in wall amplifier

[cartwright801]

Hello again. So I've got a kef soundlight system does anyone know if an inwall amplifier would be compatible with it? Something like the retro touch t2000. Or could it damage the speakers? Cheers with any help

 

[OwainDIYer]

From what I can see at
http://www.retrotouch.co.uk/audio/01521.html
the amp is rated 25 watt 4-8 ohms, so should be fine for the speakers. Note that URL points to a bundle which includes speakers.

 

[cartwright801]

Cheers

 

[Lucid]

I wouldn't take those power figures at face value. It's very easy to measure the power output in ways that vastly inflate the finial figure. The impedance, the distortion, the frequency used and whether the measurement is peak to peak power, is it sustained power (RMS... and it won't be)..yadda yadda yadda ... all these things have a massive influence on the final number, so what's on paper looks good but when you come to use it you find that the amp runs out of steam too early.

For example, 2x25W in to a 4 Ohm load is only 2x12.5W in to an 8 Ohm load.

Then there's whether it's peak to peak power which is a theoretical maximum but unobtainable in real life. Peek to peek makes it look like the amp is twice as powerful as it really is, so you need to halve the number. 12.5W is really 6.25W. But even that number isn't realistic. No amp can deliver its maximum rated power continuously. It can for a tiny fraction of a second, but after that the power falls to the RMS value which is 70% of peak. 6.25W becomes 4.3W.

Next there's the distortion figure (% THD). It's common to see weak amps measured at high THDs of 10% rather than the 0.1% THD used for Hi-Fi amps. The high distortion figure can boost the measured output by 40%. Take that back off the figure and we are down to about 3W.

We haven't even finished yet. There's still the frequency used for measurement (1kHz vs 20Hz-20kHz) and whether the power figure is under load or not.... In the end if there's no indication of how the power is measured then treat the number as bullshit. The real number will be somewhere close to 1/10th what the advert wants you to believe. 2x25W... yeah right.

 

[cartwright801]

That went right over my head...do you think I'd be safer with something like a yamaha as501 then? Like the idea of the in wall amp but not if it may cause issues + with the as501 i could have another set of speakers in the garden hooked up.

Thanks for your input

 

[Lucid]

That went right over my head...

It might well have done, but I'm sure you can understand the concept that what's claimed in the advert or web page doesn't translate to reality because the manufacturer is using smoke and mirrors, right? I mean that bit isn't too difficult to understand.

Incidentally, one of my commercial suppliers does a version of that in-wall amp. It doesn't have the fancy blue front but it's essentially the same guts inside. It is interesting to see their power rating because whatever they supply for commercial use does actually have to deliver on the promise made. Here's their figures:

Output Impedance: 4 Ohm
THD: 0.03% [my note: measured at very low distortion to ensure a usable power figure]
Frequency Response: 20Hz-20kHz [my note: measured with a signal frequency range that replicates real music]
Rated Power: 2 x 3w (RMS)

Their rating is 3W per channel. The amp from your link was rated at 25W per channel, but I said that 1/10th of that figure would be a more realistic rule of thumb number. 25W/10 = 2.5W and that's not far off what the same amp is rated at for commercial use.


Yes, you'd be safer with the Yamaha amp. The power rating of 2 x 85W is given with some reference to how it was measured: It's a minimum RMS power, and measured in to 8 Ohms using a full frequency test tone (20Hz-20kHz) and at very low distortion (0.019% THD) which all translates to a power figure that is deliverable in real terms

The only thing to add is to be aware that the KEF Soundlight system is a 4 Ohm load. That's a low Ohms number. So if your plan is still to add some garden speakers at some time in the future then try to make sure that they are at least a 6Ohm load, and preferably 8 Ohms. The reason is when driving both sets of speakers together (whether intentionally or by accident) then you really want to avoid giving the amp a hard time with two sets of low Ohms rated speakers.

 

[Sam Gangee]

That went right over my head.

In that case you either need to do a lot of reading until you understand it or else employ a professional to do the choosing (and possibly the installation).
RMS = Root Mean Square (you don't really need to know what that means - simply accept that it's a necessary factor).
THD = Total Harmonic Distortion (fairly obvious, I hope. Basically unwanted harmonic frequencies that weren't present in the original audio).

Amplifier power output is measured in Watts RMS at a specific THD (distortion) level into a specific loudspeaker impedance (Ohms Ω). Anything else is fairly meaningless.

 

[cartwright801]

Sorry last question....the yamaha amp has an impendance high and low switch. In low (a)only 4ohm (a) and (b) 8ohm. In high (a) only 8ohm (a) and (b) 12ohm. I take it i would be better having to sets of 4ohm speakers and impendance set to low so i can switch between one or two sets of speakers. If i got a second set at 8ohm id have to keep switching between high and low? Which me or the kids would forget. ...sound right?(no pun intended!)

 

[Lucid]

If done right then you shouldn't need to fiddle with the settings. The lower the Ohms of the speakers the more current they draw, and that's a bad thing. The higher the Ohms then the less current they draw, and that's a good thing. There's some other stuff going on about maximum volumes, but let's not worry about that just at this stage.

The switching on the amp deals with two things. These are: (1) what happens when driving one set of speakers on its own, and (2) what happens when driving both sets of speakers together.

The KEFs are 4 Ohms. When being driven on their own then the impedance switch needs to be set to low. The garden speakers need to a minimum of 4 Ohms but they can be more. Let's say they're 8 Ohms so the combined total is 12 Ohms. That still satisfies the requirement that the combined total be 8 Ohms or more. This means that the impedance switch can be left on Low all the time. Simple.

 

[cartwright801]

Thanks