VP2x Microphone Preamplifier Kit
All Discrete
Vintage Style
500 Series Format
Fully assembled and tested unit shown. Kit faceplates will differ.
VP2x Assembly Guide
VP2x Microphone Preamplifier Kit
In the world of professional audio, high quality microphone preamplifiers are everywhere
you look these days. Even with so many choices available, the VP2x is a unique beast that delivers
a sound all it's own. That sonic footprint is very much like the mic preamps from the legendary
mid 1970's American made, API™ consoles, better known for the "West coast" or "LA" sound.
High quality, sometimes custom component selection is crucial to achieve this sound.
VP2x Features:
•
All discrete, vintage style circuitry with 60 db of overall gain
•
Continuously variable control of Preamp Gain over a 34 db range
•
Mute switch for quiet & safe switching of microphones
•
Polarity reversal
•
20 db Pad for input attenuation
•
48V switchable phantom power
•
Fully variable, custom made output attenuator
•
Custom made, Ed Anderson EA2622 1:7 input transformer
•
Custom made, Ed Anderson EA2623-1 or EA2503 output transformer
•
Standard 2520 DOA footprint mounts in Mill-Max sockets
•
Standard 500 series compatible
•
GroupDIY 51x compatible
VP2x = VP25 or VP26:
We offer two versions of the VP2x preamp kit. The only difference between the two
versions is the printed circuit board and the output transformer. First is the VP25, which employs
an Ed Anderson EA2503 output transformer. This is the version you want if you are after a 312
style preamp. Second is the VP26, which has an EA2623-1 for output transformer duties. This is
the version you want if you are after the vintage console preamp sound.
Copyright Notice
This assembly manual, including but not limited to all text, photographs and diagrams, is the intellectual property of Classic
Audio Products of IL, Inc. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electromechanical, is strictly prohibited under International Copyright laws. The sole purpose for this document is to aid in the assembly
of the VP2x series of microphone preamplifier kits offered by Classic Audio Products of IL, Inc. Commercial use is prohibited.
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VP2x Assembly Guide
Required Tools and Preliminary Tips
Required tools and supplies:
! Good quality soldering iron with fine/small tip and cleaning sponge. The Hakko 936 with a
900M-T-0.8C tip or equivalent, is recommended. Do not use a “soldering gun”.
! #245 No Clean Solder, 63/37 alloy, 0.031” diameter. Post soldering clean up is optional, not
required with the “no clean” solder. (Mouser part #533-24-6337-8800)
! Good quality Digital Multi-Meter or DMM. The Fluke 177 or equivalent is recommended.
! 16-26AWG Wire Strippers (Mouser part #801-JIC-1626)
! Small Diagonal Cutters (Digi-Key part #MS54-ND)
! Small Needle Nose Pliers
! Phillips Screw Driver with #1 tip
! Nut driver with 1/4” socket, 5/16” deep-well socket and 1/2” deep-well socket.
! 1/16” Allen Key (wrench)
! Safety Glasses. Please protect your eyes from solder spatter and clipped component leads.
These are the only eyes you will ever have!
Optional tools and supplies:
! Desoldering Tool like the Hakko 808 almost makes mistakes fun! This is a great tool. If all
goes well, you will not need a desoldering tool for this project.
! Lead Bender to aid in bending the leads for resistors and diodes. This will help to make
your work look professional. (Mouser part #5166-801)
! Irwin Quick-Grip 1-1/2” Handi-Clamp (Useful when installing the t-pad)
! 99% Isopropyl Alcohol, Q-tips and small piece of cotton t-shirt. (Handy for cleaning up the
spatter from the “no-clean” solder. It is not necessary to clean but helps to make your printed circuit
board look more professional.)
! 1/4" Diameter Shrink Tubing (This will help tidy up the flying leads from the output transformer
and the phantom power switch)
! Heat Gun (For shrink tubing if you use it. The Hakko HJ5000 is a nice choice.)
! Panavise to aid in holding the PCB while working
! Lighted Magnifying Glass (Some of the identification text on the ceramic capacitors is pretty
darned small!)
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Assembly tips and pointers:
The VP2x series of printed circuit boards are double sided and have plated through holes.
Special care should be taken to make sure you have a good solder joint in the through hole, on
the bottom of the board and on the top of the board when possible. I have incorporated nice,
large solder pads to make soldering very easy especially for DIY beginners.
