Is there any safe way to double up speakers on my AV receiver?
July 30, 2006 8:20 AM   Subscribe

AVFilter: Is there any safe way to double up the surround speakers on my home theater system?

Any previous questions around this topic that I could find quickly spun into debates over "resistance" vs. "impedance" that I'm not able to apply to my circumstances, so I figured I'd try and ask this one straight out...

I've got a Marantz A/V receiver (SR7000) that I've successfully had hooked up to a full set of Boston Acoustics speakers for several years now. As we've moved between different houses, I've acquired two matching sets of the VRS Micro "surround" speakers (one black pair, and one white pair), and have usually just used whichever set better matched the house's decor.

In our new house, though, the room is laid out in such a way that it _might_ make sense to actually hook up both pairs at once--the room's basically a big "L", with the TV and central speakers in the central corner. I could pretty easily put one pair of surrounds at the end of each leg.

So here's my question--before I even bother trying to see if it sounds OK, is there any way to hook up all four speakers at once without overtaxing the amp? The speakers themselves are all labeled at "8 ohms", and the speaker outputs on the amp are labeled "8 ohms" as well. I could pretty easily hook each L/R pair up either in parallel or serially, but I'm not clear on the physics whether either approach is something I'd really want to do.
posted by LairBob to Home & Garden (11 answers total)
well, one thing that you could do is send an aux output on your current amp to a secondary amp in that distant part of the "L."

What you'd be doing is getting a cheap amp that will send DTS or Dolby 5.1 data to a set of speakers (to power the second set), and then just send a feed from the primary amp to the secondary amp. Voila. Surround sound all over your house.

The only negative thing I can think of would be any latency issues with a secondary amp. But I cannot believe that this would be much at all.
posted by jimmyhutch at 8:32 AM on July 30, 2006

Best answer: I can't see all your SR7000 specs on the Marantz site, but I do see that it talks about "capability for control of a second zone" which might be what you want. If you can still locate the manual, I'd read up on how that feature is intended to be used.

As for hooking up a second set of 8 ohm speakers on each channel with an existing set of 8 ohm speakers, what you create is a nominal 4 ohm load for each channel. Ohm's law says that twice the current will be wanted by the speakers for equivalent loudness in each zone (assuming linear and equal efficiency for your different speaker sets, which is probably slightly different in practice). Your amp is supposed to deliver 100 watts of power for each of its 5 channels into 8 ohm loads, but it may have enough internal resistance in each channel's amplifier to deliver something substantially less into 4 ohm loads (or there maybe internal current protection circuits that are tripped more regularly if you operate into lower impedance loads). Your unit might derate to something like 75 watts/channel into 4 ohm loads, to protect itself from high peak current demands.

In a practical sense, if you don't listen to music at high volumes, you may never experience much in the way of problems. Marantz amplifiers are well designed, stable units, that are conservatively specified and generally well made. But if you are a sonic purist, or listen at high volume levels, you may note some earlier onset of "clipping" behavior as your amp runs out of gas pushing the combined loads and perhaps even transiently trips its internal current limiters. You might also note some small muddying of the sound produced by your speakers generally, as the lower load impedance will reduce the actual damping factor your amplifier can provide. And, because parallel conventional loudspeakers represent a greater transient inductive load to the amp, there may be some loss of sonic detail if the amplifier is forced into transient instability by high volume settings.

All of this is pretty theoretical, however. In practice, long runs of speaker wire generally have more effect on the actual sonic behavior of an amplifier in a home setting than do such performance considerations. You shouldn't risk any serious damage to the amp by hooking up a second pair of speakers, if you keep the volume down.

But I would research that "second zone" feature thoroughly before I just started connecting wires.
posted by paulsc at 9:20 AM on July 30, 2006

Best answer: To simplify and endorse what paulsc said, as long as you don't crank the volume to extremely loud levels, your amp will push the power into the "harder" 4 ohm load just fine. Wire them in parallel.
posted by intermod at 9:42 AM on July 30, 2006

Response by poster: Thanks, I really appreciate that. I was aware of the "second zone" feature, but while I've never used it, I was hoping to eventually use it as it was intended and hook up something like a set of outdoor speakers on the patio. (Plus, the way the room is set up, you would never really play a separate sound source into the same space, so it would be kind of wasting that capability.)

As for limiting the volume, that should be fine--I bought the Marantz to get really nice, clear sound at "normal" levels, not to shake the windows. (The main reason to double the speakers would be to be able to fill the odd-shaped space more evenly at normal volumes, without having to crank it up.)

