Volt / Ampere on speaker

[sbirkenstock]

Hi everybody,

I´d like to build a speaker switcher with relays.
Can somebody help me with the math?
If I got a 100W amp, what are the voltages and the amperes going through the speaker?


Best regards,

Stephan

[tubeswell]

What type of amp?

[martin manning]

If the amp's rated power is 100W, that is most likely assuming a sine wave into a resistive load. A Speaker is not a pure resistance, but let's say you are running into an 8 ohm speaker. Power = V*I, or I^2*R, where R is the resistance.

For the current, 100 = I^2 * 8, or SQRT(100/8) = 3.35 A

For the Voltage, 100 = V*I, or 100/3.35 = 29.85 V

These are RMS values, so multiply by 1.414 to get the peak values 4.74 A and 42.21 V. If the amp can be driven to a square wave output, the voltage and current will be at approximately these values. You won't be switching speakers while playing, so the relay contacts should be rated for something like 5A continuous current.

see correction below

[Stevem]

You do not want to hot switch a tube amp unless you have like a 200 ohm 20 watt emergency load resistor built in, other wise the only time it's safe to do is at idle.

[sbirkenstock]

Martin,
thank you for the reply.
My result based on your calculation is slightly different.
I calculate 3,54 Ampere, which results in 20V.
Can´t figure out where the difference comes from.
Still the same relay

I heard several times, that some amps have a lot more power than the value given by the manufacturer.
To "double check" I did the following calculation:
Power = U^2 / R
For a 2 * 6L6 amp I built I got these values:
460 Volt on the plates into 4200 Ohm of the output transformer.
Equals 50,38 Watt.
Can I calculate the max power this way (for the purpose of calculating the relay at least)?

In a two speaker configuration I was thinking to switch from speaker A to speaker B, by first switch speaker B on (both parallel) and then speaker A off to always have a load.
That could avoid the extra relay to switch off the signal.


thank you guys very much,
Stephan

[martin manning]

Must have been too early for me, or the buttons on my phone calculator are too small. Sqrt(100/8)=3.54 A, 100/3.54=28.28 V. Then the peak values would be 5.0 A and 40.0 V. Your power calc for the OT primary is fine, and a little conservative since you really can't drive the plate voltage to zero, and there will be some losses across the output transformer.

Yes putting the two speakers in parallel then disconnecting the unwanted one should work. I would also mute the input signal during the switchover.