Stereo, I am happy for you that you are achieving a better sound. Mac has increased the ability of its amps to deliver current so that with a modern Mac amp you can get away with an impedance mismatch. I think the key for you is that you amp is only warm. Rules are made to be broken. But if you used a larger Mac amp at the correct impedance you would achieve a similar or better result. The impedance mismatch mainly affects the bass and this affects the perception of the overall sound. I don't recommend it.

Best

Charles

p.s. When you ask Mac, that's their stock answer.

Greg, I am happy for you that the impedance mismatch is working for you. I assume you are using the 8 ohm tap. The Blade has a nominal impedance of 4 ohm and a minimal of 3.2 so there's not much stress on the 1.25, about the same amount as with my XVX running off the 4 ohm tap with minimum impedance of 1.6 ohm.

You went from an Alexia V2 to a Blade. That's a big step down in price but apparently to you not in sound quality. Did you run your Alexia off the 8 ohm tap?

Best

Charles

I run Blades on 2 MC611 using the 4 Ohm tap.

The manual is quite clear on that "If the Loudspeaker’s impedance is in-between the available connections, use the nearest lower impedance connection." and "For the best performance and safety it is important to always match the impedance of the Loudspeaker to the Power Amplifier connections.".

I agree. However, for a 6 ohm nominal impedance modern Mac amps have plenty of current to run off the 8 ohm tap. You can't go wrong following the OM. The multiple taps allow you to have a non stressed cool running amp. Match the nominal impedance to the nearest tap or follow my previously stated rule and you will never be incorrect.:yes:

Best

Charles

I was under the impression that the output transformer protected the output device (SS or tube) by providing a constant load regardless of the output tap used.

Mike S.

I think load is related to power or current. When the current becomes excessive for the transistors, Mac has a Sentry Monitor circuit. In a short circuit the impedance approaches zero requiring the amp to produce tremendous current. A speaker like a Thiel CS5i requires an amp like a Dag that is built off a 2 ohm tap so that it can efficiently (without stress) supply the current and watts. In my opinion, Mac amps should not be used with speakers with a nominal impedance below 3 ohms. Mac amps have tremendous advantages in that when the impedance drops below the nominal impedance they can sound very dynamic and have more bass. What creating a deliberate impedance mismatch does is carry a good thing to an extreme.:yes:

Best

Charles

As much as I respect Charles and his opinion, I still believe that McIntosh gave me the correct answer that to use whatever tap sounds best and to not overthink it. Shouldn’t the people that build the amps know best?

McIntosh amp taps vs current
 

It does. Every active amplifying device, tube or transistor, has a safe operating area, or SOA. The McIntosh autoformers allow the output transistors to always run inside their safe operating area by matching the speaker load to the amplifier.

Also, every amplifying device has an optimum linear operation point, allowing for the lowest distortion with the least amount of negative feedback and generating the least amount of heat.

Autoformers allow the output devices to run in their most linear range.

It's interesting to read some on the technical side of this issue, but I have had similar situations with Klipsch speakers (impedance varies with frequency, from just below 4 ohm on the lows to 12ish on the highs). I wondered and asked this question years ago, and got an answer one a Mac related forum that either 4 or 8 was OK, whatever sounded best, but it was the 8 ohm taps that sounded "proper". "Balanced". The 4 ohm was bloated in comparison - this on properly operating MC30 and MC250. It seemed pretty obvious that the 4 ohm was definitely "off". in terms of sonics.

Look at a MC 7300 owners manual. In the back they show how a 7300 can drive a 2 ohm load from the 8 ohm tap putting out more than 800 watts a channel. If you will look at a Stereo phile review of a MC 501 you will see how well a MC 501 drives a 4 ohm load from the 8 ohm tap putting out close to 800 watts still meeting specs. So I would not worry about you driving a low impedance from the 8 ohm tap of your 600 watt Mac amp. One of the British magazines tested a 611 and put out close to 960 watt peaks across 8 ohms. So there is plenty of current there so you don't have to worry. Now crossovers can compromise woofer capabilities and this may be why you are wanting more from your Mac amps. All I can say make the speaker lines large and short as possible and make sure you have 20 amp 120 volt AC circuit feeding each amp separately.

All speakers have dynamic limits and after reaching that level the speakers compress the sound not matter how much power you have available.