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Old 23rd August 2011, 04:51 AM   #1
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Default Output Power Question

So I really want to buy and build a K-12G tube amp from tubedepot.com

Stereo Tube Amplifier Kit

Seems totally fine, great reviews and relatively easy to build unless you make some modifications which I probably will not do. However, in a review I read from audioXpress.com

http://diyaudioprojects.com/Tubes/K-...be-Amp-Kit.pdf

the reviewer claims that the output is slightly lower than 8 W/channel which is fine, but...he also says he tested the kit on speakers which are rated for much higher RMS than that! Also, in another forum I read that one person put the speakers to Infinity P162's which are rated at 150W RMS. I know that it is possible to under power a speaker and trying to bump the levels up would eventually result in clipping because there isn't enough power to move the driver rated for that high of an input, but the author of that post claims to be getting wonderful sound at high enough levels. I am a little newb and this question is driving me crazy! Any answers?

Last edited by milsman2; 23rd August 2011 at 04:52 AM. Reason: Did not fully explain the power.
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Old 23rd August 2011, 06:04 AM   #2
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Quote:
Originally Posted by milsman2 View Post
So I really want to buy and build a K-12G tube amp from tubedepot.com

Stereo Tube Amplifier Kit

Seems totally fine, great reviews and relatively easy to build unless you make some modifications which I probably will not do. However, in a review I read from audioXpress.com

http://diyaudioprojects.com/Tubes/K-...be-Amp-Kit.pdf

the reviewer claims that the output is slightly lower than 8 W/channel which is fine, but...he also says he tested the kit on speakers which are rated for much higher RMS than that! Also, in another forum I read that one person put the speakers to Infinity P162's which are rated at 150W RMS. I know that it is possible to under power a speaker and trying to bump the levels up would eventually result in clipping because there isn't enough power to move the driver rated for that high of an input, but the author of that post claims to be getting wonderful sound at high enough levels. I am a little newb and this question is driving me crazy! Any answers?
You have to use a high efficiency woofer (or a full-range speaker) with a 8W/channel amplifier, the more "dB per watt" (dB/W) is the best, ideally over 92dB/W (92 decibels at 1 watt RMS and 1 meter distance) no matter what maximum power it can handle. All speakers with the same SPL (sound pressure level) will play as loud with the same power, big or small.

Woofer like that habitually have a rigid suspension and not a soft one for "air suspension", but you need a much bigger enclosure which can be a "bass-reflex". For a 8 watts amplifier, you should use a speaker that can handle at least twice this power, a 15 watts model or more will be OK !

Some musical instrument speakers have a SPL over 105dB/W, they play very loud even at just one watt of power ... The sound pressure raise about 3dB each time the power double, so if you have a 92 dB/W speaker, at a power of 8 watts, the SPL will be about 101 dB, it is loud an off to disturb the neighbours ...
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Old 23rd August 2011, 08:06 AM   #3
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I plan to use a high efficiency woofer. I plan to use the Infinity 162's as I said earlier.

Infinity - Home Audio

I understand the concept in terms of loudness, but explain to me why I would use a speaker rated that high when I see all the time in books and forums that you should have at least 1.5*RMS for the speaker in amp power to have enough headroom to avoid clipping?
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Old 23rd August 2011, 08:33 AM   #4
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you are confusing two issues - speaker power handling capability, and speaker efficiency.

You will need a high efficiency speaker - and one without too many dips and peaks in its impedance curves...
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Old 23rd August 2011, 08:41 AM   #5
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Infinity Primus P162 Main / Stereo Speaker - Product details and features - DealTime

This speaker is pre built and 90dB 2.83 V/1 M. The RMS power handling is 150 W. How can that work with an 8 W/channel amp as I have been told to match the RMS power of the amp with the RMS power handling rating of the speaker?
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Old 23rd August 2011, 08:49 AM   #6
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don't worry about the RMS power handling. You need EFFICIENT and as flat an impedance curve as you can get. 90db is not terribly efficient - not bad, just not great. You can bet it was measured at the absolutely most advantageous point in the speakers efficiency profile, so expect a true efficiency less than this - perhaps 88db...

Is there any impedance curve data?
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Old 23rd August 2011, 09:21 AM   #7
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SoundStage! Measurements - Infinity Primus P162 Loudspeakers (11/2008)

They don't appear to be 90 db but more around 88 db. Do you have any suggestions for a better set of speakers under $400?

Also I still don't understand how you can not worry about RMS power handling. I know loudness is due in some part to efficiency, but I still fail to see how underpowering the speaker is not an issue at all, especially given that some have lower bounds for power on the data sheet?

Unless of course the theory is that the speakers can produce enough sound if they are high efficiency that regardless of RMS power handling capabilities you won't need to set the volume so high that you send the amp into clipping?

Last edited by milsman2; 23rd August 2011 at 09:37 AM. Reason: I read something new.
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Old 23rd August 2011, 10:30 AM   #8
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'Underpowering' a speaker is only an issue if you drive your amp well into clipping so generating lots of harmonics which can blow a tweeter. A 100W speaker does not require 100W, but can safely handle 100W. It will work perfectly well on 100mW, but if low efficiency it might be quiet.
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Old 23rd August 2011, 05:49 PM   #9
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Fair enough. Do you have any suggestions as to some pre built bookshelf speakers with very high sensitivity that are maybe also affordable ish?
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Old 23rd August 2011, 06:50 PM   #10
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Hi milsman2,

You say you want to buy the K12G amp which uses 10GV8 tubes and run quite hard IIRC.

The review is of the K12M, a very similar amp by the same designer but uses the 11MS8 tubes.

Using different tubes can make an amp with an almost identical topology sound quite different so it may not be a good idea to judge the K12G from a review of the K12M.

Another version of this amp to consider is the K502, again almost identical except it uses the 11BM8 tube. This is an 11v heater version of the 6BM8 tube ( also known as the ECL82 ) and many manufacturers used it to good effect in their lower powered amps.

Brgds Bill
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