Please help me understand speaker impedance

[zippy753]

I listened to a pair of Usher S-520’s (8 ohm) this weekend and think they’d be great for my small space. However, I wasn’t able to listen to the S-525 (4 ohm) center speaker. The problem is that I’m not exactly sure how using a 4 ohm center with 8 ohm fronts will affect the sound in real life or if my Yamaha V661 can even handle it.

The dealer said that I can just adjust the speaker level for the center to match the fronts. However, looking at my manual, I’m not so sure. There are only two choices for impedance in my receiver; 8 ohm or 6 ohm. Switching to 6 ohm, the manual states that I can use 4 ohm or higher front speakers but the center and surround speakers must be 6 ohm or higher. They use the word “must”.

So I’m a little confused about all this and I hope someone can help. Will I not get the most of these speakers if I buy them? Or is it not really a big deal in the real world?

Thanks for any advice.

 

[mazersteven]

Test Bench: Yamaha RX-V661 A/V Receiver
by Daniel Kumin
June 2007
DOLBY DIGITAL PERFORMANCE

All data were obtained from various test DVDs using 16-bit dithered test signals, which set limits on measured distortion and noise performance. Reference input level is -20 dBFS, and reference output is 1 watt into 8 ohms. Volume setting for reference level was -3. All level trims at zero, except for subwoofer-related tests; all speakers were set to "large," subwoofer on. All figures are worst-case where applicable.

Output at clipping (1 kHz into 8/4 ohms)
1 channel driven: 151/224 watts* (21.8/23.5 dBW)*
5 channels driven (8 ohms): 56 watts (17.5 dBW)
6 channels driven (8 ohms): 53 watts (17.2 dBW)
Distortion at 1 watt (THD+N, 1 kHz)
8/4 ohms: 0.03/0.04%
Noise level (A-weighted): -76.3 dB
Excess noise (with sine tone)
16-bit (EN16): 0.9 dB
Frequency response: 20 Hz to 20 kHz +0, -0.15 dB

*Speaker-impedance setup switch at "8-ohm" position. With switch set to "6-ohm," output was limited to about 63 watts (18 dBW).

MULTICHANNEL PERFORMANCE, ANALOG INPUT

Reference input and output level is 200 mV; volume setting for reference output level was -3.5 dB.

Distortion (THD+N, 1 kHz, 8 ohms): 0.01%
Noise level (A-weighted): -92.8 dB
Frequency response: <10 Hz to 133 kHz +0, -3 dB

STEREO PERFORMANCE, DIGITAL INPUT

Reference level is -20 dBFS; all level trims at zero. Volume setting for reference level was -3 dB.

Output at clipping (1 kHz, 8/4 ohms, both channels driven): 99/137 watts* (20/21.4 dBW)*
Distortion at reference level: 0.03%
Linearity error (at -90 dBFS): 1.3 dB
Noise level (A-weighted): -76 dB
with 96-kHz/24-bit signals: -84.3 dB
Excess noise (with/without sine tone)
16-bit (EN16): 0.8/0.25 dB
quasi-20-bit (EN20): 12.2/11.7 dB
Noise modulation: 1.3 dB
Frequency response: 20 Hz to 20 kHz +0, -0.1 dB
with 96-kHz/24-bit signals: 20 Hz to 43 kHz +0, -3 dB

*Speaker-impedance setup switch at "8-ohm" position. With switch set to "6-ohm," output was limited to about 61 watts (17.9 dBW).

BASS-MANAGEMENT PERFORMANCE

Measured results obtained with Dolby Digital test signals.

Subwoofer-output frequency response (crossover set to 80 Hz): -24 dB/octave above -6-dB rolloff point of 80 Hz
High-pass-filter frequency response (crossover set to 80 Hz): -12 dB/octave below -3-dB rolloff point of 80 Hz
Maximum unclipped subwoofer output (trim at 0): 6.6 volts
Subwoofer distortion (from 6-channel, 30-Hz, 0-dBFS signal; subwoofer trim set to 0): 0.03%
Crossover consistency: bass crossover frequency and slope were consistent for all sources and formats
Signal-format consistency: consistent for all applicable formats
Speaker-size selection: all channels can be set to "small"
Speaker-distance compensation: available for all main channels.

