Auxinput 7-1-2018
If you look at Parasound A21 specs:
250 watts x 2 into 8 ohm
400 watts x 2 into 4 ohm
750 watts x 1 into 8 ohm (bridged)
If each channel of the amp only saw half the load in bridged mode, it would want to push 400 watts, not 750 watts.
Hi Auxinput,
My previous response, while accurate, did not specifically address this comment.
As I’m sure you realize, for a resistive load P = E x I, where P is power, E is voltage, and I is current.
The increase in the maximum power rating of solid state amps which occurs when load impedance is reduced from 8 ohms to 4 ohms reflects the fact that nearly all solid state amps act as voltage sources. Meaning that they will maintain an essentially constant output voltage as load impedance varies, as long as they are not asked to exceed their maximum voltage, current, power, and thermal capabilities.
So the increase from 250 watts to 400 watts that you referred to reflects I (current) increasing substantially as a result of R decreasing, ***but reflects E remaining the same.*** And as I’m sure you realize, some amps can provide up to 2x as much power into 4 ohms as into 8 ohms, reflecting a 2x increase in current due to R being cut in half while E remains constant.
In bridged mode, on the other hand, E across the load and I through the load ***BOTH*** double (compared to stereo mode, and provided that the amp can supply the required current), while R remains the same. Therefore, since P = E x I, ideally P can increase up to 4x, as I said in my previous post.
Best regards,
-- Al
