Wiring Your FET Compressor
Much of the text and some photos have been borrowed, with permission, from the MNATS wiring guide. The MNATS guide is a wealth of information for wiring all FET Compressor revisions and general wiring info. This version is pared down and written specifically for the complete revision A kit.
Install the VU Meter
Before we load the front panel with parts, let's take the time to install the VU meter. The assembly consists of the meter, a bezel (frame), and two long headless screws and tabs. You may or may not want to remove the previously installed meter switch to complete this step, it really depends on how good you are at working in tight spaces. Start by inserting the bezel into the front of the front panel so it sits flush.
Flip the panel over and set the meter into place. You'll feel the spot where it naturally wants to sit and where the flat spots on the meter sides will line up with the slots in the bezel.
Now insert the tabs on either side. The tabs have a bit of a "V" shape and should be inserted so the "V' bottom is facing the back of the panel.
Now use the supplied screws to lock the assembly in place. Make sure the slotted side of the screw is facing up and the tapered end is against the panel. DO NOT OVER-TIGHTEN! Just lock it in place, over-tightening will bow the front VU scale plate and interfere with the needle. Most likely you won't even need a screw driver as hand tightening should secure it.
If you install the tab correctly and don't over-tighten, the screw should sit at about a 90 degree angle from the panel.
Input Circuit Wiring
Start by wiring the input (female) XLR as shown.
Use a short, thick piece of bare wire, like a clipped diode leg, to attach pin one of the XLR to chassis ground. By attaching pin one to the chassis, the chassis becomes an extension of your input source's shield. (Spoiler: you'll be doing this to the output/male as well so you might want to do it now)
When attaching the XLR input connector to the chassis itself, ensure that the body of the XLR makes electrical contact with the chassis checking at one of the bare screw sockets or safety ground. As stated earlier, you'll need to sand some paint off the chassis like you did for the safety ground. Test continuity between the chassis and the body of the XLR with your multimeter.
Start by wiring the input section of your compressor. This is how the audio gets from the input XLR to the input of the main PCB. Using the shielded 2-conductor wire, you'll want to wire Input XLR->T-Pad Input Attenuator->Input Transformer->PCB Input. The trick is that you want to have a shield connected at the input XLR pin 1 that is constant through to the input PCB but left unconnected at that end. To start, strip and solder a 12in (30cm) length of the two conductor shielded cable to the XLR as shown - shield to pin one, red wire to pin two, and black wire to pin three. This wire will slip out of the jacket if you strip an inner conductor and hold the jacket, you may have to hold the end of the conductor with pliers as you strip it.
Connect the other end of this cable from the XLR to the input of the T-Attenuator printed circuit board - red wire to the input positive and black wire to the input negative. The shield stays disconnected for the moment.
Take two 6in (15cm) pieces of two conductor cable and strip back 2in (5cm) of jacket and foil wrap.
Place a full piece of small heat shrink over each of the ground wires and shrink it with a heat gun.
Wait for the shrink tubing to cool COMPLETELY then place a full piece of small heat shrink over one of the ground leads and a full piece of medium over the other. Do not heat shrink these (yet).
Now cross about 0.25in (6mm) of the leads over one another and twist them together.
Solder the twisted wires QUICKLY making sure not to melt the un-shrinked heat shrink. You don't need to go crazy with the solder, just apply enough to secure the twisted wires. Once the joint has cooled, slide the two heat shrink pieces over the exposed wire to completely cover this wire and use your heat gun to shrink it in place. Trim the inner conductors back and strip back the jacket to expose some bare wire and cut a medium piece of heat shrink tubing in half and shrink on over each end as shown for a cleanliness.
Now connect the stripped inner conductors from your input transformer PCB, the red on positive and the black on negative. Note they are reversed with respect to one another on the input PCB.
Run the long shrink wrapped shield under the PCB. I'll explain why all this is necessary shortly.
Now, mount the input PCB to the side panel with the output side facing the rear of the enclosure using 4 stand-offs and 8 4-40 screws. Scrape or sand some of the powder from around one or more of the holes where the stand-offs mount and ensure that there is continuity between your input transformer outer can and the enclosure.
Strip back some of the jacket from the wire connected to the input side of the transformer PCB and connect it to the t-pad PCB output. Red on positive and black on negative. Take the shields from the two shielded cables that meet at the t-pad, one from the input XLR and the other from the input side of the transformer PCB and tie them together and solder them.
Take the wire attached at the output side of the input transformer PCB and strip back the jacket, trim off the shield, and place a half piece of medium shrink tube over the end.
Insert the wire into the main PCB input with the red wire on "input" and the black wire on the ground symbol.
Here is what you've just made.
Congratulations, you've done the trickiest part of the audio wiring. So what did we make here? We've created a constant shield along this input section that is grounded at one end (XLR). The shield acts as an antenna creating a low resistance path to ground for noise. It's connected at one end to avoid ground loops.
