OFFICIAL ioaudio MK67 build and support thread

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Apr 21, 2008
Northern California
This post will be evolving as the project gains traction, but for now, here is ioaudio's MK67 microphone kit which is a circuit clone of the vintage Neumann U67 microphone.



Wiring diagram:


Build related questions and technical clarifications should be posted here.

ioaudio's white market thread is located here:

Kits, bodies, and related parts are currently available at the Studio 939 webstore:

Complete documentation to follow.  This first post will be updated with general project information. 

(NOTE: The prototype pictured below is currently configured with Eric Heiserman's new HK67 capsule and Amperex EF86 tube supplied by Christian Whitmore.  The capsule mounting base has been fitted with teflon turrets to facilitate rapid capsule swapping as it may serve as a test-bed for various build configurations.)


Samples as pictured:

Love MP3

Love WAV

Summertime MP3

Summertime WAV


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Apr 21, 2008
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IMPORTANT NOTE: This DIY project involves working with circuits that carry LETHAL voltages.  The processes and build procedures demonstrated in this thread are for educational purposes only.  All work should only be performed by qualified technicians.  While the parts count in this project is low and build procedures outlined in this project straight-forward, a high voltage tube microphone and power supply is not a recommended beginner DIY project.  Schematics should be studied and thorough knowledge of all connections and components clarified prior to attempting this build.

Before beginning this project, a few strong recommendations:

1.  If you are new to DIY electronics and do not have confidence and technical knowledge to trouble-shoot the circuit when problems arise, make sure you have a solid fall-back plan.  Before purchasing expensive parts, find someone (friend or professional) local to you who can assist in person if you get stuck!  I will re-iterate that this is not a recommended project for complete beginners.  Please consider starting out with an easier low-voltage project like the VP26 microphone preamp kit from Classic Audio Products.

2.  Get a digital multimeter suitable for audio electronics that you understand how to use.

3.  Get a temperature controlled soldering iron.  Delicate electronic components can easily become damaged with even low-wattage non-regulated soldering irons.  This project also involves soldering a components mounted on a delicate acrylic board. Weller, Hakko, and other name-brand tools are excellent and recommended, but I have been using a budget Chinese regulated 70 watt iron for ~20 projects and it has worked smoothly and heats up to operating temperature in under 5 seconds and accepts genuine HAkko tips (recommended).

4.  Hakko 808 de-soldering tool.  This expensive tool can save you from hours of frustration and damaging the traces on your PCB if you need to remove components after they have been soldered in.

OK. . .  with those preliminary considerations settled, the recommended PSU configuration for this project is to modify the stock Alctron TPS-100 power supply.  As most Chinese donor microphones share this exact circuit, many builders may already have one collecting dust in a corner somewhere, and this is undoubtedly the most cost effective solution.


Our modification will render the same passive B+/regulated heater topology as the original power supply.  For builders who do not require step-by-step hand-holding, here are the modifications in list form:

1) cut the ground trace to isolate (float) the heater section ground
2) cut/remove the regulating diode for B+
3) ground the +V output node of the heater circuit
4) connect the heater supply pin to the now floated heater ground to derive negative voltage supply
5) replace R5 with suitable value for passive B+ operation in circuit (33K in this build)
6) add 25K potentiometer to the B+ output for fine adjustment

We begin by removing the 4 screws that secure the cover panel.



As the U67 does not require pattern switching from the PSU, we will remove the pattern switch.  Start by prying off the top knob cover.



If the exposed nut can be loosened from this position with standard tools, do so and remove the knob.  If it cannot be loosened, grind off the entire top portion of the plastic knob to gain access.


Once the nut is loosened, the knob pulls strait off.  Next, remove the 4 screws securing the 3 pin and 7 pin XLR jacks.  Do not cut any of the wires, we just need to move them so we can easily access the main PCB.


Remove the 3 screws that secure the PCB to the chassis.


And, we now have access to the internal wiring for modification.


From here, we can easily cut the 3 pattern switch wires close to the PCB and remove the pattern switch.


This is the portion of the ground plane we need to cut in order to isolate the heater section for negative supply.


A cutoff wheel can grind away the trace very quickly.


Next, we need to add a wire to ground the +V side of the heater circuit.  I use some console tape to secure the wire for easy soldering.



The other end of the wire will connect to this node.


The yellow wire connects to pin 2 on the 7 pin XLR.  This is our heater supply.  Disconnect this wire at the PCB.


And move it to this node to derive our negative heater supply.  Here, I am using a piece of tape again to secure the wire which makes soldering the wire in place very easy.


Here is an overview of our modified wiring so far.


We want to insert a 25K pot between this node and pin 1 of the 7 pin XLR which is our B+ supply.


Disconnect this wire and add a new red wire.  This will allow us to easily connect the pot after it is mounted.



