Axiom Home Page Forums General Discussion Technical Questions M80v2 with NAD bridged amps

John,

Thanks for your detailed input. Maybe you can help me understand where my thinking is off; my understanding is:

A bridge tied load (BTL) amplifier applies a normal signal to one terminal of the speaker, and an inverted signal to the other. If a single channel of the amp is capable of producing 20V RMS across the terminals of the M80, this equates to P = V2 / R, so in this case, 20^2 / 4 = 100W.

When connected in BTL, the M80 "sees" 20V at one terminal, and an inverted 20V signal on the other - a total of 40V RMS Using the same formula, 40^2 / 4 = 400W - four times the power. But, each channel of the amp now sees only half the load impedance (think of an imaginary center tap in the voice coil, connected to ground). The amplifier must be stable into 2 ohms, or bad things happen..

regards,
clay

Yes, I forgot to mention that in this type of setup you must remove the gold strap jumpers on the speaker terminals. Or else you would probably be shopping for new amps. There is no need for an external crossover. Its essentially the same as the bridged configuration only... different.

When I had the NAD I never thought that the amp seemed strained while driving the M80's at loud levels but once I had gotten my monoblocks (also Odyssey's) It was clear that it sounded stressed on certain passages where the mono's didnt at all, So I definetly know what you mean about having the extra power.

Brandon,
I will not forget the straps! ;^)

I ran my C370 solo before I got the C270, and at elevated listening levels you can hear the C370 strain to keep up. One of my favorite Electronica CDs is Faithless's Outrospective; adding the C270 to the mix gave the music the 'slam' that you hear when you see them live.

Very nice web site, btw.

clay

Clay, the point is just that the speaker(including its impedance, since nothing analogous to that imaginary center tap is happening)doesn't change; it's the amplifier that changes. Of course if the added maximum power capability present when bridging is actually used(unlikely), then more current is necessary for the increased power, but it's because of the increased voltage from the amplifier, not any decrease in effective "seen" impedance in the speaker. If the use of the "sees" terminology is necessary(shouldn't be), then maybe it should be said that the speaker sees twice the voltage.

I agree with you here! You're right, the issue with speaker impedance is ALWAY about the ability of the AMP to deliver the voltage.