Yamaha's RX-V661 yielded some very good numbers, including superbly low noise and distortion through its analog multichannel inputs. The digital inputs' S/N, linearity, and noise-modulation results together suggest a digital-to-analog conversion error involving the magnitude of the least-significant bit. But this was not enough to induce any audible effects on our several dithered-noise listening signals, even over headphones at reference-plus levels.

The Yamaha is equipped with a setup-menu 8-ohm/6-ohm speaker-impedance switch (I'd still like to know where they're buying these "6-ohm" speakers). The latter position apparently takes the Yamaha's supply voltage down to reduce the maximum current drawn through the output transistors, restricting output power by roughly half.

(I had no problems running high-power tests of single and paired channels into 4-ohm loads with the switch set to "8-ohm," but I'm not suggesting that anyone disregard Yamaha's directives. Honest.)

Power results otherwise were generally fine, though 5- and 6-channels-driven tests fell well short of the unit's single-channel/stereo spec of 90 watts, suggesting that total power-supply current is not sufficient to service all channels fully at the same time even when unchecked by the protection circuitry. This is not uncommon among all but the most expensive receivers, and the fact that I observed ample power in listening tests confirms that such all-channels tests, while a useful benchmark of overall potential, are not a particularly real-world model.

http://www.soundandvisionmag.com/receivers/2276/test-bench-yamaha-rx-v661-av-receiver.html

 

[mazersteven]

Am I wrong in saying that the 661 is not 4ohm stable?

How about getting another S-520 as a center channel speaker?

 

[zippy753]

How about getting another S-520 as a center channel speaker?

I could ask but I get the feeling dealers don’t like breaking a pair. He might, however, if I absolutely can’t use the S-525 center with my receiver. That’s what I need to know….anyone?

 

[Midcow2]

The Usher 525 has a little lower than average 88dB sensitivity into a nominal impedance of 4 ohms, which means it require pretty good power to drive it at loud volumes.

Your receiver is rated at 90 RMS watts per channel. The 6 ohm verus 8 ohm setting is only yo portect your receiver. It cuts your effective HT power by bridging in an internal resistor. Leave the AVR set at 8 ohms.

Now for the real answer,, normally the AVR is current limited. So looking at a maximum of 90 watts into 8 ohms yields power (90) = I^2*r where r=8 Imax = 3.35 amps maximum current before AVR shut-off so at 4ohms you have a maximum power of only 45 watts. 45 watts will probably not drive you center speaker (rated at 80 watts) at the volume you desire.

I would not use the 525 center speaker with the setup you have directly connected to the line level center speaker output. However, you can take the center pre-out and connect it to a separate amplifier and can easily drive a 525 center; that is what I would recommend.

 

[jostenmeat]

Well, I have the impression that one has to consider a combination of things. I don't think minimal impedance is the ONLY factor in deciding if an amp is capable enough.

Besides impedance, other things to consider, for instance:

- what kind of SPL you want, peaks or not. (have a meter?)
- sensitivity of speakers
- distance of listener from speakers

IIRC, the amp-knowledgable PENG was saying that the 661 had a better amp than the 663/863, or even current Onkyo entry models. Better ask him, or use the search function.

If it was me, if 4 ohms was the minimal impedance, I wouldn't be worried with moderate volumes in a moderate sized setup.

Without really knowing how much disposable income you have, or what kind of upgrade path you wish to follow (or ignore), its difficult to say.

That being said, I say get the speakers that please you most. Speakers are by far and away the most important aspect of audio, as the display would be for video. Please consider the importance of this purchase. Hook em up, and see what you think. If you feel they could use more juice, the try an ID amplifier. Return if nothing sounds different. There have a been a # of folks who did just this. Some returned the amp. Some kept it.

I hope that helps more than it confuses! Ok, I just re-skimmed your post. Small room? I bet you're fine.

 

[Adam]

The amp-knowledgable PENG is on the way...

 

[mazersteven]

The amp-knowledgable PENG is on the way...

I also PM'd Seth=L.

 

[Adam]

I also PM'd Seth L.

I didn't PM him. I just sensed his presence.

Good thinking on Seth!