Mounting the pot can be as simple as putting it inside the 3/8" diameter hold in the front plate recently vacated by the pattern switch, but I do not feel this is an adjustment that needs to be made with any frequency.  Also, exposing this trim pot will lead to accidental shifting of this critical setting when transporting the PSU.  I choose to mount the pot internally and fabricate a bracket from a piece of aluminum L-stock.

First, I drill a 3/8" hole for the pot.


Then, I cut the bracket with a mitre saw loaded with a metal cutting disc.


Next, I determine the location of the pot inside the case.  I opt to locate the end of the pot near the outside edge of the case.  This way, if in the future, I hack this case with an internal voltmeter, I can easily drill an access hole to adjust my B+ trim with a screwdriver.  But, for now, I simply locate the pot internally.


Once located, I can drill my holes through the enclosure.


I opt to use 4-40 machine screws and tap the bracket to accept the screw.


After attaching one screw, I drill straight through both the bracket and the enclosure to ensure precise alignment of parts for the 2nd screw.


Once drilled, the 2nd hole is tapped for 4-40 threads.


2 screws secure the bracket to the enclosure.



D6 regulates the B+ output.  As we want the B+ to operate passively, and the native output is close to our desired 210V target, we simply cut D6 and remove it.



Now, we have a passive B+.

Next, install and connect the 25K pot.


One red wire comes from the PCB to the pot, and the other one goes from the pot to the 7 pin XLR jack.


Natively, the PSU operating passively operates at too high a voltage even with the trim pot.  R5 needs to be raised from 6.8K to 33K.  It is located here.


The Hakko 808 cleanly and easily removes the resistor from the single sided PCB.


In actuality, I tried 3 different resistor values and different pot value before arriving at 33K R5 value and 25K for the trim pot.  What I had on hand was a 15K and 18.2K resistor wired in series to make my 33K value.


Solder the replacement resistor and trim the leads.


Max provided me with a full assembled test microphone to assist with build documentation.  With Chinese test capsule installed  in the test microphone and the trim pot set to full attenuate, I read 207.8V at the B+


A slight turn easily trims to a nominal 210V target value.


I recommend testing the PSU upon initial build substituting a 210K-260K 1W resistor between B+ and ground as a function test.  This may not yield the same end values as the actual microphone, but it will let you test and confirm the PSU is operating properly.

For now, here is the modded PSU for the MK67 project.


OK. . . it has been brought to my attention by Max that the configuration as presented to this point lacks a bleeder for the filter capacitors in the B+ section because I completely removed the pattern switch.  The best way to resolve this is to use a 1M pot inserted between the B+ and ground.  This way, the pot acts both as the B+ attenuator and the drain for the filter capacitors.  I will need to circle back and re-do sections of the build thread to reflect this change, but for now, I will move forward and put down a workable solution that does not require back-tracking.

First, we will need to remove D5




Next, we insert a 1M resistor between B+ and ground.  The easiest location is between the top of D6 and the bottom of D5 like this.



Connecting to the microphone at this point, the low end of the adjustment range is 196.6V


And the high end of the range is 203.8V


This is not high enough.  I figure a ~15K drop in R5 should allow me to trim out to 210V.  R5 is currently at 33K.


The Hakko 808 makes quick work of removal


And I replace with a 15K resistor.




Connecting back to the microphone and powering up, we now have 204V at the low end of the adjustment range.


And can trim up to our nominal 210V.


Again, I felt it was important to have a 100% functional configuration described on thread, but I will need to circle back and streamline the instructions with the more direct 1M pot solution.


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Apr 21, 2008
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With the power supply squared away and tested, we can turn our attention to the fun stuff! 

First, using wire cutters, cut the individual boards out from the PCB panel supplied in the kit.




Next, clean up the edges with a file.


There are a few clearance issues that need to be resolved on this 1st revision PCB, so mount the main PCB to the frame rails with some M2 screws (4mm length) to investigate.  It should be noted besides mounting PCB's this size screw is a replacement for 90% of the screws on typical imported microphone bodies like this SYT-5.  It is a good idea to keep a small supply on hand.  Alternately, if one of these M2 screws should strip, it is also a good idea to have a standard 2-56 thread tap and some 2-56 screws on hand since this is slightly larger than the M2 thread, and can easily and solidly repair stripped screws in these economy-priced donor bodies.


Both connection nodes of the C9 capacitor sit dangerously close to the grounded frame rails.


I opt to cut the frame rails to make safe clearance for the solder pad.  First, mark the cut location with a pen.


After removing the XLR connector base, a cutoff wheel on a Dremel or other comparable rotary tool makes fast work of grinding away the marked slots.


With the slots cut, we have comfortable clearance on both nodes of C9.



Next, install the perpendicular tube socket PCB.  It is convenient to leave the main PCB mounted inside the frame rails with the screws very lightly tightened to initially install this board because it needs to be held in place form many angles and does not have any "keyed" mounting structures built in.


In order to get the fit started, I used a piece of scrap foam that I found on the floor, but a rectangular section and wedged it into the microphone frame.


