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Tube Amps, Magnapans and Impedance

I have tubed monoblocks running in class A with the output transformers rated at 8 ohms. They are driving Magnapan 3.7s which are rated at 4 ohms. Before I bought them I called Magnapan to get the frequency curve for their speakers. The engineer advised me that they are nominally rated at 4 ohms, the lowest load they present at their lowest frequency, but the frequency curve was essentially flat at 6-8 ohms throughout the audible range. The curve he faxed was satisfactory and I have been running them this way since new.

The reason for this thread is: A friend brought over his friend for a visit and listen. When the subject of tubes and planars arose and the impedance ’mismatch’ was discussed, he stated I was missing out by not having a 4 ohm amp. I tried to alleviate his concern with the above to no avail. I stipulated that it is good electrical practice to match the load to the amp but in my case a 6-8 ohm load across the band with tubed monoblock running in class A at 8 ohms was not significantly audible if at all to justify altering the transformers. I just didn’t bother after listening.

My question to my fellow audiophiles, particularly those who have tubed amps running at 8 ohms into loads between 6-8 ohms - Magnapans being the best - what are your thoughts on the subject?

In my experience it would take a speaker with a 4 ohm rating in the midrange where the music lives to really affect the performance with an anemic amp. Anemic bass and washed out mids are sure signs of an amp struggling with its load - which I do not have. My Magnapans sing from the very bottom of their range to the top. The human voice is especially seductive.

I think he had a case of ’wish I had a system like this’ blues. I think there is a term for that. Thoughts are appreciated.
128x128keesue
keesue's avatar
keesue OP
42 posts
 
Retreived my archived file and refreshed my memory on the criteria I used to build the system. It appears my recollection was consistent with the exception of the conversation with Magnapan. They did say it would not be a problem if I wasn’t trying to fill a large room with high volumne but they did not provide the impedance curve I cited.

My notes also reminded me I consulted several knowledgeable people, including Dennis, who further went on to say it was recommended to run the Magnapans on 8 ohm output transformers as the measurements were better, less distortion and more pleasant to the ear. This is consistent with you fellas with Magnapans and 8 ohm output transformers who said the same thing. This was the determining factor in not sending the blocks back for 4 ohm transformers.

Phew! Glad that is settled in my head. My ears always said, "I told you so!" I was dreading admitting my friend’s friend was right. Ugh! (But I would have begrudgingly).

Zero Autoformers are out. Now, back to my study for some jazz. Thanks again for the great repartee! Reminds me of days when, with only one snarky comment in the entire thread! An Audiogon record!

Best to all...

P.S.:  If I do get my hands on a pair of autoformers I can audition and return, I might experiment.  That's the Audiophile in me and all of us here on Audiogon, eh?  HA!

gdnrbob
4,460 posts
 
@OP, 
Too bad you didn't post a few months ago. I recently sold a pair of Speltz  autoformers. 
There is another Audiogon member you should PM- Atmasphere.
He is well acquainted with autoformers an should be able to give you some relevant advice, as he designs tube amps/preamps.
B
richopp
760 posts
 
@OP  Nice to read your reaction to Maggies.  I have been posting for years, based on my shop-owning experiences, that they were the ones to buy if accuracy of reproduction was the goal.

No, not every room will work with them, and you do need some quality hardware--I always recommend Audio Research--but whatever sounds best to you in YOUR ROOM is fine.

Happy listening!
keesue's avatar
keesue OP
42 posts
 
@gdnrbob
Many thanks.  I'll ping him.  I have no experience with them but they are intriguing.
@richopp525

For acoustic music, Maggies are incredible.  I always tell the story of how I went to a high-end shop in Palo Alto, CA., with a colleague.  This was 1978 and I was a school teacher.  I heard a sax playing in the back of the store and wandered back for a listen.  I saw instruments hanging on the wall but no sax player.  Turned out it was Magnaplanars powered by Audio Research (with solid silver cabling) from a vinyl recording of Sonny Rollins.  I was completely stunned. I stood there and the owner came in and explained the full set up.  He was gracious enough to play serveral jazz cuts and I was in nirvana.   In no way could I even afford the cartridge!

I made it my mission to own a pair.  I finally did so in 1996 or thereabouts.  I couldn't swing the Audio Research but the custom built Cary pre and amps have served them well over the years. Peter Dahl transformers and Class A all day long.  :-)  Oh, and they will boogie too!  Granted, there is nothing like a cone for moving air for rock and roll and all the derivatives, but they can hold their own.  I am more into timbre, tone and that enveloping natural sound they provide.  Sublime...  They are not for everyone though for sure.

 

 

bdp24's avatar
bdp24
9,981 posts
 

For those who like the Maggie/tube amp combination but can’t or don’t want to spring for a high-powered amp, there is an alternative. Is everyone aware of the fact that Maggies are not the only planar-magnetic loudspeakers on the market?

Bruce Thigpen of Eminent Technology---already well-known for his linear-tracking air-bearing tonearm---found much to like in the Magnepan design, but saw a way to improve upon it: create a p-m driver that operated in push-pull fashion. All the early Magneplanar drivers were single-ended---magnets on only one side of the mylar diaphragm (if you don’t understand why that is a design flaw and weakness, a little research will reward you with the answer). Thigpen created a push-pull m-p driver over forty years ago, patented it, and uses it today unchanged in his LFT-8b loudspeaker. While he was at it, he made the driver an 11 ohm load, much more appropriate for tube amps than are the 4 ohm (and down) Maggies.

Used with a single stereo power amp, the LFT-8b’s p-m midrange driver and ribbon tweeter are combined with the 8" sealed-enclosure dynamic woofer (for 180Hz down), resulting in an 8 ohm load. But the LFT-8b is fitted with two sets of binding posts: one for the woofer, one for the planar panels, so bi-amping the LFT-8b is a breeze and allows one to use a ss amp on the woofer, a tube on the panels.

Magnepan eventually moved to push-pull operation in some of the drivers in their higher-priced models, but retained the 4 ohm impedance. There are some LFT-8b owners who bought them as replacements for their MG3.7’s, overall preferring the ET. $2499/pr shipped or from an ET dealer.

I myself am a long-time Maggie lover, having bought my first pair---the Tympani T-I---in 1972, and am a current owner of a pair of Tympani T-IVa, the ultimate realization of the 3-panel Tympani design (assuming you ignore the MG30 ;-). I very much like the ET LFT-8b, and find it to work quite well with a modestly-powered (100w/ch)---though unusually-well designed and built---tube amp in a moderately-sized room (14.25' x 22')

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