73P Assembly Guide

Safety Precautions, Warranty, and Disclaimer Improper soldering and handling of electricity can cause serious injury and damage to your property. Read and understand the instructions below before beginning your project. Follow the instructions, build carefully, and use the appropriate tools. Build at your own risk. DIY Recording Equipment, LLC is not responsible for any damage or injury resulting from the assembly or use of your kit. You are the manufacturer of your kit. It is your responsibility to turn this group of parts into a working piece of recording equipment. DIY Recording Equipment, LLC does not guarantee the success of your project and disclaims any Implied Warranty of Merchantability. Please visit the support forum for assembly support.
Damaged or Missing Parts All kits and parts are checked before being shipped to you. If something arrives damaged or if your kit is missing a part, please open a support ticket to inquire about a replacement. Missing parts will be replaced at our expense. Damaged parts should be returned for verification. If the part shows signs of use beyond what was necessary to determine that it was damaged, DIY Recording Equipment, LLC reserves the right not to replace the part.
Welcome to the 73P assembly guide.

Thank you for purchasing a 73P.

If this is your first DIY project ever, we recommend reading our Getting Started Guide.

The 73P is meant for intermediate builders who have experience with PCB soldering and using a multi-meter. Our number-one tip is to take your time. There are 180 parts in this kit and as many opportunities for Murphy's Law to come true. So don't hurry, enjoy the build, and always "check twice, solder once."

In order to catch any mistakes early, we've included test steps at the end of some of the sections. We highly recommend doing all of these tests, especially for your first 73P. If your unit fails a test, contact us via the "Help" button below and we'll help you fix it. It's much easier to find the problem when the entire unit is not built yet.

Best of luck and enjoy the process!

Required Tools top

Soldering Iron
We recommend an adjustable-temperature station. The smaller, pencil-style ones that plug directly into the wall do not have enough power to create good joints consistently.

Solder
You can use 60/40 "leaded" solder or lead-free. We recommend 60/40 because it is easier and more forgiving to use.

Wire Cutters
A good pair will be sharp and have a fine point for cutting leads down right at the solder joint. Don't use a set of large cutters meant for household wiring.

Phillips Head Screwdriver
A #1 Phillips head screwdriver.

Optional Tools top

Multi-meter
If you run into problems, a multimeter can come in handy. You can also use it to sort resistors.

Desoldering Pump
If you accidentally solder something in the wrong place, a desoldering pump can save the day.

500EXT Extender
A 500-series extender like the 500EXT will make testing and troubleshooting easier.

Room EQ Wizard (REW)
REW is an incredible piece of free measurement software for PC, Mac, and Linux. While REW is primarily known for measuring room acoustics, it's also a powerful tool for measuring analog gear. Download and install the latest version at the Room EQ Wizard website.

Resistor Sorting Sheet top

In order to save room, the resistor values are not printed on the PCBs. To identify the resistors, refer to the Component Sorting Sheet (PDF).

Interactive Build Map top

To easily find parts and keep track of which ones you have populated, use the 73P Interactive Build Map.

Power and Miscellaneous top

In this section we will populate the power supply and some miscellaneous parts that don't fit in other sections. Note that the parts of the circuit discussed in each section don't always correspond exactly to the parts assembled in that section. In this section, for example, we will populate some parts that are not discussed until later sections.

For this and future sections use all of the parts in the corresponding bag number. The bags are split up into smaller bags to make keeping track of parts easier, but the smaller bags do not correspond to individual steps.

One of the main challenges of adapting a vintage Neve circuit to the 500-series is achieving the correct power voltage. The vintage consoles ran on a single +24v rail, while 500-series modules run on dual +/-16v rails. Our solution to this is to regulate the +16v rail down to +8v and treat the -16v rail as 0v. This creates a spread of 24v between the rails (+8 to -16), which from the components' perspective is exactly the same as being powered by a +24v supply.

