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practical choice for an amp
practical choice for an amp
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Old 21st June 2007, 09:44 AM   #1
jarthel is offline jarthel  Australia
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Join Date: Jan 2002
Location: somewhere in Australia
Default practical choice for an amp

I have been looking at 805 (845/211) designs at the moment and I'm wondering if this is a practical choice for speakers such as B&W, Tannoy or anything similar.

It seems to me that people using amps like these are those with quads.

Any thoughts?

what is the minimum power you would recommend for "regular" speakers?

Thank you for the help
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Old 22nd June 2007, 01:20 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Location: Monroe Township, NJ
Paul Joppa's 102 dB. rule is a valuble yardstick. In a "typical" room, an amp/speaker combo should be capable of producing 102 dB. SPL peaks, at a 1 M. distance.

That sort of sound field is needed to handle the dynamics of large scale orchestral works. A large listening space or a fondness for "head banger" Rock requires yet more power.

Honest 90 dB. efficient speakers work with 16 WPC. Speaker manufacturer claims must be viewed with some suspicion. 8 Ohm nominal speakers with dips below 4 Ohms in the impedance curve are "really" 4 Ohm speakers. 2.83 V of drive is 1 W. into 8 Ohms, but it's 2 W. into 4 Ohms. All too often, the claimed efficiency is high by 3 dB. CAVEAT EMPTOR!

BTW, 4 Ohm nominal speakers with significant dips in the impedance curve are poor candidates as mates for tube amps. You will not find 2 Ohm taps on O/P trafos.
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Old 22nd June 2007, 01:57 AM   #3
gingertube is offline gingertube  Australia
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Join Date: Oct 2004
Location: Adelaide South Oz
Jarthel and others,
Eli's post decoded for the non-technical

You want 102dB SPL (Sound Pressure Level) at 1m

to use Eli's example:
Speaker rating 90dB/watt/metre

Doubling the Amplifier POWER is a 3dB increase in power. So with those speakers:

1 Watt amp will give 90dB SPL at 1 metre
2 Watt amp will give 93dB
4 watt amp will give 96dB
8 watt amp will give 99dB
16 Watt amp is needed to reach that 102dB "yardstick"

Note that if you use (for example) 96 dB/W/m speakers then ONLY 4 watts is needed to reach that 102 dB SPL target.

Cheers,
Ian
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