When soldering components to the PCB, apply your clean solder tip to the component lead
and the solder pad simultaneously for approximately 2 to 4 seconds. Gently apply solder to the
point where the iron, the component’s lead and PCB solder pad meet. With a little practice, you
will get a nice flow and a perfectly small yet complete “mound” of solder.
When possible, depending on the component you are soldering, try working from the top or
silk screen side of the PCB. After completing the required solder joints, flip your PCB over,
carefully clip the component leads and touch up the solder joint to the bottom solder pad if
needed. It should be noted that this bottom touch up is not a required step especially if a good
joint is made while soldering the top. I myself like this as it makes for a really nice and solid
construction. Special care must be taken to not apply too much heat as components can easily
be damaged. Excessive heat from the soldering iron or desoldering tool can lift and/or damage
the solder pads or traces of the PCB too so…BE CAREFUL!!!
Using Q-tips, a small cotton cloth and 99% isopropyl alcoholic, the small, clear spatter from
the “no clean” solder can be easily cleaned up. It seams to clean easier when it’s fresh. This
clean-up is not necessary. The clear spatter will not harm anything at all if left where it flies. It
only looks a little more “pro” if it is cleaned up.
Double check your components before starting. Desoldering from a double sided PCB with
plated through holes can be a very difficult and damaging task. Remember, “check
twice…solder once!”.
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Enough already, let’s get started!
Section 1: Resistors
1.1 Locate the zipped plastic bag with the resistors. Set the black diodes off to the side for now. You will
notice that two values have been paired up with console tape. Keep these together especially if you are
assembling more than one VP2x kit. They are pre-matched before packaging. All of the resistors for the
VP2x kit are 1% metal film, 1/4 watt.
Photo 1.1
1.2 Next, prepare all of the resistor leads with a needle nose pliers or a lead bender (see optional tools).
Use the .45” slot for all of the resistors.
Photo 1.2
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VP2x Assembly Guide
1.3 Using your DMM, identify each resistor. If you don’t have a DMM handy, you can use the resistor color
chart found on the last page of this assembly guide. As a help during the sorting and assembly process,
print out the page with the resistor color chart on it. Use the tables on the page to separate your resistors
and keep them straight while populating the PCB.
Resistors
Part # Qty
1
1
2
1
2
1
2
2
Position Value
R2
10K
R3
1K
R4, R13
20K
R5
150K
R7, R8
6.8K
R9
160
R10, R11 750
R6, R12
200
Photo 1.3
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VP2x Assembly Guide
1.4 After verifying a resistor’s value, insert the resistor into its proper location on the board. Resistors are
not polarized so direction is not important. Prop the board up at the edges or use a Panavise set up. Don’t
start soldering until you have placed all of the resistors.
Photo 1.4
1.5 With all resistors in their proper places, it’s time to solder! Solder each resistor at the top or silk screen
side of the PCB first, using the method described in the “Assembly tips and pointers” section above. Make
sure you have soldered both ends of all twelve resistors.
Photo 1.5
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VP2x Assembly Guide
1.6 Next, turn the board over on your supports or Panavise. Holding the resistor leads, trim each one with
the diagonal cutters. Trim the leads flush with the bottom of the PCB.
Photo 1.6
1.7 Grab your soldering iron and touch up each resistor lead to solder pad joint on the PCB. Use the
previous described method of heating first, then applying a small amount of solder while the tip is in place,
to form a small “mound”. Some joints will not require additional solder, only a brief heating.
Photo 1.7
1.8 Systematically work your way around the board, being sure not to miss any of the 24 solder pads for
the resistors. When you are finished, inspect both sides of the PCB verifying that all the resistor joints look
good and sound.
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Section 2: Rectifier Diodes
2.1 Locate the two diodes that were set to the side in Section 1.1. Prepare the diode leads like we did with
the resistors. These rectifier diodes are polarized and can only be inserted in one direction! The
silver band is the cathode or negative end. This silver band goes toward the direction of the small arrow on
the PCB. After inserting both rectifier diodes, solder the leads from the top like we did the resistors.
Photo 2.1
2.2 Next, flip the board over, carefully trim the four leads and touch up the bottom solder joints as
required.
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Section 3: Pushbutton Switches
3.1 I like to install the three C&K pushbutton switches next as they are closer to the board than some of
the capacitors. Locate the three switches and insert them into their respective locations. They should fit
snug. Be sure to seat them completely flat.
