Help

sbryan

I'm in need of some advice/clarification on speaker impedance and the load placed on the amplifier. My goal is to use a pair of Polk Audio Monitor 10B(s) and a pair of Series II Monitor 10(s) with my Pioneer SX980. I'll just insert here that I'm not trying to blow the windows out of the house, I just like the larger sound stage.

The issue that I'm having is that the 10B(s) are rated 6 ohm while the Series II are rated 8 ohm. I've checked the DC resistance and the two models read an identical 4.75 ohm. I understand that this is not impedance, but have read that this should get me close. That being said, shouldn’t there be a difference? I have also read that the ohm rating given to these Polks is a gift, and the two models are essentially the same being much closer to 6 ohm nominal impedances. I just don’t know how credible this information is.

I've run just the Series II through one channel, and I do experience cutting out at higher volumes for a long period. I didn’t think that this should happen as they are rated 8 ohm, but decided it probably wasn’t an issue as I would not be listening that loud. When I got the 10B(s), I hooked them up A + B, and definitely got cutting out, and much sooner at 50% volume. This makes sense as the 6 ohm model 10B(s) drop to 3 ohm in parallel.

My solution was to wire the Series II and 10B(s) in series thus increasing the impedance to a nominal 14 ohms (I actually think it's closer to 12) My question, is this safe to run? It does drop the volume, as expected, by approx. 20%

mhardy6647

Yes you can run in series and it should be safe and sane, but passing the signal though one loudspeaker and its on-board crossover is not going to improve the quality of the signal driving the second one in the series circuit

I won't guarantee that the two pairs in series will sound worse than either pair by itself -- but there's a good chance for that to be the case, I'd opine.

PS the idea of smoke testing the two pairs of speakers in parallel as the OP did isn't what I'd consider a best practice. The OP is fortunate indeed if no damage was done to that Pioneer receiver before the protection circuit acted.

Finally -- remember that the nominal impedance of a loudspeaker is just that: nominal. The actual impedance curve (impedance vs. frequency) of almost all loudspeakers is complicated. Virtually all loudspeakers are reactive loads. Resistance is frequency independent, but the capacitance reactance and inductive reactance of loudspeakers varies with frequency. The net effect is that the nominal impedance is a fairly poor indicator of the "amplifier-friendliness" of a load -- and the DC resistance is, unfortunately, essentially useless as such. The DC resistance is not necessarily indicative of the minimum impedance that a loudspeaker will present to an amplifier.

mhardy6647

Not to flog a moribund (if not quite deceased) equine, but here's an example of why "nominal impedance" ain't necessarily all that useful.

This is the measured impedance curve of an Infinity Kappa 9 loudspeaker. These were (are) notorious amplifier killers -- and the measured impedance curve provides evidence as to why.


source: http://audiokarma.org/forums/index.php?threads/question-re-wattage-how-much-does-it-really-matter.820370/page-2#post-11561130

Note that this loudspeaker manifests impedance minima below one Ohm at about 35 Hz and at about 7 kHz.

Infinity, it appears, rated the nominal impedance of these as "4 to 6 ohms".
source: http://www.infinity-classics.de/technik/manuals/Kappa_9_technical_sheet.pdf


Unfortunately, the graph above does not show the phase angle as a function of frequency, and I couldn't find a plot of phase vs. frequency, but my guess is that,
besides the low impedance at inconvenient frequencies, that these somewhat
infamous loudspeakers are also very reactive loads.

Don't try a pair of these on an SX-980!

mlistens03

I wouldn’t think so, unless the receiver is rated for that... it’ll have a much lower power rating, maybe some more THD, and I think it’ll get warmer quicker.

mlistens03

Ha yes, I remember wanting the Kappa 9.2. Then I saw a impedance chart... and I changed my mind. I think that they might even kill my Crown, which is pretty bulletproof.

sbryan

I do appreciate the feedback. Smoke testing, as stated, was never the goal but rather the outcome. The function of the protection circuit is to protect the amplifier from overheating. Albeit, I shouldn't have tried both the Series II and the 10B(s) in parallel. The receiver is 8 ohm (A, and 4 ohm (A+B), so that's on me. Doesn't explain the Series II alone though. If this is an 8 ohm speaker, this should not have been a problem right?

sbryan

For clarification: The receiver is 8 ohm (A,B) and 4 ohm (A+B)

mhardy6647

Could be an issue with the health of the amplifier or the specific loudspeakers.

The classic "non-pathological" cause for unexpected protection triggering is an inadvertent stray strand of wire bridging the "+" and "-" terminals, usually on the amplifier (although it could happen at the speaker end, too).