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

Hi All,

I ordered a set of M80s last week, after weeks of research and pondering to replace a set of JBL Northridge E100s in my 2-channel setup.

Here's my question:
I use a pair of NAD amps (C370 and C270) in BRIDGED MODE to drive my speakers (300 watt/ch). However, the nature of bridged mode halves the impedance that each amp 'sees', and with the M80, that becomes a very hungry 2-ohm load. My real question is, are the M80 truly 4 ohm loads, and has anyone else done this? The NAD manual recommends against it, saying that the thermal switches might trip at high listening levels.

I'm not sure what to do when the M80s arrive. Super-Don in tech support is asking Ian and Alan, but I wanted to see if the user community has any real-world experience. I can't be the first person to run into this...

Regards to all,

Clay - Minneapolis

Hey Clay,

Can't provide any input on bridging amps. All I can say is my Odyssey monoblocks drive my 80's to sonic bliss. In fact, the 80's are very efficient and easy to drive, most AVR's by themselves will impress.

No speaker is just ONE ohm rating across the frequency range, it varies. The 80's do not dip below 4 ohms across the freq range, I can tell you that much.

I know that EITHER one of the amps by itself will be perfectly fine with the 4-ohm load. NADs are capable of delivering huge amounts of current. Its the nature on bridging that creates the problem.

I'll start out by un-bridging the amps, using the 370 as pre-amp only, and drive the M80s with the 270 only. 'Only' in this case a 120w/ch, so I should be OK.

If anyone has any experience with bridging, I'd be interested to hear it. Alan has chimed in and recommended following NADs advice, and not to bridge due to the 4-ohm load.

In any case I'm eager to get the speakers and have a go. I ordered them on Thursday and got the shipping email on Friday, and am waiting for them to show up on the DHL tracking site! I assume Axiom uses a freight forwarder in Canada to get them to the USA, where DHL takes over. That would explain the two day delay in the tracking numbers showing up. When I get other international shipments from DHL, I can track them all the way in.

PS - Odyssey Monoblocks, eh?? nice!

For the States, the speakers would go directly to Fed Ex, then down or Purolator to DHL then down to the states.

Thanks Jason. My email said DHL so I assume they will be bringing them to the door. Just not sure how much advance warning I'll get re the expected delivery date; I travel often and would like to plan my travel around that day!

If they don't get held up too long in customs I would bet you get them in 2-4 days.

Clay, welcome. You shouldn't need to bridge your NADs to have plenty of power for the M80s, which are slightly above average in sensitivity and are by no means "power hungry", to use the term that's sometimes used without any thought behind it.

Incidentally, on bridging, although the language about the amplifier "seeing" an impedance cut is half is seen fairly often, it's technically incorrect. The windings in the speaker drivers establish the impedance of speakers and can't of course be changed by the amplifier. What's actually happening is that when bridged the amplifier is theoretically delivering twice the voltage into the speaker. Due to to Ohm's Law(I=E/R), current(I)is proportional to voltage(E)and when the voltage is doubled the current is also doubled, as it would also be if the impedance(R)was cut in half.

Having had a C370 powering my M80's at one time I can assure you that it will drive them well enough on its own. If you want even MORE power instead of bridging the amps and using them as mono blocks you can biamp instead using say the C370 for the mids and tweeter drivers and the C270 on the woofers. Thanks to the second preamp output on the C370 you can do this. The amplifier sections are identical in both of those amplifiers as I am sure you are well aware.

Haoleb,
Thanks, that's a great point. I guess that an external crossover before the amps would be ideal in this case, but is actually not needed, I could remove the bi-jumpers and send the full spectrum signal to each set of drivers, and the let the internal networks do the work. Great tip!

I'm a big believer in the having power reserves to accurately deliver transients; its what makes music 'real' to me..

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.