffler array... Ive got some details on it somewhere

  3. #3
    Steve Gonzales
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    Thank you !

    I await the details

  4. #4
    Senior Member andywin's Avatar
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    David Hafler mod to C.G. McProud arrangement

    The first step is to link the two negatives of the outer pair of speakers and provide the negative return to the amps via a third centre speaker (in series) thus achieving a mono centre.

    The problem with this setup it crosstalk degrading the stereo image. David Hafler of Dynaco came up with a solution that resolves a lot of the problems.

    His solution was to bridge the positive chanels from the pre-amps to the power amps with 22k resistors on the outputs and bridging with a 20K variable resistor

    The idea is then to use a feed from one channel only with a mono source and adjust the pot to gain minimum output from the opposite chanels loudspeaker. Swapping the input to the other channel should achieve the same results. If not then either the gain of the amps are not identical or tone/balance controls may be at fault. Next step is to connect both inputs an give it a go.

    Perhaps one of the more tehically minded members can calculate what this does to impedances etc. :dont-know

    I've attached a pic of an article from August 1965 of the Gramophone magazine to illustrate the schematics...hope you have 20/20 vision
    Attached Images Attached Images  

  5. #5
    Steve Gonzales
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    Thank you

    Very good Sir, much appreciated!

  6. #6
    RIP 2011 Zilch's Avatar
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    Hmmm. Sounds like just summing to create a center channel, which would narrow the sound field.

    A better contemporary approach would be to use a matrix stereo chip and summing to create three channels.

    Matrix chip attenuates program material identical on L & R, creating a "hole" in the middle. It's the basis of SRS stereo in TV's, for example, broadening the sound field.

    It's also why you can't hear the singer sometimes.

    It may be that one or more of them already provides the summed center.

    National Semi, I think. May take me a day or two to dig it up, if anybody's interested....

  7. #7
    RE: Member when? subwoof's Avatar
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    Look around for one of the carver "sonic hologram" stand alone processors or any of their recievers that has it built in.

    It did a bunch of L-R, L+R, etc summing ( and subtracting ) tricks - simply add one of these to your preamp outputs and feed some combination to your center channel amplifier ( always recommended ).

    Possibly using the L+R outputs to a balanced input would *leave* the vocalist !

    BTW that is one BIG center channel - it requires good technique to aim it at a couch rather than 200 pews...



    sub

  8. #8
    Senior Member andywin's Avatar
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    [QUOTE=Zilch]Hmmm. Sounds like just summing to create a center channel, which would narrow the sound field.QUOTE]

    The reverse is actually true, I've read a couple of articles of people who have tried the Hafler system and all say soundstage broadens but at the expense of pinpoint stereo imaging. Apparently it's supposedly something to do with "psychoacoustics". An Italian profesor (can't remember his name) has written a lot on this subject.

  9. #9
    RIP 2011 Zilch's Avatar
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    ENHANCED STEREO
    The LMC1982 has an enhanced stereo effect that can be
    achieved by cross-coupling reverse phase information between
    the left and right stereo channels. This feature can
    help improve the apparent stereo channel separation when,
    because of cabinet or equipment limitations, the left and right
    speakers are closer to each other than optimum.
    Enhanced stereo is created by connecting an external frequency
    shaping RC network between the OUTPUT operational
    amplifiers’ inverting inputs through an internal CMOS
    switch (see Figure 6). The external network couples 60% of
    each channel’s output to the opposite channel’s inverting input.
    This cancels a portion of the signal common to both
    channels.
    The desired 60% cross-coupling is accomplished through
    the internal 6.5 k
    W feedback resistor and an external 10 kW

    resistor. Bass frequency cancellation is prevented by using a
    0.047 µF coupling capacitor to couple only frequencies
    above 330 Hz. Switching noise is eliminated by using a
    680 k
    W resistor across the 0.047 µF. R3, R4 and C6 can be

    eliminated if enhanced stereo is not desired.

  10. #10
    John Y.
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    Klipsch, Hafler and the derived center channel

    Quote Originally Posted by andywin
    The first step is to link the two negatives of the outer pair of speakers and provide the negative return to the amps via a third centre speaker (in series) thus achieving a mono centre.

    The problem with this setup it crosstalk degrading the stereo image. David Hafler of Dynaco came up with a solution that resolves a lot of the problems.

    His solution was to bridge the positive chanels from the pre-amps to the power amps with 22k resistors on the outputs and bridging with a 20K variable resistor

    I've attached a pic of an article from August 1965 of the Gramophone magazine to illustrate the schematics...hope you have 20/20 vision
    The three channel system derived from two stereo amplifier channels was originally championed by Paul Klipsch obviosly in considedration of having two corner horns that may have been located alomg the long wall of the listening room. Using 1933 demonstrations by Bell labs as a reference, Klipsch derived the center channel by connecting the center across the 4 Ohm taps of the L & R amplifiers and connecting the amplifier commons together. In Klipsch's case, the main channels were connected in the usual manner to the 16 Ohm taps.

