I don't know why it is so hard for people to understand any avrs can drive 4 ohm speakers given the right conditions, and most budget priced power amps such as this AMC thing, or even the better known Monolith, NAD, Anthem, cannot drive 4 ohm speakers under another sort of "right conditions".
@Steve81 : You are a superb technical writer and I think Audioholics might have an article, might have be by you, if not Gene, but would you consider writing another one that will answer the frequently ask "do I need an external amp, can my avr drive my 4 ohm speaker, or 8 ohm speaker that dips to 3.5 ohm, etc.etc.etc,, and make the article easy to find, and/or link by us regular members so we don't have to keep repeating the same thing every other week? Once and for all, please...
Note that I know it may be just my wishful thinking, but it is tiring to see the same kind of questions asked so many times, and since we like to help, we have to repeat ourselves so many times. So I hope Steve, or Gene can come up with something..
Back to the point I want to make again, I think we can agree that almost every long time members, at least on Audiohlics, ASR, AVS etc., seems to know, or claim to know, even quoted Ohms law and the power formula:
V=IR,
P=I*I*R=V*V/R=V*I
combined with important known facts:
1) speaker sensitivity is often expressed as X dB/2.83V/meter, or X dB/W/meter (less common, not preferred form)
2) each time you double the distance, say from 1 meter to 2 meter, you lose 6 dB of SPL (sound pressure level)
3) while 2) above shows distance is a huge factor, most room will have a room gain of about 3 dB in the mid bass to deep bass range so all that 6 dB is not loss, especially when subwoofers are used.
4) THX standard that many movie cinema follow, is to aim for SPL of 85 dB (sort of average, "reference level"), 105 dB peak
5) most home theater users will likely feel reference level is too loud and most will likely be happy enough with 75 dB average, 95 dB peak, many just don't know, don't care what spl they are listening to, but 75 dB/95 dB, imo is a good bet. For me, it is more like 70/90, occasionally 75/95 just for the extra thrill.
6) To gain 3 dB SPL, power has to increase by 2 times, and to gain 10 dB, power must increase by 10 times, conversely, if you are happy with 10 dB lower SPL, your power requirement will drop by 10 times too.
7) Even very low impedance dips may not be an issue for even an AVR because it depends on the bandwidth of the dip, and the duration of the dip, so it depends a lot of the media contents
It is so evident from the above commonly known and understood simple electrical formula that any AVRs can drive 4 ohm speakers, one cannot generalize and has to gather up the surrounding facts. If one simply grabs one of those so called 4 ohm capable power amp such as the entry level Emotiva Bas X A series, they may or may not do justice to their speakers, and/or could have just use their AVR and would have been fine.
Here is a numercal example, using the OP's M&K S150:
sensitivity: 92 dB/2.84V/m
impedance: 4 ohm
Thanks to
@ErinH , (I hope this is the same Erin of Erin's audio corner and if not, someone will correct me) we have the needed info, instead of the silly single number, to work with:
Again, don't blindly base your decision on the single value, look at the graphs!
From this graph, you can see that phase angle is not an issue, but the dip below 5 ohms spans the range between about 120 Hz and 800 Hz could be.
Still, if the OP sits within 10 ft, and listen to no louder than 10 dB below reference, most a lot of people do, the RZ50 will do fine. It is obviously better to use a 3 channel external amp, but I am not sure about this AMC amp, their website does not provide much information, and the emphasis on "True RMS power" sounds an alarm to me, there is no such thing as RMS power, though it is widely used, so I am only alarmed by the facts that they says:
"150 Watts True RMS Power per Channel, with 240Watts RMS Power per Channel at 4 Ohm Driven"
Such wording just don't inspire confidence, and if you look into the instruction manual, there is no 4 ohm power output rating, it just says 150 W (IMF) 8 ohm, clipping power 160 W 8 ohm
Why reference "IMF", that seems like a term/standard, rarely used in specifying power amp outputs these days?
They also claim "oversized" transformer, but there is no evidence of that if you look at the size of it in the picture.
Miller & Kreisel S150 Speaker Review (erinsaudiocorner.com)
The amp is still going to help for sure, but I would highly recommend the OP go with a more mainstay 3 channel, 200 W 8 ohms, 300 W 4 ohms rated power amp such as those offered by Monolith, that may not be available in Australia. There is also the Hypex class D amp options, such as March Audio's:
Amplifiers - March Audio
My suggestion is, try the Onkyo RZ50 AVR first, you may be surprised, that it can do the job nicely, if not then go to plan B.
