DIY Factory

The DIY 12AU7 compressor Project
A project brought to you by Kent and PRR
Updated 25 March 2005

A little backround

This page is a re-work of Kent Stevenson's original page at Conditioned Response It seems that there will be some changes at the servers very soon so we have relocated it here and given it a slight edit by Kev and a general update.

Please note that PRR's original page is also presented here at the DIY Factory server but without any of the local changes and only meant as a mirror to Paul's page at his server.
PRR 12AU7 compressor

The DIY 12AU7 compressor Page

This project was the result of somewhat of a colaboration of the old Tech Talk forum and continues at the new forum, The LAB (on which I spend way too much time). In 2002 I became very interested in Vari-mu style compressors somewhat for mastering purposes but also for the cool things that it does on a drum sub. After looking through schematics on bunches of these like the DISA, Collins, Gates, Manley and the grand-daddy, the Fairchild 670, I found that the cool original ones were made with a 6386 tube as the "magic" vari-mu element in the circuit. Not all vari-mu compessors used this very expensive tube. Some such as the UA175 and the Altec 486 used a different tube on the front end. I was intrigued and thought ... " is there an inexpensive and plenitful tube that can do the job" ? After some interesting posts on the forum, gear guru PRR posted some ideas on using a 12AU7 in this spot. He tinkered with some circuit simulator stuff and came up with a schematic and some build suggestions and posted HERE.

I took it upon myself to bread board this. Just for grins here's a picture of that spagetti bowl.

I liked what I heard and thought it would make a decent project for the DIY masses. A PDF file that contains the layout and parts placement for the compressor and power supply are found HERE. The PCB is found HERE. If you are making a stereo version with pots instead of rotary switches you need to use the resistors shown but only for one side. You then connect the FROM ATTACK of channel 2 to a point after the 1k circled in green (off of that long jumper is a good spot). On the previous version I failed to mention this so if you built it and got some distortion on channel 2 you now know why. If you are using roatary switches like me you can just put jumpers in place of all the resistors circled below. Connect the headers as shown on the schematic.

This is what the underside of the main PCB looks like after etching and mounting parts

Below is an an updated version of the circuit and power supply. It reflects the suggested values for attack and release pots and meter. The updated power supply shows the 1.5V supply for the bias on the full wave rectifier. On my circuit I used an LED which I got in a bubble pack from Radio Shack. After trying several ones I found that a white one which lights up red gave me exactly 1.5V but if you try other types it will get you close.

Below are some pics of the circuit board mounted in a chassis.

You will notice that changed a few things from the schematic. Mine is wired for stereo with only one set of controls for attack and release and a dual pot for threshold. I used two Lorlin rotary switches instead of pots for the Attack and Release controls. With only one set of controls you can leave out the fixed resistors mounted on the PCB for A/R and also C1. Here is a closeup of one of the switches.

Below are the resistor values that I used based on some common time constants. Play around with these and settle on ones you like.

Here's the front of the box. I used an old crossover chassis as a temporary solution for testing purposes. The meter was from a local surplus store and the labling was weird (for radio apparently) but it worked well. I'll put the whole thing in a nicer chassis when I get it back from the studio I loaned it to for evaluation.

Original prototype

Here's a couple of units in a nicer chassis. Large lettering for the seeing impared !

I have a few comments and thoughts on this compreesor:

• As PRR mentions, this will not replace your Fairchild. It will crap out out high levels of gain reduction. That being said, most of the time engineers won't push more than 10db or so of GR and mastering guy's often just do 1 or 2 db.

• This circuit is ripe for modding: dual mono/stereo switching, buffer for the meter and opamp balanced output come to mind.

• +4 inputs and outputs would be nice in some situations such as interfacing with other "pro" gear. I have chosen to stick with the original because it makes a very cost effective project (and I'm way cheap) and there are a bunch of -10 sound cards out there like mine. Running this on a mixer's buss insert didn't seem to pose any problem as long as I watched the levels going in and brought it back on a channel. Actuall was not always necessary as most inexpensive home studio consoles run at 0db internally anyway so I was close enough in most cases without the extra gain.

• The tube output amp section does look like fun. Maybe in the future...

• Gain reduction offest fine tuning might be an option to add. It's not on my layout so my suggestion would be just to get two tubes that match as close as you can - 12AU7's are cheap! In practice I haven't noticed any major shift in the stereo image without it (Bub Ludwig might differ).

• Putting a meter tapped off C1 might be cool. It would probably show a better representation of actual gain reduction. The needle would go "up" the more GR you have. This is how I tested the comp initially and it gave an "SSL mix buss comp" feel. In the end I chose putting it on the cathode which is more "old school".

- a BIG thanks goes to PRR who is the reason this project exists.
Also to the rest of the LAB gang who put up with my seemingly endless posts on this subject.

ERRATA

Kent Stevenson ©2005

*2/6/04 added pics

*3/15/04 fixed resistor values on layout

*5/28/04 fixed phase of channel one on layout. Added stereo link info.

*03/1/05 fixed resistor value on PS in the layout. Added newer chassis pic.

brought to you by Kent and PRR and the DIY Team at :
www.diyfactory.com