RT800: 4ohm or 8ohm?

[frustrated]

I have a pair of RT 800's. I've been trying to figure out a problem that I've had for about two years now and just discovered this forum. My receivers keep "Overloading" when ever I start to turn the volume up or keep it at a medium to low level for a while. I've tried two receivers, the second I bought because I thought the first was fried!! After many conversations with different reps and tech guys, I ended up testing the impedence with a multimeter. Connecting the leads to the wires running to the speakers(individually), I got 4 ohms on both. BTW, the jumpers are on and I am using 14g wire.

What I was told was that if there is not enough impedence, the cpu in the receiver works too hard to maintain current and subsequently overheats. Now I was told if I get anything drastically under 8 ohms, I have a fried coil. But I can't believe that both speakers are blown. They both sound great! No distortion, no cracking, and good, solid bass on both.

The specs say 8ohms, why am I getting 4? I'm lost, please help.

[Frank Z]

The impedance value changes with the signal being reproduced. Your speakers are 8 ohm nominal (aka average). Which receivers have you tried? What is/was the rated continuous output?

[frustrated]

what do you mean it can change when reproduced?

The first one: Pioneer VSX D608

Stereo:100 watts per channel (20 Hz to 20 kHz, @ 8 ohm, 0.09% THD)
Surround:100W x 5 (1k Hz, .8% THD)

The new one: JVC RX 8030 VBK

Stereo: 130w per channel, min. RMS @ 8 ohm 20 Hz to 20 kHz, 0.08% THD
Surround: 130w per channel, min. RMS @ 8 ohm 1 kHz, 0.8% THD

[frustrated]

BTW, awesome home theatre and web page. Loved the intro. Ironic, my wife and I just got the same color paint to do my fun room...Tomato Red lol

[Frank Z]

Thanks and welcome to the forum, seems I forgot my manners....again!

The impedance of your speaker is not a constant, the 8 ohm value is the nominal value. The impedance can dip as low as 2ohms.

Are you maintaining enough clearance around the receiver to allow for adequate heat dissappation(sp?)? Most manufacturers recommend 6-8" above the receiver. Have you checked for a short circuit in your speaker wires with an ohm meter? Does the problem occur during 2 channel and multichannel use? Is the receiver in an enclosed cabinet?

[frustrated]

As far as the clearance...yes...now. I though about that about a month ago. I did have the DVD player, which is a laf-depth unit, on top. That got moved when I dicovered the heat one day. The wires? I've looked visually, don't know how to check with the multi-meter.

The cabinet is an entertainment center type, open in the rear with glass on the front.

And as far the your question about sound mode, I'll have to check in the stereo mode, but it happens more in the DD cinema mode, and sometimes in the prologic mode.

[Frank Z]

Remove the wire from both the speaker and the receiver. Set your meter on "Ohms" and place your leads on the speaker wire (1 on + and 1 on -) and check the meter reading. If there is no short between the 2 conductors you won't see any value displayed/needle movement. A short circuit can show up as any value greater than 0. Make sense?

Enclosed cabinets with glass doors...HMMMM. You might try leaving the the doors open next time you use your receiver and check the tempertature by hand after a normal amount of use, say after a movie or a couple of CD's. That just might be your problem.

[frustrated]

Checked...null readings on both. I'm gonna have to go with the thermal concept for now. I just took everything down in preparation for painting. But I will give it a good workout after they red goes up.

I still don't understand abot the impedence readings I'm getting. I beleive you that it can vary, but by half? I tried something else. Removed the jumpers and tested each input individually thinking that with the jumpers on, it would "half" the impedence because the circut becomes parallel after the speaker inputs. Dunno, just trying anything. What's weird is that I get the same reading as before on the bottom set of post ( I don't honestly know which part of the apeaker they go to ) and a null reading on the top pair.

I should have gotten at least something of a reading, right?

While we're on it, what the purpose behind biwiring vs biamping vs the jumpers?

[Frank Z]

I copied this from the RT800i manual. I placed the good stuff in italics, hope this helps.


