steve Posted October 30, 2005 Share Posted October 30, 2005 Need some help..I've been searching thru the archives concerning three channel listening, and I think I'm either reading it wrong or making this more difficult than it need be. Here's what I have going.. I have my preamp with -3 sets of outputs. One set goes to my 2 channel SET amp (yes, I know there are going to be the "why" questions, but bear with me) and that amp feeds my Klipschorns. I am going to have a Belle as a center speaker. I'd like to use my MC275 stereo amp as a center amp, i.e. blend the right and left channels into the one center speaker. The instructions in the Mac book show how I can make it a mono amp, but of course, that's just feeding one RCA, or balanced cable to the right amp input, then wiring the 16 ohm taps in parallel (for an 8 ohm speaker). Of course, this only gets me one channel (the right one in this case) and i want both. Somehow I feel if i hooked up both right and left RCAs to the amp and wired the speakers taps as described above, I'd short something..and I certainly don't want to do that. In the archives, I see something about a passive box mixing the R/L signals from the preamp out...would that go between the preamp and amp? If so, is there one without an attenuetor? I ask this, as my MC275 has built in attenuetors..and the less controls in the path the better. Any answers appreciated..I know I'm making this too complicated. If there's a box I can buy online that would solve this for me, please point me in the right direction!! Thanx all, Steve Quote Link to comment Share on other sites More sharing options...
michael hurd Posted October 30, 2005 Share Posted October 30, 2005 There are in fact 2 versions of the minibox, one is designed for line level use IE: between preamp and amplifier, and one is designed for use between the amplifier and speakers. Both versions output a low-level signal, that is fed to a mono amplifier or one-half of a stereo amplifier. Hopefully this makes sense, you only need one channel of the Mac to drive a center speaker. If you want to somehow use both channels, you could split the output from the min-box with a rca Y adapter, and run the same signal to both amp channels on the Mac. You must ensure that their is no electrical connection between the LF input and the HF input, otherwise you will damage the amp. As long as the crossover is split, then no problem, you just run a wire from one channel to the woofer and one to the high frequency section. Quote Link to comment Share on other sites More sharing options...
dragonfyr Posted October 30, 2005 Share Posted October 30, 2005 Here are the diagrams Quote Link to comment Share on other sites More sharing options...
Tom Mobley Posted October 30, 2005 Share Posted October 30, 2005 I've built a few of these mini-boxs. I would be happy to build you one. However, I don't think I understand why you don't just run the center channel output from your pre to one side or the other of the Mac? The Mac will easily overpower any SET I'm familiar with even with only one channel driven. Or, run the single center out of the pre to a Y cable, then to both inputs of the Mac. Bridge the Mac output per the the Mac docs and be done. I could easily build you a box, but fail to see why you need it or how exactly it would be employed. Quote Link to comment Share on other sites More sharing options...
whatever55 Posted October 30, 2005 Share Posted October 30, 2005 At one time Dynaco promoted quadraphonic sound without additional amplifiers. You just needed 2 additional speakers and of course they recommended one of their amps. I use the 3 channel verison and it works just fine. For the 3 channel you have to make sure your amp has a common ground between the left and right channels. Then you just hook up the + from the left to one side of your rear channel and the + from from the right to the other side. Depending on how it plays in relation to your mains you might need to pad the signal. Information that is out of phase will play in the rear. You will be surprised at what you hear. I will try and find they aricle and scan it. Quote Link to comment Share on other sites More sharing options...
steve Posted October 30, 2005 Author Share Posted October 30, 2005 Tom exactly my thoughts..I didn't think I'd need anymore gear..but I'm missing something here or I'm losing it in translation.. You say to run my center out fro my pre..exactly what do you mean? The preamp is a 2 channel amp..there's no dedicated "center" channel..just three L/R outputs.. is there anyway I can make this work with what I have?? Thanx Steve Quote Link to comment Share on other sites More sharing options...
steve Posted October 30, 2005 Author Share Posted October 30, 2005 and dragonfyr thanx for posting the schematics.. Michael the McIntosh users manual says to use the amp in mono, just hook one RCA up, then bridge the speaker outputs..I'm guessing this is to double the power when in mono?? I still think I'm going to need that box..if so think I will opt for the Preamp out version..RCAs only Steve Quote Link to comment Share on other sites More sharing options...
steve Posted October 30, 2005 Author Share Posted October 30, 2005 Hey Dragonfyr a question..looking at your schematics, the top one.. since I already have -3- sets of outputs on my preamp, is it necessary to run the stereo amp off of the minibox taps ? Or can I just run the center channel off of the minibox? Excuse my ignorance.. Thanx Steve Quote Link to comment Share on other sites More sharing options...
