Valve amplifiers

[Ali Tait]

My OB's are around that figure, and I find 15w/ch more than enough.

Sorry, got this wrong. I use an active system with two stereo amps, so it's actually 30w. In my set up this gives more power than I'll ever need.

[Marco]

Oh, I know, and in terms of tonality, too... Such high-efficiency speakers have a very definite sonic signature, particularly in the bass. It's a matter of whether you can live with that effect, as well as the benefits such high-efficiency brings.

Personally, I couldn't. It's all a trade-off, dude!

Marco.

[Ali Tait]

Yes it's all about compromise. But going back to efficiency for a moment, with a 3db increase in efficiency of the speaker, you only need half the amp power to get the same volume ( I think!). In the comarison we have here, there is a 6db increase, so only a quarter of the power is required to have the same headroom available, and I think you have huge amounts of headroom available Marco, way more than most folk, hence my suggestion to the OP that a high quality low power SET would be a good match to his speakers.

[Marco]

Sure, I haven't read the whole thread. What speakers has he got?

Marco.

[nightyy]

I am using Klipsch RF-7 on my primary setup (102dB), but I am also using Castle Howard S3 on a second setup, currently (Kingrex Class D blown) driven by a original Sonic Impact T-amp.
The original T-Amp has 15Wx2 I think, and the S3 only 89db, and still that is enough (10 o'clock on poti) for the setup's small room (12sqm). 2x15W easily drives small room levels on 89db speakers from what I currently see.

I plan to get better speakers in say three to five years, is it corrent to assume that speakers in the price range of 2000€-4000€ are also quite efficient ones (>94db)?

PS: The original T-Amp is rated as 2x10W @ 8 Ohms

[nightyy]

The ugly thing about amplifiers on high efficiency speakers in my opinion is that the volume dial is working within such a small range. Aren't there dials that are non-linear, and adjust 20% of the total output power withing the first 50% of the dial?

[Marco]

Hi Marco,

Welcome to AoS, btw

Thing is, you're not the OP of this thread, Chris (Sarnies) is - which is why I was rather confused, as I indeed it appears Ali was!

Marco.

[Welder]

Maybe this will help.

Very roughly 2 x increase in power will give a 3dB increase in SPL at one metre away from your speakers.

In fact, the above is virtually useless in real world applications.
Firstly amplifiers clip, often long before they reach their maximum output.
Next, this type of calculation is usually based on the nominal impedance of your speakers.

The problem is no speakers I’ve come across are exactly the nominal impedance and further, their impedance changes with frequency. As the impedance drops, more power is required. This becomes particularly irksome at low frequencies where a 1dB per Hz frequency roll-off isn’t uncommon. Your speaker bumf may give your speakers response as say 50Hz to 20 kHz but unless a) it’s been measured, and b) a dB tolerance is also given, it doesn’t tell you the roll off.
It’s not uncommon to find even properly quoted figures to be wrong, particularly with regard to the +/- dB range.
A further complication is these types of figures tend to get quoted at the standard 1Watt at 1metre distance for 2.83 Volts across 8 ohms at a fixed listening point. Go listen at 4 metres away when the impedance has dropped to say 5 ohms and not be in the exact on axis position and it all changes.

Now you add in the thorny problem of head room. This headroom needs to be within the non clipping capability of the amp. Conventional wisdom has it that 20dB to 25dB will cover the most enthusiastic transients.

Some people like to listen at “live” concert levels; that’s around 120dB for say a loud rock concert. Between 80dB and 85dB listening isn’t uncommon. Now add 20dB headroom to this and factor in where your amp starts to clip. It doesn’t take long with average efficiency speakers to run into kilowatts of power once the above is taken into account!

If you want to get it right you need measure the impedance of your speakers, preferably a plot of the impedance versus frequency.
Then you need to find out when your amp clips.

As a rough guide this may help but bear in mind this calculator works with nominal values.

http://myhometheater.homestead.com/splcalculator.html

You could of course do what many do and buy yourself an underpowered valve amp (they tend to clip nicely) and get used to listening to a bit of distortion when you crank the volume up.

[Macca]

Maybe this will help.

Very roughly 10 x increase in power will give a 3dB increase in SPL at one metre away from your speakers.

:

I thought it was a doubling in power to give 3db increase in loudness, not 10 times the power?

[Welder]

Oooops, cheers Martin