Photo 3.1
3.2 We will first solder one center pin only on each of the three switches.
Photo 3.2
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VP2x Assembly Guide
3.3 Flip the PCB over to the top side and inspect. Make sure that all three switches are fully seated with
both of their tabs touching the PCB. If need be, now is the time to press the switch tightly to the board while
heating up the single solder joint to seat the respective switch completely.
Photo 3.3
3.4 Once we have insured that all three switches are fully seated, it’s time to solder the remaining pins. I
like to follow an alternating pattern so I don’t apply too much heat to a single switch at one time.
Photo 3.4
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Section 4: Capacitors
4.1 Locate the zipped baggy with the capacitors. Separate the capacitors by type. Put all of the black
Nichicon capacitors together. Those are radial capacitors. Put the big blue one and the small tan one
together. Those are made by BC and are axial capacitors. There is one film capacitor made by Panasonic.
It is a reddish-brown in color. Last are the small lighter blue capacitors. There will be five of them. Those
are Murata ceramic capacitors.
Capacitors
Part # Qty Position
Value
2
C1,C2 1000pf, 50V
1
C3
220pf, 50V
1
C4
82pf, 50V
1
C6
27pf, 50V
Type
C0G Ceramic
C0G Ceramic
C0G Ceramic
C0G Ceramic
Manufacturer
Murata
Murata
Murata
Murata
1
C11
100pf, 50V
Film or Ceramic
Panasonic
1
1
C5
C10
33uf, 25V
470uf, 16V
Axial, Electrolytic
Axial, Electrolytic
BC
BC
2
1
1
C7, C8
C9
C12
47uf, 25V Radial, Electrolytic
120uf, 50V Radial, Electrolytic
330uf, 25V Radial, Electrolytic
Nichicon
Nichicon
Nichicon
Photo 4.1
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4.2 We will start with the five ceramic capacitors. You may need a magnifying glass to read the tiny print
on the ceramic capacitors. This picture will help you to verify the proper values.
Photo 4.2
4.3 One by one, we will insert each capacitor into its proper location. The ceramic capacitors are not
polarized so their direction does not matter. Start with C4 and then C6. Place the capacitor leads into the
through holes.
Photo 4.3
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4.4 There are multiple holes for the other three ceramic capacitors. This is to give custom builders more
options for capacitor footprint/brands. Lead spacings are 2.5mm and 5mm. All of the supplied Murata
capacitors have a lead spacing of 2.5mm. It is very important to get the capacitors placed into the
proper holes for C1, C2 and C3. See photos below. If you look closely at the series of through holes for
each of these 3 capacitors, you will see only one hole makes a connection to the signal track, the other two
connect to the ground plane for decoupling purposes.
Photos 4.4
4.5 With all five ceramic capacitors in their proper places, it’s time to solder! Solder each capacitor at the
top or silk screen side of the PCB first, like we did with the resistors. Make sure you have soldered both
leads of all five ceramic capacitors.
4.6 Turn the board over to carefully trim the ten leads then touch up the bottom solder joints as required.
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4.7 Next, we will install the two axial capacitors. In preparation, gently bend the leads of both capacitors
downward at an approximate 90° angle from the body.
Photo 4.7
4.8 Both C5 and C10 are polarized so special attention must be paid to get them facing the right
direction. The negative end of each capacitor has a black band around it. Make absolutely sure that this
black band goes to where the “-“ is at in the silk screen.
Photo 4.8
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VP2x Assembly Guide
4.9 Solder both C5 and C10 from the top of the PCB first. Turn the board over to carefully trim the four
leads, then touch up the bottom solder joints as required.
**Special Tip** Save one of the leads from the 470uf capacitor. It will come in handy later when we wire up
and install the t-pad attenuator.
4.10 Next, we will place the Panasonic film capacitor into the C11 position. This capacitor is not polarized,
so it can be inserted either way around. The supplied film capacitor has a lead spacing of 5mm. Take care
to insert it into the proper through holes. The concept is the same as it was for C1, C2 and C3, as
mentioned above.
Photos 4.10
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4.11 While holding the capacitor from the top with your finger, turn the board over to slightly bend the
leads outward. This will keep the capacitor in place until we are ready to solder.
Photo 4.11
4.12 Now, locate the remaining four Nichicon radial capacitors. All four of these capacitors are
polarized so special attention must be paid to get them facing the right direction. The negative lead
of each capacitor has a vertical gray stripe with a black “-“ symbol in it. This negative lead goes of course
the “-“ symbol in the respective silk screen. Insert C7 and C8 first. While holding both the capacitors from
the top with your finger, turn the board over and slightly bend the leads outward. Insert C9 and bend the
leads slightly over. Lastly, insert C12 and bend the leads over.