We need to horizontally align the solder pads by eye, but with the foam acting as a soft clamp, it is easy to maneuver the PCB's into alignment.


By shifting the foam forwards or backwards in the assembly, it is possible to rough in a perpendicular relationship between the 2 PCB's.  Once aligned, solder one of the center pads.


Check for alignment with a straight edge.


If the alignment needs adjustments, it is easy with only one solder joint to heat the joint and shift the PCB especially when much of the "holding" is accomplished by the foam.  Once confirmed, solder another tab.


And check alignment again.


The PCB can now be removed from the frame rails and the rest of the tabs soldered.  Do these very slowly allowing each joint to cool before soldering the next one. 


With these large solder joints that require lots of solder, as the puddle hardens, it has a tendency to shrink and pull the PCB's out of alignment.  If you use a lot of solder on these connections (which is a good idea for strength), you may hear the board making crackling noises as the solder joints solidify.

If after soldering all of the joints, you find the boards not perpendicular due to the solder connections shrinking, firmly apply pressure in the desired direction to the PCB like this.


And run your soldering iron across the joints re-melting 3 pads at a time and allowing them to cool while holding pressure.


After traversing all the way across all of the pads like this, you will find you can slowly and methodically move the assembly back to perpendicular.  As you are only "releasing" a small section of the assembly each time and the rest of the solder pads are still secure, the board does not come grossly out of alignment and slowly moves towards the desired angle.


Max has stated that this kit is designed in a way that DOES NOT require cleaning solder flux residue off of the PCBs for 100% functional build.  All of the HiZ sections of the circuit are floated on teflon isolators, acrylic board, or soldered free-floating in the air.  It is a personal preference of mine to build electronic kits cleanly especially premium kits like this.  This is one of the points where you cannot go half way.  It is way messier to half-way clean a board than it is to simply leave the flux and opt not to clean.  Partly cleaning will only serve to spread a sticky film all over the boards, so make a decision one way or another.  For me, people are watching  :eek:, so it is best not to embarrass myself.  I also like for anyone who opens up one of my builds to see the best solder joints I can make, and neatest wiring I can produce at the time, and sparkling clean PCB's completely free of flux.  So for this build guide, I will be cleaning everything.  Due to the complex nature of the physical assembly, some thought needs to be put into the sequence of assembly to allow for efficient cleaning along the way.

Use 90% or higher isopropyl alcohol for the job.  This can be found at most drug stores and the pharmacy section of some supermarkets.  75% which is also widely available simply does not have the strength to efficiently dissolve flux.


The ideal situation is to be able to submerge the entire PCB and scrub with a toothbrush.  As the PCB attachment joints are some of the largest and dirtiest in the is project, and the grooves created between the joints very hard to mechanically access, clean this section first before installing other components by submerging and scrubbing.


As the flux residue dissolves into the alcohol, it will become sticky and start to leave residue.  I use small quantities in a shallow dish and change it often so my solution remains fresh.  After scrubbing, I wipe the boards with fresh paper towels and cotton Q-tips.



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Apr 21, 2008
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Next, populate the main PCB. 

Resistors are not sensitive to isopropyl alcohol.  Transformers and capacitors ESPECIALLY the polystyrene ones are extremely sensitive and should not be touched at all with solvent.  So, I sequence the assembly to maximize my opportunities to dunk and scrub before installing the more delicate components which need to be cleaned around very carefully.


Begin with the main board resistors.

R14 =  150K
R20 = 60R
R19 = 10K
R18 = 470R
R9 = 50M


Carefully double check the value of each resistor.  R9 is too high a value for my multimeter to read, so I rely on correctly identifying it via stripe markings.


Confirm all components are correctly placed.


Bend the leads to secure them in place, flip the board over and solder.


Trim the excess leads close to the PCB.


Next, we populate the opposite side of the same board.

R15 = 3.3M
S2 = jumper wire
R13 = 2.2M
R12 = 2.2M
R11 = 6.8K
R10 = 15K



Flip the board over, solder, and trim the leads.


With the resistors installed, it is a good opportunity to scrub the flux residue.


Why not build with immaculately clean boards?  ;D


Next, populate:

R4 = 820K
R5 = 2.2M


Next, reference the wiring diagram in the 1st post of this thread, and begin attaching the designated wires to the perpendicular tube socket PCB.  For clarity, I use 22 gauge silver teflon wire the same color wires in the diagram.  The "wire kits" in the studio 939 webstore contain the necessary colors.

For quick reference, here is the wiring diagram again.  The image on the 1st post is higher resolution.



And here are the required wires installed.  Refer often to the wiring diagram to confirm everything is in the right place.


This is a good opportunity to clean the PCB again.


Next, install the tube socket.  The pin sockets can sometimes be very stiff when new, so it is a good idea to insert a test tube before installing the socket to break in the connector a little bit.  This will prevent damage to the microphone when the socket is installed and a tube inserted for the 1st time.


Position the tube socket. . .


. . . and solder one pin only.


