Another 2CH mic pre

[Dave G.]

I finished assembling this in January, and with the limited time I have to work with it, it took awhile to fix my problems.

When I first turned it on I found noise was a problem. It was reading 36db below full scale, plugged straight into a Delta 44. So I applied for a building permit to lower the noise floor.  I found the PS was under filtered, so I added a couple capacitors in the first stages of the PS. This gained me 6db. After Checking everything I could, I suspected the heater supply was the source. After building a DC PS for the heaters The noise was gone.

Even with the noise channel 1 sounded great, without the noise CH1 sounded GREAT. Everything I hoped for and more.

CH2 on the other hand was a little too dark for my taste. It also had what I can only describe as a bump about every 3 seconds. I went through the circuits with a fine tooth comb, and could not find the cause. The thing is it never showed up on the scope. I only get an hour or two a week in the studio to work so it took a month to find the problem. One day it dawned on me to put the scope on the secondary of the OT. And there it was, my elusive bump. At this point I knew it had to be coming from the Buffer for the VU meter. The buffer was tucked in behind the low voltage PS. So I unsoldered it and the picture tells the rest of the story.

 

[Dave G.]

I remember one of those opamps falling out during assembly. Being hard to reach behind that PS I must have not inserted it right.

 

[Dave G.]

I was a little hesitant about using cheep output transformers, but read good things about Edcor. I am very happy with them and may use them again on another project.

Here is the schematic

Thanks again for all your help

 

[shabtek]

nice!

no internal pics?

 

[Dave G.]

I took a couple internal pics, but they ain't pretty. Here's one.

The top right is the low voltage PS, for phantom power & VU buffers.
Below that is the High voltage PS, and just to the left you can see the heat sink on the regulator for the heater supply.
Both channels are on the same turret board on the left.

I may take steps someday to brighten CH2, and add a little gain.
CH1 has more gain than I'll ever need.

Dave G

 

[carlmart]

Where did you get those TM-200? I couldn't find them at Edcor.

 

[Dave G.]

Where did you get those TM-200? I couldn't find them at Edcor.

Back in "73" I was playing with a group from school, and a group from church. We Bought a system together. Part of it was a Bogen MX30A, with 4, tm-200s. I ended up with it and turned it into a guitar amp back in the 90s.
Having those transformers is what started this project.
Here's the amp.
The grey amp on the bottom right is one my dad built in the 40s.