BI-AMPING OR BI-WIRING THE RT800i
IF YOU DECIDE TO BI-AMP OR BI-WIRE,
YOU MUST REMOVE THE FLAT METAL
JUMPERS BETWEEN TERMINAL POSTS.
FAILURE TO DO THIS COULD RESULT IN
DAMAGE TO YOUR AMPLIFIER AND
LOUDSPEAKERS.
Bi-amping allows you to use separate
amplifiers for the high and low frequency
sections of your loudspeaker to achieve
greater dynamic range and lower distortion.
After removing the jumpers, connect the
speaker wires from the high frequency
amplifier outputs to the upper set of terminal
posts on each speaker. Follow the same procedure
for the low frequency amplifier to the
lower set of terminals as shown in Figure 2.
Remember to maintain correct wiring polarity
(+ to +, - to -) in all connections.

Bi-wiring provides small but noticeable
improvement to the overall transparency of
the speaker. Separate speaker wires are run
to the low and high frequency drivers from a
single amplifier. After removing the jumpers,
connect one set of speaker wires to the
upper terminals on each speaker and one set
of heavy gauge wires to the lower terminals.
Connect the other ends of both wire sets to
the amplifier outputs as shown in Figure 3.

Click
here to see the entire manual and the figures that ar referenced.

[Kenneth Swauger]

Hello,
Thanks for posting your question, read the impedance article in issue #9 of the Speaker Specialist:
http://www.polkaudio.com/home/library/newsp/
It, hopefully, will help understand what a speaker's impedance means.
Regards, Ken

[frustrated]

Ken, thanks for the link. It provided some insite. However, I'm still a little confused about the lack of a reading on both top pairs of posts on the speakers. I got 4ohms on the bottoms (once again still confused as to why it wouldn't be 8) and nothing on the tops. Could this mean that something is blown because there is no reading?

In the middle of painting the room, so I can't do any testing of the sound right now.

Also, any known issues with the JVC RX8030 that you know of, whether with the amp or impedence issues with these brand of speakers?

Thanks again.

[Kenneth Swauger]

Hello,
I'm glad the article was helpful. The reason there wasn't any DC resistance measurement is because of the speaker's internal crossover. The upper set of binding posts connect to the high pass filter section of the crossover. One of the components, in this portion of the crossover, is a series capacitor. A capacitor is not a conductor, in fact its not even a semi-conductor. This is why capacitors have Voltage ratings instead of power ratings, current doesn't flow through them, including the small amount of current flowing through the DC resistance circuitry of your meter. So, when you place your meter on the lower set of binding posts (the low pass filter section), there is a conductive path and the current can flow and the indicator, on your meter, reacts. But the upper binding posts are not conductive to DC current flow and no meter reading. The high pass section, of your speaker, has lower and lower impedance (AC resistance, if you will) as the frequency increases. This means the capacitor can charge and discharge many thousands of times per second, responding to the incoming audio signal, allowing a current to flow through the tweeter, but not your meter.
Regards, Ken

[frustrated]

Ok Ken...breathe! That was impressive. So what I gather so far is...barring any type of physical problem with the speakers (which there doesn't appear to be from the previous findings) the overheating is produce by inadequate ventilation.

Well, until I finish painting the room I won't be able to check. I will definately let you all know how it goes. So until the next state of confusion, thank you both very much.

[Frank Z]

Your very welcome!

[hectorh]

I have a different approach to this problem because it sounds like a very old problem. The polks are 8 ohm speakers and the Amp can pump out the power but how about the speaker wires. 14 guage may not be enough!!! I remmeber my first amps/speaker buys when I would spend the cash on the equipment and go cheap on the speaker wires. I had problems with signal cutoffs and distortion. I also learned from my early experiments that to never close up the amp in a cabinet that totally enclosed the amp. The heat could easily cause the amp to malfunction.

[jcaut]

The RT800, at least the "i" version, dips down to nearly 4 Ohms at a couple of frequencies, according to my measurements. In fact, it's below 8 Ohms at most frequencies. But that's not unusual.

Anyhow, your speakers aren't the problem. Either of those receivers should be able to drive them without shutting down. Speaker wires that are too small wouldn't cause the receiver to shut down either. On the contrary, unless there is a short, the smaller wires will cause increased resistance which would reduce the amount of current being delivered, which would make the receiver run cooler (less power delivered to the speakers).

Try improving ventilation!

[frustrated]

The painting is complete and the room is back together. After a nice warm up of some stereo music, I cranked it up and then went to an explosive movie. (despite the wife's argument) All's good.

Answer: Ventilation. So far as I can tell. I moved the DVD to a different shelf and it seems to be the trick. I'll have to find out about whether or not I have the "I" series.

Thanks again for all the help.