FET/RACK (Older Version)
Step 2 - Building the Power Supply

Posted by Hairball Admin on May 01 filed in Build Guides


This build guide combines instruction for building the rev A and rev D complete and partial kits.  Building the different revisions is very similar and the differences will be noted on this page. You may see a blue rev A PCB or black rev D PCB in the images, don't let that fool you, instructions apply to both PCBs unless otherwise noted.

Please note: If you're building a partial kit you may not have all the optional parts included in the complete kit and some of your components may vary slightly.

Building the Power Supply

I’ve probably said this a million times: STUFF, WIRE, AND TEST YOUR POWER SUPPLY FIRST!  You can avoid huge headaches by completing this section in full before stuffing the rest of the PCB.  A faulty power supply can damage everything that is connected to it, so make sure your power supply is working before moving on. On the MNATS PCBs, the power supply section is clearly marked with a box. 

The power supply section contains resistors, a regulator, diodes, polarized capacitors, and a wire connector.  If you are unsure how any of these parts are oriented, please skip ahead to  “Stuffing the PCBs” below and review how each component is inserted.  The 1 watt 1.1KΩ resistor dissipates a lot of heat.  Solder this resistor slightly raised off the PCB to avoid heat damage to your PCB.

Also note, the regulator footprint has a line on one side.  This line indicates which side the regulator tab should sit on.  

After installing the power supply components, including the wire connector, double check the orientation of the polarized capacitors, diodes, and the regulator.  Included is a clip-on heat sink for the regulator.  Once attached, you’ll be ready to start assembling the enclosure!

A fully stuffed rev D power supply is pictures above.

Enclosure Preparation

Before beginning the important step of wiring the FET Compressor power supply, we need to start with some assembly. Start with your enclosure back panel.  This first step is the most critical of all, preparing the safety or chassis ground.  Don’t mess around with this step, do it, and do it well.  The purpose of the chassis ground is to provide a direct path to ground for your enclosure.  It has some shielding benefits, but more importantly, it has life saving benefits.  Should any connection in your power supply fail and make contact with your enclosure, it will be come electronically charged.  With no chassis ground the next person to touch your enclosure will create a path to ground and be subject to serious or fatal injury.  If you have your enclosure properly grounded, any dangerous levels of electrical contact will create a near zero resistance to ground tripping your onboard or circuit fuse. I’ll be using one of the pre-drilled holes as my chassis ground, while the MNATS guide that follows will use a counter sunk bit to create a custom hole.  Either method will work.  What is important is that you have a dedicated hole for ONLY the chassis ground, the connection is secure, and you have a near zero resistance path to ground.  

Using some sand paper or a small Dremel grinding bit, remove the powder coat from around the hole directly next to the IEC inlet.  Be sure to get right down to the bare metal and clear 1/2” around the hole.  While you're at it remove the powder from around right XLR input and output mounting holes, output transformer mounting holes, and one of the input transformer mounting holes. Next, take the enclosure side panels and loosely attach them using the supplied 6-32 pan screws.

This enclosure is like any other component that has a lot of interconnected parts.  Start by loosely assembling the parts, then tighten all of the screws after all of the parts are loosely connected. Next grab the two PCB rails.  These are the two rails your PCB will mount to in your enclosure.  Why does the PCB sit on rail when it can be mounted to the bottom of the enclosure?  The answer is simple. Should you ever need to service or troubleshoot your PCB, simply remove the bottom and top of the enclosure for complete access to the top and bottom of your PCB.

Rail preparation, note the image above has the standoffs in the holes on the far right, for this build you should use the middle hole. Attach four of the aluminum standoffs to the rails using the supplied 4-40 screws.  Two in each of the MIDDLE circular holes and two loosely in each of the MIDDLE slotted holes (which is different than the images).  Next attach them loosely to the bottom of the side rails.  One in the bottom circular holes and one in the slotted holes using the supplied 4-40 screws and nuts. Next you’ll temporarily place either the top or bottom panel (they’re the same) on the enclosure.  You’ll attach this panel using nine supplied 6-32 flat headed screws.  Attach these screws tightly. This panel will help set the placement of the side panels.  Once it’s securely fastened, tighten the side panels and the rail in the circular hole at the rear of the enclosure.