Photo 4.12
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VP2x Assembly Guide
4.13 Now, solder all of the leads from the bottom of the PCB. There will be five capacitors (ten leads) to
solder. Don’t forget about the Panasonic film cap! After soldering, trim the leads as close as you can to the
bottom of the PCB.
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Where we are at so far…
Top of PCB, Progress so far
Bottom of PCB, Progress so far
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Section 5: Discrete Op Amp Sockets
5.1 Locate the zipped plastic bag that contains the hardware. Dump the contents out on a paper plate or
something similar so you can easily sort and locate the six Mill-Max sockets.
Photo 5.1
5.2 Situate your PCB with the silk screen side facing down. Drop the six receptacles into the holes as
shown. Please take notice that there are seven plated through holes and only six receptacles. One
is not used as there is no circuit connection to that pin, if that pin exists on your discrete opamp. If you own
a vintage Melville or Huntington, there is no need to clip the pin.
Photo 5.2
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VP2x Assembly Guide
5.3 Solder the sockets from the bottom of the PCB only. You will have to move the tip of your soldering
iron around the large pads to heat evenly. If you are using a very small tip on your soldering iron, it is easy
to actually lift the socket slightly, using only the soldering iron tip, to help heat the entire solder pad. Solder
will find it’s way into the plated through hole. You will see evidence of this after you turn your PCB over to
inspect. Solder from the top if you need to. This should not be required if done well from the bottom.
Photo 5.3a
Photo 5.3b
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VP2x Assembly Guide
Section 6: 25K CCW Gain Potentiometer
6.1 Proper installation and alignment of the Bourns 25K potentiometer is a little involved. It will require a
few steps, but will yield perfect results. Thanks to our friend Volker Mayer, better known as [silent:arts] for
the original idea! First, locate the Bourns 25K potentiometer. Remove the mounting nut and set to the side.
Insert the potentiometer into the R14 position. Do not solder anything at this time!
Photo 6.1
6.2 Locate the aluminum L-bracket and the hardware from step 5.1.
6.3 Install the #4-40 undercut flat head screws and 1/4" standoffs into the two rear mounting holes. Fully
seat the standoffs and tighten securely.
Photo 6.3
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6.4 Install the remaining two #4-40 undercut flat head screws and 1/4" standoffs into the two front
mounting holes. Thread the standoffs only partially on, just to the point of where the bottom of the screw is
flush with the top of the standoff.
Photo 6.4
6.5 Position the L-bracket in a way that the mounting screws straddle your supports.
Photo 6.5
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VP2x Assembly Guide
6.6 Angle the PCB assembly so that the pushbutton switches clear the rectangular cutout in the L-bracket
while simultaneously guiding the Bourns potentiometer into its hole.
Photo 6.6
6.7 Drop the rear of the PCB over the two rear mounting screws.
6.8 Gently hold the PCB tight to the back of the faceplate portion of the L-bracket while making sure the
PCB is flush with the top and bottom. Thread two #4-40 nuts onto the two rear mounting screws (no lock
washers are used at this time).Tighten with a 1/4" socket and nut driver.
Photo 6.8
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6.9 While gently pinching the front of the PCB to the L-bracket, tighten the front two mounting screws with
a #1 Phillips screwdriver.
Photo 6.9
6.10 Thread the two #4-40 nuts onto the front mounting screws and tighten with the socket and nut driver.
Photo 6.10
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6.11 While holding the potentiometer from behind, center the threaded shaft in the mounting hole. Hold
securely whilst threading the mounting nut onto the shaft. Tighten with a 5/16" socket and nut driver.
Photo 6.11
6.12 Finally, solder…the three exposed pins of the gain potentiometer’s mounting bracket from the top or
silk screen side of the PCB. Follow the sequence in the pictures below. The #4-40 nut must be removed to
complete the third (front) pin.
Photo 6.12
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6.13 Unfortunately, we now need to remove the PCB from the L-bracket. So, remove the mounting nut
from the Bourns potentiometer. Next remove all four 1/4" nuts from the #4-40 screws. Unscrew the front
two mounting screws until they are below the PCB. No need to remove them completely. Gently lift the rear
of the PCB and slide it away from the L-bracket. Set the L-bracket assembly to the side for now.