Hardhead Posted October 30, 2005 Share Posted October 30, 2005 Steve-- You said that you have 3 L/R outputs on your preamp. Do you mean 3 SETS of L/R outputs or 3 outputs? Are the 3 outputs labeled in any way? Is there any chance you could post a picture of those outputs to resolve the issue clearly? If it's necessary to build a mini-box to derive a center channel, all you'll need is a couple of resistors of a fairly specific value, 5 RCA jacks, a little wire, and a box to put it in. It isn't hard to build, or you can have Tom do it for you. You'll need to keep the level of the center channel 6-7 dB lower than the outer speakers to keep the imaging and stereo effect right. If you can tell me the source impedance of your amp, I can tell you what value resistors to use so that the -6 or -7 dB level of the center speaker is constant when you turn up or down the outer speakers. Otherwise you have to adjust the level of the center speaker all the time, and that can be a hassle. Let us know about those outputs. Quote Link to comment Share on other sites More sharing options...
Al Klappenberger Posted October 30, 2005 Share Posted October 30, 2005 Steve, The center channel "mixer" can be very simple if your center power amp has a gain control on it. All the resistors can be fixed. I used 33K Ohms from Left and right to center and 12K Ohms from center to ground. The values are not critical. The 12K is really not even necessary but helps isolation between left a right a little. I am still using this scheme, but it's located in the same box with my subwoofer crossover. Al K. Quote Link to comment Share on other sites More sharing options...
Tom Mobley Posted October 30, 2005 Share Posted October 30, 2005 What Hardhead said. If there's three sets of outputs what is the third one if not some kind of derived center? Can you post the make and model of the pre or a pic of the back showing the connections? I got your email, haven't answered it yet pending clarification of these questions. Quote Link to comment Share on other sites More sharing options...
steve Posted October 31, 2005 Author Share Posted October 31, 2005 Tom the preamp is a Mcintosh C2200 tube..the outputs are 3 stereo outputs..one always on, the other two can be switched in/out with switches on the front..again to clarify..three sets of two..L/R on each set.. The output impedence on the unbalanced outputs of the preamp is 250 ohms, if this could be fixed so I wouldn't have to adjust volume. Thanx all for answers Steve Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 31, 2005 Share Posted October 31, 2005 Build the minibox with a volume control. If you look at the schematic Dragon posted, the schematic diagram on the output side of the minibox is by way of a variable resistor (aka volume control). a 25K linear taper pot would work fine. It's very easy to adjust the center channel to just blend with the main L/R speakers, and then stop. You can sit in the usual listening position, and have someone turn the control up or down until a good blend of all three speakers is achieved. Since the center will be getting both a hard left and right signal, an inline crossover can be used to roll the frequency of the center off to about 150 Hz, and this will prevent the possibility of bloated or exaggerated bass coming from the center channel. Erik edit: different if the center channel amp already has a VC on the input. Quote Link to comment Share on other sites More sharing options...
Al Klappenberger Posted October 31, 2005 Share Posted October 31, 2005 Erik, "Since the center will be getting both a hard left and right signal, an inline crossover can be used to roll the frequency of the center off to about 150 Hz, and this will prevent the possibility of bloated or exaggerated bass coming from the center channel."Where did you get this from? There is no exaggerated bass created by a center channel! The only thing you may need to do is check the phasing of the center. Since it's not the same exact type speaker as the flanks, the polarity may be reversed. I found I needed to do this back when I was running two Belles with a Cornwall in the center. The bass was much better with the Cornwall reversed.Setup is easy. I used pink noise (or FM tuner between-station noise). Just adjust the center level until the noise comes evenly distributed across the room. The correct adjustment will be obvious!Steve,The C2200 is one of Mac's new designs. They droped the center output after the C27 or C28 models. My C34V doesn't have it. Their "classic" tube models all had it, I think.Al K. Quote Link to comment Share on other sites More sharing options...
