Imitation is the sincerest form of flattery, part 2

When we last saw our project, I needed to fine tune the LCR filter component values to arrive at the filters I desired. A surprisingly large amount of time was spent experimenting, trying to get all of the parameters to line up where I wanted them. I quickly found out that the DCR value was integral to both the filter frequency and its range (boost/cut). The Low Shelf filters required some large inductors indeed. MCE Magnetics provided those at a great price. I settled on two of the 950mH inductors and three of the 380mH inductors. These were configured as low shelf filters with centers based at 51Hz, 70Hz, 123Hz, 177Hz and 363Hz. For the High Shelf filters, I kept the stock capacitor value but swapped the stock capacitors for Poly-Film capacitors made by Kemet, which resulted in filters centered at 7.5kHz, 9kHz, 10kHz, 12kHz and 14.5kHz. I adjusted the resistor values to best match the +/-15db range that I desired. Note that these values can also be substituted in the JLM EQ Mod kit, which is kind of in the +/-18db range, but vary from filter to filter.


Now that the LCR values were sorted out, I wanted to clean things up a bit. Looking around on eBay, I found a nice set of premade prototype circuit boards that would fit the bill. I was able to fit both the High-Mid and Low-Mid filters on one circuit board and the Low Shelf on another. The High Shelf filters, being only capacitors and resistors, were mounted on the wafer switch daughter board. The components were installed with point to point wiring, utilizing each components built in leads.

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Once the Filter Sets were built and tested, I used double sided tape to attach the small board to the open area left on the PM2000 channel strip, where the unused original parts were removed. I mounted the two channel strip PCBs on a section of 13mm birch plywood.  Added four 600ohm:600ohm transformers scavenged from the same PM2000 and their necessary wiring.

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Concerning the power supply, when I first tested this circuit, I used a linear supply. But I didn’t need one that big. I found a good deal on some single 24vdc and 48vdc switching supplies. Mounting two of the 24vdc supplies and wiring them as a bipolar supply with a virtual ground between, provided plenty of clean power in a smaller size with less heat. I added the 48vdc supply to provide phantom power for the preamp circuit. All three were mounted on another piece of birch plywood.


After wiring everything up and testing, I started looking for a rack case to mount them in. And of course, eBay came to the rescue. One new 3RU aluminum case was on the way. The case arrived just before I cut the plywood panels, so I was able to cut them to just fit inside the case. Mounting the circuit boards was easy, drilling the holes for the controls, not so much. Sadly I don’t have a CNC mill at home, yet. So the holes were laid out with a dial caliper and a center punch. 22 holes and 4 dimples on the backside. After making sure everything fit, I painted the faceplate satin black and applied PTouch labels. Add a few holes in the back for XLRs and IEC and it’s ready for delivery to it’s new owner.

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