blakep82
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| posted on 22/11/08 at 12:00 AM |
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OT any one know how to work out the power output of an amplifier?
does anyone know? is there any kind of formula? P=IxV or anything? i have a feeling its not as easy as that....
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mediabloke
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| posted on 22/11/08 at 12:20 AM |
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It's been a while since my elec. eng. course, but IIRC:
Peak power (assuming no losses) is usually considered as:
P=V x I
But as the output is considered a sine wave, power is more accurately calculated as V x I x 0.707
(root mean square or RMS). Hence audio amp output being shown in W RMS.
HTH 
Francis
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blakep82
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| posted on 22/11/08 at 12:22 AM |
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cheers fella 
definitely helps, but how would i measure the voltage and current of the amps outputs?
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mediabloke
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| posted on 22/11/08 at 02:32 AM |
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Google threw up this page that might be most useful:
Clicky linky
If the amp has a part no., it'd probably be quicker & easier to Google it.
You'd either need to use an oscilloscope or meter with a "True RMS" range to measure the voltage across / current through a load
(e.g. with a speaker connected) to calculate it.
But bearing in mind your measurements would need to be carried out at max volume, it'd probably be best to choose some chewns that your
neighbours enjoy.
[Edited on 22/11/08 by mediabloke]
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blakep82
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| posted on 22/11/08 at 10:25 AM |
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no serial numbers on this amp, its from (i think) 70s, and doesn't even say who made it. i took it to a shop once coz i needed new speakers and
the guy was asking a lot of questions about the amp i didn't know the answers to so it took the amp in. he sort of grimaced said it was
particularly powerful...
i don't have an oscilloscope... don't know anyone who has, so i guess i'll just have to leave it, or get a new power amp and new
speakers 
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flak monkey
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| posted on 22/11/08 at 11:05 AM |
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Take the lid off and have a look for the power amp transistors (there will be 2 or 4 of them), they'll be connected to a big heat sink. Note
down the numbers off them and you should be able to find out what they are.
Stick a pic up, someone might recognise it. Lol
David
Sera
http://www.motosera.com
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blakep82
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| posted on 22/11/08 at 11:26 AM |
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here's the, er, 'beast'
 
Description
it does say 'eagle' on the front. googled it, but doesn't bring up
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MikeRJ
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| posted on 22/11/08 at 11:57 AM |
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Wow, I used to have that exact model many, many tears ago! It blew up in style unfortunately. I think it was around 30 watts RMS per channel, not
hugely powerful but the large speakers of the day tended to be a lot more efficient than modern bookshelf speakers.
You can get a rough idea of a power amps output by measuring the supply rails with the output loaded, and then subtracting a volt or two for the
losses in the output stage. RMS power = (V*V) / (8*R) where R is the impedance of the speaker, and V is the peak to peak voltage delivered to the
speaker (which will be a bit less than the supply rails).
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blakep82
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| posted on 22/11/08 at 02:18 PM |
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lol never expected to find anyone who had one the same
found it up in the loft many many years ago when i was a lad (well, about 7 years ago maybe) when i bought a pair of technics 1210s and started
mixing. to start with i just plugged my mixer into the scart socket on my 14" telly lol found that amp in the loft and started using it. its
just a little past its best though...
30W rms is near enough for me! near enough to know i want a better amp now 
sadly, after putting it to the back of my mind for years and years, i really REALLY want some Cerwin Vega speakers, they say they're comaptable
with low power amps, but i think 30w per channel amp, with 210w per speaker is pushing it just a little lol so, new amp it is!
ah, it probably won't buy any for a while anyway... its nice to dream
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MikeRJ
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| posted on 22/11/08 at 05:27 PM |
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You can pick up some nice amps for relatively little money these days, e.g. have a look for Arcam Alpha range on ebay.
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blakep82
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| posted on 22/11/08 at 06:07 PM |
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i've seen a 300w per channel (don't know if thats RMS though.. probbaly not. its probably peak) for £109 which i thought was quite
pleasant
http://www.htfr.com/more-info/MR131698
only one input, but then i've got the mixer to take care of that.
http://www.htfr.com/more-info/MR223844
mmmmm
quite like this too
http://www.htfr.com/more-info/MR249335 
and these for the decks
http://www.htfr.com/more-info/MR224598
lol, if only i had a spare £550 or whatever it all works out to!
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02GF74
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| posted on 22/11/08 at 10:14 PM |
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^^^ as you suspect it is not that easy. supply voltage to amp is a rough guide but that does not take into account distortion. It is the clipping of
the output by turning the volume know to 11 that kills speakers, so you are more likley to blow speakers with a low power amp whacked up to max than
with a more powerful amp.
that amp looks like it should be on the antiques road show! .
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blakep82
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| posted on 23/11/08 at 02:54 AM |
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aye, it gets distorted at about 1/3 of the way....
the amps def past it, don't know if i can justify spending all that money on a new amp and speakers just yet. i've spent a total of about
4 hours on the decks in the past year. really poor...
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Ben_Copeland
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| posted on 23/11/08 at 08:35 AM |
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Check out your local Richer Sounds and their website, they always seem to have good bargains on and are usually willing to knock money off for a sale
Ben
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Roadster
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flak monkey
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| posted on 23/11/08 at 12:53 PM |
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Roksan Kandy L3 is my amp of choice, not cheap at £600 though. They do sound stunning though.
http://www.roksan.co.uk/kan_stereoamplifier.html
Also recently upgraded an old Quad 405 power amp for a friend, and after running it in it sounds fantastic coupled to a Musical Fidelity pre amp.
Total cost was about £300.
Possibly the best way to buy Hi-fi is secondhand. If you stick to the higher end stuff (Arcam, Exposure, Roksan, Quad, Cyrus etc) you can't go
far wrong, and you will usually pay 1/3rd of new prices.
Remember power isnt everything, if you actually put a power meter on the amp, you will find that even with only 30W you will very rarely even get
close to pulling it. With the higher power you just have more in reserve for sudden dynamic changes in music. You'll find most of the hugely
expensive amps arent even 100W, more than enough for the lounge at home. Unless you are building a PA
David
Sera
http://www.motosera.com
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