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I was given a pair of vintage (brand new unopened) speakers which I am hoping to use with a wall mounted remote controlled amplifier as part of my home cinema. Here are the specs.

  • Speakers: Wharfedale 8.1s rated at a nominal impedance of 6 Ohms and power handling of 100 Watts
  • Amplifier: ADASTRA WA-210 wall mount digital amplifier 8 Ohms 2 x 10W

I am not intending to wind the volume up but would like a decent level of sound. What sort of performance should I expect?

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    \$\begingroup\$ Have you read the amp manual? It says you should not connect speakers with impedance below 8 ohms, and your speakers are below 8 ohms. It might work, or the amp might not like it and burn up, damaging the speakers along with it. \$\endgroup\$ – Justme Dec 23 '20 at 9:59
  • \$\begingroup\$ You'll do no harm as long as you don't wind the level up to the point of obvious distortion. So try it. Big speakers can be reasonably efficient and Wharfedale made a name for themselves (long before this model) with the E series, giving 90dB (at 1 metre) from just 1W. \$\endgroup\$ – user_1818839 Dec 23 '20 at 11:49
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Amplifier manual says speakers below 8 ohm impedance should not be used, and thus the 6 ohm speakers are not compatible with it. The amplifier can refuse to work due to overcurrent limit.

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The big problem here is that it look like the rated sensitivity of these speakers is 86 dB SPL (under normal test conditions--1 kHz, 1 watt, measured at one meter on axis in a standardized size of room). That's not the lowest sensitivity rating I've ever seen (by quite a stretch), but it's definitely on the low side.

So, depending on how loud your idea of "decent level of sound" actually is, and to some extent, how large and reflective of a room you're planning to install them in, there's a decent chance you may end up disappointed.

A lot here depends on your listening habits though. If you're planning on listening to something like hard rock or heavy metal, I foresee pretty serious disappointment in this combination. There's simply no way you're going to get the volume levels most people want for that kind of music.

Even with a more powerful amplifier, these just aren't the speakers for that kind of music. They won't play very loud, and they won't produce deep bass.

On the other hand, if you mostly listen to jazz or classical, and especially if you want it primarily as background music at fairly low volume so you can carry on a conversation without it intruding...they may work out really well.

As far as the impedance mismatch (amp rated for 8 ohms, speakers rated at 6) it's quite unlikely to be a major issue. The impedance of a typical loudspeaker varies pretty widely with frequency at best, and the difference between 6 and 8 ohm nominal impedance is actually fairly small. Again, it's going to come back to listening habits though. If you're running the amp right to the ragged edge, the reduced impedance could lead to a problem--but you don't have to reduce volume much to reduce that substantially.

Title Question

The preceding is largely addressing the specific speakers mentioned here. It kind of hints at the situation, but doesn't really directly address using a small amplifier to drive large speakers.

To state the situation directly, the size of the speakers doesn't matter nearly as much as their sensitivity. Just for example, if I connected the same 10 watt per channel amplifier to (much larger) Klipschorn loudspeakers, the situation would be drastically different. It would have no difficulty at all producing what most people would think of as loud music--even thunderously loud music with quite deep bass. The difference is that Klipschorns have a rated sensitivity of 105 dB SPL for one watt of input (again, 1 kHz, one meter, in a standard sized room). They're sensitive enough that you don't need to feed them a whole lot of power to get a lot of output.

And as a horribly over-simplified rule of thumb, speakers that are physically larger will tend to have higher sensitivity. So, the combination of a small amplifier with large speakers is definitely not destined to end in tears. In fact, a lot of the problem here is that the speakers he was asking about honestly aren't particularly huge. The same amplifier with larger speakers that had a higher sensitivity rating could easily work out quite a bit better.

Summary

This combination is not going to play very loud. It mostly comes down to two questions: what kind of music do you listen to, and how loud do you want it? If you want loud music with deep bass, you'll almost certainly be disappointed. If you want "soft" music at fairly low volume, it's likely to work out a lot better.

But a different amplifier would/will only mitigate that to a fairly limited degree. Regardless of the amplifier you use, these speakers simply aren't every doing to produce really loud music or (especially) deep bass. Just not going to happen.