+16v power passes first through F1, which is a self-resetting fuse. This protects the 500-series rack in case there is a short on the 73P. Then it passes through a regulator circuit made up of R44, D3, and T9. R44 and the zener diode D3 set the voltage to 9v. The transistor T9 acts as a buffer, providing the current the circuit needs. T9 also drops the voltage about 1v, giving us +8v. The small capacitor C28 filters noise out of the power rail. Power is fed to the gain blocks of the preamp through R19 and R42. The net N goes to the PRE 1 and PRE 2 stages, while M goes to the Output. Finally, the big capacitors C11, C12, and C27 provide reservoirs of current for the preamp circuitry.

Bend Resistors top

Bend the resistor leads 90 degrees at the body so they can be inserted into the PCB.

Sort Resistors top

Unlike other projects you may have built, the 73P does not have all the part values printed on the PCB. Where they are not specified, reference the parts list in that step of the guide, the Resistor Sorting Sheet, and/or the Interactive Build Map.

To identify resistors, you can either read color code printed on them or test them with a multi-meter set to read resistance. (Pro tip: to measure resistors quickly you can remove the probes from your multi-meter and place the leads of the resistor directly in the probe sockets.)

Reference
Value Color Code
R15, R16 4.7k yellowpurpleblackbrownbrown
R17 1k brownblackblackbrownbrown
R18, R21, R22 6.8k bluegrayblackbrownbrown
R19, R44 270R redpurpleblackblackbrown
R20 100R brownblackblackblackbrown
R42 12R 1W Markings: 12R 5% 1W
R43 1.5k brown greenblackbrownbrown

Place Resistors top

Place the resistors into their respective positions on the PCB. As you place each resistor, bend its leads against the bottom of the PCB so that it stays in place during soldering.

Place all of the resistors flush to the PCB except for R42. For R42, leave about 1/8" of space between the resistor and the PCB. This will allow air to flow better around this resistor which generates significant heat.

Solder Resistors top

Solder the resistors to the PCB. Observe good soldering technique: heat the pad and lead for 2-3 seconds, apply a small bit of solder, and continue to heat the pad for another 2-3 seconds. The finished joints should be shiny and should have just enough solder to cover the pad entirely.

Trim Resistors top

Use clippers to trim away the excess leads. Clip as closely as possible to the joint without clipping the joint itself.

Populate Diodes top

Place each diode so that the line on the component matches the line on the PCB. Double check the orientation, then solder and trim.

D2
1N4004 diode

Markings: 1N4004

D3
9v zener diode

Markings: 85C 9V1

Populate Small Capacitors and Fuse top

Place the smaller capacitors and the fuse in their respective places. These parts are not polarized and therefore can be placed in either direction. Solder then trim the leads.

C26
.01u cap

Markings: 10n 250V BC

C28
.1u cap

Markings: BC104

F1
Self-resetting fuse

Populate Large Capacitors top

These capacitors are polarized, so they must be placed in a certain direction. The positive lead is slightly longer, while the negative lead is marked with a stripe on the body of the capacitor. Place the capacitors with the positive lead in the pad next to the "+" marking on the PCB. Double check their orientation, then solder and trim.

C11, C12
470u cap (x2)

C13
100u cap

C27
1000u cap

Place TRS Jack top

Remove the nut and washer from the TRS jack and set them aside. Place the TRS jack. Hold it in place with tape or by bending the leads, then solder and trim.

J1
TRS jack

Place 48v LED top

Place the red LED in the LED1 position on the bottom side of the PCB. Orient the LED so that the longer lead goes in the "+" hole. Notice that the leads have a flat segment about 1/4" from the LED body. Insert the LED until the bottom of this flat segment, then bend it towards where the front panel will be. Bend the leads against the other side of the PCB, but do not solder yet.

LED1
Red LED

Solder 48v LED top

Next we'll ensure the alignment of the LED by putting on the front panel. Place the PCB in the bracket and fasten it with at least two screws from bag 8.1. Attach the front panel with the washer and nut of J1. Confirm that LED1 is sitting flush with the front panel, then solder and trim the leads. Then remove the front panel, take the PCB out of the bracket, and put the screws back in bag 8.1. Set the washer and nut for J1 aside until step 8.3.

