Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Here is some insight that may help.

206.173.189.124

Posted by Dave B on May 20, 1999 at 11:19:02

In Reply to: Hey Victor or any other expert- your thoughts on a ckt design quesion pls posted by jk on May 20, 1999 at 10:31:11:

jk,

What non-inductive resistor values are you looking for? I assume in the 1k to 100k range.

Typically, parallel wired resistors are preferred. Do you know why? There are 2 benefits that I can think of, just off the top and one of those benefits directly applies to why you shouldn't series those Vishay BMFs.

While the "bulk metal foil" resistor is a good one, it is very limited in it's resistance range because the resistor element is NiCr sheet, trimmed into a serpentine pattern to achieve Rval. This is purely limited by the geometry of the pattern and the bulk sheet resistance of the particular alloy NiCr used.

Interestingly, I've got 2 related projects going on at work, one is a BIG version of the Vishay unit but intended for high power current sensing. Packages in the 25W to 250W range and are flat and heat sinkable. I re-engineered the thermal path and reduced the temp rise from the element to the heat sink from 114 deg. C to 0.3 deg. C using some new, available technology in materials.

Now, to your need. About 2 months ago, Jon Risch and I privately discussed the current drawbacks to wirewound resistors. Jon pointed out that he knows the steel end cap in the construction is one of the biggest sonic problems for audio use. This set me off to search for an alternate material that still allows the requisite welding operation but hopefully can eliminate the dissimilar metals in the construction. I have found an excellent resistance wire and have ordered the same alloy in strip form to punch the end caps out with and then ordered the same alloy in 18ga for the axial lead portion. Most of the requested values I got from speaker designers at HiFi99 were in the 1 to 100 ohm range, 5W size. This is easily achieved using non-inductive winding techniques. If the value gets too high, I have to use VERY fine wire diameters which are very difficult to handle and keep from breaking, especially keeping in mind that I have to wind to twice the req'd Rval to non-inductively wind.

If all works out accordingly, we should be releasing a new line of Audiophile Grade Resistors later this year if I get the thumbs up sign from key audio designers after I sample them. Thanks to Jon's observations and keen understanding and my search for the "right" kind of materials, we may have a solution here.

Dave Bender
VP of Engineering
Ohmite Mfg. Co.

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