Confirm that the socket is sitting flush and even to the PCB.  If an adjustment needs to be made, it is very easy at this point to re-heat the single solder joint while pressing down on the tube socket to ensure uniform seating to the PCB.


Solder another pin and re-confirm alignment.


Then, solder the remaining pins.


As these large joints generate a lot of flux residue, now is a good time to clean them holding the PCB upside down and brushing from the bottom.



Next install:

C2 = 10nf
C7 = 10nf


C2 requires a teflon turret to be pressed into the PCB.


The lead should make a half turn around the turret for secure connection before soldering.


A wire also needs to be attached to the turret.  Prepare the wire by tinning the lead and bending a small hook at the end.


Install the wire on the teflon turret, solder the joint, and cut off the excess capacitor lead.



Optionally, you can trim the teflon turret length on the back side of the PCB.


C2 and C7 installed.


Cleaning the joints now becomes a little more tedious because the capacitors do not particularly like isopropyl alcohol.


Next, solder a wire to the open grid pin on the tube socket.  I use some tape and a block of wood to hold the wire in position.


Next, install:

C4 = 10nf
C11 = 10nf



And make a good cleaning on all of the new solder joints.  Make sure to clean well this time because many portions will become hard to reach after the transformer is installed.


Max's custom wound BV.12 is the heart of this kit.


Position the transformer on the PCB.  Note: the label text on the transformer should be opposite or upside down from the PCB text.  Make sure to orient the transformer properly.



Solder ONLY one joint on the transformer at this time.


It is vitally important that the transformer is seated flat onto the PCB.  If an adjustment needs to be made, re-heat the single solder joint while pressing the transformer tight to the PCB.  Other parts rely on precise placement of the transformer so double check and make sure it is seated all the way.


Solder another joint and re-confirm alignment of the part.


Once confirmed, solder the remaining pins and clean the bottom of the PCB with isopropyl alcohol.  Make sure to keep the solvent off of the transformer and only brush the bottom of the PCB.


BV.12 installed!


Next, install:

C9 = .47uf
C17 = 80-160mmf  (polystyrene)



C9 should be positioned to clear the transformer pins.


VERY carefully clean the solder joints under the transformer making sure not to get alcohol on the transformer.


C9 and C17 installed.  Note: it is critically important not to get alcohol on C17 as it is a polystyrene capacitor and particularly sensitive to solvents.  The part can be destroyed by exposure to isopropyl alcohol.


Next, locate the upper PCB and install:

R7 = 680K
R6 = 680K
C6 = 10nf
R3 = 820K
C5 = 10nf
C3 = 10nf



Clean the bottom of the upper PCB and place it on top of the transformer.


This kit aligns very precisely, so the initial steps of careful PCB alignement with a straight edge pay off now as the transformer and 3 PCB's come together.


Locate the long screws and nuts to secure the top PCB to the assembly.


And install all the way through the transformer and lower PCB.


Install the nuts on the bottom side of the lower PCB.


Confirm alignment of the upper PCB and solder all of the tabs.



Carefully scrub the solder connections clean with isopropyl alcohol making sure not to get alcohol on the sensitive components.


At this point, the PCB assembly is complete.  From here the build gets pretty interesting so hang on for the ride.  I think I will need to use another post to begin the high impedance sections.




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Apr 21, 2008
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NOTE: I opted to use silver teflon wire for the the high impedance section I am about to begin building.  Ideally, several of the wires need to free float in the air, but if this fails for whatever reason, it is a good thing to have the added insurance of the best insulating material of the teflon wire.  This safeguards the integrity of the robust isolation design in this kit.  Also, there are many correct ways to route the connections in this section.  I opted to use a method prioritizing strong mechanical connection between the leads and stable soldering position.  I am sure there are other methods that would look more elegant, but I hope this suggested procedure can at least be a starting point.  Study the wiring diagram and read through each build section before starting.  Take your time and all should go well.

OK. . . switching to low key background for a second since the white background really wasn't working for this part.  Begin the high impedance section by locating the acrylic board.


Peel the protective membrane off of the board.


We will be referring quite frequently to the wiring map provided in the 1st post.  It may be a good idea to print it if you have a color printer or open the image separately in another window for reference.  The photos in this section may not be adequate to verify all connections.



C10 = 560mmf

Thread the lead back up through the extra through hole.



Next, install the remaining components on the acrylic board:

C8 = 10nf
R8 = 400M
R2 = 60M
R16 = 60M
R1 = 400M
C1 = 10nf


Turning the board over and accessing from the bottom, we begin by making a connection between C10 (polystyrene clear) and C1.  We will leave the lead on C1 strait and wrap the lead from the styrene capacity around it to make a solid physical connection.


Next, pull the lead from C1 diagonally to the 400M R8 and twist the leads together to make a strong connection.


Here is a different angle.  We want to diagonal lead to sit flat to the bottom surface of the acrylic.



Solder the junction between C1 and C10 and cut off the excess.