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  • \$\begingroup\$ Top answer, I totally agree. These speakers may be "big" by modern standards, but in their day they were classed as "bookshelf" speakers, and getting half-decent bass out of those always means low efficiency. \$\endgroup\$ – Guy Inchbald Dec 23 '20 at 12:06
  • \$\begingroup\$ The speaker sensitivity of 86 dB SPL per watt at 1m is not impressive, but I guess the loudness is subjective and opinionated. Rough calculations tell me that 86dB is about one lawnmower. Pushing 10W to single speaker equals 96 dB SPL from single speaker. Push 10W to both speakers, and that's 99 dB SPL, which almost equals to one jackhammer at 1m. I don't think it is necessary to go any near the amplifier limits to get loud music, in contrast to what you say. \$\endgroup\$ – Justme Dec 23 '20 at 12:39
  • \$\begingroup\$ I get it that it is not a great amp but had to go with this amp because ....The ceiling mounted projector I watch movies on is being driven by an Amazon Fire TV Stick via HDMI. I need to amplify the line level 3.5mm audio out from the projector to some speakers. I dont want a messy run of cable from the ceiling to a big amplifier at ground level and back again up the wall to speakers. I am not looking for loudness because I have a separate HiFi set-up for good music. I am either going to get some 8 ohm speakers or try the Wharfedales at low volume and see what happens. Thanks for all the help! \$\endgroup\$ – British Raj Dec 23 '20 at 16:55
  • \$\begingroup\$ @Justme: In theory you're absolutely correct. In reality, not so much. Even rock music (where recordings are generally pretty heavily compressed) has a pretty high ratio between peak volume and average volume, so to keep from clipping on the peaks, you need to keep the average volume quite low (and if you do clip on the peaks, you'll probably destroy a tweeter). \$\endgroup\$ – Jerry Coffin Dec 23 '20 at 17:49
  • \$\begingroup\$ The other big problem is that the measurement is normally done at 1 kHz, but in most music, the vast majority of the power is at much lower frequencies, and these speakers won't reproduce those frequencies nearly as efficiently. \$\endgroup\$ – Jerry Coffin Dec 23 '20 at 17:54
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You probably won't run into problems with that large ratio between amplifier and speaker rated power. However, high power speakers tend to be fairly inefficient, so you may not end up with the volume you hope for.

Paradoxically, a big amplifier with small speakers is OK. As you turn up the volume, the speakers begin to distort badly, it sounds horrible, and you back off.

The biggest problem comes when you have speakers two or three times the amplifier power. You turn it up, the speakers aren't at full travel so are not distorting significantly, so you turn it up a bit more. Then the amplifier clips, generates a swathe of high frequency distortion, which passes straight through your crossover and fries the tweeters, which are typically rated at a fraction of the power of the whole speaker system (done that!).

With the 10:1 ratio between ratings, your speakers should get away unscathed, even if you let the amplifier clip.

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    \$\begingroup\$ ...Unless you are my brother, and you borrowed my very nice, 30W bookshelf speakers, and you hooked them up to your 200W amp, and you either did not notice (because of the thrash metal you were playing) or you did not care (same reason) that they were distorting. Smoked the voice coils in both tweeters. Little ****ard! \$\endgroup\$ – Solomon Slow Dec 23 '20 at 17:13
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Looking at the amplifier it only admits to 2x 10W (max) power, and the speakers mentioned in the manual are 5W (RMS) 10W (max) so it looks like a fairly weak amp - I'm guessing some generic cheap (probably class-D) amplifier IC in there, the sort you'd find in a cheap bluetooth speaker. I'd put money on the amp being 5W RMS per channel or lower.

Given it's a wall mount 38mm depth box there's not much room for substantial power supply either so this thing is not going to be banging out the choonz - it's designed for quiet background music:

Thank you for choosing the Adastra WA-210 wall amplifier for your discreet audio installation.This unit is designed to provide a complete integrated solution for background music in rooms or corridors.

Howver, even 5W of power can be window-rattlingly loud with an efficient speaker, although a cheap low-power amp is going to sound like hot garbage very quickly as you turn up the power. It's up to you if you care about that.

The impedance mismatch is unlikely to matter, the manual forbids <8 Ohms so up to you if you risk it - you're unlikely to damage the speakers and they're the valuable part of this setup.

The speakers are decent units, I find it amusing you call them retro as to me they're modern bookshelf units - at best studio monitors - while "proper" speakers have at least an 8-10" woofer. If you can find an old hifi separates unit / amplifier that actually manages 25W RMS or better (charity shops/thrift stores etc.) you could have a VERY sweet sounding setup with a real kick.

For info - the "peak" ratings on amps are very misleading - RMS is always the more truthful measure. A peak rating is like saying your car can do 200mph... when dropped from a helicopter.

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  • \$\begingroup\$ I agree that the amplifier is missing the important audio spec of "continuous output power at low distortion into 8 ohms per channel". \$\endgroup\$ – Audioguru Dec 23 '20 at 19:26
  • \$\begingroup\$ TBH the amplifier is missing a lot of specs, but then the manual does say it's only for background music in corridors so they're "managing expectations" quite heavily there. \$\endgroup\$ – John U Dec 24 '20 at 16:08

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