6.14 Turn the PCB over and solder all seven pins belonging to the Bourns potentiometer.
Photo 6.14
6.15 Lastly, turn the board back over so the silk screen is up. Solder the fourth mounting bracket pin
(closest to the pushbutton switches) from the top of the PCB.
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Section 7: EA2622 Input Transformer
7.1 (Optional step) For good measure, I like to use a small piece of double faced tape between the
PCB and the input transformer even though I have placed no copper tracks directly under the transformer.
The material measures .05” or about 1.25mm. The double faced tape will help in holding the transformer in
place while soldering. A piece of fyberoid fish paper will work also.
Photo 7.1
7.2 Locate the EA2622 input transformer. Take note of the pin nearest to the black dot that is on the side
sticker. This pin is also marked on the bottom of the metal can with a dot. This is pin #1. It is very
important to make sure the transformer is orientated correctly.
Photo 7.2
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7.3 Carefully align the pins with the plated through holes. On some occasions, some of the pins may need
to be slightly adjusted so they line up correctly. Use a needle nose pliers and gently straighten them. The
pins can easily be broken so be careful. Do not force the transformer down if things are not aligned
properly. Seat the transformer fully.
7.4 Turn the PCB assembly over with silk screen side down. The double faced tape from Step 7.1 will
keep the transformer in place if you elected to use it. If not, simply put the assembly down on your table.
The input transformer will be the tallest component. Make sure the bottom of the transformer is being held
parallel to the PCB. Solder all eight pins of the EA2622 transformer.
Photo 7.4
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Section 8: EA2623-1 Or EA2503 Output Transformer
8.1 Locate the output transformer that came with your kit. An EA2503 comes with the VP25 kit. An
EA2623-1 comes with the VP26 kit. The first step is to prepare the flying leads. You can print out the
following table to use for a guide. You must double check the transformer dimensions to make sure
your print out is scaled properly. If not, you will have to make adjustments accordingly. Simply align
your transformer over the paper, individually pull the leads straight out from the transformer bobbin and cut
to the length designated for each respective color.
**Special Tip** Save the lead cutoffs for use wiring the t-pad and the 48 volt toggle switch.
8.2 Using your wire strippers, remove about 1/8” (3mm) of insulation from the end of each of the
transformer leads. Proceed carefully so you don’t cut any of the wire strands. Twist the end of each lead so
they are small, neat and compact.
Photo 8.2
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8.3 Tin each exposed lead with solder. Preheat the wire with your soldering iron, then apply a small
amount of solder using only enough to evenly coat the twisted wire.
Photo 8.3
8.4 Locate the remainder of the hardware that was set aside in step 5.1. Slide a #4-40 flat
washer onto
each of the longer #4-40 pan head screws. You will have 3/4" long screws for an EA2623-1 or 7/8” long
screws for an EA2503. Also, separate the other two washers along with 2 of the #4-40 nuts and two #4-40
lock washers. Keep this hardware handy so you can get to it easily.
Photo 8.4
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8.5 Insert the two screws with washers into the mounting holes of the transformer. The screw heads
should enter the holes on the side of the laminations where the leads come out.
Photo 8.5
8.6 Pick up the transformer with a finger on each screw head to keep them from falling out. Slide a
remaining flat washer onto each screw.
Photo 8.6
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8.7 Next, pick up your PCB assembly and slide it over the 2 mounting screws. The mounting screws will
enter the silk screen side of the PCB first.
Photo 8.7
8.8 Insert a lock washer and nut onto each screw. Only finger tighten them for the time being.
Photo 8.8
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8.9 You may only get one nut to physically spin around before hitting the transformer bobbin, especially
with an EA2623-1. If that is the case, hold the nut in place with your thumb, turn the PCB assembly over
and then snug-tighten the screw with your #1 Phillips screwdriver.
Photo 8.9
8.10 While looking at the PCB assembly from the top, rotate the transformer so that the laminations are
nice and parallel with the silk screen. Hold the transformer in place while tightening the screws from above.
Thanks to the lock washers, the screws will tighten rather nicely without the need to hold the nuts with a
tool. Take care not to over tighten.
Photo 8.10
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8.11 (Optional step) If you have some shrink tubing handy, this is a nice place for a couple of pieces
that are 1/4" diameter and about 1/4" long. I like to gather some of the leads together and put the shrink
tubing over them before soldering the leads in place.
