Axiom Home Page Forums General Discussion Technical Questions Best way to run two channels off one pre-out?

I want to experiment with my center channel. To bore those who know and to inform those who don’t, I am running two M22’s right now for my center channel. I am going to order a VP150 and play a bit. I want to mount the VP over my screen and keep the M22’s where they are, under the screen about 24” apart. I am using a 3808 for a pre-amp and I have ten, 200W / 8 ohm channels to use. I want to power the M22’s off one channel and the VP150 off another channel. The problem is, the 3808 only has one center channel pre-out. I’ve read (don’t recall where or who) that it is not a good idea to use a Y off the pre out. I don’t know how else I can do this without using a Y.

I know that many of you will think this is a dumb idea, and it most likely is. I just want to experiment as I don’t think anyone has ever done this before… I will more than likely just end up buying a second VP150 and mount it under the screen to have one over and one under, but for now, I just want to settle my curiosity.

So….. is using a Y a bad way to do this? Is there another way to do this?

I don't think it's dumb, per se. You'll just be splitting your pre-out voltage in half. As long as you're connecting each side of the split to a different amp, I don't see what problems can come from it. As you know, you can compensate for the voltage drop during calibration.

Mike, you might have read something I commented on in the past, but you don't recall the point clearly enough. The problem with Y-connectors isn't going one into two(or more), it's going two into one. When you try to combine two voltage outputs into one input simply by using a Y, the dumb voltage doesn't know that it's supposed to go only into the connected input. Instead it also runs up the other branch of the Y into the other output, causing at least distortion, and possibly damage.

So yes, Mike; no one has ever done that before. Your success with the experiment will assure your undying fame in the annals of Axiom.

John's answer is very confusing to me, but as I understand it, running 2 channels off a pre-out has to be how Axiom expects people to use the A1400-8 to run 2 center channels, right?

In other words, I would think that Peter's got the right answer here.

Yes, Peter has it right.

Once his tall wife puts her stamp of approval on it, then it will be official.

My wife is of average height.

Here .

So in summary: dividing is OK, summing is bad.

Dividing: One output driving two amps. (One sub-out to two subs.)
Summing: Two outputs being combined into one amp. (L&R outs into a mono amp.)

You assume too much John. The Axiom forums are not my only source of reading material. My question was specific to using a Y off an output, not to combine two outputs. I have read that that is not a good idea. Two outputs into a Y would make no sense, in any application. So what’s with the condescending tone anyway?

Peter - Thanks. That's what I thought, but wanted to verify.

Hi michael_d,

I don't think JohnK was being condescending; he was just differentiating between the two so as to be entirely clear.

It isn't a dumb idea to split the voltages from one pre-out, sending them to two separate amplifiers. It just lowers the output voltage to each but if there's enough gain in the preamp stage for the single pre-out, you should be able to compensate.

As to summing two pre-outs with a Y-connector, I get lots of private emails from customers who propose to do just that, and, as JohnK explained, it's not a good idea and likely to result in circuit damage.

Regards,
Alan

I have tried the latter(two in to one). As Alan stated, not a good idea and notwithstanding the possible circuit damage issues, it significantly cuts the volume to each speaker. It is OK, though, to run the Y-connector out of the pre-amp but to TWO connections on the amp so each speaker can have its own hook-up. I have tried it this way BUT it will provide MORE volume with the two combined than the one speaker on its own so the question is if you still wish the third speaker in the chain(however it is connected), it will be very difficult to balance the volume with the other two.

Ooooh! Undying fame. I'm tempted to buy an old receiver just for the occasion.

I used to split the outs of my preamps to be able to run a powered sub long before a pre came with two sets of outputs. Worked just fine.

Mike, I usually wouldn't take the time to reply to someone incensed over a slight to them that didn't actually occur, but I'm surprised that after all this time you don't know me better than that. What was intended in that second paragraph was humor, not condescension. It was simply a playful follow-up on your thought that your proposal was something that had never been done before in combining Axiom speakers(not even by Jay and Mojo!).

As to the actual point involving the splitting of one output or combining two outputs with a Y, as Alan pointed out, the combining has been the subject of questions to him. Also, I can recall at least two occasions on the forum when Rick Widner and I cautioned someone about this, and so I thought this might have been the previous discussion about Y-connectors that you were referring to.