Next twist the leads of R1 and R16 together.


Use a piece of black 22 AWG silver teflon wire, strip and tin a lead, and form into a hook.


Install this wire flat to the acrylic board at the twisted R1, R16 junction.  A capacitor sits underneath this position, so it is important to place the wire tightly to the PCB for clearance.


Run the wire over to C8 and twist the leads together.


Solder and cut the excess.

Next, twist together the leads from R1 and C1.


Solder and trim the excess leads.


Our acrylic board is now fully populated and the on-board connections completed. 



The next phase of the build will be making the connections from the main PCB section to the acrylic board.


We begin with the 2 wires that are isolated on the main board.  The green wire is our grid connection coming directly off of the tube socket and isolated from the main PCB.  Ideally, we will need this wire to connect to the acrylic board without touching anything else except the acrylic board.  Cut this wire to length, strip, tin, and create a hook.


A piece of tape secures the wire in place for soldering to the end of R8 and the diagonal lead.  I have left this connection twisted together but unsoldered to this point.  For my routing, I opt to position the wire flat to the acrylic board.


After soldering, trim the excess leads.



Confirm the wire path routes cleanly and does not contact any other components or the main PCB.


Next, prepare the blue wire coming off of the teflon turret.  This wire also is high impedance right from the teflon turret, so it will also need to be cleanly routed like the green wire.


Solder the joint and trim off the excess.


Next, we will route this black ground wire from junction 2 on the main PCB to the end of C8.


Twist the leads together and solder.




Next, prepare the white wire at junction 4 on the main PCB.  This will route to R16.


The connection may be a bit hard to see with the clutter around it.


Here is a closer look at the joint.


Solder and trim the connection.



Next, prep this red wire and route it to R8.


Solder and trim the joint.



One more red wire remaining to connect.


Route this wire to R15, twist and solder.


The acrylic high impedance board is now connected to the main PCB.


Double and triple check all of the connections carefully against the wiring diagram.

The next step is to mount the acrylic board to the main PCB assembly.


Locate the screws, nuts, and spacers included in the kit.  One of the screws is nylon.


The nylon screw goes in this hole where a pad comes into close proximity or contact with the screw.


Carefully route and guide the wires and attach the acrylic board.


Three key points:

1) the green and blue wires in the upper left corner of the image below are floating and do not touch anything except for the bottom of the acrylic board.

2) the diagonal lead running along the bottom of the acrylic board is not contacting any wires along the entire run.

3) None of the solder nodes or component leads exposed at the bottom of the acrylic board should touch anything (wires or components).  They should only contact air and the acrylic board.



Use a flat tip screwdriver to manipulate the wires into proper position if necessary.

With the acrylic high impedance board installed, we need to make our wire connections on top of the board that go to the switch board and capsule above.  First, prep a blue wire by cutting the mid-wire insulation and exposing a portion of the conductor.


We will attach this to R2.


Next, tin the end and make a hook at the end of a green wire


And connect it to the junction of C10 and C1.


Next, connect a yellow wire to the end of C1.



The upper side wiring on the high impedance board should look like this.


For normal builds, this should be good to go, but because I have a very high likelihood of using this microphone for various test configurations, I choose to use teflon isolators in the microphone frame's top plate.  Normally, the front capsule will connect directly to the C1/C10 junction.  If using a teflon isolator, this is a jumper wire, so change my wiring on my build to 2 wires at that junction.



And the main PCB assembly is complete at this time.





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Apr 21, 2008
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This is an optional step, but next, I prepare the chassis to install 3 teflon turrets on the top plate.  I begin by removing the 2 screws holding the top plate to the frame rails and the capsule mount.


Next, I mark the location of my turret holes.  It is important to consider the location of the switches that will go directly below this plate.  By making my holes directly next to the standoff screws that attach the switch PCB but leaving a little space for the diameter of the standoffs, I ensure I will not hit the switches.


Center punch the holes and then make a small hole to start.



Then, follow up with a larger drill bit to fit the teflon turret.


I am using Keystone 11251 turrets because they are large diameter and seem to hold much better in the top plate than the smaller turrets especially with hand-drilled holes.



In order to clear the switch PCB, I cut off a small section of the lower turret.


Next, locate the mounting hardware for the switch PCB.  These should be included in the SYT-5 body kit.


The top and the bottom of the switch PCB look very similar.  Make sure you have the correct board orientation or else the switches will not match the microphone sleeve markings.


Check and make sure the turrets will safely clear all 3 switches below.


I will re-iterate that teflon turrets are an optional step.  It is perfectly fine to pass the capsule wires straight through the top plate and make the necessary connections to the specified connection points on the wiring diagram directly.

At this point with the top plate removed, it is an excellent opportunity to raise the capsule mount to bring the capsule to a more optimal position inside the headbasket.  I opt to raise it 8mm.  In the previous prototype, I drilled and tapped the top plate to accept the 1st Gen SYT-5 capsule mount which readily receives an M3 hex standoff to raise the saddle.  For the gen 2 bodies, a spacer of some kind is the easiest way to accomplish this.

