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VT-73 Hybrid Tube Mic Preamp Assembly Guide

11. RACK MOUNTING THE VT-73 MIC PREAMP MODULE

For rack mounting, we recommend a FiveFish PSU-2448 Power Supply

Kit and Power Transformer to supply reg/- voltage and +48V

phantom power to your SK-1DI module.

You may also mount the VT-73 Mic Preamp inside a custom 1u 19” rack case. Just supply the preamp module with the necessary

+/- voltages, and connect to XLR male and female jacks. Use the wiring guide below. You can use a 15-pin EDAC connector, or

solder the connection wires directly to the holes provided on rear of the board.

+48V DC

-16V to -18V DC

GND

+16V to +18V DC

CHASSIS / GND

XLR OUT + HOT

XLR OUT - COLD

XLR IN + HOT

XLR IN - COLD

GND

Summary of Contents for VT-73

Page 2: ...posted online on the Internet in whole or in part without the expressed written permission of FiveFish Audio This document is solely provided to the kit builder of the VT 73 Hybrid Tube Mic Preamp Copyright 2017 FiveFish Audio fivefishaudio com Assembly Guide Revision 1 2 201760804 ...

Page 3: ...ier This assembly guide is a work in progress and we re continually working to improve it As always we welcome the input of the DIY community if you have any comments suggestions or additional information that you think will be helpful for inclusion in this assembly guide If you have any questions that are not covered by this assembly guide please feel free to contact us at www fivefishaudio com o...

Page 4: ...n personal injury Grounding one conductor of a two conductor outlet is not sufficient protection This mic preamp module is designed to be plug in into an API 500 series rack or compatible unit DC Power is supplied by the 500 series rack to this module It is strongly suggested that you use a lab power supply the first time you power your mic preamp project If you made any mistake during assembly of...

Page 5: ...n 48V short circuit protection to protect the preamp from damage The output of the First Stage is fed to the Second Stage Tube amplifier for additional signal processing This Second Stage also generates the Second Order Harmonics added to our original signal Second Stage output is controlled by a second potentiometer that can limit the output level of the Mic Preamp A Third Stage amplifier allows ...

Page 6: ... small to medium sized soldering tip is required to solder the small parts You may use Lead free or 60 40 lead based solder 2 2 Cutter You will need a cutter to cut component leads and wires 2 3 Solder Sucker Pump If you made a mistake and need to desolder a component you ll need one 2 4 Multitester Used for measuring resistance continuity and voltages 2 5 Magnifier Lamp I recommend a clean and we...

Page 7: ...g the wrong component into the wrong location Keeping the components in their zip bags also reduces chance of small parts getting lost or misplaced Best practice is to solder the smallest and or shortest components first Then solder the taller and larger components next Big bulkier components like switches jacks big capacitors are soldered last NOTE The photos shown on this assembly guide are Rev ...

Page 8: ...017 FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 1 Solder all 1 4 watt Resistors ERRATA Solder a jumper wire across RTUBE resistor location See Section 4 0 for other ERRATA corrections R20 R21 R22 ...

Page 9: ...reamp Assembly Guide 3 2 Solder all Signal Diodes and Rectifier Diodes IMPORTANT Diodes must be installed in the correct orientation Observe where the white band is located on the board silkscreen marking and orient the Diodes to follow the same direction ...

Page 11: ...iveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 4 Solder the 3 IC Sockets TIP One side of the IC socket will have a notch Position this notch on the left side or on top for use as a guide for Pin1 location ...

Page 12: ...c Preamp Assembly Guide 3 5 Solder the 2 PDIP Relays Take note of the Relay part number that came with your kit and follow the next step IMPORTANT Insert Relays in proper orientation Match the white band on PCB silkscreen with the Relay s line markings ...

Page 13: ...oltage Regulator at IC4 location You can use the heatsink provided and mount it vertically or bend the legs and secure 7812 metal tab with machine screw and nut flat on the PCB board If your kit came with A18W K Relays Do NOT install the 7812 Voltage Regulator Instead solder a JUMPER wire across pin 1 and pin 3 of IC4 location ...

Page 15: ...pacitors These capacitors have polarity markings one lead is and the other lead is ERRATA Use 47uf 50V for C25 location IMPORTANT Observe the proper polarity when inserting these capacitors Match polarity with PCB silkscreen markings All negative leads of electrolytic capacitors are orientated facing downwards ...

Page 16: ...right 2017 FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 8 Solder Q1 Transistor IMPORTANT Observe proper orientation and the direction transistor is facing Follow the PCB silkscreen markings ...

Page 17: ...e LED colors any way you want But for consistency use the same color arrangement on all mic preamp kits you ll build A typical color arrangement may be GREEN for phantom power switch BLUE for 20dB Pad YELLOW for polarity reverse RED for 12dB Boost IMPORTANT LONG LEG of the LED must be oriented at the BOTTOM position as shown NOTE LEDs are installed from the bottom Bend LED legs as shown on photos ...

Page 18: ...FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 10 Photo showing how the 4 LEDs will show through the Aluminum cut out panel Note Do NOT attach the panel yet NOTE LEDs are mounted at the bottom of the PCB ...

Page 19: ...ke sure they re mounted horizontally level and not crooked or mis aligned TIP You may use masking tape to hold the switch secure in it s place while you solder Solder only one of the switch terminals and review if the switch is still horizontally level If everything looks good solder the rest of the switch terminals ...

Page 20: ...Copyright 2017 FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 12 Solder all medium sized 220uf electrolytic capacitors IMPORTANT Observe proper polarity and orientation of and leads ...

