Magnepan 20.7 midrange resistor experiments.

[Beet Farmer]

The last post about resistors was a number of years ago. For 3.7 Maggies.
I thought I might write about my experiments with the midrange resistors for Magnepan 20.7 in particular. I have been playing with them for a number of months.
The midrange resistor mainly changes the TONAL BALANCE of the mids. No resistor, brighter, adding resistance warms up the midrange sound, plus bringing it closer to a seamless connection to the bass panel.
I tries and immediately discarded the stock ceramic one ohm resistor. It is terrible.
I bought a variety of Mundorf 10 watt wirewound. From 0.33 / 0.68 / 1.0 / 1.5 / 2.2 ohm pairs, and some quads, so I could used parallel and other resistances.
I felt with the Mundorf parallel to get 0.75 was good to start with, and also tried the 1.5 ohm alone. The 1.5 was clearly 'too much' leaving the sound rather muddy and dull. With other changes, particularly adding a lot of Furutech AC duplex, the balance seems to favor back around the one ohm level.

After awhile I decided to 'go for it' and bought some Duelund CAST.
I placed a 5watt Duelund CAST 0.75 on the tweeters, along with a home made version of the "Al Sekula choke tweak"* (a choke) on a dowel with 30 turns of Kimber wire. This has been the tweeter mainstay since. I really have no complaint using it. (and actually place it inside soldered in place, I feel that confident it is right)
The midrange I have varied from below one ohm (0.90), to 1.05 ohm (pairing a 2.0 and 2.2 CAST) , then 1.1 ohm (using two 2.2 ohm Duelund CAST.

One sad discovery is one cannot safely 'hang' Duelund resistors by the resistor wire from the Magnepan 20.7 midrange holes. The leads being highly angled off the long resistor body, and stressed, may break off right where the wire enters the resistor, rendering the resistor useless and not repairable (the very fine pure silver wire of the Duelund is easily work hardened, and soon will just seem to let go) . So I made a little 'carrier' of a dowel and some wire so the weight of the resistors wrapped onto and carried by the dowel, and by some 18g wire held on the dowel. This is particularly helpful with multiple resistors.

The sound improvement using the Duelund resistors is a real upgrade in clarity. Finding the exact right impedance.. Hovering near one ohm, but I am finding I like the 1.1 ohm. I am going to further try 'three resistor' combinations. Which will allow 1.067 ohm, and 1.085 ohm values. I find the differences to be noticeable, and worth investigating to find just the right balance.
The multiple resistors also allow greater current flow.. Two parallel seem better for this than one. So I will find out if three does more? or two are just as good.
The sound mainly seems to be the lower mids which gain from the slightly over one ohm. Below one ohm (like 0.90 ohm) the upper mids gain ascendancy.
The higher value Duelund are on their way as I write.. So perhaps next week I can see how they perform.


* the "Al Sekula Choke tweak" can be found by Googling the words.

Added: I had a few odd Duelund Cast laying around, so I made up a set using (1.3 ohm parallel with a 10 ohm to give) 1.15 ohm total. I decided to make it and install them. First impression more midbass, which would be expected. Just put them in, so need some break in time to decide if want to go 'more' higher impedance? or somewhere less, between 1.1 ohm and 1.5? I also can go as high as 1.2 ohm..