Low-impedance speakers with transformer-coupled amps?

Boomzilla -- Sat, 10/17/2009 - 12:34

 I've read that low-impedance loudspeakers (typically defined as those with a nominal impedance of 4-ohms or less) are not well served by amplifiers that have output transformers.  The reasons that have been given include:
1.  Poor current transfer due to the transformers (low-impedance speakers draw more current) and
2.  Poor damping factor due to the high output impedance of the transformers relative to the low loudspeaker impedance
Despite these issues, my dealer says that my McIntosh amplifier (mine uses the McIntosh "autoformer" output transformer) will sound spectacular with my Thiel 4-ohm loudspeakers.  I haven't been able to test this claim yet (my MC352 is in the shop getting shipping damage repaired).
So my question is:  All other things being equal (yeah - I know they never are, but this is a thought experiment!), will an amplifier without audio output transformers perform better with low impedance speakers than one with audio output transformers?  If there are, in fact, real world issues that negate all or part of the above theories, please enlighten me.  
Of course, once my amplifier comes back from the shop, I'll listen for myself.  If the sound isn't satisfactory, my "plan B" is to buy an (output transformerless) integrated amplifier.  Also, if anyone has personal experience with low-impedance speakers and McIntosh autoformer amps, I'd LOVE to hear of your experiences.  Thanks!  Boomzilla

Robert Harley -- Mon, 10/19/2009 - 09:23

I don't have any experience with McIntosh amplifiers with their different approach to output transformers, but your generalization about tube amplifiers and low-impedance loudspeakers is generally correct. There are, however, cases of tube amplifiers sounding wonderful with current-hungry loudspeakers (Magneplanars, for example). The only way to know is to try the combination.

Carlos Souza -- Sun, 11/15/2009 - 01:31

 Boomzila, I see that you have a Mac Mc352, a solid state amp with autoformers.
What you have read about tube amplifiers with output transformers does not apply to Mac SS amps with autoformers, it is a totally different ball-game. Your dealer is correct, Mac autoformers actualy help better match the output stage to the load, the amp is able to deliver more current into low impedance speakers.
I used to own an Mc402 (SS, 400W/channel, autoformer) and drove Gallo Reference 3.1 speakers with it.
For practical / matching purposes the Gallo 3.1 is a 4 ohm speaker through most of the bass and midrange and is a current-hungry speaker. My MC402 grabbed the Gallos and shook them by the tail, when connected to the 4 ohm tap. The downside (or the upside, if you like a more "forgiving" sound) is a slightly smooth / velvety sound quality, the famous McIntosh signature sound.
With your Thiel speakers (I own a pair of 3.5s), you may want to try the 2 ohm tap, it works miracles, because most Thiel speakers dive to 2 ohms in the bass. Also, the McIntosh smooth sound is very synergistic with Thiel speakers.
I hope this helps

Boomzilla -- Tue, 11/17/2009 - 16:25

 Thank you VERY much for your reply, Carlos.  I hope to have my McIntosh in the system by this weekend & will post again after a listen.

 A good sense of humor makes it ALL sound better!

Mark Adams (not verified) -- Tue, 11/17/2009 - 20:43

I recently purchased a pair of Magnepan MG-12s, and took them home and plugged them into my also newly acquired Pioneer 1019 A/V receiver. The sound is not what I heard out of the Maggies at the dealer. I am wondering if I have the wrong receiver and would like some advice on a replacement. The Pio does not have a pre-amp output (except for a sub), so I cannot utilize an amp with it. I am new at pursuing good home audio, and would appreciate any advice. Thanks, Mark 

Carlos Souza -- Tue, 11/17/2009 - 21:22

Mark, the Magnepan MG12 is a 4 ohm speaker. Maggie speakers thrive on power and current (read tghe FAQ section at the Magnepan site), 100W at 4 ohms is the minimum, most owners use 200W amplifiers.
Your receiver has the following specs:

Continuous average power output of 90 
watts* per channel, min., at 8 ohms, 
Front (stereo) . . . . . . . . . . . . . . . .90 W + 90 W 
Power output (1 kHz, 6 Ω, 0.05 %, 1 ch driven)  120 W per channel 
Guaranteed speaker impedance: 16 Ω to 8 Ω, 
less than 8 Ω to 6 Ω (setting required)**

**On page 36 of the owner's manual, Pioneer provides a setiing to drive 6 ohm speakers, however, there is no setting compatible with 4 ohm speakers.
This inability to drive low impedances is a pretty common problem of A/V receivers, the designers must cram lots of circuitry in a manageable chassis, so some money is saved on the power supply and power transistors.
If you still have the time, I suggest that you return the receiver and look for one with at least 100-150W into 4 ohms, not 6 ohms.
Good luck

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