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    Posted: 24 December 2018 at 11:11am
Just a question on the output devices on a amplifier

is it right thinking can you generally estimate the power of an amplifier by its out-put devices,?


I understand its not just that, also ie power-supply voltages rails and probley more than I know about

but generlay speaking , ie amp we don't know the wattage , for example amp has maybe 10/20/30 output devices, can we work out the power/wattage

thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2018 at 11:53am

More output transistors means greater load stability under low impedance loads.

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Elliot Thompson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2018 at 11:54am
As a rough guess,yes,also a lot depends on the devices themselves,as few good quality ones will out-perform many lesser ones...

If it's an old school amp,you can generally make an educated guess to the power from the size of the transformer.


Edited by jbl_man - 24 December 2018 at 3:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote efinque Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2018 at 1:53pm
I read on another thread a coarse rule of thumb is 1,4 x output power (I've seen a similar formulae in a guide/article on amplifiers)

For example a T.Amp E-800 has ~1kW output and 1,5kW power consumption.


Edited by efinque - 24 December 2018 at 4:10pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2018 at 3:32pm
Supply voltage determines maximum available power.  Output transistors wont't make any power but they withstand certain amount of current / power dissipation. More parallel connected output transistors = more current they can handle without break down, which is critical especially if driving low impedance load. 
Ofcourse, power supply must be capable for giving enough current too!

Many cheaper amps may give lots of power for 8ohm/ch loads but when 4ohm load is connected, power supply will sag and there is not much "extra" power (which in ideal world, would be doubled like 500W/8ohm -> 1000W/4ohm). 
Just like one of my amps, Carlsbro powerline 2000.  It has whopping 18 output devices / ch (15Amps each) but power supply is way under rated.  500W/8ohm and only 700W/4ohm. 
Tamp TA1400 has only 6 devices/ch but gives same output power.




 



Edited by Tonskulus - 24 December 2018 at 3:39pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RC1 Sound System Quote  Post ReplyReply Direct Link To This Post Posted: 24 December 2018 at 10:02pm
Best way to guess an analogue amp's power is to start by looking at the power supply.  Measuring the DC voltage rails with a multimeter will give you an idea of it's theoretical maximum peak voltage output, then you need to deduct a margin for sag under load.
 
Sinewave RMS wattage is calculated by squaring the peak voltage and then dividing by twice the load impedance.
So, if an amp had say voltage rails of plus and minus 100VDC, then theoretical maximum output into 8 ohms would be 100 x 100 / 16 = 625w RMS, or into 4 ohms 100 x 100 / 8 = 1,250w, or into 2 ohms 2,500w RMS.
 
However, an amp's power supply voltage will sag under load, plus there will be losses in output devices and emitter resistors that increase under load as well, plus there may be current limiters that protect the amp from low impedances, so the actual wattage will be less than these theoretical figures.  The extent to which this sag applies under load will be unique to each amplifier and depends on lots of factors.  It can be calculated with fairly basic electronics, but a gut reaction looking at how overbuilt the amp is would probably give some idea.  For a typical amplifier that is two ohm stable you may find the 625/1,250/2,500 actually sags down to more like 550/900/1,300 or so perhaps.
 
The total wattage rating of the output devices does not really give any accurate idea because the power that an amplifier can dump through the devices must be carefully in accordance with the device's safe operating area curves, not the advertised maximum power rating of the devices.  However, as Elliot wrote above as a general rule more devices will allow the amp to be more stable and reliable driving heavy loads.  Also, a big transformer and storage capacitor will help in that regard as well.
 
So, as per my comments above, I think you would get a better idea measuring the amp's voltage rails and then estimating how much sag it might suffer according to how overbuilt it is or isn't....
 
Oh, and as I'm not on here often MERRY CHRISTMAS EVERYONE!


Edited by RC1 Sound System - 24 December 2018 at 10:03pm
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