Problem with JVC-powered 2ch rig

nadams

Hi guys,
I'm running a JVC RX-558v receiver and a pair of Polk 5jr and a pair of MA 3dx speakers. The Polks are on the #1 channels (of course) and the MAs are on the #2 channels. I have replaced the woofers in the MAs with 4ohm Rockford Fosgate Punch subs for kick, plus the old foam surrounds in the original speakers were rotted. All four speakers are sitting in front of me, though the MAs are laying on their side stacked on top of each other on the floor under a desk. Therefore, I do not have a direct line of hearing for the MAs. So I decided that the Polk tweeters could do a good enough job on their own and disconnected and removed the original MA tweeters.

Following so far? Good. I originally powered this rig with a Pioneer SX-450 receiver (vintage, 15watt/ch), and it did just fine, but I was afraid I was going to hurt it (the front panel lights were getting pretty dim sometimes... those Fosgates just love power), so I replaced it with my more powerful (100w/ch) JVC amp today. I had a bugger of a time trying to get things evened out with my equalizer, and decided to turn off the MAs for a while and just listen to the Polks. As soon as I turned off Ch 2 I was blasted with the tweeters of the Polks. After regaining my hearing, I turned Ch 2 back on and immediately the tweeters cut off. I'm wondering if maybe this is a problem of resistance on the channels... Time for another paragraph.

I am still using the original crossover in the MAs, and my question is, when using a crossover, does a 4 ohm sub + a 4 ohm tweeter = 8ohm load? If so, is this imbalance of loads on the amp what's causing the tweeters to cut out? The amp is rated for 8-16ohm loads... therefore a 4 ohm load from the MAs could affect things, if I'm understanding this correctly.

Also, I noticed that I had to turn the amp way up in order to get an acceptable amount of volume out of it. And, I had none of these problems when running it with the Pioneer amp. However, the back of that amp reads- "A,B -- 4 ohm or more/speaker" "A+B -- 8 ohm or more/speaker". I took this to mean that maybe it didn't care much that the MAs were only a 4ohm load.

If all this is true, and the JVC amp just wants the 4ohm load, what can I do to increase the load to 8ohms without adding the tweeters? Can I just go to Rat Shack and buy a 4ohm resistor and tack it between the positive and negative wire for the tweeter, or wouldn't it be able to handle the power? Thanks in advance for your replies.

nadams

RuSsMaN

So basically, you are running the MAs as subs for each channel, is that what you are trying to acheive? And the Fosgate woofs in the MAs are 4 ohm woofers, correct?

nadams

Originally posted by RuSsMaN
So basically, you are running the MAs as subs for each channel, is that what you are trying to acheive? And the Fosgate woofs in the MAs are 4 ohm woofers, correct?

That is correct... I am only running the MAs for bass response because I'm not happy with the 6.5" mids that are in the Polks alone. The Fosgates that are in the MAs are 4ohm woofers, and the tweeters that came out were 4ohm as well. The MAs can be viewed at www.king-nerd.com/dood/images/system/mvc-011f.jpg.

I have thought about replacing the crossover in the MAs with something more suited to these subs, but they sound fine as they are.

BTW- when I said that the tweeters cut off... I didn't mean that they totally turned off, they just get much quieter. And yes, this receiver does have a step-down when both sets of speakers are on (it goes to 80w/ch), but this is a dramatic change, it's not just from the power drop.

Suggestions?

nadams

RuSsMaN

First off, the xover in the MAs probably isn't affecting the Fosgate woofer. It is probably passed somewhere between 2000 and 4000 Hz, way beyond the woofers range.

Now, onto the issue at hand. We have 1 MA, and 1 Mon 5 for each channel. Wire from the POS on the amp to the POS on the MA. Then wire from the NEG on the MA to the POS on the MON 5. Finally, wire from the NEG on the MON 5 back to the NEG on the amp.

This will put the speakers in series, and give you >8ohm on each channel (assuming the Fosgate runs approx 4, and the Mon 5's run approx 6-8).

Cheers,
Rooster

nadams

Thanks, RuSsMaN, I'll try that right now.

nadams

nadams

Okay, they're now hooked up as you described. I definately had to turn the treble up to get anywhere near what the Polks alone were putting out. I don't know if this just means that the RF subs are drowning out the tweeters (which I don't think it is) or that it didn't actually help :-/. Maybe I do have to have those tweeters hooked up in the MAs. Doesn't make sense to me. The only thing I could come up with was the resistance thing.... which it doesn't appear to be. Can anyone come up with anything else? Maybe you have some more ideas up your sleeve, Russ? Thanks,

nadams

RuSsMaN

I've been thinking about this more. With the tweeters disconnected on the MAs, you might have something funky going on in the network, with the circuit incomplete.

Try the wiring scheme above, but switch the MAs on the MON 5's. Wire the the POS on the MON 5 first, tell me what happens.

Cheers,
Rooster

nadams

Okay, I did one channel like you said except with the speakers switched around, and it still did it.... Maybe I should just hook the damn tweeters back up to the MAs and forget about it... or replace the crossover with and inline single speaker xover. The xover that's in there right now is just an inline one... there's no real xover "network". Thanks again,

nadams

nadams

To clarify some things-

I'm doing this testing with my treble level at "0" on the amp and bass at "6" on the amp (ranges are -10 - +10)

After doing the last test, I quickly unhooked the setup and just plugged the Polks into the amp alone and the difference was night and day. As soon as I did that, the tweeter just came alive. It was bright and clear.... whereas with the series setup or just the way I had it originally, it was muddled and dark.

nadams

nadams

This is where I smack myself on the forehead and say "DAMN, YOU IDIOT!"

I hooked up one of the MA tweeters and hooked the speaker to ch 2 (with the 5jr on ch 1). Started out with only the Polk on, it was nice and bright... turned the MA on, and it died off.... so apparently it does it even with the tweeter there. It didn't do it with the Pioneer amp because it doesn't drop off the power if you add the other set of speakers.

Ah, but now I take the forehead slamming back, because the tweeter wasn't bright when it was hooked up in series with the MA, running at full power.... any thoughts on that one, Russ?

nadams

RuSsMaN

Ok, one more thought. With the tweets in the MAs disconnected, temporarily connect the tweeter pos and neg to complete the circuit, and wire as I intially stated above. Now we are just doing this on the 'A' terminals, correct?

Anything?

Doing the A/B setup with each speaker on their own set of terminals, the Fosgated woof might just be hogging too much current. If it's dipping down below 4ohm, the power supply in the amp may be over-taxed. I'm willing to bet that is what it is going to boil down to. The Pioneer worked, right?

Cheers,
Rooster

nadams

Well, that appears to have done the trick. I wired them in series (Polks wired directly into the POS of the amp), hooked up the MA tweeter and the Polk tweeter was noticibly brighter.

One more question. How can I get the resistance of the tweeter, without the tweeter? Or should I just forget about it, hook the tweeters back up, and be happy?

And yes, the Pioneer worked fine, but it was stable down to 4 ohms... This JVC amp is pretty much bottom of the line... I bought it about 3 or 4 years ago for about $140.

Specs-

100 watts per channel, min. RMS, driven into 8 ohms, at 40Hz to 20kHz with no more than 0.8% total harmonic distortion

Anyway.... thanks once more Russ, your assitance is greatly appreciated.

nadams