Place Switches top

Place the switches and use a small screwdriver to bend a couple of the leads against the bottom of the PCB. Solder and trim.

SW1-SW4
DPDT switch (x4)

Populate Relay top

Place the relay so that the white line on the body aligns with the line on the PCB. Bend some of the leads against the bottom of the PCB to hold it in place. Double check the orientation, then solder and trim.

U2
DPDT relay

Populate Header and Socket top

It can be tricky to mount headers and sockets exactly perpendicular to the PCB. Our preferred technique is:

  1. Fill a single pad in the middle of the header's footprint with solder.
  2. Place the header and press gently against the solder joint while heating the edge of the soldered pad.
  3. After the solder liquifies and the header snaps into place, remove the soldering iron and hold the header in place until the solder cools (about 5 seconds).
  4. Now solder the rest of the pins of the header from underneath.

For the X2 header, make sure plastic tab aligns with the line on the PCB.

CON2A
8-pin socket

X2
4-pin header

Assemble Heat Sink top

The two large transistors T8 and T9 will be attached to the same heat sink. Therefore, they must be electrically isolated so they don't short to each other through the heat sink. Assemble the parts in this order, from left to right:

  1. Screw
  2. Shoulder washer, place the narrower part inside the transistor
  3. Transistor 1
  4. Insulator pad
  5. Heat sink (with the two pins facing the same direction as the transistor leads)
  6. Transistor 2
  7. Black washer
  8. Nut

Tighten the screw just enough that the assembly holds together.


Insulator pad


Shoulder washer


Screw


Washer


Nut

T8, T9
TIP3055 transistor (x2)

H1
Heat sink

Place Heat Sink Assembly top

Place the assembly so that the side with the insulator pad and shoulder washer goes in T8. Align the assembly so that all the parts sit nicely in the PCB, then tighten the screw. Solder and trim the transistors.

Test Power top

For this and all following test steps, connect and power up the 73P before testing and power down and disconnect after.

  1. Set your multi-meter to read DC volts. (If your meter has multiple DC ranges, set it to 200v DC).
  2. Probe the 0v test point with the black probe. For all future tests with the multi-meter, the black probe goes on 0v.
  3. Probe the V+ test point with the red probe. The voltage should be within 1v of 24v.
  4. Engage the 48 switch (3rd from the top). LED1 should illuminate.

Did your 73P pass the first test? Great! Proceed with confidence. If not, double check your solder joints, the resistor values, and the orientation of D3. If these are all correct, please contact us via the "Help" button below.

Optional: Request Soldering Feedback top

We invite you to pause at this point and send a photo of your solder joints for feedback. Click the "Help" button below and send us an email with the subject "Soldering Feedback" and a hi-res photo of the bottom of your PCB. We'll take a look as quickly as we can (within a few hours Mon-Fri) and send a thumbs up or a tips on how to improve.

Direct Input top

The DI circuit is a simple buffer based around an ultra-low noise JFET. R28 sets the input impedance at 1M Ohms, which is ideal for instrument pickups. R29-R31 set the DC operating point of the JEFT, and C16 and C14 keep this DC isolated from the rest of the circuit. The JFET, T5, takes the high-impedance instrument input and turns it into a low-impedance output that can be sent to the preamp. R25 and R26 attenuate the input signal 35dB to microphone level.

Populate Resistors top

Place the resistors in their respective places, bend the leads, solder, and trim.

Reference
Value Color Code
R23 2.7k red purple black brown brown
R24, R27 470k yellow purple black orange brown
R25 4.7k yellow purple black brown brown
R26 47k yellow purple black red brown
R28 1M brown black black yellow brown
R29 2.2M (brown) red red green gold
R30 100k brown black black orange brown
R31 120k brown red black orange brown

Populate Other Components top

Populate the diode, transistors, capacitors, and relay. The diode, electrolytic capacitors, tantalum capacitors, transistors, and relay are polarized. On the tantalums the positive lead is longer and marked with a "+". Orient the transistors according to the outlines on the PCB. The positive Also note that the two transistors are different. T5 is silver on top and the footprint on the PCB is filled in.

