Sunday, August 4, 2019

Sound Workshop Series 34 EQ rack project - FINISHED!!!!!!

EQ Rack Project
Sound Workshop Series 34 EQ rack project
In a former life, I was a full-time record producer/recording engineer. When one of the main studios I worked out of closed, I got 8 channel strips from their Sound Workshop Series 34 console, a good 80s-ish workhorse console. My gole was to take the EQ modules out of the strips and rack them up as a single 8 channel EQ outboard unit.

Several years ago I racked up just the 8 mic preamps from those strips into an outboard chassis. All it required was a dual rail 16 volt power supply with phantom, a chassis, and some connectors. The outputs are unbalanced, but I have A/D converters that switch from +4 to -10 so I just use the converters on -10db making it a really simple project. Here's a pic:



mic preamp
Sound Workshop Series 34 mic preamp innards

Mic Preamp
Sound Workshop Series 34 mic preamp done
Easy peasy. But my next project was much tougher given that I don't have much electrical engineering experience. I wanted to take the 8 EQ modules from the channel strips and rack them up. No problem if all I wanted was 8 channels of unbalanced in and outs on the EQs, but that would make it tough to connect with other gear in the studio (compressors, limiters, etc...). I wanted balanced ins and outs.

To accomplish that, I needed to build balancing and unbalancing boards that would also run on dual rail 16 volt power supplies (since every part of the Sound Workshop console was dual rail 16 volt). Here's the schematics I found (sorry I don't have a credit for where they are from. I even tried a reverse image search to find the originator.):



Each relied on an op-amp that cost about $5 with about $1 worth of capacitors. Pretty simple circuit. However, 8 channels of each would cost about $100 total. That plus the chassis and power supply was starting to add up. But oh well.

So I built the balancing boards as point to point wiring on a breadboard, wired everything up with the EQ modules and the in/out jacks using another Five Fish Audio adjustable voltage dual rail power supply, the same one I had used for the mic preamp project. However, every time I powered the thing on, I blew the fuse.

After doing some thinking and talking with the owner of Five Fish Audio, I think I had too much current draw with the 16 op-amps from the balancing and unbalancing boards plus all the op-amps on the 8 channels of EQ. The three options were: go unbalanced and scrap the balancing/unbalancing boards; or go down to 4 channels total; or add a second power supply. In the end, I chose to add a second power supply. Now, each supply covers 4 eq modules, 4 unbalanced to balanced converters, and 4 balanced to unbalanced converters.

Sound Workshop Series 34 EQ rack prjoect - modules up top (front), balancing and
unbalancing boards on the left, two power supplies on the right, in and out
jacks on the bottom (rear)

I wired this whole thing up and...tons of problems. Here's a rough list: Channel 2 had no output, but when I changed which pin I was using as the output pin from the multi-pin connector on the EQ module, I could get sound to pass, but it was distorted and the EQ functions didn't work. Channel 6 had a low-frequency pulse (referred to as motorboating because the frequency of the sound is too low to hear but it causes a square wave like distortion that makes a repetitive putt-putt type sound). Then half the channels didn't seem to be working at all.

I figured out that there was a problem in the power supply rail going to channels 5-8, so that was an easy fix, just a bad connection. Channel 2's problem turned out to be a resistor on the module that had come lose and that was easy to fix, so now its normal pin 4 output from the multi-connector worked which reenabled the eq on the output.

EQ Rack project
The 8 EQ modules from the console

The motorboating on channel 6 was really tough to figure out. I ended up retracting every connection through the output unabalanced to balanced converter which I traced it to. Even after checking and redoing all the solder joints I couldn't fix it. I tried swapping op-amps and that didn't help. Finally, I just built another balancing board (and reused the op-amp since that wasn't the problems) which only cost $1 in caps. That didn't work, but it did change the motorboating sound. So, after some more tinkering, I just decided to build yet another balancing board for $1 and THAT one worked perfectly. Something wasn't right in one of the caps I think.

By this point, all 8 channels were sending signal perfectly, the eq was working, levels were the same across all 8 channels, the stereo imaging was fine when I used any two in combination, etc... All I needed to do was clean up the wiring mess inside and put the top on. When I fnished with that, I did a final check and...channels 5-8 wouldn't power up. I figured it was a fuse, only it wasn't, exactly. Turns out the fuse holder was really cheaply made and one of the pins that connected to the line to the power receptical had completely come out. I swapped in a higher quality fuse holder and everything was perfect! 8 channels of Sound Workshop Series 34 EQ in a single chassis.

EQ Rack Project
Sound Workshop Series 34 EQ rack project - DONE!

Channels 1 and 2 with the green knobs were modified years ago to sound like an SSL J series console and are very smooth on the top end. Channels 1-4 all have variable "Q" mid and low-mid options and channels 5-8 have low/high pass or bell-curve options for Q on the high and low channels. This makes all 8 channels semi-parametric, but channels 1-4 do have fully parametric mid and low-mids.



And just for fun, here's a little shot of it with both my racks of outboard gear:


🚺

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