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Ultimate crossover resistors

Posted by madisonears (A) on January 6, 2019 at 16:35:01

I have tried almost all of the resistors suitable for passive crossover duty and not been wholly satisfied with any of them. From the cheapo cement wirewounds to Duelund solid carbon, they all impart a coloration of various qualities from dullness to brightness to edginess to grayness. You probably have a favorite, but consider this alternative.

I have always liked the sound of Vishay/Dale RN. When used in line level applications, they do not impart much of their own sound compared to many boutique style parts. All the RN's I have used are non-magnetic, which I believe is one factor in the ability to accurately pass a signal. While only very slightly diminished in the uppermost frequencies, they exhibit what seems to be a realistic tonality, or neutrality if you prefer, and an uninhibited sense of space. They are surpassed in this quality only by the much more expensive Vishay bulk metal Z-foils, which are also slightly more transparent.

While RN's are available in a vast array of values, they can dissipate only 3/4 watt or less, which makes them unsuitable for speaker level applications. Or does it?

My latest tweak is building a ladder of parallel RN65's capable of handling tweeter level power, and it is a complete success. Two buss wires of 20ga solid OCC copper form the legs of the ladder, with (24) 73.2 ohm resistors soldered in parallel to provide a total resistance of 3.05 ohms. The beauty of this approach is that one can arrive at precisely the proper resistance for padding the tweeter by adding or removing a resistor or two.

Here is how to do it. Determine the required power dissipation of your tweeter padding resistor. Use whatever value you have now, or calculate it by dividing the RMS output power of your amplifier by ten. If you have a hundred watt amp and you really use all of that power (few do), you would need a resistor that can dissipate 10 watts. My amps are only 65 watts, and I opted to make ten watt resistors. At a half-watt per resistor, that means I need at least 20 resistors in parallel to dissipate 10 watts. I went for 22, just to be on the safe side, and so I would have a couple extra in case I wanted to remove a few to tune the padding. Now determine the value of your existing resistor and find the inverse of that. My crossover design calls for and I was using Mundorf MOX 3.3 ohms, so the inverse is 0.303. Divide that by the number of resistors you require (mine is 22): .303/20 = .01377. Find the inverse of that number, 72.6, and that's the value in ohms of each resistor you'll need in parallel to make a single resistor of 3.3 ohms. ALL the resistors must be within 1 or 2% (RN's are 1% tolerance) of the same value, or the highest values will dissipate more power than the smallest, and that might lead to failure. If the exact value you require is not available, you should choose the closest. I used 73.2 for the calculated value 72.6. A few ohms will not matter because you will still need to tune the pad by removing or adding a resistor or two. Buy a few extra. The cost per piece is generally in the thirty to fifty cent range, so this is not an expensive tweak.

I used 20ga OCC solid copper for the ladder legs because it's the optimum size wire for transmitting tweeter-level power and high frequencies, and it's excellent wire. I bent the legs of each resistor to form a U and stuck both legs into a piece of styrofoam to make a compact row of resistors, about 4" long, with the legs pretty much in two straight lines. Then I soldered the pieces of stripped wire to each resistor leg, leaving a couple inches excess on the opposite ends of the ladder to allow attaching to the crossover. You can push and pull on the legs while soldering to ensure good contact between them and the wire. There is no need to wrap the leads around the wire if you're careful while soldering. It's okay to leave a little excess solder on the joint for strength. Clip the resistor leads only after they're all soldered securely and all the joints are smooth. Leaving the iron too long on any joint might liquify the one next to it, so you've got to get in and get out, then let it cool for a few seconds before moving to the next one.

Took about an hour each and breathed a lot of fumes and melted the styrofoam a bit around the resistor leads (you could use any soft material such as balsa, clay), but it sure sounds good. The highs are so smooth and open. If you know the difference between cymbals that sizzle and zing and cymbals that ting and then shimmer freely in space, then you know what I'm talking about. I allowed a couple hours playing time for the solder joints to settle, then I ended up adding a couple more resistors to each ladder (reducing the pad level to 3 ohms) because the highs sound so good I want more not less. This does not significantly affect the crossover point.

Peace,
Tom E
berate is 8 and benign is 9

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