D1
1N4004 diode

T4
2N7000 transistor

Markings: 2N7000

T5
LSK170 transistor

Markings: LSK170>

C14, C16
22u tantalum cap (x2)

Markings: 226F

C15
.1u cap

Markings: BC104

C17
100u cap

U1
DPDT relay

Test Direct Input top

  1. Calibrate REW with a multi-meter.
    1. Connect a TRS cable to the first output of your interface. This output might be called "1" or "L."
    2. Open the Generator in REW and send a 60Hz, -10dBFS signal to the plug.
    3. Set the multi-meter to read AC volts. When testing AC, it does not matter which probe goes to which point.
    4. Probe the Tip and Ring of the plug and note the value.
    5. Press "Calibrate Level" and enter the value.
  2. Switch the dB unit to dBu. Then set the generator to 60Hz, +4dBu.
  3. Connect the TRS plug to J1
  4. Place the probes on the DI and 0v test points. Your meter should read about 0.6V.

Balanced Input top

This section does the job of receiving the balanced input from the microphone and converting it to an unbalanced signal that can be used by the rest of the preamp. This is delicate work because any noise introduced here will be amplified many times by the preamp. This is why the input transformer X1 is in a metal can to shield it from interference.

Some parts of this circuit block we populated already in sections 1 and 2, but we'll explain them here. R15, R16, and R17 make up the LINE input. These resistors form a voltage divider that attenuates the input signal 20dB and increases the input impedance to ~10k Ohms when the LINE switch is in. Phantom power is sent to the mic input through R21 and R22. R20 and C13 give the phantom power a bit of a soft start when the switch is engaged. And R18 sets the brightness of LED1. The Lo Z switches the wiring of the transformer primaries from series (default, normal impedance) to parallel (low impedance). The transformer performs several functions: it eliminates hum by isolating the grounds of the input and the rest of the circuit, it amplifies the input signal 6dB (9dB with the Lo Z switch in), and it converts the balanced input signal to unbalanced. The capacitor C39 makes the frequency response of the transformer more linear by damping ringing.

J1 feeds into the DI circuit and also triggers a switching circuit. T4 is a transistor configured as a switch. When a plug is inserted in the DI jack, it changes the state of the transistor, which in turn changes the relay U1. By default, with nothing in the DI jack, U1 sends signal from the input transformer to the rest of the circuit. With a plug in the DI jack, it takes the signal from the DI circuit instead.

Place Capacitor top

Locate the rectangular transformer sub-board. Place the capacitor in the C39 position and solder. This and the rest of the silver, polystyrene capacitors in the kit are not polarized. Make sure to trim the leads very close to the solder joint, as we will be installing the input transformer on top of them.

C39
180p cap

Place Header top

Place the longer, straight pins of the 8-pin header through the transformer board so that the black plastic posts sit flush with the PCB. Double check that you have inserted it on the correct side of the board: the plastic posts should be sitting in the X1B position on the same side of the board as C39. Solder and trim the leads close to the joints.

X1B
8-pin header

Place Transformer top

Remove and discard the two screws from the brown side of the transformer next to the pins.

Place the transformer on the opposite side of the board from C39 and X1B. Pay close attention to the pin numbers printed on the transformer and on the PCB. Double check that they are aligned before soldering. You can put the transformer in backwards. And if you do it's difficult to remove and expensive to replace.

Attach the board to the transformer with the two black screws. This will require a bit of force, as the screws are cutting the threads. If the board starts to bend, back them off a bit. Then solder the and trim the transformer pins.

Transformer screw (x2)

Place Transformer Assembly top

Place the transformer assembly on the motherboard so that the exposed pins of the header sit in the X1A footprint. Solder a couple pins of the header from the top of the board to hold it in place. Then flip the board over and solder all of the pins from the bottom side. Since the pins do not protrude through the bottom of the board, there is no need to trim.