Ratio Circuit
Although the ratio and meter PCB show a footprint for a molex style header, I've decided not to use them in this kit because: a) they require a specialized tool; and b) are not as dependable as soldering directly to the PCB.
Start by cutting the wires below to length.
| Pin Number | Wire Color | Wire Length |
| 1 | Brown | 5in(12cm) |
| 2 | Black | 3in (7cm) |
| 3 | Violet | 8in (20cm) |
| 4 | Green | 3in (7cm) |
| 5 | Grey | 8in (20cm) |
| 6 | Shielded | 8in (20cm) |
For pad 6, strip the jacket back from the shielded wire and clip off the ground wire and black wire. We only need the red wire on this end. Place half a medium shrink tube over the end to prevent the cut wires from shorting to the red wire.
Solder each wire to the ratio PCB in the order shown in the table above.
Attack and Release Potentiometers
During this next step you'll want to mount your control pots to the front panel. You'll note that the pots have a locating tab that is not used on our front panel. This tab will prevent the pots from sitting flush on the panel. You can simply cut them off with your cutters. Also the attack, output, and release bushing is slightly smaller then the hole. Just center them by eye.
Mechanically secure a resistor and film capacitors in parallel to the attack potentiometer as shown. Refer to this table for their values.
| Rev A | Rev D |
| 1.2KΩ Resistor | 470Ω Resistor |
| 0.012uF Capacitor | 0.022uF Capacitor |
Next, attach a new yellow wire around 6in (14cm) long to the counter-clockwise lug.
The rev A attack pot shown above. Secure a 270KΩ resistor (both revisions) to the release potentiometer (5M). Attach a new brown wire about 6in (15cm) long to the CCW lug. Next, attach the brown wire from the ratio PCB assembly we made in the earlier step. Solder all wires to the pot.
Prepare one end of a 13in (33cm) long piece of a new shielded wire as you did earlier. Leaving only the red conductor and adding half a piece of medium shrink tubing. Cut a new piece of 3in (7cm) red wire. Attach both to the remaining attack pot lug. Ignore the attack switch wiring for now.
Take the other end of the shielded cable attached to the attack pot and clip away the black conductor and cover with half a piece of medium shrink tubing.
Attach the other end of the shielded wire coming from the attack pot CW lug to the main PCB - shield to ground and the inner conductor to pad 7.
Shown above is the shielded cable from attack pot to main PCB. Connect the black wire from the ratio PCB pad 2 to lug "B" of the SPDT switch on the back of the Alpha attack pot.
This is the "normally open" (NO) lug of the switch, that is, when the pot is turned in any but the most counter-clockwise position this lug is open circuit with respect to the common lug.
Cut a new piece of 8in(20cm)shielded wire. Strip the jacket on both ends and cut off the black conductor leaving only the red conductor and shield on both ends. Leave these about 0.5in (1.5cm) long and cover both ends with a half piece of medium shrink tubing. Attach the ground from one end to lug "B" of the attack pot along with the black wire from the ratio PCB. Attach the red inner conductor of the shielded cable to lug "C" (common) of the SPDT switch. Next, run the green wire from the ratio pcb assembly pin 4 to lug "A" of the SPDT switch. Solder the wire in place.
Here is a diagram of the release and attack wiring for review.
Take the other end of the shield of the shielded cable to the ground pad next to pad 22 on the main PCB. Solder the inner conductor of the shielded cable to pad 22 on the main PCB.
Solder the violet wire from the ratio board connector to pad 20. Solder the brown wire from the release pot to pad 18. Solder the grey wire from the ratio board plug to pad 21. Solder the yellow wire from the attack pot to pad 19.
Output and "0" Set Potentiometers
Cut a new two conductor shielded cable 12in(30cm) long and attach it to the output potentiometer (250K). Attach the shield to the CCW lug.
Prepare the last wire from the ratio board connector (pad 6) as follows. Strip a fairly long piece of the outer insulation. and clip off the black conductor.
Shown above is the other end of cable from pad 6 of the ratio PCB with the black conductor cut. Slip a piece of 1/6 shrink tube over the long shield and heat it. Cut the inner conductor somewhat shorter and strip the end of it.
Now attach this assembly to the output pot. Tie the two shields together and solder them in place. Secure the inner conductor of the shielded ratio board wire to the CW lug of the output pot. Solder all connections.
Attach the output pot to the front panel. Solder the other side of the output pot two conductor shielded cable to the main PCB. The shield goes to pad 16. The CW lug goes to pad 15 while the wiper (center) lug goes to pad 17.
Blurry pictures kind of suck. Here is a diagram.
Note that the pads are not in numerical order!
