What's My Problem?


OK, here's the situation . . . . hoping some of you with more knowledge and experience than I have can help me out.

On 2-channel listening, my system sounds great at low levels -- say at 9 o'clock or less on my VPC-1 passive preamp volume. Lots of openness and air, good imaging, lots of space around instruments. Of course, dynamics and bass suffer, but that's to be expected.

Between 9 and 12, the sound starts to get harsher and the soundstage begins to close up -- orchestral stuff sounds much more confused and congested. Above 12 o'clock, it's really not worth listening to.

These aren't very high levels -- 9 o'clock is my "late night with the wife sleeping down the hall" listening level, and 12 o'clock doesn't get Verdi's "Requiem" to real-life SPLs.

My first assumption is that my amplifier just doesn't have enough juice. But the RB981 puts about 200 wpc into a 4 ohm load, and I would think that would be enough to get to at least decent levels, even with my admittedly power-hungry NHT 2.3As . . . .

Alternately, I thought that maybe the 9000ES/RB981 combo wasn't ideally suited for a passive preamp. I understand that component matching is critical here, but I'm not really clear on how it works . . . . the volume gets loud enough with no problem, it's just that the quality suffers.

Then again, it could simply be "louder=more annoyance from digital harshness," and I need to replace the 9000ES with a better Redbook CDP. But it seems to me that if the CDP was to blame, the soundstage and "airy-ness" wouldn't change much as the volume increased.

Suggestions would be greatly appreciated!

Thanks!

Pat
tsrart
Hey Tsrart,

I agree, it is the amp that's coming up short. The Rotel RB-981 (130w @ 8ohms) is a fine amp but, most truly muscular amps will double their power at 4 ohms. You are only getting an additional 70wpc at 4 ohms, that's not enough. I had a similar situation when I owned a Krell KAV-500 and a pair of Revel M-20. The Krell, in its time, was considered a brawny amp at 100 wpc and a substantial toroidal transformer. The Revels are, as Warrenh mentioned above, a rather power hungry speaker, 86 db at 4 ohms. At moderate levels the sound was fine but, if I tried to crank it, the soundstage would begin to muddle and then collapse completely. I upgraded to a Plinius SA-250MkIV, a bit of overkill, but I did not want to have to upgrade my amp again and I believe, with this amp, I have accomplished my goal. For you, this may be a little more then you will want to get into but there are other excellent sounding amps with the power to handle your situation. It's also important to understand that not all 130 wpc amps are created equal. As I mentioned above, there are amps that will double down a 4 ohms. So you may want to look for an amp in that power range that will double down, that will should do the trick. I think you would be headed in the right direction if you were to consider an amp upgrade and it might not be as expensive as you might think. Good luck and happy hunting......John
Before rushing off and spending money by changing amps, I strongly recommend you examine your room setup. The Rotel is a reasonably decent amp and the NHTs are not that power hungry. In an unsympathetic room the NHTs can tend towards harshness.
Does you still have the problem if you take the passive out of the system and use the lexicon as your pre? As Gregm says the impedance of a transister based passive changes as volume changes and if you have very few steps on your volume control you could drive your amp to clip at a very low setting on the volume control. Is that passive a good match for your cdp,amp and cables?
Sounds like a mis-match to me. I don't think it's power-starved clipping at all, but a disparity between the output impedance of the passive pre-amp and a low amplifier input impedance. If the RB-981 has an input impedance of less than 47K ohms (26K, 32k ?), then it very well could be the source of distortion at moderate volume settings. This could also be made worse by long interconnects and the 86db efficiency of the NHT's. Just runs out of steam...
Wow -- thanks for the great response!!

I've got some more info:

I had an idea that I SHOULD have had earlier, but didn't think of until I saw Warrenh's post mentioned changing amps OR SPEAKERS. Well, I have three other pairs of NHT speakers in my HT rig, all of which are 8 ohm and easier loads to drive than the 2.3As. I picked the 2.1s, which are full range and with almost the same driver complement as the 2.3As, but with a slightly shorter cabinet and an 8 ohm impedence.

I moved out the 2.3As and put the 2.1s in the same spot -- didn't tweak them, and the 2.1s don't have the NHT Spike Kit bars on the bottoms, either, but I figured it would give me an idea.

It did . . . .

The 2.1s absolutely blew the 2.3As away. The soundstage started big and got BIGGER as the volume increased, the harshness was noticeably decreased, and the bass was much more solid, in spite of the fact that the 2.3A is supposed to go lower . . . .

So I think we can safely say that the problem is some sort of issue with the Rotel driving the 2.3As. I'm going to play around some with your other suggestions, including going back to A/B through the Lexicon. Gs5556, I think the input impedence of the Rotel is 27K, although I am using 2 foot interconnects with a fairly low capacitance between both CDP/preamp and preamp/Rotel.

I'm going to be annoyed if it's the preamp . . . . just bought that, and I really like the sound (or lack thereof) with it in the system as opposed to the Lexicon . . . .

I'll keep everyone posted, and again, thanks VERY much for all the help!

Pat