Here's a speaker Ohms question that might not have been asked

[zeke=]

I am playing around with (comparing) receivers and speakers that I have accumulated. The last find was a NAD amp model C372, with a separate NAD tuner, and a pair of wall mount Vienna Acoustics Waltz Grand speakers. Amp and speakers rated at 4 Ohms. It does not sound good; weak and flat. Pretty much unbearable, even at volume. Then, I hooked up the VAWG pair to an old Harmon Kardon HK3485 and they sounded brilliant. The 3485 is an 8 Ohm output on each pair of R &L channels, A and B terminals. Just using one set (A) for the test; 8 Ohms driving 4's.

The HK receiver also runs just about anything I throw at it even hooked up to a 5 channel selector with 3 sets of small 8 Ohm bookshelf speakers on just one A or B output (not the best Idea, but I run all these at very low background levels). Everything sounds about the way you would expect given the difference in speaker pairs. So bonus points for the HK receiver do-all unit.

But I'm led to believe the NAD works well with 8 Ohm speakers using, again, one channel A or B. Just falls on it's face with the 4 Ohm pair. I don't have any other 4 Ohms speakers and combining will skew the results. I think you can imagine why I am puzzled. No one component seems defective, in need of caps or other service AFA I can tell. I did finally hook up some Bose bookshelf models to the b side of the HK to switch back and forth between speakers and they compared well, no dip in sound quality either way,

Let me simplify the question, any ideas why that first combo was so bad? Does that NAD need A and B in use at all times?

 

[swechsler]

Let me simplify the question, any ideas why that first combo was so bad? Does that NAD need A and B in use at all times?

It happens that some amps just don't sound great with some speakers, and specs don't tell the whole story. To clear up a misconception, the NAD is not a 4Ω amp and the H/K is not an 8Ω amp. Transistor amps generally have pretty low impedance, but I couldn't find the rating for the output impedance of either amp published. But then, it's not generally published for most consumer amps. The NAD is certainly 4Ω rated, and the H/K may be as well, but that's just the impedance that they can handle without blowing up or overheating. It's possible that the VA speakers have a very low impedance dip at one frequency that the NAD is unhappy with, but I could find neither an impedance graph nor a minimum impedance rating for them (the rating of 4Ω is typically the nominal impedance unless otherwise specified).

In the past, H/K did advertise their ability for high current output (see: Ohm's law), but the HK3485 is well past their heyday from the 1970s and 80s. FWIW, from what I've been able to find online, both of your amps appear to be about the same age. I don't know how you determined that the NAD wasn't in need of service, though, since simply looking at components is not really enough to determine their health. However, at less than 20 years old, it seems unlikely that either amp would need recapping simply due to age.

One other thing to clarify: the A and B outputs are electrically identical; when both outputs are selected, they will be connected and it would be the same as if you had connected both pairs to the A output.

 

[zeke=]

Thank you. So 2 pairs of speakers connected to A, or A and B, combine for a 2 Ohm load? Then a pair of 8 Ohm speakers all powered would be closer to the 4 Ohm "rating."

Yes, you are correct in that I'm only blown out dust gently. I have a fundamental understanding of electronic circuitry but I normally play with old point to point vacuum tube stuff and I can clearly see where the paper caps are and those that are definitely swollen and shorted. I will do electrolytics on solid state, but I don't have an LCD meter and don't do SMD caps. So, no, I don't know anything about the NAD except what it does plugged in.

I will have to do a little more experimenting. Maybe the mismatch is why I got the NAD and VA for free. They sound terrible together. It's worse than only one channel in that the sound is not bright at all. The louder you play it the more you notice that it won't sound any better no matter what.

I guess I don't understand why a low impedance would sound bad but if you put less load on the amp it wakes up. Is has to be what you say, they just don't like each other.

 

[lini]

(...) So 2 pairs of speakers connected to A, or A and B, combine for a 2 Ohm load? (...)

Per channel - yes. most usually. However, there also are some integrated amp and receiver models, which in case of A+B operation run the two speakers per channel in series (rather than in parallel), so that the impedances add up instead.

Greetings from Munich!

Manfred / lini

 

[ConradH]

4-ohm speakers should use a bit heavier wire, but that's not usually an issue unless it was really marginal to begin with. Something I learned while building DIY amps is that transistors can/will have a gain falloff as the current goes up. See "beta droop." Most amps show an increase in THD at higher currents because of it. To get around that you can use better output devices or more devices in parallel to keep the current in each device low. IOW, more output devices than would be needed just for the rated output power. Almost everything that affects performance gets worse at higher currents. So, it's no surprise to me that some amps don't do as well with 4 ohms instead of 8.

 

[zeke=]

I didn't get a manual but I did find a spec sheet. From the sheet:

The circuitry automatically senses the impedance characteristics of the loud speaker and will then adjust its power supply settings to best cope with that specific load. PowerDrive topology is a practical approach to enable an amplifier to easily deal with musical dynamics and difficult speaker loads. Thus we have the highly desirable characteristics of high dynamic power and low impedance drive capability in one affordable package.

I guess I need not worry about impedance after all. Find speakers it likes. And it does say on the back that connecting a 2nd pair adds the impedance respectively. So with four 4 Ohm speakers it says it will operate at 8Ω.

Even though this equipment came as a lot I'm going to assume now that it wasn't necessarily used as the same lot. They kept other equipment when they moved downsizing. I got some surplus of this and that. I won't from this point on try and figure out how it was used. I'll just try different speakers on the NAD and use the elsewhere. I think I have a great place for them using an entirely different receiver than either one of these. Just need some more base and I don't prefer sub woofers. I might have some old 10" around that I can check the cross overs on and let them be the low down.

Thanks to all. Good lesson to learn. I'm an old retired carpenter that doesn't hear all that well anyway. I need some bright mid range and a little base, but not enough to bother others. KISS.