Poor Man's HPF and switched attenuator
« on: March 17, 2013, 03:06:28 PM »
In my first all tube mixer I included a high pass filter (HPF) as the client asked for it to remove room rumble when recording live. So I implemented a simple second order RC filter with non-coincident poles - which basically means I cascaded two RC circuits with identical values. I tediously hand wired all the capacitors to a 2 pole 6 way Grayhill switch on all six channels and I am pleased to say it worked fine.

A few days ago I needed something similar for another project and I did not fancy doing all that hand wiring again so I thought about a PCB version. It then occurred to me that I already had a PCB with a 2 pole 6 way switch and a bunch of capacitors on it called the poor man's EQP1A PCB. I dug out the schematic and sure enough a simple reconfiguration turns it into a HPF. Here's a pic of the original poor man's schematic and how it is hooked up to make the EQP1A EQ.

At the bottom left you can see the schematic of the switch PCB. To make the HPF you populate it with caps in the usual way and short out the resistors in series with the switch wipers. Then your input goes into T2 (a switch wiper) you connect T1 and T3 together and also connect a resistor from their junction to ground and you take your output from T4 (the other wiper). The first set of caps and the resistor from T1/T3 to ground form the first half of the filter and the second set of caps plus the load on T4 forms the second half.

A while back, for another project I am working on, I needed a stepped attenuator and I realised it is quite easy to make one of these from the poor man's PCB as well. You can use a 1 pole 12 way Lorlin. Instead of fitting caps, you wire resistors from one cap to the  next using the inner connections of each cap. So you fit a reisitor from C1A inner to C2 inner, from C2A inner to C3 and so on. The bottom connection of the switched pot is C1 inner and the top is C12A inner.



'The only people not making mistakes are the people doing nothing'


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