Next remove the top/bottom panel and place the PCB on the standoffs using the supplied 4-40 screws. The PCB will set the position of the standoffs in the slotted holes as well as the rails in the slotted side panel holes.  Tighten these screws.

Now our enclosure is secure and we’re ready to finish the chassis ground.  The chassis ground requires a thick gauge wire.  Think about it.  If the chassis ground is ever called upon, it will pass a lot of current quickly before the fuse blows.  The last thing we want is our wire frying and disconnecting before the fuse blows.  Cut off 4in (10cm) of green 16AWG wire. I’ve started by attaching one end of the safety ground wire to one of the supplied toothed lugs.

Now attach the other end of the safety ground wire to the “ground” terminal in the IEC inlet.

There is no need to cover this ground connection with shrink tubing.  Finish the connection by securing the toothed lug to the hole cleared of powder using the supplied screws from your enclosure hardware bag.  You could also add a touch of a “Loctite” thread fastener to secure the connection.  However, make sure that it does not interfere with the lug and enclosure contact. For added safety consider covering the screw head with a sticker or writing a note by it.  The last thing you want is someone unintentionally loosening the safety ground.

The enclosure is powder coated so the contact made between the panel surfaces is not enough to create electrical continuity throughout the assembled enclosure.  Continuity is achieved through the machine screw connections between the top/bottom panels and side/back panels.  The top and bottom panels have counter sunk screw holes that are free of powder.  The side and back panels have machine screw inserts that are also free of powder.  The screw head makes contact with the top/bottom at the countersunk hole and contact with the side and back through the threaded insert.  To confirm your safety ground is working, connect the bottom panel securely to create electrical continuity between the sides, back, and bottom. Now grab your DMM and turn it to the resistance (Ω) Setting.  Place one probe on the ground lug and place the other probe in each of the side and back panels top threaded inserts.

Place your black lead in one of the threaded screw holes and your red on the ground lug. Confirm that your resistance reading is less than 1Ω between all panels.  If you have a larger reading you may need to tighten your connections or clear some additional powder.

Assembling the Meter Switch

Before wiring the power supply, let’s get the meter switch assembled.  This switch selects between gain reduction metering, +4 VU metering, and +8 VU metering.  Most important is our “OFF” switch.  This is important to note.  The FET Compressor, like the original, does not have an “ON” switch, but rather a switch that when activated turns the unit OFF.   The unit is turned ON by activating any of the 3 metering options.  Let’s start by grabbing our meter PCB. WAIT!!!  You may have received a meter and ratio PCB with your MNATS PCB.  DO NOT USE THESE PCBs.  They are for a rotary switch version of this compressor.  You are building the classic version with push button switches.  Use the PCBS that have "Hairball Audio" written on them.

Do not use the PCBs shown above for this build.

Shown here is the meter PCB that you should use. Your PCB may or may not have a black solder mask, that is not important. This PCB only requires two components.  A 3.6K resistor that is in series with the output and the +4 switch and a 8.2K resistor that is in series with the +8 switch. Install those two resistors then add the meter switch.  The switch is inserted with the smaller pins entering the top of the PCB.  The power switch is not mounted to the PCB.  Ensure the switch is flush to the PCB surface.

Now grab one of your switch brackets from the enclosure screw bag.  The switch needs to be inserted in a particular direction to ensure the holes will line up with the blind standoffs.

Once you’ve established the bracket orientation, insert your switch and mount them using the supplied 4-40 screws.

There is a bit of play in the mounting holes.  Try and keep them centered, but you may have to go back and adjust the positioning after you mount it in the front panel. The switch will most likely not sit at a perfect 90 degrees in relation to the bracket.  Now is the time to fix that.  Have a good look at the switch and determine if it’s sitting a little off from 90 degrees and fix the orientation by holding the bracket and applying some pressure to the switch. You want to get as close to 90 degrees as possible.