Gain top

This style of mic preamp handles gain in a unique way. Instead of simply varying the gain of an amplifier, it controls a combination of attenuation, feedback, and switching amplifiers in and out of circuit. This is why for this section we have a rotary switch and lots of resistors rather than a single potentiometer.

At gain settings 0-50dB the signal passes only through PRE 2 before going to OUTPUT. The resistors attached to steps 0-35dB create voltage dividers to attenuate the input signal before it is amplified by PRE 2. At 45dB and 50dB R61 and R62 increase the gain of PRE 2. At 55dB, the switch sends the signal to PRE 1 before PRE 2. R54-R56 attenuate the signal between the preamp stages. Finally, at 70dB, both preamp stages are engaged with no attenuation between them.

Populate Resistors top

Locate the GAIN sub-board. Place the resistors in their respective places, bend the leads, solder, and trim.

Reference
Value Color Code
R45 1k brown black black brown brown
R46 1.8k brown gray black brown brown
R47, R50, R56 3.9k orange white black brown brown
R48, R55 3.3k orange orange black brown brown
R49 2.2k red red black brown brown
R51, R52 12k brown red black red brown
R53, R54 2.7k red purple black brown brown
R57 510R green brown black black brown
R58 68R blue gray black gold brown
R59 4.3k yellow orange black brown brown
R60 6.8k blue gray black brown brown
R61 330R orange orange black black brown
R62 120R brown red black black brown

Place Rotary Switch top

We are going to take a lot of care in soldering the rotary switch. This is because, like the input transformer, it is expensive (about $50 at the time of writing) and almost impossible to remove once soldered. Follow the steps below to make sure the switch is on the correct side and perfectly flush with the PCB.

Remove the plastic covers from the rows of pins. Place the rotary switch in the larger notch in the front of the bracket with the pins facing up. Fasten the switch to the bracket with the washer and nut. Discard the lock washer. Place the GAIN board over the pins of the switch so that the side that says, "All parts other side!" is visible.

Now solder a single pin near the middle of the rotary switch. Remove the assembly from the bracket and visually confirm that the switch is perfectly flush to the PCB. If the switch is not flush, put it back in the bracket to hold it in place, reheat the single solder joint, and press down on the board until it snaps into place. Once you've confirmed that the switch is flush, solder the rest of the pins and trim the leads. Finally, check that you have soldered all the pins and that there are no solder bridges between them.

SW5
12-position switch

Place Header top

Place the side with shorter pins in the CON1B position on the same side of the board as the switch and the rest of the components. The long pins should face up and reach just a bit taller than the switch. Double check that the header is on the correct side of the board, then solder.

CON1B
8-pin header

Test GAIN top

  1. Optional: If you have clip probes for your meter, solder leftover component leads in the TP1, TP2, and TP3 test points. The next steps will go faster if you can clip to those test points.
  2. To make this step easier with two hands, attach the large knob from bag 8.2 to the GAIN switch. Use the included screwdriver from bag 8. The position of the knob pointer doesn't matter for now.
  3. Set the meter to resistance. If there are multiple ranges, set it to 20k.
  4. Put the black probe on TP1 and the red probe on TP2. Now check the resistance at each position. They should be within 10% of these values:
  • 1 (counter-clockwise): 6.9k
  • 2: 4.8k
  • 3: 4.9k
  • 4: 5.1k
  • 5: 6.1k
  • 6: 12k
  • 7: 12k
  • 8: 12k
  • Now move the red probe to TP3 and check the remaining positions:
    • 9: 2.7k
    • 10: 5.4k
    • 11: 8.7k
    • 12: 12.6k
  • Remove and set aside the knob and screwdriver until section 8.
  • If any of the readings are wrong, double check your resistor placement and soldering. See the schematic at the beginning of this section for which resistors are related to which positions on the switch. In the schematic the bottom is position 1 and the top is 12.

