100W enough for B&W CDM 9NTs?

[Blindamood]

:?: Until recently, I was running my Pioneer Elite VSX-45TX receiver (100W x 7) with B&W 602s, 601s, and CC6 speakers (and Velodyne sub) in a 5.1 arrangement. I have been very pleased with this setup for both multi-channel music and movies. Recently, however, I received an small influx of cash and decided to upgrade the fronts to CDM 9NTs and the center to a CDM CNT. With this new arrangement, I have had the receiver shut down on me twice now, in movie scenes with hard-hitting bass (e.g., loud explosions). I'm guessing the caused is the low minimum resistance (3 ohms) of the 9NTs.

So, until the next cash influx, when I am able to power the fronts with a new 2-channel amp (for example, the Rotel RB-1080), I'm considering how to prevent this problem in the meantime. One solution I've considered (in lieu of just turning the volume down) is to set the speaker size to Small, and set the crossover frequency to 50 Hz. Does this sound like a reasonable solution? Will this prevent the receiver shutdown? I would really like to use the 9NTs full-range, but don't want to damage them with inadequate power.

Thoughts? I appreciate any comments or additional suggestions you can provide.

[David Meek]

Hi Brad, welcome to The Guide!

One quick thought: have you set the Pioneer for operation with 4-ohm speakers? There should be a toggle switch on the rear of the unit to choose between 4- and 8-ohm impedances.




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[Blindamood]

Actually, there is a toggle switch between 8 and 6 ohms. However, the 'nominal' rating for the 9NTs is 8, so that's what I picked. What does selecting this option do? How would this affect the rest of the speakers, which work fine at 8?

[will1066]

Leave the setting at 8 ohms, Blindamood. The B&Ws are rated at 8 ohms. I'm not really sure what's happening, however. The Pioneer should not have problems driving the B&Ws. I don't think changing the crossover or the speaker size will do anything. Something's amiss. Just curious, what volumes are you listening at?

[Blindamood]

Well, fairly loud, but not unreasonably loud (have used it louder on previous occassions with other speaker setup). On the Pioneer, it was in the 14-15 range.

[David Meek]

I'm guessing the caused is the low minimum resistance (3 ohms) of the 9NTs.

If the speakers are rated at 8-ohms but can dip to 3 (if I'm reading your posts correctly), then yes, the high intensity passages could be causing your unit to go into protection mode if you are listening fairly loudly. As a simple test, shut the receiver down, switch to the 6-ohm setting, power the receiver back up and play the same problematic passages at the same volume and see if the unit goes into protection. If it does, then yep there's something more serious afoot.




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[Blindamood]

So, what exactly changes in the receiver when switched from 8 ohms to 6 ohms?

[jaakan]

a minimum 3 ohms load is not good for that Pioneer.
It doesn't need to be in loud to go in to protection mode,
which doesn't fully protect.
try Star War EP1 : start of the pod race.

power off, flip down the front panel, hold down the speaker(s) button
should say 16,8,6 ohms ... it uses a different set of resisters.


if your rev keeps going into protection mode you'll need to get it fixed soon, trust me, Because it will break soon.

Does it go in to protection mode with the speakers set to small?

What you should think about doing?
1. get a power amp
2. get a Rev that can deal with a 3 ohm load
Yamaha's upper end can
Rotel's can too
Pro/pre + power amp is your best bet

[brucek]

So, what exactly changes in the receiver when switched from 8 ohms to 6 ohms?

The lower the impedance of the speaker you use, the more current they will draw from the amplifier, and the more heat will be generated.
From a technical standpoint, that's the simple answer.

Some amplifiers, that can't tolerate low impedance loads, have the selectable switch on them that allows lower impedance speakers to be used. In reality, this switch simply alters the amount of power supply (rail) voltage available to ensure your amplifiers transistor output stage doesn't overheat when using lower impedance speakers. It doesn't change any impedance with regard to the output and it really has no bearing on what ratings for nominal impedance of the speakers you have attached to it. It's a simple matter that by lowering the output voltage available across the speakers, the lower the amount of power is dissipated.

Certainly the specified impedance load for a set of speakers is average over the audio range. This impedance changes greatly depending on frequency. It's interesting to look at the frequency vs impedance graph of many speakers and you would be surprised how low the impedance gets at some frequencies. The ohm rating is nominal. Many manufactureres will also spec the lowest impedance for their speakers.

Many larger, more expensive amps are able to handle tough loads and don't require any alteration to the available power supply maximum.

The result of using a load that is lower than allowed for an amplifier will be in the form of heat and a thermal shutdown if taken too far.
A test of the lower load, while monitoring the amplifiers heat sinks temperature, would tell you pretty quick whether that load is OK or not. You'll know, long before you do any harm to your amplifier, if it will tolerate a "specific" set of speakers. Your amplifier obviously cannot handle your speakers at the volumes you are subjecting it to.

Off-times the power supply can't supply the current required to the speakers with a full rail and low impedance load.
But, there is also the common situation where the power supply will indeed supply the current required for a low impedance load, but, the output transistors, given the amount of heat sinking provided is insufficient and will result in a thermal shutdown.

I strongly suggest you switch your amplifier to the lower setting and avoid this thermal shutdown you're experiencing. The final solution is to replace the power amplifier.

brucek

[Blindamood]

Thanks everyone for the information / advice. I've switched the receiver to 6 ohms as a temporary measure, but am definitely on the lookout for a good two-channel amp to drive the 9NT fronts in the long run.

[John Holmes]

I drive M&K's (4ohm) with my 27TX Elite. And have played them at or near reference level a few times. I routinely play them between -15 and -10. I have never had any shutdown. My rears drop into the 4 ohm region too. I do run everything small though.

I have heard this issue come up before with the Elites. I know they a rated stable into a 6 ohm load. I just don't get how a THX certified piece, cannot handle a 4 ohm load but, THX speakers are usaully 4 ohm speakers???

Blindamood, I hope you get a handle on this soon.




"I came here, to chew bubble gum and kickass. And I'm all out of bubble gum!!!" My DVD's

[Blindamood]

Well, from reviewing other posts, I'm wondering now if it has to do with the fact that I had the receiver plugged into a surge protector (Monster Cable HT800). I'm kinda nervous about plugging directly into the wall, because my house actually got a direct lightning strike a couple of years ago, damaging several appliances/components. However, this model may not be sufficient to support the kind of power required by this receiver, and maybe it couldn't get the power it needed during a loud surge...

So, I've plugged it directly into the wall for now, and will see if I get a repeat of the shutdown. Thanks for the feedback!

[Blindamood]

So far, so good (with receiver plugged directly into the wall). Did a bit of 'stress testing' last evening, and experienced no shutdown. Appears that I was just choking the receiver by using the surge protector?!

[David Meek]

Brad,

Some PC/SPs do choke the flow of current enough to affect equipment. That's been a recurring topic of discussion at forums all over the 'net. I'm glad that appears to be the cause. If it is, I strongly recommend replacing the Monster with another PC/SP that doesn't "current limit" you - those lightning strikes, can cause you major grief, ya know?


If you do want to pursue another PC/SP, I highly recommend the VansEvers line. I've been using a Model 100/14 for several years with very good results. Plus Mike (VansEvers) is a great guy to deal with. FYI, the Model 100 doesn't show on his web page. If you are interested, and need a 14-outlet solution, ask him about it - he may still have some of the chassis in stock.




David - HTGuide flunky
Our "Theater"
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[John Holmes]

Blindamood,

Glad you found the problem. And I happy to hear it was an easy fix.




"I came here, to chew bubble gum and kickass. And I'm all out of bubble gum!!!" My DVD's