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Topic: AV receiver for 3-Ohm (min) impedance speakers (Read 8325 times) previous topic - next topic
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AV receiver for 3-Ohm (min) impedance speakers

Hi guys. I know similar questions have been asked in this forum before, but I hope you'll understand me wanting to get this right before spending any money:

I've come into possession of a pair of B&W Nautilus 804s and I'm looking for an amp for them. I'd like to go the AV route because of the extra functionality they provide out of the box (Bluetooth, Airplay), and the possibility of adding a sub later on.

The 804s are rated at 8 Ohms, but with a minimum impedance of 3 Ohms. Will I be OK with a standard 8-Ohm amp, or do I need one that's rated for 4 Ohms? If possible I'd like to avoid spending a ton of money on a top-of-the-line amplifier with far more channels than I'll ever need...

And as for power, I'm guessing something around 100W should be more than enough? I don't listen to music particularly loud, although the room is pretty big (5m x 6m, with 3.5m ceilings).

Full speaker specs:
Nominal impedance: 8 Ohms (min 3.0 Ohms)
Power handling: 50W - 200W into 8 ohm on unclipped programme
Sensitivity: 89dB spl (2.83V, 1m)
Frequency response: 45Hz – 20kHz ± 2dB on reference axis

Re: AV receiver for 3-Ohm (min) impedance speakers

Reply #1 –
Most energy is in the bass, where these dip down to below 4 ohms, so I would get an amp/receiver that can drive 4 ohm speakers.

Btw, the difference between 50W->100W and 100W->200W is each time +3 dB SPL (maybe a bit less at high SPL due to compression).
"I hear it when I see it."

Re: AV receiver for 3-Ohm (min) impedance speakers

Reply #2 –
Since the speaker is nominally 8-Ohms, I wouldn't worry about  it.    Most manufacturer's don't bother disclosing the minimum impedance, so with most speakers you'd just be happy knowing they are "8-Ohms".

The receiver/amplifier might overheat with a maximum power (or near maximum power ) constant test-tone at whatever frequency the speaker impedance dips to minimum, but the odds are pretty good that the speaker would be damaged first.

And, real program material usually has a peak-to-average ratio of about 10:1, with the energy spread-around the frequency range (mostly in the lower & mid-frequency ranges).    The bottom line is, you are unlikely to be stressing the amplifier.

Quote
And as for power, I'm guessing something around 100W should be more than enough?
100W is enough for most "home situations".     But, it gets very complicated...   There are just too many variables...   The loudness you like to listen, the amount of bass you like, the sensitivity/efficiency of your speakers, the size and acoustics of the room, the dynamics of the music...  

Re: AV receiver for 3-Ohm (min) impedance speakers

Reply #3 –
Quote from: xnor on
Most energy is in the bass, where these dip down to below 4 ohms, so I would get an amp/receiver that can drive 4 ohm speakers.
Yep. My parents have an Onkyo TX-NR616 that is specified into 4 ohms (the smallest model to do so at the time) and actually seems to be doing some secondary-side switching when you set the load accordingly - heat emission was substantially reduced after doing so. Not the most powerful or fast unit, but gets the job done. I am under the impression that things have rather gotten worse since then, since a current model would not even feature the most basic version of Audyssey any more but rather their own room EQ. I guess someone figured out that licensing costs add up, while sales figures are declining...

My favorite brand would have to be Pioneer right now, at least since it seems they finally got their act together when it comes to user interfaces (which historically were generally awful) - technically their stuff already was good before that, notably beefy power supplies. Yamaha also tends to be reliable, though power supplies on their lesser home theater receivers can be pretty wimpy and I'd recommend a midrange model at least. (Note that impedance selection on their receivers only seems to be a current limiting adjustment, while their integrated amps tend to have a slider switch at the back that allows selecting secondaries.)

With the margins on AV receivers being as slim as they are, expect no overdesign. They'll typically tend to wring as much power out of as few power transistors as possible, right on the limits of SOA. In the interesting of keeping self-heating under control, bias currents are not likely to be too generous either, making distortion figures a bit less good than they could be, especially into lower-impedance loads. Transformer sizing may also be on the skimpy side, though this tends to show mainly in multichannel operation, while power into 4 ohm stereo loads would still be ample. From memory, you may find something like >100W/ch into 4 ohms stereo and 6x (or 8x) 20-30 W / 6 ohms multichannel.

I am a bit out of the loop when it comes to stereo AV receivers. Stereo has been seeing a bit of a comeback, so there may be more now these days than the Kenwoods one was seeing a few years ago.

Re: AV receiver for 3-Ohm (min) impedance speakers

Reply #4 –
Quote from: sirdavidoff on
The 804s are rated at 8 Ohms, but with a minimum impedance of 3 Ohms. Will I be OK with a standard 8-Ohm amp, or do I need one that's rated for 4 Ohms? If possible I'd like to avoid spending a ton of money on a top-of-the-line amplifier with far more channels than I'll ever need...
In lieu of an actual impedance measurement, yes, you would be much better off getting an amp capable of driving 3-4ohm loads.
Since the 804s are obviously used, seems you are not averse to such, so perhaps a late model upper mid-tier AVR from Denon, Onkyo or Yamaha should be fine, as many are rated for/capable of driving lower impedances. As long as they have the features you need.

cheers,

AJ
Loudspeaker manufacturer

Re: AV receiver for 3-Ohm (min) impedance speakers

Reply #5 –
Many thanks for all the advice! It sounds like I might be better off staying on the safe side and getting something rated for 4 ohms. Of the AVRs below, is there one in particular that would be better suited to my 804s?

Onkyo TX-NR646
160 W/Ch (6 Ω, 1 kHz, 1% THD, 1 Channel Driven, IEC)
175 W/Ch (6 Ω, 1 kHz, 1 Channel Driven, JEITA)
Rated for speakers 4 or 6 Ω–16 Ω
http://www.uk.onkyo.com/downloads/2/8/3/4/5/ONKYO_TX-NR646_datasheet_EN.pdf

Denon AVR-X2200W
95 watts per channel (8 ohms, 20 Hz~20 kHz, .08% THD). Provides ‘operational stability’ down to 4 ohms
http://usa.denon.com/us/product/hometheater/avreceiversht/avrx2200w

Yamaha RX-V679
90 W per channel (6 ohms, 20Hz-20kHz, 0.06 % THD, 2 ch driven)
150 W per channel (4 ohms, 1 kHz, 0.9 % THD, 1 ch driven)
150 W per channel (8 ohms, 1kHz, 10% THD, 1ch driven)
http://europe.yamaha.com/en/products/audio-visual/av-receivers-amps/rx/rx-v679_g/

Marantz SR5009
100 watts per channel (8 ohms, 20Hz~20kHz, .08%THD). Can ‘safely drive speakers down to 4 ohms'
http://us.marantz.com/us/Products/Pages/ProductDetails.aspx?CatId=avreceivers&ProductId=SR5009

Pioneer SC-1224
560 W Multi ch Simultaneous Drive (8 ohms, 1 kHz, THD 1.0 %)
200 W/ch (4 ohms, 1 kHz, THD 1.0 %, 1 ch Driven)
160 W/ch (6 ohms, 1 kHz, THD 1.0 %, 1 ch Driven)
http://www.pioneer-audiovisual.eu/sites/default/files/datasheets/english/SC-1224.pdf