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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.
keesue

Showing 3 responses by bdp24

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bdp24
10,095 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')

bdp24's avatar
bdp24
10,095 posts
 
Roger Sanders makes two versions of his power amp, one for ESL's the other for magnetic-planars such as Maggies. It is very well regarded in the Magnepan community. Check the Planar Speaker Asylum for reviews.
bdp24's avatar
bdp24
10,095 posts
 
I don't know whom at Magnepan you talked to, but the 3.7 "6-8 ohms throughout the audible range"? Nope, it is 4 ohms over most that range, dropping to 3 ohms at some frequencies.

The original version of the Tympani T-I (which I owned in the 70's) was a nominal 8 ohm load, but Magnepan re-engineered later Tympanis to 4 ohms (my Tympani T-IVa are), and all subsequent Maggies have been as well.

Fortunately the load is---unlike ESL's---almost purely resistive. It's not a difficult load, but Maggie lack of sensitivity coupled with it's modulus of impedance (the fancy term ;-) makes it a power sponge. While tube amps do work with Maggies, high-power tube amps are costly to buy and own. Low sensitivity/impedance loudspeakers eat power tubes for breakfast, lunch, and dinner. Lovers of Maggies and tube amps solved that dilemma in .6 and earlier models (which have parallel cross-overs) by bi-amping---using a high power amp on the bass driver, a medium on the midrange drivers and tweeters. In the .7 models Magnepan switched to series cross-overs, which cannot be bi-amped without surgery on the speaker.

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