    PRE 1 top

    The PRE 1 and PRE 2 sections are the amplifiers that provide the lion's share of the 73P's gain. Both are essentially the same circuit. The only differences between the two are the values of the feedback (R1 and R63) and PRE 1 has an extra resistor, R4 to increase the gain.

    The PRE 1 and PRE 2 amplifiers each consist of just three transistors. T1/T11 and T3/T10 amplify the voltage, while T2/T12 is an output buffer that amplifies the current. The capacitors C9/C32 and C10/C36 AC-couple the input and output, preventing DC from entering or leaving the circuit. The rest of the components act to set the bias points of the transistors, and keep them stable with lots of feedback and filtering.

    R5/R71 sets the gain of the amplifier, which in it's normal configuration is 18dB. At the 45 and 50dB gain settings, the GAIN switch adds resistors in parallel with R71 on PRE 2 to increase the gain to 23 and 28dB. PRE 1 has a fixed resistors, R4, added to set the gain to 30dB.

    Populate Resistors top

    Populate the resistors in the PRE 1 block on the motherboard. Note that R4-R14 should be placed on the bottom of the PCB. This is to leave room for the gain switch which goes above the PRE 1 block.

    Reference
    Value Color Code
    R1 15k brown green black red brown
    R2 120k brown red black orange brown
    R3 51k green brown black red brown
    R4 91R white brown black gold brown
    R5 1.8k brown gray black brown brown
    R6 470R yellow purple black black brown
    R7 1.5k brown green black brown brown
    R8 10k brown black black red brown
    R9 5.1k green brown black brown brown
    R10 68k blue gray black red brown
    R11 47k yellow purple black red brown
    R12 2.2k red red black brown brown
    R13 390R orange white black black brown
    R14 33k orange orange black red brown

    Populate Transistors top

    Place leads of the transistors through the nylon spacers (it doesn't matter which of the four holes in the spacer you use). Then place the transistors and spacers in positions T1-T3. The polarity of these metal-can transistors is indicated by a small tab on the body. Make sure that the tab matches the drawing on the PCB, then solder and trim the leads.

    T1-T3
    BC109C transistor (x3)

    T1-T3
    Transistor spacer (x3)

    Populate Capacitors top

    Note that the electrolytic and tantalum capacitors are polarized. On the tantalums the positive lead is longer and marked with a "+".

    C4, C7, and C8 are meant to be installed flat to the board, but we are going to install them upright to save space. To do this, bend one lead down against the body of the capacitor so it is facing the same direction as the other lead. Then place them with the body inside the circles on the board.

    C1
    150u cap

    C2
    22u cap

    C3
    470u cap

    C4
    1500p cap

    Markings: 1500J

    C5
    100p cap

    Markings: WIMA 100/100-

    C6, C9, C10
    22u tantalum cap (x3)

    Markings: 226F

    C7
    680p cap

    Markings: 1000J

    C8
    1000p cap

    Markings: 680J

    Place Socket top

    Place the header socket as you did in step 1.14.

    CON1A
    8-pin socket

    PRE 2 top

    In addition to the amplifier circuitry described in the previous section, the PRE 2 block contains the output trim control. VR2 is the output trim potentiometer on the front panel. VR3 is trimpot for finely matching gain between two units.

    Populate Resistors top

    Populate the resistors on the PRE 2 sub-board.

    Reference
    Value Color Code
    R63 18k brown gray black red brown
    R64 120k brown red black orange brown
    R65 68k blue gray black red brown
    R66 470R yellow purple black black brown
    R67 51k green brown black red brown
    R68 47k yellow purple black red brown
    R69 1.5k brown green black brown brown
    R70 10k brown black black red brown
    R71 1.8k brown gray black brown brown
    R72 5.1k green brown black brown brown
    R73 390R orange white black black brown
    R74 2.2k red red black brown brown
    R75 33k orange orange black red brown

    Populate Components top

    Populate the other components, except for the potentiometer VR3 and header CON2B. As you did in the last section, pay close attention to the polarity of the transistors and polarized capacitors. The trimpot VR2 should also be placed according to the outline on the PCB.