Tom Mobley Posted October 31, 2005 Share Posted October 31, 2005 OK, now I see what the extra outputs are about. You could turn on a set of those, run the output to a custom mini-box. It would have L & R in, an attenuator for convenience (allowing you to use the remote on your MAC pre to adjust the volume without having to hop up and adjust the Mac center amp every time) and two identical mono center outs. Essentially the same thing as as running a Y-cable at the back of the center amp. Remaining details are what kind of appearance do you want on the minibox? Most of the ones I've done were in $5 plastic boxes, not very pretty. Al's little aluminum one is cool, I could probably find a similar one. Choice of box will affect the cost of the deal. Quote Link to comment Share on other sites More sharing options...
PrestonTom Posted October 31, 2005 Share Posted October 31, 2005 Steve and others, The mini box is pretty simple and inexpensive. I would urge you to make sure it includes an audio pot so you can more precisely set the level. As Al implied the level for the center does not need to be very loud, certainly not as loud as the L & R speakers. I am leery of using Y-cords to create a summed center. Depending on the configuration, it is possible to get a summed signal to connect to the extra (center channel) amplifier and at the same time you can inadvertently end up also sending a summed (mono) signal to the left & right speakers. I have seen folks do this when using an extra pre-amp set on 'mono". The summing was done early in the circuit of the "center" pre-amp and all the L&R's were tied together and "went back" to the "stereo" pre-amp. The stereo separation was lost. Good Luck, -Tom Quote Link to comment Share on other sites More sharing options...
michael hurd Posted October 31, 2005 Share Posted October 31, 2005 I think you might have interperated it wrong, he mentioned he wanted to use both channels on the Mac for the center channel. This is where I mentioned he could split the signal from the center output of the mini-box, and use one channel of the Mac to drive mid-hi, and the other to drive the LF portion. Quote Link to comment Share on other sites More sharing options...
steve Posted October 31, 2005 Author Share Posted October 31, 2005 I'd like to keep it as simple as possible. Tom, (Mobley)..a plastic box is alright with me. It'll probably be out of view anyway. One with a volume pot sounds like the way to go, in case I use an amp that has no input controls in the future. I'd also like to come off of the preamp on it's own set of outputs, into the minibox, then one output from the minibox to one input on my center amp. I have enclosed a page from my Mac owners manual. It appears that if I do it this way, then I'll run jumper wires from my right speaker outputs to my left speaker outputs, then one speaker wire from the left speaker output to my center speaker. And like Mr. Preston said, I shouldn't feed a mono signal back to the main L/R speakers with the minibox correct? I'll eventually use the MC275 for my mains, and probably rewire my SET for the center channel, as it can be used for mono also. Tom, let me know about building the box please. Thanx Steve Quote Link to comment Share on other sites More sharing options...
Al Klappenberger Posted October 31, 2005 Share Posted October 31, 2005 Steve, A bit of advice here: Don't fiddle with straping the Mc275 for mono to run the center. Use it for the flank speakers right away. Use the SET amp for the center. Back when I had a Cornwall between two Belles I used to set the center amp a -2 dB below the flanks. The Cornwall is rated -7 db below the Khorn and Belle (98 versus 105 db). This puts the center at 9 dB below the flanks which mean it will require 9 dB less power from the center amp for a Belle in the center of two Khorns. This means that the center amp will only need to put out about 9W to keep up with a 75W / channel amp on the flanks! Al K. Quote Link to comment Share on other sites More sharing options...
sfogg Posted October 31, 2005 Share Posted October 31, 2005 Al, "Where did you get this from? There is no exaggerated bass created by a center channel!" There can be using this passive method. The entire bass from both the L and R channels will combine in the center and be reproduced by it. That centers bass will couple with the bass from the L and Rs more then in the higher frequencies (I think it begings to occur below the 1/4 wavelength distance between L and C and C and R) and that can cause some bass bloat. It can also occur from room influences. Reproducing bass from the dead center of a room is going to make the room modes for the bass from the center worse because of the equal distance to both side walls... that will make the modes stronger.... IOW larger peaks or nulls because the modes to each side wall will be the same. Tom, "The summing was done early in the circuit of the "center" pre-amp and all the L&R's were tied together and "went back" to the "stereo" pre-amp. The stereo separation was lost." Any of these passive approaches reduce stereo seperation and placement. You no longer have the ability to reproduce hard right and hard left in a mix... it will be reproduced as right and center or left and center. Shawn Quote Link to comment Share on other sites More sharing options...
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