8.11.A If you are assembling a VP25 kit with an EA2503 transformer, insert Brown, Red, Orange and
Blue through one piece of shrink tubing. Next insert Green, Yellow, Violet and Gray through the other piece
of shrink tubing. Do not apply heat to the shrink tubing yet!
8.11.B If you are assembling a VP26 kit with an EA2623-1 transformer, insert Green and Brown through
one piece of shrink tubing. Next insert Red, Orange, Blue and Black through the other piece of shrink
tubing. Do not apply heat to the shrink tubing yet!
Photo 8.11.B
8.12 Time to solder the leads by completing two at a time. Insert two of leads into their respective silk
screen marked plated through holes. Flip the board over and solder. Repeat until all leads are soldered.
Inspect the leads and trim them close to the PCB if they protrude above the solder mound.
Photo 8.12
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Section 9: T-Pad Output Attenuator
9.1 Locate the custom Clarostat 3-deck potentiometer. Remove the nut and lock washer and set to the
side. Locate the capacitor lead that we saved back in step 4.9. Insert this lead through the center solder
lugs of all three potentiometer decks. Leave one end of it sticking out about 1/16” (1.5mm) beyond the
solder lug.
Photo 9.1
9.2 Solder the capacitor lead to the solder lugs starting with the center one. Apply your soldering iron tip to
the lug and lead simultaneously for about 2 seconds. Bring in your solder so it flows nicely around both the
lug and capacitor lead. Repeat for the other 2 lugs and then trim the long end of the lead back with your
diagonal cutters. Take care not to heat up the lugs too long or you may damage the attenuator!
Photo 9.2
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VP2x Assembly Guide
9.3 Locate the leads we saved from the output transformer back in step 8.1. Three of these will be used to
make the t-pad connections. We will refer to the three decks of the attenuator as A, B and C. A being the
closest to the shaft. Cut a piece of Blue lead to 1-7/8” (47.5mm), cut a piece of Black or Gray lead to 1-1/2”
(38mm) and cut a piece of Brown lead to 1-3/8” (35mm).
9.4 Using your wire strippers, remove about 1/8” (3mm) of insulation from one end and 1/4" (6mm) of
insulation from the other end of each lead. Twist and tin both ends like we did in steps 8.2 and 8.3.
Photo 9.4
9.5 Using your needle nose pliers, bend the longer end of each lead forming a small hook.
Photo 9.5
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VP2x Assembly Guide
9.6 Mount the attenuator vertically in your Panavise. Hang the three leads from the solder eyelets. Blue
goes to deck C, Black or Gray to deck B and Brown to deck A. Connections to the solder eyelets must be
exactly as shown or the attenuator will not function properly. Check twice, solder once!
9.7 Solder the three leads to the potentiometers being careful not to heat the lugs too much.
Photo 9.7
9.8 Use a small Handi-Clamp to temporarily hold the attenuator in place on the PCB. The solder lugs need
to be pointing towards the pushbutton switches as shown.
Photo 9.8
9.9 Insert the three leads into their respective silk screen marked plated through holes. The through hole
labels correspond to the potentiometer deck labels. R1A (Brown) goes to hole A, R1B (Black or Gray) goes
to hole B and R1C (Blue) goes to hole C. Flip the board over and solder the three leads. Inspect the leads
and trim them close to the PCB if they protrude above the solder mound.
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Section 10: 48V Phantom Power Toggle Switch
10.1 Locate the remaining leads we saved from the output transformer back in step 8.1. A short piece of
Red and Orange will be used to make the toggle switch connections. Cut one piece of each Red and
Orange lead to 1-7/16” (36.5mm).
10.2 Using your wire strippers, remove about 1/8” (3mm) of insulation from one end and 1/4" (6mm) of
insulation from the other end of each of lead. Twist and tin both ends as we have done previously.
10.3 Using your needle nose pliers, bend the longer end of each lead forming a small hook.
10.4 Locate and open the factory sealed bag containing the toggle switch. Put the hardware to the side
for later. Mount the toggle switch vertically in your Panavise similar to the method we used for the
attenuator. Keep the side with “7101 C&K” facing you. The orientation of the switch is not crucial. I just
prefer to see the label.
10.5 Hang the Orange lead from the center solder lug and the Red lead form the left solder lug. Solder the
two leads to the switch being careful not to heat the lugs too much.
Photo 10.5
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VP2x Assembly Guide
10.6 (Optional step) If you have some shrink tubing handy, this is another nice place for a piece that
is 1/4" diameter and about 1/4" long. Position the shrink tubing over both leads.