Because he happens to be local to me, I contacted forum member Shaggy who dropped by to pick up a few miscellaneous mic parts the other day and has a machine shop!  I threw a quick drawing together on sketchup and he was gracious enough to produce a Delrin spacer to my specs on his lathe and drill press.


If any of you keep up with Shaggy's microphone builds, he is a meticulous craftsman and, well, it's a bit silly but the spacer is better machined than the entire microphone body.  It's like using a Ferrari when all you need is a skateboard.

In order to insert this spacer, I will need longer M2 screws for the capsule mount.  I use 12mm length stainless steel M2 screws.




With the capsule mount secured, I re-attach the top plate to the frame rails.


Next, install the switches on the switch PCB.  The top side of the board can be identified by the lettering being in the correct order (A, B, C, -  D, E, F).  the bottom side will have the letter order in reverse.


Switches will mount on the top side of the PCB.


Note: S3  or the pattern switch is the 3 position on-off-on switch.  The other 2 switches do not have a center position and are on-on.

Lay all 3 switches onto the PCB and flip the assembly over to solder.  The switches with the isolated contact lugs are a bit harder to align.  I recommend pushing them inwards towards the center of the PCB so the 2 lugs so the switch can center on the board.


While pressing the switch towards center, solder one lug on each switch.


Next, temporarily install the switch PCB to check switch alignment to the cutouts on the microphone outer tube.


The 3 position pattern switch is most critical because there is the least amount of margin available for mis-alignment.  Here, it is nearly perfect, so no adjustments are necessary, but if we do need to adjust, it is much easier to do with only one lug soldered in on each switch.


With alignment confirmed, solder the remaining lugs and clean the PCB from the bottom.



While it is unlikely the switches have shifted from soldering, I reconfirm switch alignment after the switches are completely soldered.  No changes here, so we can move forward.


Note: when installing the switch PCB, a way to minimize the "slack" in the board is to press the board against the 2 screw side when tightening the screws.  This will guarantee a consistent position.  Also, by pressing firmly this direction, it places the 3 position pattern switch a little bit further towards the outside of the microphone which gives the shaft a little bit more room to swing.


For wiring up the switch PCB, refer often to the wiring diagram in the 1st post of this thread.  All of the required connections are clearly laid out there.

Prepare a new yellow wire by stripping the end, tinning, and creating a right angle.


Secure the wire against floating lug "B" on the pattern switch (S3) with some tape and solder into position.  After soldering, I feed this wire through the center hole as it will go up to a capsule connection.


I realize at this point that I forgot to install a wire to the acrylic board, so I prep a white wire by stripping the end, tinning, and creating a hook.


This will attaches to C1.


Next, trim and prepare the yellow wire coming off of the polystyrene capacitor C10.


This wire will connect to the floating center lug (E) on the S5 switch.  Secure the wire with tape to the switch PCB to hold it in place while soldering.


Next, trim and prepare one of the 2 blue wires coming off of R2.


Solder this wire to lug "D" on the S5 switch.


Here is how the wiring looks so far. . . next, prepare the white wiring coming off of C1.


We will attache this to lug "A" on the pattern switch.  At this point, I am using any available method to align the wires for soldering.  In this case, I am using a spring clamp shimmed with a piece of wood underneath.


Next prep one of  the green wires coming off of C1 on the acrylic board and solder to lug "C" on the pattern switch.



Next, locate 3 wires coming from the tube PCB.  black (ground), yellow (8), and red (9).  These wires connect to solder lugs 1, 9, and 8 on the S1 switch.



After installing the wires, I made a decision to route the wires on the "inside" of the PCB due to clearance concerns with the outside tube.  This was easily accomplished by temporarily removing one of the standoffs securing the acrylic board. 


At this point, the switch PCB is completely wired in.


Remove the top plate of the microphone body and connect the yellow wire that is attached to the center of the pattern switch (lug B) to one of the teflon turrets.  This wire will connect to the back capsule.  We previously fed this wire up through the center hole of the switch PCB.



Next, install the switch PCB to the top plate with the screws and standoffs provided in the SYT-5 body kit.



The top plate can then be re-connected to the frame rails and the PCB positioned inside the microphone body.


4 M2 screws attach the main PCB to the frame rails.  As the boards are pressed into their final positions, adjust the switch PCB wires so that all of the ones coming from the acrylic board or connected to free-floating switch pins are free-floating and do not touch other wires.  For this build, it was barely possible to accomplish this, and i had to do some poking and prodding with a small flat head screwdriver to move the wires into cooperation.


Two wires remain.  The blue wire goes to the capsule backplate turret, and the green wire goes to the front capsule turret.  These two wires should be routed to free float and not contact anything except for the turret teflon turrets.



At this point, the top portion of the microphone wiring is complete.