Does the serial/parallel hook-up make any difference?
posted by LairBob at 9:49 AM on July 30, 2006

Response by poster: (Oh, and thanks, intermod--you must have posted your answer just as I was typing that last question.)
posted by LairBob at 9:50 AM on July 30, 2006

Hook them up in parallel. A series hookup gives you a 16 ohm load into which, your amp will never develop full power, and which will present weird enough reactances that you will probably notice degraded sound.

If you're interested, back in 1961, Popular Electronics published an article on "The Sweet Sixteen," which was a home brew speaker system, designed around 16 inexpensive 8 ohm small speakers, internally wired series/parallel (series to create 4 banks of drivers at 64 ohms each, then the 4 banks paralleled to bring the total array impedance back down to 16 ohms). It was a hugely popular home hifi project, and my Dad and I built several versions of it, that sounded pretty good for the day and the 10 watt tube amps we used.

What made the series/parallel wiring necessary was the need to match the impedance of all those driver elements to the 8 or 16 ohm outputs of tube amps popular back in those days. But what made the series/parallel wiring idea work work as well as it did was the fact that all the driver elements were, for practical purposes, identical.
posted by paulsc at 10:26 AM on July 30, 2006

Best answer: Hook them up in parallel. A series hookup gives you a 16 ohm load into which, your amp will never develop full power, and which will present weird enough reactances that you will probably notice degraded sound.

I agree about paralleling them, but..

Assuming linearity, it doesn't make any difference if you chose series or parallel. The first limit you will hit (the first significant non-linearity, if you like) is the maximum output voltage of the amplifier. Serial is worse because it will require increased output voltage.

The problem with paralleling them is that you may be asking the amplifier to supply either more current or more power than it is able to. In the worst case this could cause a failure, but it is far more likely that the amplifier will switch into some kind of over-current or over-temperature protection mode. It might turn off for a few minutes, it might just sound really distorted, or you might not even notice. One more note about this.. Protection modes are a safety features, you should avoid running the amplifier into those modes if possible. This goes double if the load is out of spec.

Go ahead and parallel speakers, but ideally you should take it slow. Keep volumes low for the first hour or two. Then increase the volume a little and test it out for a couple more hours, etc. If you ever hit a protection mode, back off.
posted by Chuckles at 12:22 PM on July 30, 2006

Wiring speakers in series is bad because the impedances of the speakers will interact with each other. And since a speakers' impedance varies non-linearly with the frequency of the audio signal it is reproducing, it will definitely mess up your signal.

Stick with parallel.
posted by pmbuko at 6:51 PM on July 30, 2006

I don't think that is correct pmbuko, even if speakers are fundamentally highly non-linear (everything is, in the end), near the operating point they have to act pretty linearly or you will hear distortion instead of music..

Anyway, don't you model a speaker + enclosure system with linear circuit elements? (Anyone have a good link to that kind of model?)
posted by Chuckles at 7:16 PM on July 30, 2006

Dynamic (voice coil + diaphragm [cone]) type loudspeakers of any size are a significantly complex load, especially at low frequencies, near their mechanical resonance frequencies. A primary function of a well designed baffle in some classic designs like a bass reflex enclosure (ported Helmholz resonator), or an acoustic suspension enclosure is to flatten the natural free air cone resonance peak of the speaker by loading it to a larger volume of air mechanically than it can encounter by itself. But the most efficient mechanical enclosure, the folded horn (like the classic Klipschorn) is only partially effective in doing this. The electrical load presented by the best designed systems is still highly reactive, and the smoothing effects of even the best driver/enclosure systems are significantly complicated by passive crossover networks commonly used to pad driver efficiencies and restrict working frequency ranges for individual drivers in multi-driver speaker systems.

One of the most important benefits of solid state amplifiers over their older tube cousins was the achievement of extremely low internal impedance through use of large amounts of negative feedback, which gave the solid state, direct coupled amplifier a vastly superior damping factor. High damping factors are valuable as an aid to controlling loudspeakers with high mechanical mass, and therefore, large reactance.

Wiring speaker systems in series is not a good idea for many reasons, not the least being that doing so defeats most of the design measures taken to control reactance, and minimize resonant distortion.
posted by paulsc at 9:18 PM on July 30, 2006

Okay, I'm convinced..

pmbuko: the impedances of the speakers will interact with each other.

Yes, exactly, I shouldn't have questioned you! It isn't non-linear though :P

So, if you have two same model speakers, what does putting them in series accomplish..

paulsc: But what made the series/parallel wiring idea work work as well as it did was the fact that all the driver elements were, for practical purposes, identical.

Well, if there are manufacturing variations, it seems like you would create a sort of average response, which might actually be a feature. Heh..

Sorry about the misunderstanding, and pointless derail.
posted by Chuckles at 10:46 PM on July 30, 2006

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