Establishing power needed for your speaker.

[WMcD]

From time to time, and recently, we get questions about how powerful an amp is needed for K-Horns, or any speaker.

Part of the problem is that most people don't have a means of determining what sort of power is going into their speakers on peaks. I suppose a peak reading meter system or oscilloscope would tell this but this is a tall order.

It would nice to know whether the present amp we have is being driven, or might be driven, to 1 watt, or 5 or 10, or even 0.1 watt on peaks.

I'm going to try the following at Christmas when I have time to fool around. Maybe others will want to experiment and we can compare notes.

Some test CDs have a zero dB track. This is a sine wave at typically 1000 Hz which recorded at the maximum level on any CD. Therefore it establishes the peak any program can deliver out of your CD player. Yeah, odd that this is called 0 dB.

It should be possible to use that to drive the output of your amp (and input to the speaker) to a reference level, adjusting the volume control. If you have an AC volt meter, you should adjust the volume control on the amp so the voltage delivered to the speaker is 2.8 volts rms (square root of 8) while playing the 0 dB track. That is a nominal 1 watt into 8 ohms. With a K-Horn, it should be very, very loud.

Back tracking a bit, you may know that power into the speaker is referenced as

Power = Volts*Volts / 8 ohms. (Technically a voltage sensitivity, but it is what is used so that variations in speaker impedance is not in the picture. We assume it is 8 ohms.)

So the required voltage = square root (power * 8)

So, for 1 watt, volts = square root (1 *8) = 2.82 volts

For 5 watts, volts = square root (5*8) = 6.3 volts

For 10 watts, volts = square root (10*8) = 8.9 volts

For 0.1 watts, volts = square root (0.1*8) = 0.89 volts

In any event, if you have an ac volt meter which reads rms out of the amp (rms is the usual) you can establish a reference knob position (or digits on some read outs) on the amp where the maximum CD level output is 1 watt, or whatever you choose from the above list. I'm gonna mark the knob pointer with a pencil mark.

Then you can play your favorite CD music at that knob setting. You'll know that the maximum music peak possible on CD is at whatever watts. The actual recorded max may not be up to that available max. I'd expect that pop recordings are crowding that max overall. On the other hand, classical orchestral recording reserve the peak for crecendos (sp?).

This technique may answer suspicions about power. E.g. I have a 10 watt amp. If things are adjusted for 5 watts max, and the output on CDs at that setting is more than adequate with the K-Horns, then I know there is little to be gained by going to a bigger amp. At least as far as power.

I'll let you know.

Gil

[Soundthought]

Seasons' greetings Gil,

Sounds like a fun experiment.

Count me in.

It would be interesting to actually see peak outputs at varying intervals with an established 1watt/1meter reference point.

Especially with some of the newer mixing formats available.

I believe this same experiment was discussed not too terribly long ago.

Although, i'm not quite certain.

I will try it with my RF7s and Fortes, and you can try it with your Forte IIs and Quartets.

I'll try to keep notation, unless of course, I get thoroughly enveloped in the experience.

It's been known to happen a time or two.

I think the technical term for it is "spacedicus outicus"

BTW. Where can I get a test disk with the 0 decibel track?

[sunnysal]

let us know what you find out. regards, tony