    T10-T12
    BC109C transistor (x3)

    T10-T12
    Transistor spacer (x3)

    C29
    680p cap

    Markings: 680J

    C30
    100p cap

    Markings: WIMA 100/100-

    C31
    150u cap

    C32, C34, C36
    22u tantalum cap (x3)

    Markings: 226F

    C33
    22u cap

    C35
    1500p cap

    Markings: 1500J

    C37
    1000p cap

    Markings: 1000J

    C38
    470u cap

    VR2
    5k trimpot

    Populate Potentiometer top

    Place the potentiometer. Bend the two outer leads against the PCB, then solder and trim.

    VR3
    A10k potentiometer

    Place Header top

    As you did in step 4.3, place the shorter pins in the CON2B position from the bottom of the board. Double check that the header is on the opposite side of the board from the other components, then solder.

    CON2B
    8-pin header

    OUTPUT top

    The OUTPUT section provides more voltage gain and the output current needed to drive the next device in the signal chain. The output is another three-transistor amplifier. T6 and T7 provide voltage gain, while the big power transistor T8 provides the current. T8 and X2 form a transformer-coupled amplifier. The VR1 trimpot sets the bias of T3 for the lowest distortion and highest headroom. R39/C24 set the gain of the amplifier. The OUTPUT block provides 18dB of gain, with 4dB of this provided by the output transformer.

    Populate Resistors top

    Populate the resistors in the OUTPUT section on the motherboard. Leave about 1/8" of space between R41 and the PCB as you did with R42.

    Reference
    Value Color Code
    R32 2.2k red red black brown brown
    R33 3.3k orange orange black brown brown
    R34 56k green blue black red brown
    R35, R38 1.2k brown red black brown brown
    R36 18k brown gray black red brown
    R37 33k orange orange black red brown
    R39 1.5k brown green black brown brown
    R40 68k blue gray black red brown
    R41 47R 1W (red) yellow purple black gold

    Populate Components top

    Populate the remaining components. Pay close attention to the polarity of the transistors, polarized capacitors, and trimpot.

    T6, T7
    BC109C transistor (x2)

    T6, T7
    Transistor spacer (x2)

    C18
    22u tantalum cap

    Markings: 226F

    C19, C20, C24, C25
    100u cap (x4)

    C21
    330p cap

    Markings: 330J

    C22
    220p cap

    Markings: 220J

    C23
    5000p cap

    Markings: 5000J

    VR1
    5k trimpot


    Solder Jumpers to Transformer top

    Begin by tinning the ends of the black hookup wire. Do this by heating the exposed wire and then adding a bit of solder. Wrap the ends of the wire around pins 6 and 7 of the transformer. Then heat the pin and wire for about five seconds, then add solder.

    Find a bit of leftover component lead (you probably have them lying all over your work surface). Wrap this lead around pins 2 and 4 of the transformer, then solder.

    Solder Wiring Harness to Transformer top

    Begin by removing the ends from the wires. Twist as you remove them so that the strands of wire are twisted together, then tin the leads of the wiring harness.

    Solder the wiring harness to the transformer according to the chart below. Pin 1 of the wiring harness is red, pin 2 is next to pin 1, etc.

    Harness Pin > Transformer Pin
    1 (Red) 8
    2 5
    3 3
    4 1

    After soldering, gather the wires together and fasten them together with the wire tie. Don't over-tighten the wire tie; make it just tight enough to hold the wires together. Clip the extra length of the wire tie.

    Final Assembly and Testing top

    Mount Motherboard top

    If you soldered leads to the test points earlier, trim them down to the solder joints now.

    Mount the motherboard to the bracket with three of the black, pan-head screws. The screws go in the two mounting holes on the far right side of the board, and the one just below the output transformer cutout. Do not install the standoffs yet.

    Fasten the input transformer to the bracket with the two black, flat-head screws.

