VC528 "Missing Link 2"

Two stage API console channel with high and low pass filters

The overall goal of the VC528 is to achieve the complete and lovely sound of a vintage API console channel without the cost of the rest of the console. It is definitely a little something different for the 500/51x format. Basically, it is a two stage console channel with the vintage circuitry of my API board.

The first stage is a balanced, transformerless receiver. Everything is true to the original console circuit. This entire first stage can be bypassed with shunt jumpers or simply left of the build if one desires.

Next in the block diagram comes the fader. Due to space constraints and a request from a friend, I have chosen to use a 24 position Elma with a 1K overall value. Original faders in my board were 600ohm. The fader has 12dB of gain in hand to keep with the vintage API norm. Since the majority of the upper range gain steps are in 2dB increments, I have used a stepped Grayhill for a fader trim that is in .5dB increments.

Next in line is a 6 position stepped LP filter. This filter was not in my board. I have however implemented the same passive philosophy that they used for the HP filter that was in my board. Since it was easy to implement, I have added a switch to select 6dB or 12dB per octave for the filters. Very smooth and natural sounding, by the way.

I follow this filter stage with a 3 transistor discrete voltage follower using 5087/5088’s. This buffer circuit is very reminiscence of the buffer amp used 4 times in the 550a. Very handy indeed.

Next is a 6 position stepped HP filter. This filter can also be switched between a 6dB or 12dB slope per octave. Again, the overall philosophy is the very same that was used in my old board.

The last stage is the post fader booster amp. Again, the same circuit as found in the vintage console. I have modified the feedback R and C to provide unity gain when using the entire VC528 circuit.

The VC528 modules have been given a few nicknames by some happy users. They have been referred to as “Audio Bacon” as well as “The Missing Link"

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