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Output power for a power amplifier
Output power for a power amplifier
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Old 9th September 2006, 02:46 PM   #31
EWorkshop1708 is offline EWorkshop1708  United States
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Location: Florida
Lets try a new equation......different numbers More watts.
4 ohm Speaker.

How much power output at 4 ohms, with power supply rails that are +/- 70V unloaded, but may drop to 54V under full load. Load goes to ground rail, so it is not bridged.

Tell me if this is right?!?!?

54V rail minimum.
-4V because of voltage drop of amp at clip.
50V peak output
*.707 to get RMS
35.35V RMS output

35.35 squared = 1249.6225 / 4 ohms = 312.4WRMS or 624.8W Peak

Is this accurate?
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Old 9th September 2006, 03:34 PM   #32
MikeB is offline MikeB  Germany
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Yes, that's correct.

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Old 10th September 2006, 09:33 AM   #33
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Hi Prog,
a properly specified 8ohm speaker does not match this description
speakers go down to 4ohm and below in different frequency areas
If it goes this low it should be specified as a 4 to 8ohm speaker or at worst a 6ohm speaker.

It is generally accepted that the minimum impedance of a speaker (anywhere in the frequency range) never falls below about three quarters to two thirds of the nominal impedance. Almost no-one builds to the DIN standard.

Speakers are a difficult load and they stress the output stages of amps particularly if the sinks are allowed to get hot. Wrongly specifying the speaker impedance can only exacerbate the problem.
But if retail amplfiers were designed to work well, rather than to a barely acceptable budget then the whole industry would have a better reputation, resulting in an ability to drive even difficult loads whin specification. Never EXPECT an amp to perform well when outside it's specified limitations, although some are very capable.
regards Andrew T.
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Old 10th September 2006, 09:44 AM   #34
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Hi Ew,
I have an amp as bad (no, slightly better) than your example.
It is a very poorly specified example with a complete imbalance between the various stages of the amplifier. Yet it sounds quite nice into a speaker load about twice the specified minimum.

I would expect a well balanced design, if properly implemented, to achieve a peak voltage output into the specified load to be MUCH better than 20V less than the quiescent rail voltage. Mine is +-69Vrail falling to 55Vpk (into 8r) and to 49V (into 4r)and I think that 14V/20V loss is terrible.
The pair of transformers (per monoblock) are VA underspecified by a factor of about 1.5 to 2.

But your number crunching is correct, just aim higher.
regards Andrew T.
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