Apply pressure to set the switch at a 90 degree angle.

Now mount your switch and bracket into the meter switch bank slot on your front panel using the supplied 4-40 screws.  Again, there is some play and room for movement.  You want the switch to sit square and have clearance around all sides.  This is achieved by adjusting the switch to be at a perfect 90 degrees from the bracket and by adjusting the switch/bracket and switch/panel standoff screws.  It’s not hard.

Wiring the Power Transformer

Start by mounting the power transformer.  It’s mounted to the hole on the side panel in the back right rear of the enclosure.  Supplied with the transformer are two foam pads, a mounting plate, screw, bolt, and nut. One foam pad is placed between the transformer and the side panel, the other is placed on the opposite side (the “top”), then the mounting plate.

Power transformer from left to right - metal cap bracket, foam pad, transformer, foam pad.

If you want to get into DIY, you need to learn how to wire a power transformer.  I’ll warn you now that at no point will we tell you how to wire your transformer primary based on lead color.  There is a simple and good reason for this, there is no common color coding between transformer companies or even models.  The color code is always on the side of the transformer.  Let’s learn how to read it! A power transformer has two windings.  These are literally two long pieces of wire, each wrapped around an iron core.  Transformers do a few things, but this transformer's job is to “step-down” your 120/240V mains to something closer to what is required for the compressors power rails.  The transformer has nothing to do with converting AC to DC, that happens in the power supply.  All voltages that we refer to on both the primary and secondary are AC voltages.

Shown above is a simple transformer with two windings.  The primary winding is on the left and the secondary is on the right.  Each winding is labeled with a dot on one end.  This dot indicates the start of the winding.  This is important when using transformers with multiple windings and for matching polarity.  You may note that the transformer in the Hairball kit also indicates “0V” or “25V/0.6A” at each end of the secondary windings (image below).  IGNORE THESE, THEY ARE VERY MISLEADING IN OUR APPLICATION.  All that is important when looking at the secondary is that you see that you have two 25V windings and the start of each winding is labeled by a dot.

The FET compressor requires a secondary that has two separate (dual) windings.  One for the +30VDC rail and one for the -10VDC rail.  The primary winding needs to be rated for your local mains voltage (120/240).  Because Hairball Audio ships all over the world, we’ve selected a transformer with dual 120V primary windings as well.  As you’ll see, they can be configured to accept 120V or 240V mains.  For these reasons your transformer has dual primary, and dual secondary windings.

Let’s start with the secondary.  It’s wired the same way regardless of mains voltage.  It’s a “center-tap” or “split secondary”.  The end of winding 1 and the start of winding 2 are tied together and connected to the CT pad on the main PCB (image above).  This creates our 0V reference. 


Again, these labels are misleading, ignore them.

That leaves two wires and two pads labeled AC.  Connect one wire to one pad and one wire to the other.   Either wire to either pad, orientation is not important.  When wired this way the secondary’s outer leads have the same AC voltage (the winding’s rating - 25VAC) when measured referencing the CT.  Now you have two 25VAC supplies.  One will be rectified and regulated to create the 30VDC rail, and the other will be rectified to a negative voltage and reduced to -10VDC.

On the primary winding there are two choices.  You can wire in parallel for 120V mains, or in series for 240V mains.  Though polarity does not apply with this secondary configuration, for the sake of consistency connect the start of the winding to the IEC L or live terminal and the end to the IEC N or negative terminal.

Below is a primary wired for 120V mains.  It’s in PARALLEL.  The start of each of the windings are connected and soldered to L, and the ends are connected and soldered to N.

Shown below is a primary wired in series.  The start of winding 1 is connected to L.  The end of winding 1 is connected to the start of winding 2 and are only solder to each other and covered with heat shrink.  The end of winding 2 is connected to N.

