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Old 21st June 2007, 06:49 PM   #1
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Default A Tube amp for a Hostille Tube Enviornment

Hello Diy Audio community,

I have recently purchased, then repaired a pair of Magnaplanar SMGs. I love their sound, and their character, however, when it comes to listening to them through my EL34 SET, it just doesn't cut it...

I knew from the begining that I was going to have to build an amplifier for these speakers the moment I bought them. After repairing them I was certain that these speakers need a really nice amplifier to really sing.

I am exploring both a Tube, and a Solid State options, but I am more comfortable. In choosing an amplifier, I have to look at the characteristics of the speakers, which becomes a Real when you match these Speakers with tubes:

Magnaplanar SMG
Two-way speaker, crossover at 800hz
Frequency Response: Flat from 20hz-20khz
Impedance: Flat at 4 ohms from 0hz-40khz
Sensitivity: 88db

Now, none of these characteristics are tube friendly.... Which is what makes this such a challange.

I wanted to ask the Memebers of the DIY community if they know of any tube designs which would make these speaker sing. Any seggestions or Ideas?

-Moose
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Old 21st June 2007, 07:02 PM   #2
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I've owned most every type of Maggies there are and they all sounded great with a 25 to 50w push pull amp. You can rebuild one of the Dynaco amps or build something from scratch. There are many schematics available on this forum.
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Old 21st June 2007, 08:27 PM   #3
SY is offline SY  United States
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Something in the Dyna Mark III power class will work well. I had MG-1 and SMG running off some modified Dynas and it was a fine combination, if not exactly a powerhouse.
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Old 21st June 2007, 11:25 PM   #4
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Actually those specs aren't too tube-unfriendly except for the low-ish impedance. At least the Maggies had a reputation for rather constant impedance across the band, so that the higher output impedance of a tube amp wouldn't cause huge shifts in frequency response. I would argue that this is a case where you might prefer a traditional feedback tube amp, which will have a low enough output Z to drive these painlessly. In other words, an SET with a 3 ohm output Z, even a relatively powerful one, ain't gonna cut it. In the old days, the marriage of Maggies and ARC amps was famous - in fact for a while ARC used to distribute Maggies. SY's suggestion of a Mark III or something like it is a good start . You should probably look for at 50 watts to make these sing unless you sit really close and listen to harpsichords exclusively. The feedback loop should give you an output Z of 0.5 ohm or lower. 1 ohm might even work. There were lots of classic KT88/6550 designs that meet these criteria, with an EL34 design a possibility too. You can either refurbish an old amp, or build from scratch using some of the designs posted in threads here.
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Old 22nd June 2007, 12:45 AM   #5
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Most Maggies had an impedance some where around 4 ohms pretty consitantly across the band. So a push pull amp with a output Z around .5 ohms, less would be better, at 50w or so on 4 ohm taps worked very nicely. Even a big S.E.T. wouldn't be sound good becouse the output Z is to high as Brian indicated.
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Old 22nd June 2007, 01:58 AM   #6
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Moose,

A Harman/Kardon Citation II refurbished ala Jim McShane is an ideal mate for the "Maggies". 60 WPC continuous and about 120 WPC instantaneous provide the necessary "grunt". For a tube amp, the damping factor is extraordinary. The O/P trafos in the "Duece" are among the VERY best ever wound.

I'm lucky enough to own a "CitaShane". Mailed fist inside a velvet glove is a good description.
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Old 22nd June 2007, 05:07 PM   #7
kevinkr is offline kevinkr  United States
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I've owned MG1.4, and MG1.6QR as well as borrowed SMG and found all to be comparatively easy to drive with moderate power PP amplifiers.

I drove the MG1.4 with a heavily modified Citation II, and MC30s. I drove the MG1.6QR with the modified Citation II, and 0fdbk 300B pushpull triode amplifiers. I found the performance with the triode amplifiers to be quite good despite the relatively high source impedance which was over 1 ohm on the 4 ohm tap.

In my experience the maggie diaphragms are quite well damped all on their own and don't seem to need particularly high damping factors, what they do need is plenty of current.

The MC30s did an extremely good job driving the 1.4's, perhaps even a little better on the bottom end than the Citation II.

The collapse of my business forced the sale of the MG1.6QR, and my subsequent fascination with low power SETs pushed me in the direction of Onken style bass cabinets and JBL horn mids and tweeters. (All diy of course) I will never forget how bowled over I was with the bass quality when I first got the MG1.4s.
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Old 22nd June 2007, 05:30 PM   #8
bigwill is offline bigwill  United Kingdom
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Surely the flat input impedance would mean that output impedance would not matter much?
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Old 22nd June 2007, 05:47 PM   #9
kevinkr is offline kevinkr  United States
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A beefy tube like the 6550 or KT88 seems to be a good choice in an amplifier driving these. I did achieve very good results with the 300B in pushpull as well, but this a more expensive harder to implement solution, and frankly I suspect not quite the optimum choice for Maggies. (At least my design wasn't) By this I mean that the presentation seemed a little too "romantic" on the top end, although the bass quality was very good - all this to the best of my recollection as I sold the 1.6's back in 2000.
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Old 22nd June 2007, 06:02 PM   #10
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Quote:
Originally posted by Eli Duttman
Moose,

A Harman/Kardon Citation II refurbished ala Jim McShane is an ideal mate for the "Maggies". 60 WPC continuous and about 120 WPC instantaneous provide the necessary "grunt". For a tube amp, the damping factor is extraordinary. The O/P trafos in the "Duece" are among the VERY best ever wound.

I'm lucky enough to own a "CitaShane". Mailed fist inside a velvet glove is a good description.
Thanks for the kind words Eli!

I have a number of customers that use Maggies and "Citashanes". I own AR-9s myself, 87 db, 4 ohms, drops to 3.2 ohms worst case. They drive them just beautifully.

The upgrades to the amp significantly improve the bass (along with other areas). The feedback I get is that the "Citashane" amps will drive darn near anything. They have 4 ohm taps too. And peak power is in the 140 watt to 150 watt range, so there's a ton of reserve oomph!
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