D-Space Studios Electronics Projects folders

» 2007_DiY_DAW_mixer_project
» 2010-2011-mixer-work
» 2010-THAT1200-and-THAT1600-IO-PCB
» 2010-building-THAT-input-and-outputs
» 2010-mixer-proto-www31mar2010update
» 2011-2010-MixerWork-to-19Jan11
» 2011-2010-mixer-work-current
» 2011-Jan-Mixer-enclosure-rework
» 2011-switch-attenPCB
» 2014-mixerProgress

This is a digital audio workstation mixer that I had been building/redesigning since 2007.
The goal is to make a 16-channel class-A conducting channel mixer that utilizes Jensen 990 or API 2520 style Discrete OPAMPS made by various shops.

The first try in 2007 didn't work out so well do to excessive grounding issues (too many virtual grounds derived from voltage dividers); so those parts were repurposed into invididual mic/line preamps (see the "9" Preamp on main page).

In 2009/2010 Jeff at Classic Audio Products Inc. (was of Illinois) had built and made PCBs available for an active combinding amplifier (ACA) based upon and compatible as a drop in replacement within vintage API recording consoles.

After buying some of these PCBS, I set to work in greatly reducing the complexity of my design, eliminating LED meter boards and a Left/Right/Center assign PCB and carried forward.
Using Jeff's PCBs I modifed the circuit to a non-inverting configuration and tested/reworked to reduce channel crosstalk noise.

Signal Flow
The TRS jack input "front end" uses transformers from Cinemag, Altran, and Ed Anderson to debalance the signal and provide galvanic isolation. This feeds individual channel amps (990 and 2520 style DOAs), attenuation switches, a Left/Right channel bus, followed by a buffer, drivers + master attenuators for two set of signal rebalancing transformers from Cinemag and Jensen transformers.

Dual inline package (DIP) integrated circuits are not used in the design, but I had made several breadboards utilizing THAT Corp InGenius input and output drivers at one point which fit snugly within a 1 rack unit prototype. These are great circuits on a chip, and easy to implement, but my goal was to have input/output transformers to do the balancing acts for this particular design.

Linear PSU supplies +/-24VDC, +/-16VDC, and aux voltages provide several amps per rail. This PSU designed by Group DIY 51x Alliance (originally for GDIY 51x series rack), in its own enclosure. Photos of the beefy PSU are around here somewhere, check the index.

Many thanks and respect to Dana, Jeff @ ClassicAPI, Joe @ JLMAudio, David @ Cinemag, and John @ The John Hardy Company for moral encouragement, troubleshooting and design ideas.

Of course, deep regards to makers of the many discrete OPAMPs (DOA) used: John Hardy Company, Illinois, USA: APP Studio/AMS Micro, Italy: JLM Audio in Brisbane, Australia: Forssell Technologies in California, USA: and Millennia Media in California, USA.

Switches by Electroswitch and Grayhill.

Enclosure manufactured by Par-Metal Products , NJ.

Progress continues as of Feb 2014.