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Old 9th May 2016
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rvwainscott's Avatar

Dueling Banjos - now with twice the fun


A friend of mine is going to New Orleans and asked me if I
could build him a stereo mic pre so that he can go record
some drums. He wants something for overheads.

Normally, I like to dwell on something like this for several
months. You know, gradually gathering things up in a
random fashion until enough stuff appears to build whatever
thing I've had months to think about. It's how I work.

I do not have months but, instead 3 days so cue up the Dueling Banjos
as I'm about to go as Redneck as you can get while still having
something that is pretty solid.

Lets Begin!

These are the first things I grabbed. I've got a whole tub of
Cinemag input and output transformers. Things I've experimented
with over the years. I also notice a power supply from a
Black Box Analog Design HG-2. If I use it I'll still need phantom
and it would require 3 transformers to run. Never mind that.

I also have the main circuit board from an HG-2 that I ruined
to where I couldn't offer it up to the public. Everything I
would need for a stereo tube mic pre was in perfect working
order so I'll modify it.

I find copies of the circuit boards that I do plan to use and tape them
down so that the holes in the circuit board match the holes I've drilled
into this metal that will become my case.

I do not have any time to order anything so I am going to harvest
perfectly good working parts out of mistakes I've made. To do this,
I use my antique angry cauldron solder pot. Set the board on it and
the pieces pop out fast and clean.

I took that protoboard stuff that I had just drilled holes in and
attached the output transfomers, standoff's, load resistors to it.
Notice the added standoff for stability. Classy huh?

I have to make this line level processor into a mic pre. I have
to change some values and I need some extra parts and switching
so what better place than to just glue them to the bottom of the
circuit board. I've seen worse.

I thought I would bolt this in so I would feel like I had done
something other than sort trash.

I thought it a good idea to do the rear plate because seeing the
ins, outs and the power connections would help me to focus as now
I had to work in that direction.

I'm slightly out of order here but I wanted to show the pile of stuff
and I only took photos of it like this at this stage.

I stuffed this power supply with harvested parts and got this
toroidal out of a bin and I wired up and tested the power supply.
Those two things will give me 6.3 volts, 12 volts, 48 volts & 360 volts.
I have a stand alone switching supply that will give me +/-16 volts.
I do not like to use switching supplies but the things that need that
power are filtered really well.

I found a faceplate we didn't use because the holes were too small
so I drilled them out and bolted in some switches.

I did a test to see that the switching supply would power those
opams and that those input transformers were actually wired up
correctly. In the original design, those opamps are just buffers but
now they give me a 10X gain.

Now the power supply gets wired in.


It always comes together fairly fast at the end so long as I test things
along the way. Of course, I stumbled across some problems when I first
fired it up but most of that was Atmospheric Resistor kind of stuff.

In the end, this pre has a pentode stage, triode stage, high pass filters,
air and a drive switch. The L/R sides track within 1/2db of one another.

Thanks for visiting.