Since you were actually referring to someone elsewhere who had the notion that even splitting a single output with a Y was a bad idea, the comment first would be that it's been done probably millions of times with no problems when wired correctly. Splitting the output in this way creates a parallel circuit with the two input resistances. A relatively simple discussion(no condescension implied by that description)of the results can be found here . Note that a parallel circuit isn't a "voltage divider"(that's what a series circuit is)and as illustrated in fig.3-38, each input receives the same voltage as the source outputs. There's no apparent factual support for the concern of this other poster and you can proceed with confidence.

John… That’s why I asked. I must have been overly sensitive this morning. For some reason, (which is not real clear to me now) when I read your response it just hit a nerve and it felt like you poking fun at me. I read it a few more times and it kept stringing the nerve. You’ve never been anything but nice to me before, so it really took me by surprise. After reading it again now, I think I just read too much into what you wrote and was being a pussy. Sorry about that and thanks for the clarification. I ordered a VP150 today. I’ll post my impressions after I’m done screwing around with things.

When you get tired of the VP 150 send it down my way. Heck Mikey I'll even pay shipping

You may change your mind when you find out how much shipping is from my house..... Ricky.

So I've heard

One more question. What does a person do if one of the center channels is a bit louder than the other, but they are both being driven off the same pre-amp out put? Some sort of in line resister to lower the voltage to one of the amps?

I would calibrate them independently and just see how well they match up. If you have the M22 co-located you should see a bump in there output. The VP 150 is a bit more sensitive and will draw a bit more from the amp but as I see it the difference should be very minimal with the M22s set up the way you have them.

How do you know one is louder than the other? My M2s are above and below the screen. I could not tell you if the sound is anchored higher or lower on screen. It just comes from the screen.

A question I've often thought....

If one (me!) were running a VP100 and a VP150 in parallel off of the AVR, how does one (me!) calibrate the two speakers independently?

Before you get the RIGHT answer from on of the smart guys if you knew which of the centres played louder, couldn't you run this off of the first centre but with an attenuator? I'm asking, not saying it's going to work or not.

With what shall he attenuate it, dear Liza, dear Liza?

Calibrate the 150 with the 100 disconnected and then check to see how much more the 100 reqires to match the level of the 150, I think I found they are within 1-2 db of each other and never noticed a difference when I ran them in parallel with one above and below my TV.

Mike, as some of the replies indicate, you probably don't do anything if one is just "a bit" louder. The image shift probably won't be noticeable and adding resistance to pad the louder one down shouldn't be necessary.

Can a resistor of somekind be used at the 2nd centre(louder)? or perhaps an L-pad?

Again, smart people will be along shortly, but I think a resistor will not attenuate evenly across the spectrum.

On second thought, maybe I ought to just leave well alone.

And here I just said nice things about you in another thread!!!

Actually I was referring to Adrian. You know he PMed me several pages (14!) of weird looking electrical wiring diagrams and schematics of different things I *may* want to try on the two centers ... I don't believe Boeing had that many drawings on the 767.

Lol, I went to the Shemp Howard Technical Institute.

Mornin' Adrian!

Oooops...sorry! I thought it had to do with my "smarter people will be along" comment....just before you showed up!

Morning Rick! Here's an interesting article about L-pads with attenuation charts. The general consensus from the smart guys here is that if your centres are within a couple of dbs, don't bother, but if it was a noticeable amount, you could use an L-pad(confirmed with one of the 'smart guys' via pm).

Oh!! I'll send you the installation instructions, better clear your browser first.

I believe we're going to roll with that consensus and save the browser.

Also...

Are you sure this wasn't the school Adrian ... Shemp Howard Institute of Technology school?

I don't know. But you can email the Dean of the school ...Moe@3knuckleheads.com for more info

Did you mean the "Shemp Howard Institute of Technology"? I went there too this morning...

I bet their college cheers are interesting, if they're anything like the ones we had at Notre Dame:

"WE ARE ... N.D."

"Let's go Irish!

"Here come the Irish!"

"WE HATE N.D. because THEY S*CK!!

"WE ARE SHEMP TECH"

"NYUK...NYUK...NYUK!!"

Let's see some accompanying choreography.