With the turrets labelled and the PCB secured, the assembly should remain largely undisturbed for some time assuming we do not have any mistakes or failed components inside :)


One more test fitting confirms that the switches are still aligned with the outer tube slots.


Next, install wires on the 7 pin XLR insert.  To match with our power supply:

pin 1 = B+
pin 2 = heater
pin 5 = audio +
pin 6 = audio -
pin 7 = ground


Pin 7 (ground) is bridged to the chassis tab at the connector.


Remove the 4 M1.6 screws securing the bottom piece of the microphone frame and install the 7 pin XLR insert.


Leave enough wire lead to back the XLR insert out if repairs or changes are needed.  I estimate it is possible to neatly leave 1/2 to 1 inch of wire than I have in the photo below and still route the wires very neatly.


Solder the wires to their designated lugs at the bottom of the microphone PCB.


Clean the solder lugs with isopropyl alcohol being careful to avoid the capacitor and transformer.


Neatly route the wires and re-install the base piece.



At this point, the microphone is completely wired up and ready for a tube and capsule.





Next, I make my 7 pin XLR cable.  I am using Gotham GAC7 cable which contains 2 large gauge conductors.  I use these on pin 7 and pin 2 of the cable as these are my heater and ground.  For this cable, I do not connect the shield to the ground pin so all of the conductors are straight through on the cable.  The grounding scheme can be determined at the PSU or in the mic body.



The solder lugs are close and there are a lot of wires.  Carefully verify with a multimeter that there are no shorts in the cable.

Next, for function testing, install a known-working capsule that you are not afraid to destroy.  This happens to be a capsule from a stock Alctron MK47 microphone.


Also, install an EF86 equivalent tube that you are not afraid to destroy.



Double check that all connections are as they should be, and fire up the microphone!  If all goes well, there should be no smoke and everything should work correctly.  This build functioned flawlessly right away.  It is raining in Northern California tonight, so I have wider line voltage fluctuations than normal.  The passive B+ supply will fluctuate with the line voltage.  This is normal.


With the microphone confirmed working, it is now time to replace the function test components with high quality pieces for the final build.

I am using a Telefunken manufactured EF86 tube that is branded for Sonotone.  For this mic, I simply emailed Christian Whitmore and requested the best type EF86 he had on his current shelf, and he asked if I was concerned about the branding.  I was not and only care that the tube is original factory spec and a good sample so he sent the Sonotone.


As a baseline reference in this build, I will be using a new production Neumann K870 capsule.  These are available directly from Sennheiser.



The saddle that came with the donor body had holes that were just slightly too widely spaced for the original Neumann capsule, so I used a small drill bit to lengthen the screw hole slots on both sides towards center.


Use M1.6 screws which are not included with the capsule to secure the capsule.


A backplate connection tab is also not included with the capsule, so I stole one from another cheap Chinese capsule in my drawer and attached a purple 28 gauge stranded silver teflon wire to one of the screws connecting the capsule to the saddle.

It is critically important to keep splashing solder flux and hot vapors off of the capsule when soldering.  Protect the installed capsule with a plastic bag when soldering near it.


The leads on the capsule only extend far enough to reach the teflon turrets, so if you plan to use an original capsule, I recommend installing these.


With the final tube and capsule installed, the microphone is complete.  We will need to re-trim the B+ voltage with the new components and then we should be able to put the microphone into service!



Category 5

Well-known member
Jul 24, 2004
Palm Beach, FL
Good stuff as always Chunger.  I have bee using the same teflon wire as you and love it but notice two things - 1,  it can become brittle and break at the solder joint if exercised too much, particularly the lower strand count version  and 2, it can be tough to strip

I have been using an x-acto and the rolling technique to strip but it's difficult when wires are already attached and also "may" be responsible for issue #1 since it's possible I am scoring the strands in parts when stripping.

Any tops for stripping this superb wire?

I love it otherwise.  It seems to suck solder straight through the air!  lol

BTW - Did you get my email about these mics?


Active member
Feb 7, 2014
Rhode Island
Category 5 said:
Good stuff as always Chunger.  I have bee using the same teflon wire as you and love it but notice two things - 1,  it can become brittle and break at the solder joint if exercised too much, particularly the lower strand count version  and 2, it can be tough to strip

I have been using an x-acto and the rolling technique to strip but it's difficult when wires are already attached and also "may" be responsible for issue #1 since it's possible I am scoring the strands in parts when stripping.

Any tops for stripping this superb wire?

I love it otherwise.  It seems to suck solder straight through the air!  lol

I noticed the same thing about breaking, but man it solders up nice!



Well-known member
White Market Member
Apr 21, 2008
Northern California
I generally use a utility knife blade to cut the shielding circumference as well.  The wire strippers just don't work well on the teflon wire.  I had problems with wire break early on in my U87 build (dual PCB version) but that was with really thick 20 gauge wire.  I'm using 22 gauge in this build and mind my the way the wire orients as it comes off of the solder joint.  I try to make my sharp bend if necessary after the joint because the wire is stiffer than many others.