    Pan-head screw (x3)

    Flat-head screw (x2)

    Mount Output Transformer top

    Place the transformer in the gap in the motherboard with the side with the pins 7, 5, 6, 8 facing up. Fasten it to the bracket with the larger, silver screws and nuts from bag 8.1.

    Transformer screw (x2)

    Transformer nut (x2)

    Attach Switch Caps top

    Press the caps onto the switches, according to their labels on the PCB (POL is ∅). Orient the caps so that when the 73P is standing upright in a 500-series rack the text will be right-side up.


    Switch cap (x4)

    Mount Sub-boards top

    Install the two taller standoffs in the mounting holes that overlap the PRE 1 section and the shorter one in the hole next to CON2A. Then mount the sub-boards and screw them to the standoffs with the remaining black, pan-head screws. Do not install the nuts, front panel, or knobs yet.

    Pan-head screw (x3)

    GAIN standoff (x2)

    PRE 2 standoff

    Mount Front Panel top

    Place the front panel on the bracket, paying close attention to the 48V LED. Then fasten the panel loosely with the nuts of the switch and potentiometer and the nut and washer of the DI jack. Press all of the switches and reposition the panel if any of them are sticking. Then, when you have ensured all four switches can move freely, tighten the nuts.

    Turn both knobs completely counter-clockwise. Then place the knobs so the pointers align with the most counter-clockwise part of the silk-screen legend. Fasten them in place by tightening the set screws with a flat head screwdriver.

    TRIM nut

    GAIN knob

    TRIM knob

    Final Test and Calibration top

    Test Output top

    The first test is very simple, we are just going to make sure the 73P is passing signal from input to output.

    1. Set all switches OUT, gain to 0dB, and TRIM fully clockwise.
    2. Connect inputs and outputs to the 73P.
    3. In the REW Generator, set the frequency to 1kHz, change the unit to dBFS and set the output to -10dBFS.
    4. Open Levels in REW. You should see your -10dBFS test signal on the Out meter and approximately the same level coming back on the In meter (we will calibrate this precisely in a moment).

    If your 73P is not passing signal, first check your test setup by connecting the output of your interface directly to the input. If this is working, check the mechanical connections to the sub-boards and X2. Make sure nothing is plugged into the DI jack and that the TRIM is set clockwise. Finally, check the placement and soldering in the OUTPUT section.

    Calibrate Output Transistor Bias top

    In this step we will set the bias of T8 for the highest headroom possible.

    1. Set your multi-meter to read DC volts. (If your meter has multiple DC ranges, set it to 20v DC).
    2. Probe both sides of R41. It does not matter which probe goes on which side.
    3. Trim VR1 until the meter shows 2.5VDC.

    Calibrate Gain top

    1. Keep your connections the same as the previous steps.
    2. In the REW Generator, set the level to -60dBFS.
    3. Set all switches OUT, gain to 50dB, and TRIM fully clockwise.
    4. Open Levels in REW. The In meter be within a few dB of -10dBFS.
    5. Trim VR2 on the PRE 2 board until In reads as close to -10dBFS as possible.

    Test Features top

    Finally, we will go through each feature one-by-one to make sure your 73P is working perfectly.

    1. Keep your connections the same as the previous steps and set the REW level back to -10dBFS.
    2. Start with all switches OUT, gain at 0dB, and TRIM fully clockwise.
    3. Polarity: Toggle the ∅ switch IN and OUT. As long as the IN level stays the same, the switch is working correctly.
    4. Lo Z: Engage the Lo Z switch. The In level should go up by about 3dB.
    5. 48: We already tested the phantom power in section 1. Do not engage the 48 switch while the 73P mic input is connected to your audio interface.
    6. LN: Engage the LN switch. The The IN level should drop by -35dBFS.
    7. Gain: Test each step by first reducing the REW level by the gain amount for that step, then switching to that step. For example, for the second gain step reduce the REW level 20dB from -10dBFS to -30dBFS, then switch to position 2. The In level in REW should be within 2dB of -10dBFS for each step.

    Finished! top

    All good? Congrats on finishing your build! Have a question or problem? Drop us a line.

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