bruno2000
Well-known member
This all started when I got a really good deal on some PC mount Grayhill switches. I've always been a fan of the "In Your Face" API sound, so I set about to build some clones, using these switches. I wanted the EQ to be the 4 band 550b version, but unfortunately, the switches were only 6 positions, so one frequency from each band had to go. The boost / cut switching arrangement in the APIs is pretty clever, but requires one whole gang of the switch to determine which circuitry is affected. My solution (since I had only 6 position switches) was to set the dB level with the rotary switch, and to select the boost or cut with a toggle switch. Most of us from old school would find the frequency we wanted to cut by boosting it, then switch to cut. My solution seems to address that, but I'll find out how well it works in practice.
Input from several local engineers was solicited, and many said that they really liked the filter that is in the 550 and 550a, so I added that as well.
If I was going to build an EQ, I might as well add a mic pre, so the 312 circuit was added. In "normal" mode, the chain is: Input XF > DOA > Output XF > T-Pad > EQ (2 DOAs) > Output XF. There is also a "Split" option that allows the mic pre and EQ to have their own independent ins and outs.
This is the same 7" x 2" format that I used for my Dual Mid 1084 project, and not the 500 / 51X series
http://www.groupdiy.com/index.php?topic=40610.msg503046
Once again, FPE did a great job on the front panels,
The missing Green knob caps are on their way here, and there are 4 empty rack slots ????????
Here's a top view. Thanks to Jeff for the transformers, and to Gary for the DOAs.
Here's a view with the "cap board" folded back and the top showing. At the top left is Jeff's T-Pad with a 4th pot added concentrically for the mic pre gain.
The EQ curves are spot on compared to the 550b. There are several options on the board, like input transformer impedance select, blocking caps on the transformer primaries or not, and what type, and of course DOA selection. We'll be listening to how they sound next week, trying all of the options, after I get their permanent power supply built.
More to come.
Best,
Bruno2000
Input from several local engineers was solicited, and many said that they really liked the filter that is in the 550 and 550a, so I added that as well.
If I was going to build an EQ, I might as well add a mic pre, so the 312 circuit was added. In "normal" mode, the chain is: Input XF > DOA > Output XF > T-Pad > EQ (2 DOAs) > Output XF. There is also a "Split" option that allows the mic pre and EQ to have their own independent ins and outs.
This is the same 7" x 2" format that I used for my Dual Mid 1084 project, and not the 500 / 51X series
http://www.groupdiy.com/index.php?topic=40610.msg503046
Once again, FPE did a great job on the front panels,
The missing Green knob caps are on their way here, and there are 4 empty rack slots ????????
Here's a top view. Thanks to Jeff for the transformers, and to Gary for the DOAs.
Here's a view with the "cap board" folded back and the top showing. At the top left is Jeff's T-Pad with a 4th pot added concentrically for the mic pre gain.
The EQ curves are spot on compared to the 550b. There are several options on the board, like input transformer impedance select, blocking caps on the transformer primaries or not, and what type, and of course DOA selection. We'll be listening to how they sound next week, trying all of the options, after I get their permanent power supply built.
More to come.
Best,
Bruno2000
