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Amp Designs That Are Able To Drive 4/2 Ohms Load

I have noticed that some amps like the Krell Evo 302 will output the exact wattage when halving the impedances ie. 300W/600W/1200W into 8/4/2 ohms respectively. On the other hand, some amps like the Bryston 7B-SST does not output the exact wattage when halving the impedances ie. 600W/8 ohms to 900W/4 ohms or the Plinius SA-100 (100W/8 ohms to 180W/4 ohms)

Can someone enlighten as to where the 'missing' watts have gone for amps that do not output the exact 2X the wattage when going from an 8 ohm to 4 ohm load? What are the criterias in amp design that determine which route to adopt, and what are the pros and cons of each design?

Thanks in advance.
ryder
dave_b's avatar
dave_b
3,159 posts
 
Crappy designs can't deliver stable power below 4ohms...even the Krell 400xi can put out exquisite power and control down to 2ohms!!
cford
66 posts
 
Watts = voltage x current
Watts = voltage squared / resistance
Watts = current squared x resistance

The max voltage of a amp is limited to its internal DC supply voltage.
If the wattage does not double when the resistance is cut in half then the amp is not able to output the required current.
electroid
283 posts
 
When an amp can't double its power as the impedance halves, the power supply is to blame. I have a CJ MF-2500A and it doubles down to two ohms.

ET
starcon
98 posts
 
the critical factor is the power supply. There is a reason a big amp has a huge power supply. also number of output transistors and size of capacitors can be big factors. Great amps also tend to weigh more than lesser amps. (of course there are class d amp-IMO are a complete waste of space)
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