Power Requirements

[teej]

Hi all, I work with a busking trio - and we're recently questioning whether or not our generator is powerful enough for our gear. Can someone please explain to me how to work this out (I'm pretty numerate, but electricity is a bit of a closed book, and the other 2 are no better).

btw - our generator's a Honda Cycloconverter EX7 (700 watts max output)

and we use it to power
1x '78 Roland Cube 100w (guitar)
1x Yamaha Stagepas 500w (1x vocal, upright bass)

But the wattage used to describe the output of the amps is different to the wattage used to calculate their draw, correct? (f'rinstance the pa only draws 65w as I understand it).

Hopefully...

[bremen]

[quote name='teej' post='398979' date='Feb 3 2009, 02:36 PM']Hi all, I work with a busking trio - and we're recently questioning whether or not our generator is powerful enough for our gear. Can someone please explain to me how to work this out (I'm pretty numerate, but electricity is a bit of a closed book, and the other 2 are no better).

btw - our generator's a Honda Cycloconverter EX7 (700 watts max output)

and we use it to power
1x '78 Roland Cube 100w (guitar)
1x Yamaha Stagepas 500w (1x vocal, upright bass)

But the wattage used to describe the output of the amps is different to the wattage used to calculate their draw, correct? (f'rinstance the pa only draws 65w as I understand it).

Hopefully... [/quote]

Any amp will take more power from the wall (or generator) than it delivers into the speakers, but it won't take it all the time.

So though your PA is rated at 500W, if it's amplifying a sine wave at full power it's probably taking 800W from the mains. The vocal and upright bass aren't anything like a sine wave however, so with the amp clipping on peaks the average power going into the speaker is going to be more like 50W, ie 80W from the mains.

Approximately :-)

So your generator is probably OK, so long as you're not playing drum&bass. Presumably it's got an overload protection cutout? If it hasn't tripped out, it's OK.

[teej]

[quote name='bremen' post='398988' date='Feb 3 2009, 02:43 PM']Any amp will take more power from the wall (or generator) than it delivers into the speakers, but it won't take it all the time.

So though your PA is rated at 500W, if it's amplifying a sine wave at full power it's probably taking 800W from the mains. The vocal and upright bass aren't anything like a sine wave however, so with the amp clipping on peaks the average power going into the speaker is going to be more like 50W, ie 80W from the mains.

Approximately :-)

So your generator is probably OK, so long as you're not playing drum&bass. Presumably it's got an overload protection cutout? If it hasn't tripped out, it's OK.[/quote]

Thanks, bremen, that's certainly encouraging. We're nowhere near clipping, and we're playing blues from the '30s-'50s.

I'd still like to know if there's a formula I can use to work this thing out in future...

Edited February 3, 2009 by teej

[bremen]

[quote name='teej' post='399081' date='Feb 3 2009, 04:00 PM']Thanks, bremen, that's certainly encouraging. We're nowhere near clipping, and we're playing blues from the '30s-'50s.

I'd still like to know if there's a formula I can use to work this thing out in future...[/quote]

There are a lot of variables. Most amps are 70 to 80% efficient, Class D are more efficient. And the ratio between maximum and average power depends very much on the instrument and what style you're playing.

Rule of thumb: Fuse doesn't blow = happiness. Fuse blows = get a bigger generator!