2 stage mic preamp
The VP28 is a full fledged 2 stage mic preamp for the 500 series format. The module is heavily based on the full channel in my old API console. Not only the mic preamp circuitry, but the fader and poster fader booster amplifier circuitry is also on board.
This preamp was actually the original idea for my first preamp before the VP26 was ever released. It has been some years now and I really don't recall exactly why I changed directions and went with the VP26. I think it was mostly due to a friend telling me that the world already had too many 500 series preamps and why would I create another one. That made me think that a 2-stage API style pre may not be received so well. I think he has been proven wrong a few thousand times already!
Circuit Design and Topology
The preamp or 1st stage is nearly identical to the VP26 with a few exceptions. The original Mute switch has been replaced with a Mic switch. When engaged the module is in Mic mode. This is the same mic preamp circuit as a VP26, up to the t-pad. When disengaged, the preamp is in Line mode. This switch basically inserts a U-pad at the module's input changing the input Z to 10k along with dropping the level by 35dB. The signal still follows the same path as a mic signal so you get the benefit of the EA2622 input transformer in Line mode. The preamp section has the typical VP26 spec, 60dB of gain.
The other main switches are Polarity flip, Pad which is the typical -20dB mic input pad and of course 48V for phantom. All four main switches have their own LED. The only red LED on the faceplate is for phantom. Easy to spot from a distance. See, I listen to you guys!
The next addition is drBill’s requested Signal Present indicator. The only green LED on the faceplate. This is a full wave rectifier circuit feed from immediately after the 1st EA2623-1. The RC4558 IC is the dual opamp for this. The circuit is HiZ and does not effect the sound or performance of the preamp section at all. Some will care less about this. Others will say it's by far the best bang for their buck ever spent. Literally, it costs about a dollar! I think it's a very handy tool especially when your signal gets lost somewhere in the elusive patchbay.
Our electrons leave the "VP26 section" and would typically go thru the t-pad and then out. Since there is no t-pad attenuator on the VP28, the signal heads to a stepped "Channel Fader". This 12 position Grayhill switch emulates the full throw fader from the console. There is 12dB of gain "in hand" at the fader giving us a total of 72dB for mic gain.
After the fader we have a two pushbutton HP filter array, implemented the same way as the console and as in the VC528. Left switch in, 40Hz, right switch in 80Hz, both in is 160Hz. I also carried over the -6dB/-12dB per octave switch from the VC528.
Then we are off to the 2nd 2520 gain stage and the 2nd and final EA2623-1. From the fader to the output is more or less the same signal path used in the VC528. So, the VP28 is a complete VP26 preamp with the 2nd half of the VC528 following the HPF from the 553F. A mouthful indeed.
I have chosen to only offer the stepped gain version for the VP28. The biggest reason is the definitive Unity Gain setting when in Line mode. The front panel artwork is designed around this and changing to a pot would though it out the window. The lowest preamp gain step is 4dB and the remaining steps are in 3dB increments. The fader is in 2dB boost increments and 4dB cut increments. Doing the math, there are a number of positions where the overall gain will be the same, but the gain staging is completely different. Each one will sound slightly different. Many tonal options to be found here.
Last thought, I fully believe that "less is more" does not apply to discrete electronics and great transformers. "More is more" and is typically better to my ears.
Module is available as a DIY project only.
No info about availability.$285 submitted Oct 2nd 2014, 16:55 by grahamspice | last Change Jan 26th, 15:56 by modulargrid