Cut new pieces of yellow, white, and orange wire 3.5in(9cm) long and solder the "0" set potentiometer as shown. Note the that PCB wire order should be in the same order as the 0 pot wire placement. Use the extra nut supplied as a spacer behind the pot bracket.
Meter Circuit and Output
Cut wires to the lengths listed in the table below and twist them in pairs: Red/Blue, Orange/Green, and White/Black.
| Pin Number | Wire Color | Wire Length |
| 1 | Blue | 20in(50cm) |
| 2 | Red | 20in(50cm) |
| 3 | Orange | 20in(50cm) |
| 4 | Green | 20in(50cm) |
| 5 | White | 8in(20cm) |
| 6 | Black | 8in(20cm) |
How do you twist wires anyway? It's easy and will greatly reduce noise.
Make sure these red and blue VU meter wires are long enough to route them well clear of the input circuit on the main PCB or you will have problems with feedback at high gain.
Attach the black and white wires from pins 5 and 6 (both marked VU) from the meter board connector to the VU meter as shown. The polarity is not important but be reasonably quick with your soldering. There are wires internally connected at the other end. Heat this for too long and they'll drop off.
Solder the green and orange wires from pins 3 and 4 (marked 28 and 29) of the meter board connector to pads 28 and 29 of the main PCB. Pin 3 of the meter board connector goes to pad 28 of the main board and pin 4 of the meter board connector goes to pad 29 of the main PCB.
Attach the wires from the output transformer to the main board as marked. It is virtually impossible to screw up this step if you simply look at the markings on the board or refer to the correct overlay available on the MNATS site noted earlier in this guide.
Don't refer to outdated or incorrect drawings when wiring this or any other step. Just use the proper verified colors printed directly on the board. From the top, they are:
Brown
White/Black
White/Red
Black
Grey
Violet
Take the red and blue twisted pair. Connect blue to "X" and red to "Y" on the meter PCB. Solder to the male output XLR the same way you did with the input connecting lug 1 to the ground tab with a piece of cut diode lead. Now solder the blue wire from the meter board pad X and the blue wire from the output transformer to the output XLR lug 2. Next solder the red wire from the meter board pad y and the red wire from the output transformer to the output XLR lug 3. Pin 1 is soldered to the tab just like the input XLR to connect the XLR to the enclosure.
You'll notice that you have two wires remaining on the output transformer orange and yellow. These are simply tied to one another to create a series connection. Use a piece of small shrink tubing to avoid shorts.
Solder one end of your remaining 16AWG green wire to the ground pad (- lead) by C25. The rev A has a dedticated pad for this.
Solder the other end of this wire to a lug and attach it with star washers to a convenient chassis ground point (not the ground for the IEC).
Wiring an Incandescent Meter Lamp
The most efficient way to power the VU meter lamp is directly from the power transformer secondary AC current. There is no need to rectify (change to DC) it as the incandescent lamp will happily run from this AC current.
However, the voltage at the transformer secondary is surely higher than we need to power the VU lamp, which is ordinarily a nominal 12 Volts. Even then, the lamp will last longer and probably look better when running from a lower voltage. The light from the VU meter lamp can be blinding in an otherwise darkened studio environment.
So a resistor is needed to drop the voltage. In order to select a resistor we need to know a few things:
- The transformer secondary voltage under load
- The current drawn from the lamp
- The voltage we would like to see at the lamp itself
The voltage of a single secondary winding is easily measured if you have finished the previous wiring so far. Set your meter for AC Volts and measure from the center tap at the terminal strip to either of the secondaries. Using the Avel-Lindberg toroidal transformer sold by Hairball which is already hooked up and powering the rest of the circuit, the measurement is 30.4V.
Measuring the current drawn from the lamp in the meter sold by Hairball gives a reading of 89mA.
Powering the lamp at around 11 Volts seems to give a decent glow.
So 30.4V - 11V = 19.4V. We need to drop 19.4 Volts across the resistor.
19.4/.089 = 217.99. So we need a value of around 218 Ohms for the resistor. Choosing a standard value close to that we settle on the supplied 240 Ohm 3 Watt resistor.
Wire one side of the VU meter lamp terminals to the center tap connection of the power transformer secondary. This is the same connection that feeds the CT pad on the main PCB. Slip a piece of insulation over one side of the 240Ω power resistor. Cut the other end of the resistor lead short and mechanically secure a wire to it by creating a hook on each side. Solder this connection and slip a piece of heatshrink over the junction. Shrink the heatshrink over the solder joint and exposed wire.
Loosen either of the two outside terminals of the power transformer secondary terminal strip and insert the wire from the prepared assembly and tighten. Attach the other end to the remaining terminal of the VU meter lamp.
Leave the resistor suspended in the case without touching other wires or components. It will get hot!
Your wiring is now complete!
DO NOT plug in your compressor and try to pass audio to see if it works. It does not work. The unit NEEDS to be calibrated to work so let's do that now!