Page 21: ...h Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 13 Solder the large C12 3300uf capacitor NOTE Solder 100pf capacitor in parallel to C12 from the bottom side of PCB Solder 100pf capacitor across C12 from behind the PCB ...

Page 22: ...Copyright 2017 FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 14 Solder the Bourns Potentiometer Make sure to mount potentiometer level and flat as possible with the PCB ...

Page 23: ...older the 10K Potentiometer as shown below Take note orientation of the wires and how they go into the PCB IMPORTANT Use as short as possible pieces of wire to connect potentiometer to the board to minimize RFI noise pickup Connect top and middle terminals of potentiometer together ...

Page 24: ...put Transformer NOTE Do not just duplicate the orientation of the transformer you see on this page Instead rely on the keyed PCB pins of the transformer It only goes easily correctly in one direction If the transformer pins don t align with the PCB holes turn the transformer around and try again ...

Page 25: ...dapter board is then soldered to the main board using right angle headers making the tube orientation vertical once the 500 series preamp is inserted into the rack First step is to solder the current limiting resistor for the LED Next solder the Right Angle Headers to the board as shown Turn over the PCB and solder the headers from the component side of the board Take note that the shorter leg is ...

Page 26: ... the proper orientation The Longer leg of the LED goes to the hole marked with a sign Do not solder the LED yet B Insert and solder the Vacuum Tube socket Solder all 7 pins to the board C After you ve soldered the Vacuum Tube socket position the Red LED into the center hole of the Tube socket Then solder the LED leads Figure 3 18 A Figure 3 18 B Figure 3 18 C ...

Page 27: ...e complete Vacuum Tube PCB Adapter assembly to the main board and solder the right angle headers NOTE The vacuum tube can have varying degrees of microphonic vibrations that can be heard with your audio signal To reduce these vibrations insert the rubber tube damper over the glass body You may also tie and secure the tube to the PCB using a short string or cut piece of rubber band to further reduc...

Page 28: ...2017 FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 20 ERRATA Solder a 100K resistor to bottom of PCB as shown in photo below Make sure you also have a 100pf capacitor soldered in the location below ...

Page 29: ...FiveFish Audio VT 73 Hybrid Tube Mic Preamp Assembly Guide 3 21 Install mounting spacers to PCB using the Aluminum Male Female Hex Spacers and 4 40 nuts Then mount PCB to aluminum sled using the provided small screws ...

Page 30: ...front panel Tighten nuts of potentiometers securely IMPORTANT Do not overtighten nuts to avoid cracking the Acrylic Front Panel Then install knobs by tightening the set screw on knob NOTE Anodized 0 125 aluminum CNC engraved Front Panels are also available as an upgrade option Contact FiveFish Audio for details ...

Page 31: ... Assembly Guide NOTE VT 73 Mic Preamp Kit comes with a 3mm Transparent Gray Acrylic front panel The lighted LEDs show through very nicely through the tinted transparent front panel 48V Phantom Power 20dB Pad Polarity Reverse 12dB Boost Output Trim Gain Control ...

Page 32: ...Preamp Assembly Guide 3 23 Insert the 3 IC chips into their sockets There is a proper orientation for each chip Take note of the half circle cutout on the Chip Body and orient the IC chip as shown in the photo below IC1 THAT 1510 IC2 OPA2134 IC3 LME49710 ...

Page 34: ...on 1 1 4 1 Use a wire jumper across RTUBE location 4 2 Use 470K for R22 location 4 3 Solder 100K resistor at bottom of PCB as shown in Step 3 20 4 4 Use 47uf 50V for C25 location 4 5 Solder a 100pf capacitor in parallel to C12 from the bottom side of PCB as shown in Step 3 13 4 6 Use 100K resistor for R20 location 4 7 Use 5R1 resistor for R21 RPOT location ...

Page 36: ...ges 5 2 Check if all diodes are installed in correct orientation 5 3 Check if all polarized electrolytic capacitors are installed in correct orientation 5 4 Check if all relays are installed in correct orientation 5 5 Check if jumper to bypass IC4 is required only required if using A12W K Relays 5 6 Check if IC1 IC2 and IC3 are inserted in their correct locations and oriented correctly 5 7 Check i...

Page 43: ...when I first turn ON the unit This is normal The vacuum tube needs to warm up before all DC voltages become stable and settle down to their optimum values Please allow 60 90 seconds for the tube to sufficiently warmp up before using the preamp After this alloted time the sratchiness on the bottom potentiometer will disappear There is no sound coming out but only during the first few seconds when I...

Page 44: ...your preamp is not functioning please review Section 4 ERRATA and Section 5 PRE FLIGHT CHECK In addition here are some approximate typical voltage readings around the Vacuum Tube section If your voltage readings are several orders of magnitude wrong please re check for any missing parts or parts in the wrong location ...

Page 45: ...r fine sand paper or something similar to scrape off the oxidized film on the terminals that may prevent making a good electrical connection If you re getting incorrect voltage readings around the tube section see previous page AND you ve verified all parts are soldered in the correct location and you don t have any other build mistakes try re seating the Vacuum Tube from it s socket and re insert...

Page 46: ...tom power to your SK 1DI module You may also mount the VT 73 Mic Preamp inside a custom 1u 19 rack case Just supply the preamp module with the necessary voltages and connect to XLR male and female jacks Use the wiring guide below You can use a 15 pin EDAC connector or solder the connection wires directly to the holes provided on rear of the board 48V DC 16V to 18V DC GND 16V to 18V DC CHASSIS GND ...

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