VP28- CAPI- Classic Audio Products, Inc. built by Billy Pratt of Audibility Recording - ABW Reviews
Channel: Audio Builders Workshop
Date: 2017-09-15

"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 fed 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. "- http://capi-gear.com/


"There is a ton of excellent and highly regarded DIY gear out there, but so much of it only comes in pieces. We know that not everybody can solder and that’s where we come in.
We may offer many services in the audio industry, but we enjoy building DIY gear the most. If you’re as much of a gear slut as we are then you understand that looking at gear is almost as much fun as listening to it. This deep-seated philosophy of ours holds build quality and presentation as the highest priorities in all of our items; because clean, thoroughly checked builds lead to electrical and mechanical integrity and longevity.

Our company recognized the potential of DIY gear only after several months of research, intensive forum browsing, reading customer testimonials, and listening to blind shoot outs posted online. We decided to begin our personal preamplifier collection by building some for ourselves. A few years and 750+ hours worth of solder fumes inhaled later we have built over 30 different models of preamplifier and microphone kits from several of the highest regarded DIY suppliers."   
- http://www.audibilityrecording.com/di4u/