 

[PENG]

I just checked the specs of those speakers here:

http://www.usheraudiousa.com/products/loudspeakers/usher-series/s-525-center-channel

S-525
2-way system tweeter 1" (UA025C-10), mid-bass 5" (KSW2-5029R)
Sensitivity 88 dB @ 1 watt / 1m
Nominal impedance 4 ohms
Frequency response (-3 dB) 50 Hz ~ 20 kHz
Power handling 80 watts
Crossover frequencies 2.38 kHz

S-520
2-way system tweeter 1" (UA025-10), mid-bass 5" (KSW2-5029B)
Sensitivity 86 dB @ 1 watt / 1m
Nominal impedance 8 ohms
Frequency response (-3 dB) 52 Hz ~ 20 kHz
Power handling 75 watts
Crossover frequencies 2 kHz

The S-525 has lower impedance but it is also 2 dB more sensitive than the S-520 so if you feed them with the same power the U-525 would produce a lower sound level by only 1 dB. It will theoretically take approx. 26% more power to level match it with the S-520.

Keep in mind small speakers are typically not too sensitive. They need relatively (to larger speakers) more power to play loud, yet they cannot handle too much power, in this case the S-525 can only handle 80W maximum. That may still be tough for the 661 if you push the S-525 all the way to its maximum power handling capability. So it all depends on how loud you want to play those little Ushers and whether you have a subwoofer to take care of the power demanding low frequency signals.

 

[zippy753]

Wow, I’m impressed with the help I’m getting on this forum. Thanks for all the replies! I do have a sub and I’ve read that it’s especially important for these Ushers. I won’t be turning the volume up too loud because I have neighbors – maybe once in a while. But it seems the consensus is that these speakers aren’t optimal for my setup. I like the Usher because it sounds great for such a small speaker – and I can get a good price. But my heart isn’t completely set on them. I’ve also been looking at B&W 685 and HTM62 that sound great to me and maybe a better option.

Keep in mind small speakers are typically not too sensitive. They need relatively (to larger speakers) more power to play loud, yet they cannot handle too much power, in this case the S-525 can only handle 80W maximum. That may still be tough for the 661 if you push the S-525 all the way to its maximum power handling capability. So it all depends on how loud you want to play those little Ushers and whether you have a subwoofer to take care of the power demanding low frequency signals.

Are you saying that the specs are only part of the picture and having a sub will make it easier to drive the Usher front and center? Or, do you think these speakers really need an amplifier to get the most out of them?

Thanks again everyone.

 

[PENG]

PENG was saying that the 661 had a better amp than the 663/863, or even current Onkyo entry models. Better ask him, or use the search function.

Thanks for reading that post of mine. I wasn't sure if the 661 had better amp than the 863. I would have thought that the 661, 663, 863 all had the same amps and power supplies, that was until I read the lab measurements for the 863 taken by the same reviewer who tested the 661 last year.

To confuse us further, H.T. Mag just tested the 863 also, and got much better numbers than S&V:

http://hometheatermag.com/receivers/708yamrec/index2.html

Here's what was said:

"This graph shows that the RX-V863’s left channel, from CD input to speaker output with two channels driving 8-ohm loads, reaches 0.1 percent distortion at 164.8 watts and 1 percent distortion at 184.1 watts. Into 4 ohms, the amplifier reaches 0.1 percent distortion at 224.4 watts and 1 percent distortion at 270.6 watts. "

Their 5 & all channel numbers were equally dismal, consistent with those obtained by the S&V reviewer. So HT mag's numbers for the 863 are consistent with those of the 661 obtained by S&V. I am beginning to think that may be something went wrong in the S&V's recent lab test on the 863.

Back to the OP's question, I also think the 661 should do fine with those speakers, especially if he uses a powered sub to take care of the bass.

You really can't do much about it because those little speakers cannot take too much power anyway.

 

[PENG]

Are you saying that the specs are only part of the picture and having a sub will make it easier to drive the Usher front and center? Or, do you think these speakers really need an amplifier to get the most out of them?

Thanks again everyone.

Other important factors are your room dimensions and acoustic characteristics, how loud you play your music and how far do you sit from the speakers. Specs are useful too, especially when it includes a set of frequency response curves, impedance and phase angle curves.

 

[mazersteven]

I like the Usher because it sounds great for such a small speaker – and I can get a good price. But my heart isn’t completely set on them. I’ve also been looking at B&W 685 and HTM62 that sound great to me and maybe a better option.