10.7 Insert the two leads into their plated through holes. The Red lead will go to the through hole labeled
1 and the Orange lead to the through hole labeled 2. It will be helpful to use a small piece of double faced
tape to temporarily hold the toggle switch to the top of the attenuator.
Photo 10.7
10.8 Flip the board over and solder the two leads. Inspect the leads and trim them close to the PCB if
they protrude above the solder mound.
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VP2x Assembly Guide
Section 11: Final Assembly!!!
11.1 Locate your L-bracket assembly and position it on some sort of work support. You should have the
four mounting screws and standoffs already attached to the L-bracket from Section 6. Make sure the two
undercut head screws closest to the front of the L-bracket are unscrewed so their threads do not protrude
above their standoffs. Remove the Handi-Clamp from the attenuator. While holding the PCB assembly at a
slight angle, guide the attenuator shaft through it’s mounting hole while simultaneously guiding the switches
and Bourns pot through their respective holes. Be gentle. Don’t force anything.
Photo 11.1
11.2 Repeat steps 6.8 through 6.10. This time you will put a #4 lock washer onto each of the four
undercut screws, before threading the nuts on.
Photo 11.2
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VP2x Assembly Guide
11.3 Locate the aluminum face plate that was supplied with your kit. Position it over the protruding shafts
of the potentiometers. Locate the nut for the attenuator. Thread this nut on the attenuator shaft keeping it
loose for the time being. You can use the lock washer if you so choose. I do not use them for my
assemblies.
11.4 Position the assembly vertically, with the face plate facing up, in your Panavise.
Photo 10.4
**Special Tip** By applying a bit of console tape over the end of your sockets, you can tighten the nuts to
the face plate with a much lesser risk of scratching or marring the face plate.
**Special Tip**
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11.5 Thread the supplied nut onto the shaft of the Bourns potentiometer. Finger snug it just above the
face plate. Center the face plate over the Bourns shaft by sliding the face plate left to right to estimate the
center. Use a nut driver with a 5/16” deep-well socket to securely tighten the nut.
Photo 11.5
11.6 Insert the attenuator’s locating lug into the slot in the L-bracket. Finger snug the nut just above the
face plate. Adjust the bottom of the face plate left to right, making sure it is parallel with the L-bracket’s left
side. Use a nut driver with a 1/2” deep-well socket to securely tighten the nut.
Photo 11.6
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VP2x Assembly Guide
11.7 Locate a lock washer from the toggle switch hardware. Slide the lock washer onto the toggle switch
shaft. Insert the toggle switch into the hole through the L-bracket and faceplate. Thread a nut onto the
toggle switch shaft finger tightening it to the face plate. Make sure the Red lead is at the left-side lug of
the toggle switch when viewing from this perspective.
Photo 11.7 & 11.8
11.8 Securely tighten the toggle switch nut using a nut driver and 5/16” deep-well socket. Take special
care to make sure the switch body is parallel with the face plate.
**Special Tip** A small amount of dishwashing soap may come in handy for the next step. A touch on the
white plastic shaft of the switches will make the installation of the switch caps easier.
11.9 Next we will install the pushbutton switch caps. To start the process, depress all three switches so
they are in the engaged position. I like the switches to be in this position when the caps get their final push
on. Start a black cap on the top or “Mute” switch. More often than not, the fit is pretty tight. Too tight to push
on with your thumb. I use the rubber coated handle of my needle nose pliers for some leverage. The white
cap will go on the middle or “Polarity Reverse” switch and the last remaining black cap will go on the “Pad”
switch. Support the rear of the PCB assembly and be careful not to damage anything.
Photo 11.9
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**Special Tip** Before installing the knobs inspect them to see if there is any white paint from the
indication line smeared where it shouldn’t be. A little Xylene or Goof Off on a cloth rag will clean this right
up. Don’t rub to hard or long. Just lightly brush it with the rag pulled over your finger tip.
11.10 Now onto the aluminum knobs. Back both of the set screws out of the 1/2" pointer knob so they do
not protrude into the shaft hole. While holding the preamp unit so the top edge is facing you, slide the 1/2"
pointer knob onto the Bourns shaft. Ever so slightly snug one of the screws with a 1/16” Allen wrench.
Slowly and gently rotate the knob counter-clockwise. Stop when you will feel the knob reach the end of its
rotation. Loosen the set screw that was snug and rotate the pointer so it is pointing just above the first gain
indication dot. Snug the set screw with a little more torque. Slowly rotate the knob fully clockwise, making
sure the pointer rotation is even between the first and last indication dot. Securely tighten both set screws
for this knob after making any necessary adjustments.
