Crawls Backward (When Alarmed)

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Building a Baffle Step Compensation Box, Pt. 1

Before we get back to the Super/Pro/Bandmaster tweed amp build, we go on a diversion.

You may recall that after I built my Fostex 126En-based fullrange speakers, I cooked up a baffle step compensation circuit to use with them.   I recently put the speakers in my main system and needed to build a stereo baffle step circuit and a box to house it in.  So I rounded up the parts and set to it.

It's a simple circuit - just a 10k pot, a capacitor whose value is calculated on the size of the speaker box, and a resistor.  I have a Hammond 1590 (I think...) box to put it all in.

The value I need for the capacitor is 17nf, or .017 uF. 

I have a bunch of Russian paper-in-oil capacitors I'm trying to use in projects to get an idea of how they sound, and I happen to have some .033s.   I start using the old noodle and figure that I can put two of the caps in series and hopefully come close to .017.

I go through a bunch of caps and find two that add up to 16.9nf in series, and another pair that add up to exactly 17.0.  What good fortune!

Now it's off the the trusty drill press to drill a hole for the potentiometer and the input and output jacks.

The pot will control the amount of cut from about 624 hz upward.  If you speak audio, you will recognize that the circuit is really a shelving filter - it will cut up to 6db from that shelf frequency and higher.

With the box drilled, I sand it with 220 grit sandpaper in preparation for primer.  Not too exciting, but required for a decent finish.

Then it's off to the spray booth (aka the backyard) to hit it with a couple coats of primer.  I let the primer dry "less than an hour" so I can put the topcoat on.  The primer can says something to the effect of "apply additional or finish coats before one hour or after 24 hours."  I have no idea why this is the case, but I don't want to wait 24 hours, so I go back with the finish coat after about 50 minutes.

I do let the box dry overnight before I move onward and put the hardware in place.  This means the phono input and output jacks and the "cut" pot.


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