I guess it's a 2 edged sword.  The stiffer wire allows you to nudge them into place in tight spaces so they do not touch each other in the HiZ sections, but they are harder to work with and you need to mind the strain a bit more at the solder joint.

I suspect moving down to a slightly thinner stranded 24 gauge wire would give you a lot more flexibility and tighter turn radius on the wire and lessen the likelihood of breaking.

I'm more comfortable with teflon wire especially in situations like this where isolation is critical and the assembly a bit intricate.  The pre-built reference sample that ioaudio sent to me does not use teflon wire, so I am assuming it is 100% optional.  Perhaps he can chime in on the issue of how "messy" you can get with the build and still derive maximum HiZ performance from the kit as laid out.

My assumption is if Acrylic isolation board is used, for those sections, I would assemble it so that all critical wires are truly free-floating and any incidental contact is with materials that perform better than air or acrylic for isolation (ie. teflon).


Well-known member
Jun 7, 2012
What type of wire was used in the reference sample from Max?

I agree the teflon wire is very hard to strip. I wound up having a bunch of those strands break off and get stuck in the skin of my finger, and had to pull them through. That was no fun


Well-known member
May 11, 2005
I just used standard PVC sleeved wire like the originals.
I experimented with silver/teflon but had problems with wire break - moved the wire 5 times back and forth and it would break.

Touching other components on the HiZ section is not a problem - most insualtion issues are surface related.



Well-known member
Apr 6, 2013
If you're willing to invest a bit (as it's pretty dear), what you guys need is this:

Makes a quick meal of PTFE insulation with very clean cuts, and you can swap the blades once they wear out.

Haven't looked back since.


Well-known member
Jul 11, 2010
Lincoln, Nebraska
^ those strippers are really nice for braided shield coax etc.

I like the Molex auto stripper, sorry no link.

Has anyone tried a "hot stripper" on the Teflon stuff? 

Category 5

Well-known member
Jul 24, 2004
Palm Beach, FL
Banzai said:
If you're willing to invest a bit (as it's pretty dear), what you guys need is this:

Makes a quick meal of PTFE insulation with very clean cuts, and you can swap the blades once they wear out.

Haven't looked back since.

Got it.  It works pretty well except it takes the AWG24 nozzle to properly strip my AWG22 PTFE wire.  Very clean strands though.  The only time I can't see it working is with very short wires such as exposed ends when soldering tube mic cables.  Thanks for the recommendation.  It'll find MUCH use on my bench.


Well-known member
Apr 6, 2013
Category 5 said:
Got it.  It works pretty well except it takes the AWG24 nozzle to properly strip my AWG22 PTFE wire.  Very clean strands though.  The only time I can't see it working is with very short wires such as exposed ends when soldering tube mic cables.  Thanks for the recommendation.  It'll find MUCH use on my bench.

Exactly, very useful tool to have lying around. For anything that's PVC like mic cables I just use a basic stripper like the one below. Saves me from dulling the blades of the fancy one, and it strips backwards for the very short wires like you describe.

And definitely no need for thermal strippers when you have the two of these.


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Well-known member
Jul 11, 2010
Lincoln, Nebraska
Here's the Molex auto stripper.  It has a depth fence (proper term?) so you can strip the same wire after wire.  Not cheap but a beast.

The hot stripper was simply a 120V:6.3V filament transformer with a special wire that was shorted across the secondary, can't recall the metal, it would get hot but not orange or light at all.  No insulation stood a chance.  Roll the cable over the edge of the hot wire and it melts it right off, teflon, pvc, etc...  even the dreaded cables we used for some hipot testing of rail car stuffs... the sharpest of coax razor strippers would baaarely cut.  Hot stripper was like hot knife into butter.


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Well-known member
White Market Member
Apr 21, 2008
Northern California
Small techincal note on B+ current:

Telefunken EF86 is drawing .888mA - .894mA
Valvo E80F is drawing 1.015mA - 1.03mA

both trim to 210V on the PSU as currently described but be aware, the PSU needs to be re-trimmed after tube swapping.

My yield on the E80F tubes is not good from the various sources discussed in related threads at ~$20 per tube .  I have 1 out of 3 quiet tubes so far which makes it essentially a $60 alternate tube.  I have not tested them in studio yet against the Amperex and Telefunken EF86's, but am burning in a couple to prepare.

Category 5

Well-known member
Jul 24, 2004
Palm Beach, FL
Good info Chunger.  I got a few e80f here but came across some ef86 telefunkens in sealed boxes so I am very eager to compare these.  I wanted to try e804s too. 

Will get with you to get some mics after I recover from Black Friday.  Lol. 

What brand were your e80f?  I know they're technically all the same as the Valvo but it's possible different brandings were binned differently. 

Also my MK47 is being quiet with tubes the MKU47 didn't like so I'm thinking noisy or bad tubes aren't simply bad, but something else may be making them behave as such.