# Converting Watts to Volts

Discussion in 'Amps and Cabs [BG]' started by Petebass, Feb 20, 2004.

1. I remember a thread where someone (possibly Bob Lee) explained how to convert the output of an amp from Watts into Volts. I had a search but I can locate it.

Can someone run me through it again?

Alternatively, can someone tell me approximately how many watts converts to 300V? I'm guessing it would be a lot.

2. It depends upon the impedance of the speaker you're using. The formula is Voltage (V) equals the square root of the Power (P) multiplied by the Resistance (R).

So for a 4 ohm speaker, 300 watts times 4 ohms equals 1200, then take the square root of that to get 34.6. For an 8 ohm speaker, the voltage would be 300 watts times 8, which equals 2400, then take the square root of that to get 49 volts. For a 2 ohm speaker, 300 times 2 equals 600, square root of that is 24.5 volts.

The corollary to this: power equals voltage times current. Rearrange that formula: power divided by voltage equals current. So a 4 ohms speaker draws, at 300 watts, (300 divided 34.6) equals 8.7 amps. A 2 ohm speaker, at 300 watts, is drawing (300 divided 24.5) 12.2 amps. That's a lot of current, and that's why amps have trouble into 2 ohm loads.

3. Depends on impedance.

so watts=volts^2/ohms

and square root of (watts x ohms) = volts

swapping around, that means multiply the watts times impedance, and take teh square root of the result.

so your 300 V would be 5625 watts at 16 ohms for instance

it's probably been covered before, but so what.

4. Oops, I misread original post, I thought he wanted volts equalling 300 watts, not watts equivalent to 300 volts. Me bad. Formulas still apply, just adjust numbers accordingly.

5. That's exactly what I was after. Thanks.

So to get to 300V, it would be 22,500 watts with a 4 ohm speaker and roughly 11,500 watts for 8 ohm.

The reason I ask is because I'm being sneaky tonight. I hate doing sound and playing bass at the same time, especially if I don't get a chance to tune the foldback. I'll get 5 mins to do so tonight. I had an idea and it works on my small PA here at home, I just wanted to check that I wasn't going to do any damage to the club's bigger PA.

My Multimeter can measure frequency. I connected my multimeter to the speaker vis the parallel output, and deliberately induced feedback. The multimeter told me exactly which frequency was the culprit so I cut it at the EQ. I kept doing it until a bulk of the problem frequencies were gone, and the overall mic volume increased substantially without feedback.

Now that's all well ang good at 100w, but the amp at te club tonight is 1,100 watts per channel. My multimeter is good up to 300V so it looks like it should be OK. 1,100w, with either an 8 or 4 ohm speaker, of nowhere near 300v (94v and 66v respectively).

I hope this works. Can anyone see a reason why it won't?

6. Yes. Inducing feedback on a 2x1,100 watts system will be earsplitting. And if done so for more than a fraction of a second, tweeters, possibly midranges will blow.....

7. I agree. Which is why I i like to get it all out of the way before we start playing.

I agree. The accepted method for tuning foldback around here is to bring the volume up very gradually and very carefully until you notice the onset of feedback. As soon as you hear it coming on, cut the volume before the feedback gains momentum, edjust your EQ, then repeat the process until all problem frequencies are tamed.

I didn't get a chance to try out the multimeter idea. Partly because there was no time and partly because I didn't need it. Whoever set up the foldback last did a great job. 2k was too hot but I didn't need a multimeter to identify that one - it's a common problem frequency. Once I sorted that out we had very loud and very clean foldback all night. The singers were raving about it and trying to give me credit I didn't deserve.
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8. It nice to see everything went fine without any feedback (intentional as well as unintentional).