Ceiling speaker choice for existing Onkyo 5.1 amp

[Munzy]

Hi all,

I am just planning the setup of speakers for our property renovation and could do with some advice.

Years back I bought an Onkyo HT-R393 5.1 amp which I have used for home cinema and really like. I have since acquired another identical Onkyo amp. I will be setting one up in the cinema room using the standard Onkyo 6 ohm surround speakers. I’m thinking about using the spare amp to run ceiling speakers around the downstairs for music.

The amp is rated at 100W output per channel @ 6 ohms. I am thinking about splitting the five channels to give me 10 speakers across the open plan downstairs as part of a single zone. I will keep the subwoofer channel for a passive sub box.

In looking at speakers for the ceilings I have lots of choice depending on budget but it’s the spec I need help with.

The Polk RC60i seem to get good reviews in a quality/price trade off. Trouble is they are 8 ohm speakers leaving me with the choice of wiring two per channel in series to give 16 ohms per channel (amp can handle 6-16 ohms) or to wire in parallel to give 4 ohms which is out of spec for the amp.

I will usually have the speakers on at a fairly low level allowing people to talk comfortably over the noise (slightly louder than your average restaurant background music). Occasionally I’ll want to ramp it up for a party.

Would it be safe to match 4 ohms per channel (2 x 8 ohm speakers in parallel) to the 6 ohm Onkyo given the usage above?

Would a 16 ohm per channel setup (2 x 8 ohm speakers in series) leave me with barely audible sound?

Have I calculated the power output in the series-wired scenario correctly? Onkyo amp outputs 100W @ 6 ohms therefore the channel output becomes 38W @ 16 ohms however this is split across two speakers so each speaker is outputting a piddly 19W?

Should I scrap the whole idea and look at something else?

Thanks in anticipation!

 

[Lucid]

You're starting on the basis that the Onkyo specs are true to life. Sadly, they're not. It's the same for all AV Receivers.

If your Onkyo were really 100W/ch, and remembering that the receiver has an amplified sub channel sharing in the power pot, then you'd have a mains power consumption somewhere in the region of 6 x 100W. The spec sheet shows us a mains consumpution of 380W. Since an amp can't magic up more continuous power than it gets from the mains, then there's something contradictory about the speaker output wattage versus the power consumpution.

As a rough rule of thumb (and this works for all AV receivers), take the mains consumpution (380W), then knock off about 10% for running the preamp features and for heat loss (now 340W) then divide that by the number of powered channels (6 in the case of HT-R393). This gives us roughly 55W per channel.

Onkyo's own spec sheet quotes 65W/ch in to 8 Ohms @ 0.7% THD with just two channels driven. When driving fewer than the total number of powered channels, some of the unused power can be diverted to the live channels. So in the case of Onkyo's own claim, there's roughly-speaking a bonus of 10W per channel gained from the unused channels. The reason why all the power doesn't divert is the current limit of the output transistors per channel. They're a natural bottleneck.

55W per channel in to 8 Ohms converts to 27.5W per channel in to 16 Ohms. This means each speaker gets a shade over 13W. That doesn't sound like a lot, but it's enough for casual listening, and though it won't blow the roof off at party time, you'll still be able to generate enough volume through all 10 speakers that folk should have a good time.

The thing with wattage is that you don't need a lot to create a reasonable level of sound. Look at the Polk spec. The efficiency is 89dB/W/m. That means that measured from one metre distance, and fed with a single Watt of power, the speaker will generate 89dB of sound. When you run your test tones or auto set-up wizard, the amp plays at somewhere in the region of 80-85dB per channel. 89dB from 1 Watt is louder than that. When people are milling around underneath the ceiling speakers, it'll be loud enough.

What'll be really important for you - and I can't stress this enough - is making sure you get as much of the amp power to the speakers as possible. This means minimising your power losses through the cable.


There are three things that kill power in cables. We start with resistance, and this is governed by two things: cable cross sectional area (thickness), and the material that the cable is made from. The remaining third element is cable length. All of this is simple physics. Make thin cables, constructed from aluminium (even with a with a copper coating), and have long cable runs, means you can wave goodbye to 50% or more of your amp power. Here are some numbers:

We'll use 100W at the amp end to keep things easy to relate back to percentages. We'll say the speakers and amps are well matched; 8 Ohm throughout, and we'll use 20 metres as the cable length. This means the only main variables are thickness and material.

With a 100% copper cable of a diameter of one square millimetre, our power at the speaker is 85W.
Aluminium carries only 60% of the power compared to copper; so an alu-based cable gives us just 51W

Increase the cable thickness to 1.5 sq mil gives us around 91W (54W for alu)

Increase the cable thickness to 2.5 sq mil gives us around 93W (56W for alu)

A 4 sq mil cable gives 96W (58W for alu)


There's a Law of diminishing returns in effect. The change in efficiency from 1.5mm to 2.5mm wouldn't justify the extra cost unless the cable length was longer. 1.5mm of 100% copper is the sweet spot.

Be careful when buying cable. There are a lot of crafty resellers who try to hide what the cable is by using acronyms such as CCA or by talking about the benefits of skin effect as a way to justify the choice of a copper coated cable. I've even heard "well, aluminium is used in power lines so it must be good". It's all B.S.

Skin effect is marginal at best and only starts to happen at frequecies even bats can't hear. As for alu in power lines, yes, at the sort of high tension voltages used in the National Grid then the aluminium/steel combo cable is fine. But for home speakers we aren't transmitting at 40,000 to 100,000 Volts, so the same rules don't apply.


If this and any other posts helped you, then pay the authors the compliment of hitting the thanks button. It shows your appreciation and marks the reply for those looking for answers in the future.

 

[Munzy]

@Lucid Thanks for taking the time to read through my questions but even more to review the Onkyo specs and come back with such an informative and broad answer.

I genuinely don’t think I’ve ever received such a great response on a forum!

Brilliant!

Dom