Photo 11.10
11.11 Follow the same procedure in Section 11.11 for the 5/8” output attenuator knob. This knob is easier
to set if the preamp unit is facing right way up. Make sure before final tightening of this knob that the
bottom of it does not drag on the mounting nut below. You will probably have to raise the knob up slightly.
11.12 The only step now remaining is the colored knob inserts. Depending upon which color you have
chosen, there may be a thin protective film coating to be removed from the top of the insert. Lightly scrape
the top edge with your finger nail to start and remove the clear film. Once this has been done, use the
same technique to expose the PSA on the back of the insert. Position the insert into the appropriate knob
and press firmly to insure a good adhesive bond. Repeat for the remaining knob. If there is a slight marring
on the front of the insert where your fingernail marked the surface, a drop of Goof Off on a rag with even
this out. Use very light pressure as it is very easy to permanently damage the colored surface!
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11.13 (Optional step) If you have utilized shrink tubing, now would be the time to position the pieces
evenly along their wires and apply a little heat with your heat gun.
Congratulations…besides some simple testing…that’s it! Continue on to section 12 for testing.
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Section 12: Quick Startup Testing Procedure
12.1 The tests for section 12.1 will be preformed with the preamp not connected to a power source. Also,
no discrete op amp should be installed. Use of a digital multi-meter set to measure resistance is required.
**Special Tip** If your DMM does not measure any resistance during these section 12.1 tests, don’t worry,
everything is probably fine. The resistance is just beyond the range of your DMM, which is a good thing!
Anything well below a 200K Ω reading, will require finding the source of the problem.
12.1.a Probe between the “-V” and “+V” sockets for the DOA.
Your reading should be a high resistance, over 200K Ω.
12.1.b Probe between the “-V” and “C” sockets for the DOA.
Your reading should be a high resistance, over 200K Ω.
12.1.c Probe between the “+V” and “C” sockets for the DOA.
Your reading should be a high resistance, over 200K Ω.
12.1.d Probe between the “C” and “O”(Output) sockets for the DOA.
Your reading should be a high resistance, over 200K Ω.
12.1.e Probe between the “+V” and “O”(Output) sockets for the DOA.
Your reading should be a high resistance, over 200K Ω.
12.1.f Probe between the “-V” and “O”(Output) sockets for the DOA.
Your reading should be a high resistance, over 200K Ω.
12.2 The tests for section 12.2 will be preformed with the preamp connected to a power source. A 16V
bipolar supply is recommended for 2520 style op amps. Still, no discrete op amp should be installed. Use
of a digital multi-meter set to measure DC < 400V is required.
12.2.a Probe between the “+V” and “C” sockets for the DOA.
Your reading should be approximately +16V DC.
12.2.b Probe between the “-V” and “C” sockets for the DOA.
Your reading should be approximately -16V DC.
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12.3 The tests for section 12.3 will be preformed with a discrete op amp fully seated in the Mill-Max
sockets and the preamp inserted into a 500 series rack.
12.3.a Set the Output Attenuator knob to its full clockwise position. Set the Gain knob to full
counter-clockwise. Engage the Mute switch. Connect a dynamic microphone to the preamp input.
Disengage the Mute switch. Slowly bring the Gain knob up until you hear the source. Verify the
operation of the Output Attenuator. Verify the operation of the Pad switch and also the Polarity
Reverse Switch.
12.3.b With the same initial settings as 12.3.a, connect a condenser microphone to the preamp
input. Engage the 48V Phantom power switch. Disengage the Mute switch. Slowly bring the Gain
knob up until you hear the source.
**Special Tip** If you notice a scratchy noise when rotating the Bourns Gain potentiometer, turn it back
and forth three or four times throughout its full range of rotation. The noise should disappear almost
immediately. This sometimes happens when the potentiometers are brand new. No need to be concerned,
this is normal. The pots are fully sealed.
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Qty: 1 Position: R2
Value: 10K
Qty: 2 Position: R7, R8
Value: 6.8K
Qty: 1 Position: R3
Value: 1K
Qty: 1 Position: R9
Value: 160
Qty: 2 Position: R4, R13
Value: 20K
Qty: 2 Position: R10, R11 Value: 750
Qty: 1 Position: R5
Value: 150K
Qty: 2 Position: R6, R12
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Value: 200