making an eq boost/cut switch using Elma 04-1130

OK-I feel like a real knucklehead, as I have been reading through the various threads on attenuators and have not seen the light....

I am replacing some Bourne 10k linear pots installed into a pair of eq boards with 24 step Elma switches....and I need to find a calculator to determine resisitors values for the zero position, boost and cut. I only want ~6 db in either direction, so .5 db or 1db steps are what I'm after.

Would anyone be so kind as to help out and point me in the right direction? There must be a thread on this already...

for the curious the boards were apparently made by 'T de P' for someones [defunct] custom mixer project, and I got them on Evilbay.....after tracing them out, they are exactly the same as the control section of his 825 (schemo on Gyraf site), minus the low and high shelves. I intend to implement similar to this: http://forsselltech.com/schematics/EQ1A.PDF
Shame the elmas won't fit in the direct mount scenario..

> There must be a thread on this already...

Not for what you are asking. Calculating the swinging-input EQ with any accuracy is a total brain-drain. Even if I could calculate it, I'm not sure I would. Simulation might be a guide, but the easiest path is to bite it and see. Wire it up.

First divide the problem. There is no "center" position on a 24-throw switch, yet you surely want one. Call it a 23-throw switch: 12 up, center, 12 down. That leads to average 0.5dB steps so the "loss" of one throw is no practical problem. Wire throws 24 and 23 together (unless your switch has a limit-pin).

You know a 10K linear pot works. How do you build a linear "pot" with a many-throw switch? Put equal resistors between throws, that add up to the pot value. At the end the pot wiper hits the end-terminals, so we need 22 resistors between 23 poles. We want them to sum to 10K, so 10K/22= 454.5Ω each. Tip: the "10K" value is not very critical, you could use a 7K or 15K pot and hardly be able to measure the difference. Me, I'd round-up to 470Ω 5% resistors, though 2% parts might give a little more center accuracy.

I said the average step would be 0.5dB. But this type of EQ is NOT linear with pot rotation. The gain equation is mind-bending and not linear nor anything simple. However I don't know why uniform 0.5dB steps are needed. In fact I think the "error" will be in the direction of small steps near the center and large steps at the extremes, which is probably a Good Thing.

You say you want +/-6dB. This is NOT set by the pot. It is set by the ratio of the 10K input and feedback resistors against the impedance of the EQ tank network. For the L-C-R resonators, the boost at peak is nearly set by the resistance (actual resistor plus coil resistance). Fred thought this problem was so interesting, and so under-written, that he wrote a paper. Study that. Fred will be along to quiz you.

Counting on thumbs, the tank resistance for 6dB is 10K. That suggests much larger coils (and smaller caps) than we would use for a +/-20dB EQ. Which is a drag, because I see you already have your coils.

Tack-solder a pile of 470Ω resistors to a switch and see what it does. (You can wire-up just one "pot", results will not be hugely different when you add more.) I suspect the Q will be low unless you re-design the tanks. If the steps are too small in the center and too big at the extremes, try some 680Ωs in the center and 270Ω at the ends. (That's why I say tack-solder: tin the terminals and resistors, and tack it just enough to stay connected on the bench, not like it was going on tour. After you swap resistors to happiness, you will re-do it with good joints that will survive abuse.)

Many many thanks!

I will do up one switch and report....