The B&W CM series might also be an option. IMO the sound characteristics of the CM series is more like the Usher's then the 685's. JMO

 

[zippy753]

Thanks again everyone, I think I understand better – I may not be able to get the full potential out of the 4 ohm center but it might not matter because it depends on my space and how loud I play my music. There are a few “ifs” I’ll need to think about. I just wish it were easier to judge speakers before I buy. The dealer offered to have me bring my receiver, so I’m pretty sure he’ll break open the center box to test it there and reassure me.

Mazer, I haven’t heard the CM series, they look great, but they’re just a bit more than I’d like to pay. I’d like to keep it under/around $1k for L/R/C speakers.

 

[mazersteven]

I’d like to keep it under/around $1k for L/R/C speakers.

Look ask anyone around here. The best part of your Home Theater experience is blowing your budget.

 

[TLS Guy]

I just checked the specs of those speakers here:

http://www.usheraudiousa.com/products/loudspeakers/usher-series/s-525-center-channel

S-525
2-way system tweeter 1" (UA025C-10), mid-bass 5" (KSW2-5029R)
Sensitivity 88 dB @ 1 watt / 1m
Nominal impedance 4 ohms
Frequency response (-3 dB) 50 Hz ~ 20 kHz
Power handling 80 watts
Crossover frequencies 2.38 kHz

S-520
2-way system tweeter 1" (UA025-10), mid-bass 5" (KSW2-5029B)
Sensitivity 86 dB @ 1 watt / 1m
Nominal impedance 8 ohms
Frequency response (-3 dB) 52 Hz ~ 20 kHz
Power handling 75 watts
Crossover frequencies 2 kHz

The S-525 has lower impedance but it is also 2 dB more sensitive than the S-520 so if you feed them with the same power the U-525 would produce a lower sound level by only 1 dB. It will theoretically take approx. 26% more power to level match it with the S-520.

Keep in mind small speakers are typically not too sensitive. They need relatively (to larger speakers) more power to play loud, yet they cannot handle too much power, in this case the S-525 can only handle 80W maximum. That may still be tough for the 661 if you push the S-525 all the way to its maximum power handling capability. So it all depends on how loud you want to play those little Ushers and whether you have a subwoofer to take care of the power demanding low frequency signals.

Here is the impedance curve of one of an Usher driver the closest I can find to the one in the S-525.

Note that although this is rated as an 8 ohm driver, the impedance is only a hair over six ohms from 100 to 400 Hz. Now there is where the power demand is highest. Now if you put two drivers in parallel as in the S-525 the impedance will be just over 3 ohms.

Unless the OP plays it very softly, or limits voltage by employing the 6 ohm switch then I think he is putting the center channel output devices of the center channel amp of his receiver at risk.

We have this problem endlessly. The manufacturers specs are no help. They really do need to publish the curve, so you can see the impedance where the power demands are highest. Unfortunately impedance dips more often than not coincide with the area of maximum power demand.

I have come across several speakers recently that are rated at 8 ohm nominal and they had impedance dips across a significant part of the spectrum below four ohms.

I think you can pretty much say that a speaker with more than one woofer is in fact 4 ohm or less, no matter what the spec sheet says, unless the manufacturer will publish a curve.

The next issue is that these current limited receivers are becoming a bigger and bigger problem overtime. They limit choice, as in this case. On the few occasions I have heard Usher speakers, I have been pleased by them.

If I was the OP, and he likes the Ushers, he should buy them and consider a three channel amp to drive his front speakers. Unless he can get an impedance curve of the speaker he wold consider as an alternate he has no assurance he won't have the same problem, no matter what the spec sheet says.

 

[Seth=L]

Are the drivers on the Ushers high excursion? Does high excursion factor in with impedance dips? It would seem they would be somehow related? Sorry, I just never addressed this before.

So long as the Ushers don't go much lower than their average impedance rating the RX-V661 should do fine for all but the largest of rooms. Of course speakers that size aren't intended to fill large rooms with sound anyway.

In the event that the receiver is running out of steam, which I doubt it would ever become a problem, you could always add some amplification.

 

[mazersteven]

In the event that the receiver is running out of steam, which I doubt it would ever become a problem, you could always add some amplification.

Thank You Seth=L.

 

[Seth=L]

Thank You Seth=L.

You have 661 thanks....RX-V661....weird day.