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Can lower power amps safely drive 4 ohm speakers ?

Well I tried the 104/2's with the AU-5900 and the heatsink did start to get uncharacteristically warm, but slightly warm more than hot. Normally it feels cold or ambient temperature. I think it's OK.

Treble is missing some sparkle, despite putting new ferrofluid in them, but also the midrange is a bit shouty, even unpleasant on some music. Maybe the issue is the mids being too prominent. Anyway I'm going to recap them to see if this improves the balance
 
Dom123 said:
Well I tried the 104/2's with the AU-5900 and the heatsink did start to get uncharacteristically warm, but slightly warm more than hot. Normally it feels cold or ambient temperature. I think it's OK.

Treble is missing some sparkle, despite putting new ferrofluid in them, but also the midrange is a bit shouty, even unpleasant on some music. Maybe the issue is the mids being too prominent. Anyway I'm going to recap them to see if this improves the balance
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Be very careful about recapping any Kef reference speakers. Kef specially matched the crossovers to the driver set. There is a reason they are labeled Right and Left with sequential serial numbers. If you really need to update the crossovers, recommend you use a Falcon Set designed for Kef 104/2s. But honestly, unless those speakers were really abused, those crossovers should be good.

You are describing the sound of a system without synergy. And that description also is why I have a spare high current amp in my collection. If I need to send one of my Amps in for servicing, the Yamaha M40 will do the job if I don't push things too hard. It drove the Kef 107s for a few months and they sounded good. They sounded much better when I put them on the Bose 1801, where they reside right now. Sometimes, I feel I could use double the power I have.
 
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Its more complicated with impedance, because its an ac characteristic, not DC. Speaker crossover circuits have inductors and capacitors which behave very differently with ac, then they do with DC, they have 'reactance'. For example, its easy to take two resistors, say 8 ohms each and add them in series to get 16 ohm, whether ac or DC. But if we take one 8 ohm resistor and one 8 ohm inductor, and add them together in series, we dont get 16 ohms, we get 8 + j8, which requires some understanding of basic ac circuit theory and mathematics- polar and rectangular coordinates. Just like in the real world, 10 + 10 can equal 20, but not if I am pushing a heavy refrigerator with 20 lbs of force, and another person is pushing the same amount, but in the opposite direction, in this case 10 + - 10 = 0.

Impedance varies over the audible 20-20K frequency range, thus its useful to have the chart showing its response from the speaker manufacturer. Many speakers are 4 ohms, its avg value rating, but all of them have their particular rises & dips over the freq range, which is why many folks have different results with their Sansui amps, whereby those speakers that tend to dip low- below 4 and going toward 2 ohms are likely to stress an amp so to speak.
 
Dom123 said:
Hi Everyone,

In my quest for vintage audio perfection, I am interested in some vintage speakers that are rated at 4 ohms, for example KEF 104.1's.

Can anyone confirm if it will be OK to run these from my collection of small Sansui amps, or should I stick to 8 ohm speakers ? I have an AU555a, AU317, AU517, AU4900, AU5900 & AU7900 as well as a couple of 771 receivers.

Many thanks in advance for any advice
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those are all medium output receivers and amps. You may want to check out the sensitivity measurements more than the Impedance. the sensitivity number will tell you how loud the speaker will play with a given power input. The KEF 104s seem to do better with 200 watt per channel amps that can easily handle a 4 ohm load or lower.
 
I'm listening to my 104.2's on my AU-7900 and notice the heatsink is getting quite warmish, but not hot, whereas on my Celef speakers, rated 8 ohm, the heatsink remains cool with no perceptible change in temperature even at loud volumes. So these speakers are definately stressing the amp more than my other speakers. (I have a few different pairs, all nominal 8 ohms).

I presume "warmish" is not a major problem as long as warmish doesn't become hot ......

But I sorted the issues with the tweeters and the sound of these 104.2s is great with my AU-7900. I really enjoy them. A big smooth sound that encourages turning up the wick a little.
I suppose running them on my recently aquired AU-777 is completly out of the question as it only refer 8ohm loads in the manual. Definately don't want to risk any damage to it.
 
Unfortunately it's not a simple yes or no answer. My 45W Samsung receiver claimed it could drive 4ohm speakers, but running my 6ohm 89Db sensitivity Pioneer S-T300 was too much for it. Nominal speaker impedance ratings don't tell the whole story, you need to look at an impedance sweep. My Acoustat Spectra 1100s are rated at 6 Ohms, but have an impedance dip to 1.1 Ohms. Infinity Kappa 9s are rated at 4-6 Ohms as well, but have an impedance dip below 1 Ohm, and developed a reputation as amp killers. There is no way I personally will run either off of any receivers, let alone a low powered one. But a lot also depends on volume level music type, jazz at low volumes, and metal at high volumes with loudness turned on, are very different demands. I also use Crown's amplifier power calculator to determine how many watts you truly need, just put in distance, desired volume and amp headroom, and speaker sensitivity (the true one, not the ones certain manufacturers inflate), and just remember that if sensitivity is rated as 2.83V@1M, it is 1W at 8 Ohms, and 2W at 4 Ohms

https://www.crownaudio.com/en-US/tools/calculators
 
Thanks for the answer. My understanding is that these speakers were designed to have a constant 4ohms across the spectrum and shouldn't dip below.
But I feel the AU-7900 has no trouble driving them. The heatsink gets midly warm (maybe 45°C or so) but only after an hour at fairly high volume. Back in the day when a UK magazine did a mega test of many amps, they measured 79watts into 8 ohms and 132watts into four ohms for the AU-7900, although they judged the power loss into 2 ohms to be too great, and recommended only using a single pair of 4 ohm speakers.
 
Dom123 said:
Well thanks for all the replies. Some very interesting comments. My fun comes from buying cheapish vintage gear, mainly Sansui, usually not working, fixing it & enjoying.
Most of these old Sansui amps sound absoultely fantastic to me especially on my old Celef (8ohm) speakers. They are all recapped/restored following the great advice and comments available on AK.

I listen to music for about 1 hour each evening pretty loud, mainly Jazz. None of my amps ever get beyond mildly warm to say the least, even playing loud.

I love my Celefs, but need some decent back-ups just in case. Been through a whole string of old B&W speakers, as well as MA, Celestions etc. and everything dissapoints, either too bright, too dull, too shouty....... I even have a pair of modern Fyne Audio f302's, which I don't think sound that good to me, dull sounding (it could be my 60year old ears).

An opportunity has come up to buy a pair of KEF 104/2s in Mahogany for a very very reasonable price. The cabinets need some work, and the bass drivers some new foam donuts, otherwise the seller claims the tweeters are working fine. I'm thinking new ferrofluid, new donuts and I'm good. Can see about recapping later.

So I'll give them a go and see how I get on. The AU-517 is rated 65W (8 ohms) and my AU-7900 rated 80W (8 ohms) so should be plenty powerful enough. This is just for my own personnel listening, certainly never going to be partying...
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I like "shouty", know exactly what you mean
 
Well yes I was listening to them yesterday and the midrange is very present & indeed a little too present, indeed a bit "shouty". In two minds about the 104.2.......maybe listening in fairly near field conditions isn't optimal.
 
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