    Early on, Hafler chose to use the three speaker setup where the center is placed in series with the L & R channel returns, the L & R being connected to the taps representing twice the impedance of the individual speakers - all speakers of the same impedance. Thus, each side saw a series impedance mated to the proper amplifier taps. Hafler admitted that stereo imaging was reduced by crosstalk from the opposite channel and recommended moving the L & R farther apart than normal and the use of a blend control to assist in reducing the crosstalk to regain proper imaging.

    In a letter from Dynaco, dated July 21, 1961, Mr. William Phillips states "It is not possible, under any circumstances, to derive a third channel from the preamplifier alone, no matter by what means". Pretty strong conclusion to my inquiry at that time of bridging the preamp channels with two 33K Ohms and the derived signal from the center of this bridge feeding a center amplifier. Their proposed solution was to have the L & R 16 Ohm taps bridged by two 500 Ohm resisters, the center of which would connect to ground through a 100 Ohm audio pot, providing the adjustable source to the center amplifier. This became their documented solution to the problem of derived center channel.

    Later on, Dynaco produced a low cost Quad device providing for L & R rear matrixed signals without, as claimed, degradation of the stereo signal. This was around the time that SQ, QS and other four channel schemes were proposed. It was also the time that I obtained my four JBL L100's, which I have to this day. I mostly used the Sony SQ system with a Crown D150 on the fronts and a Crown D75 on the rears. I never bought a Dyna Quadaptor, but I wish I had gotten the schematic just to see how they derived their rear signals.

    John Y.

  11. #11
    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by John Y.
    In a letter from Dynaco, dated July 21, 1961, Mr. William Phillips states "It is not possible, under any circumstances, to derive a third channel from the preamplifier alone, no matter by what means."
    Yeah, well, no op-amps back then, and he probably never heard of a unity-gain buffer, either.

    Seems like once you derive a Matrix subtraction signal, you can feed it to a center channel.

    Gonna watch for that Carver hologram thingy, I am. Could be fun....

  12. #12
    Senior Member Flodstroem's Avatar
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    "Ambiophoni"

    Hi Steve

    In Sweden in the sexties and seveties this idea was called "ambiophoni". The idea was to try to get som new information from the tracs of common stereo vinyl LP´s

    There was a common belief at that time when using the stereo-headphones there was allways thougt the stereo-image was much better than when using the common stereo loudspeakers. The bad image was probable due to some cancellations of the program materials when reproducing the "L" and "R" program in the same room.

    The effect of cancellation could be compensated in a way by adding a third or even a fourth channel as described by member John Y, Zilch etc. Most famus for "ambiophoni" in the USA at that time was probable "Carver" with their model C-4000.

    My own practical solution at that time was to use a balansed input line amp (without balaced output) or a balanced input power amp (a mono-block). I connected (bridged)one of the "R" outputs fro the pre-amp to one of the balanced "hot" input and connected the other "L" outputs from the pre-amp to the reminding "hot" balanced input. What was amplified in the third amp was the electrical differensies between the two channels (you can get "two" different "differencies", positive or negative depending on whitch way you put the "R" and "L" outputs from pre-amp to the two balanced inputs.

    The positive effect of using balanced inputs was it did´t ad any extra crosstalk. you could allso balance the volume of this third channel to not to be to loud regardles of the two front speakers ("L" and "R").

    The image of a third channel is strongly dependent of the recording of the original program material and most of the "studio-recordings" are useless.
    Absolutely the best effect to demonstrate the "ambiophoni" is with live-recordings.

    I should never forget a very special occasion when hearing the public clapping there hands and whistling in rear of me when the "band" was in the "L" and "R" in the front of me when listening to a live recording by Rolling Stones. It was like a live experience at that time.

    Best Regards
    Flodstroem

  13. #13
    Senior Member andywin's Avatar
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    Good site for info

    This link should explain all... and no more mentioning nasty things such as Op-Amps and other new fangled devices that have no heaters and don't glow in the dark.

    Seriously though the only true stereo + centre mono would be if Mercury released some of their early 3 track recordings on DVD without any mixdown. It still does'nt guarantee it will sound good though.

    http://www.dai.ed.ac.uk/homes/cam/gadgets/hifi.html

  14. #14
    Michael
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    Its really not that complicated. Any amp that can be bridged can run like this.

    Its also referred to as 'tri mode' and 'phantom mode'. You connect your two main speakers to the amp as per normal, but connect the third as though it is running in bridge mode of the amp.

    However there is one catch. If the two main speakers are 8ohm, then the third must be 4ohm. Or the other way around, if the two main speakers are 4ohm, the third must be 8ohm.

    I've found that tougher amps can run 8ohm loads all round, or even 8ohm stereo loads with a 2ohm bridge load. But to play it safe follow the method in the 3rd paragraph

  15. #15
    Steve Gonzales
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    Simply OUTSTANDING!!!!!!!!

    Thank you to EACH and EVERYONE that responded to my question!! I think I have a grip on what to do now. MUCH appreciated!

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