So how do you translate this to your power transformer?  It’s very simple!  Look at the side of your power transformer and you should see your windings with the lead color displayed.  By referencing the lead colors and what you’ve learned here, you’re now prepared to wire your power transformer. Now for a little “tough love”.  If you still have an issue understanding how to wire your power transformer, DIY might not be right for you.  If you insist on moving on, have a professional wire your power section. If you are at all serious about DIY electronics, and I hope you are, please take the time to read the full MNATS guide and review the Mark Burnley wiring diagrams.  The information included on these web pages will teach you all of the crucial basics of transformer selection and wiring for any project you may attempt down the road.  You will be thankful you did. Alright, let's wire this bastard. Some of these images are in black and white.  Why?  Some people are lazy and prefer to just look at pictures rather than learn something.  If someone chooses to ignore the previous section and wire based on the lead colors in these images they may do damage to themselves and their projects.  If you're not 100% sure, go back and read this section from the beginning. Start by wiring the L and N connections above (based on your mains voltage) to each of the two middle power switch lugs and cover each with half a piece of shrink tubing.  Which goes to which is not important in this step.

Note: I am in the United States and have a mains voltage of 120V, therefore I wire my primary in parallel.  If you refer to the diagram above, you'll see that this means that I have two wires connect at each lug.  The two "L" wires and the two "N" wires.  If I lived in an area that has 240V mains, I'd wire in series.  If you refer to that diagram you'll see that two of the wires are connected to each other and only one wire connects to "L" and one to "N".  For 120V mains you'll want to use two half pieces of large shrink tubing and for the single wire 240V main you can use  two half pieces of medium shrink tubing to cover these connections.

Now take your black and white 16AWG wires and twist them together.  Connect the black wire to the "L" terminal on the IEC inlet and cover with half a piece of large shrink tubing.

Connect the white wire to the "N" terminal on the IEC inlet and cover with half a piece of large shrink tubing.

Now connect the other end of this white and black pair to the terminals closest to the front panel on the switch.  Attach the black wire on the same side as the "L" lead(s) from the primary.  These are the lead(s) marked with a dot.  Connect the white wire on the same side as the "N" lead(s) from the primary images. Cover both with half a piece of medium shrink tubing.

Looking at the image above will help you understand the mechanical function of the "Off" switch.  The FET Compressor, like the original, has an "Off" button rather than an "On" button.  When +4, +8, or GR metering is selected the off button is out.  The middle and lug closest to the panel are connected closing the power circuit and allowing for current flow.  When you turn the unit off, the button slides in and the middle is connected to the empty lugs furthest from the front panel opening /breaking the current flow. To wire the secondary, let's start by grabbing the power connector and hardware.

Using the nut and bolt in the top hole, locate the four screw holes that form a rectangle.  Secure the connector to the chassis using the top hole closest to the front panel.

The terminals across from one another are electrically connected and wires are secured with a screw.  Connect the secondary CT in the middle and the the 25VAC connections on either side.  Now cut two 10in(25mm) pieces of red 22AWG wire and one 10in(25mm) piece of black wire and twist them together.  Connect the black across from the CT and the red wires on either side.  Confirm all connections are screwed and secure.

Connect the other end of the wires to the terminal blocks on your main PCB.  Black to CT and one red wire to each AC pad.

We're done!  

Well not really, we need to test the power section to confirm it's working correctly. Start by removing the fuse holder from the IED inlet and placing a fuse in the holder.  Insert the fuse and holder back into the IEC inlet.  YOUR UNIT MUST BE POWERED ON WITH THE FUSE IN PLACE FOR THIS TEST.

Grab your multimeter and set it to read DC volts.  Place the common lead on the CT terminal of the secondary (at the power connector or PCB header) and place the lead on the +30V lead marked on the PCB.  This is near the big 1W resistor (R87).

You should get a reading of +30V DC.  A tolerance of  +/- 5% is fine, anymore than that and you need to check for errors.

Keep your common lead on the CT terminal and use the other lead to test the -10V DC point by CR6.

Again look for a DC voltage that is within 5% of -10V DC.

If all your readings look good, you've successfully built your power supply and can move on.  Don't continue until you have successfully completed this step.  

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