Hi, madmike.
Let's get a few things settled:
A. In the formula you used, V=P/I, I is the current, not impedance. So dividing 45 by 8 is not correct.
B. A speaker's impedance is called that because resistance is not the proper term for a device that handles changing frequencies. Impedance is the measurement of "apparent" resistance to a changing signal. For speakers, their impedance is calculated while driven by a 1KHz signal. Since audio largely falls in the range of 20Hz to 20KHz, the impedance is different throughout the range. Since a speaker is largely a long thin wire wound around a form of some kind, it is basically a short circuit, so avoid DC across the speaker.
C. A DMM set to ammeter function is pretty much a dead short. If you put this ACROSS an amplifier output you will either blow a fuse (in the meter or amp) or blow the output stage.

Using a single LED per channel isn't very satisfactory for tracking audio output. An LED operates in a very limited voltage and current range. It's not good at showing power output throughout the entire power range. That's why most audio power meters you see on stereo amps use multiple LEDs.

Using a bulb is probably the better idea. However, they're tricky in that a cold filament is a dead short. After you apply power and let it heat up this resistance goes to a safe resistance range, though it's different from bulb to bulb. You'll have to guard against that. Perhaps you need to apply a small "startup" current to preheat the filament while isolating the amp from it. Or, better yet, put a power transistor to drive the bulb. This largely isolates the amp's output from the bulb, but not entirely.

Hope that helped.
kenjj

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All my pencils *used* to have erasers!

 

Thanks for your reply,
A) oops!! silly mistake
B) You learn something new every day
C) I was lucky

What about using an op-amp to the bulbs?? would this be safe for the amp?

Are there any circuits I could use where the bulb is not directly connected to the amp? (like a relay circuit or potential divider?)

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I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

 

A power transistor makes a pretty good light bulb driver, without mechanical parts that wear out in a relay.

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Uncle $crooge

 

Thanks could you possibly advise me on what type I require??

and is it used like a normal transistor where the input goes to the base, the device (bulb in this case) goes from the +Ve to collector and the emiitor to 0v??

and I presume I would need one for each output?

Would the attatched circuit work?

Attached Images

power transistor.PNG (8.5 KB, 7 views)

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I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

 

The DC output of each output wire is about 6.5V. So your transistor would be turning on the light bulb all the time, and the bulb will flicker at the audio frequencies which is too fast to see any variation.

You would need to use an opamp to shift the DC level and drive the power transistor.

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Uncle $crooge

 

what voltage should I set the op-amp at to get the best response?

nevermind, i will use a variable resistor

so that is all I need then, op amp> power transistor> bulb

would the circuit in my previous post work by using an op-amp before the transistor?

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I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

 

I have decided to completely change what I was going to do.... I am going to mount 2 or 4 speakers inside the case and i have just baught a dual 4" blue cold cathode with sound module so I will use that and put the mic near one of the speakers

This way is cheaper, easier and there are less things to go wrong

Possibly a few minor glitches but apart from that, it should be ok

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I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike