Balanced Input/Output Assembly Guide

Safety Precautions, Warranty, and Disclaimer Improper soldering and handling of electricity can cause serious injury and damage to your property. Read and understand the instructions below before beginning your project. Follow the instructions, build carefully, and use the appropriate tools. Build at your own risk. DIY Recording Equipment, LLC is not responsible for any damage or injury resulting from the assembly or use of your LINE2AMP kit. You are the manufacturer of your LINE2AMP. It is your responsibility to turn this group of parts into a working piece of recording equipment. DIY Recording Equipment, LLC does not guarantee the success of your project and disclaims any Implied Warranty of Merchantability. Please visit the support forum for assembly support. Damaged or Missing Parts All kits and parts are checked before being shipped to you. If something arrives damaged or if your kit is missing a part, please contact me directly via this contact form to inquire about a replacement. Missing parts will be replaced at my expense. Damaged parts should be returned to me for verification. If the part shows signs of use beyond what was necessary to determine that it was damaged, DIY Recording Equipment, LLC reserves the right not to replace the part.
Thank you for purchasing a Building Block kit. The Building Block kits were designed to be easy to use, but they do require some electronics knowledge (or a willingness to learn) to implement successfully. Since the possibilities for use are endless, this guide will focus on providing you with the basic information you need to research and design your own project.

Schematic and Circuit Description

The Building Block 1 & 2 circuits do little more than carry audio and power to and from the ICs (U1), which do account for most of the circuits' performance.

Balanced Input Circuit

Balanced Input Schematic

The network composed of C1-C3 filters out noise above the audio spectrum. C4 blocks any DC from being passed to the next stage; it's relatively large value (100uF) ensures that it will not create a filter in the audio range with the input impedance of the next stage. If you prefer not to have the electrolytic capacitor the signal path, you may replace it with a jumper at your own risk. CB1 and CB2 perform the important "housekeeping" task of power-supply bypassing. These capacitors reduce noise and provide small amounts of local storage for supply current. U1 is a dedicated balanced line receiver chip. Any of the pin-compatible offerings from THAT Corporation, Texas Instruments, or Analog Devices may be used. The THAT 12xx series of chips boast the best specs and have been widely adopted in the pro-audio world.

Note: The + and - inputs for the Balanced Input PCB are marked backwards. To maintain the correct polarity of your signal, you should connect the positive input to the pad marked "-" and negative input to the pad marked "+". We apologize for the inconvenience. This mistake will be corrected in the next revision of PCBs.

Balanced Output Circuit


As in the Balanced Input circuit, CB1-CB2 provide local power-supply bypassing and C1 blocks potentially harmful DC from entering the circuit. The capacitors C3 and C4 prevent DC offset at the output of the circuit. The inductance of ferrite beads F1-F2 react with the capacitance of C5-C6 to shunt RFI noise to the chassis, away from the audio circuit. U1 is a balanced line driver IC. The THAT Corp. 1646, TI DRV134, and SSM2142 are all pin-compatible.

IC Choices

(Click a part number to see pricing and availability.)

Balanced Input ICs

Manufacturer Part # Gain Comments
THAT 1240P08-U 0dB Conventional line receiver
THAT 1243P08-U ±3dB Conventional line receiver
THAT 1246P08-U ±6dB Conventional line receiver
THAT 1200P08-U 0dB High CMRR
THAT 1203P08-U ±3dB High CMRR
THAT 1206P08-U ±6dB High CMRR
THAT 1250P08-U 0dB Low cost
THAT 1253P08-U ±3dB Low cost
THAT 1256P08-U ±6dB Low cost
Texas Instruments INA134PA 0dB Conventional line receiver
Texas Instruments INA137PA ±6dB Conventional line receiver
Analog Devices SSM2141PZ 0dB Conventional line receiver

Balanced Output ICs

Manufacturer Part # Gain Comments
THAT 1646P08-U +6dB
Texas Instruments DRV134PA +6dB
Analog Devices SSM2142PZ +6dB

Bill of Materials

Balanced Input

Qty Part # Value Type
1 U1 Balanced line receiver THAT 12xx, TI INA13x, SSM2141
2 C1, C2 470pF Ceramic or film, non-polarized
1 C3 47pF Ceramic or film, non-polarized
1 C4 100uF Electrolytic, non-polarized
2 CB1, CB2 .1uF Ceramic, non-polarized

Balanced Output

Qty Part # Value Type
1 U1 Balanced line driver THAT 1646, TI DRV134, SSM2142
1 C1 100uF Electrolytic, non-polarized
1 C3, C4 10uF Electrolytic, non-polarized
2 C5, C6 100pF Ceramic or film, non-polarized
2 CB1, CB2 .1uF Ceramic, non-polarized
2 L1, L2 Ferrite bead  


 Begin by inserting the 8-pin IC socket into position, making sure to align the notch in the plastic with the notch on the PCB labeling. Then flip the PCB over while holding the socket in place and solder the socket's pins.
 Once the socket is in place, you can populate the rest of the PCB at once. Insert the capacitors/ferrite beads into their respective places and bend their leads against the back of the PCB so that they stay in place when flipped over. Then flip the PCB over and solder the leads. None of the capacitors are polarized, so they can be inserted in either direction.
 Trim the excess leads with a wire cutter. Trim the leads as close to the solder joints as possible without clipping the joints themselves.
The End?
Congrats! You've successfully populated the Balanced Input PCB. Now it's time to implement it in your circuit--you're the man now, dog!

Connecting to the Outside World

Once the PCBs are populated and tested, they are ready to be wired up to the "outside world." Each PCB contains three sets of pads for connecting the Building Block to other circuit blocks.

Balanced Input

  • The audio input should be a balanced, professional (+4dBu), line-level audio signal. You can use a 3-pin XLR or TRS 1/4" jack.
    • Hot (+) goes to XLR pin 2 or TRS "Tip"
    • Chassis goes to XLR pin 1 or TRS "Sleeve." Pin 1/Sleeve of the jack should also be connected to the physical chassis.
    • Cold (-) goes to XLR pin 3 or TRS "Ring"
  • The audio output should be connected to the input of an unbalanced circuit, or a TS (2-pin) TRS jack.
    • goes to the next circuit's audio input or TS "Tip"
    • Ground (unlabeled on PCB) goes to the next circuit's audio ground or TS "sleeve"

Balanced Output

  • The audio input should be an unbalanced, professional (+4dBu), line-level audio signal. You can wire the balanced output directly after an unbalanced circuit, or to a TS (2-pin) 1/4" jack.
    • goes to the source device's audio output or TS "Tip"
    • Ground (unlabeled on PCB) goes to the source's audio ground or TS "sleeve"
  • The audio output can be connected to a 3-pin XLR or TRS 1/4" jack.
    • Hot (+) goes to XLR pin 2 or TRS "Tip"
    • Chassis goes to XLR pin 1 or TRS "Sleeve."  Pin 1/Sleeve of the jack should also be connected to the physical chassis.
    • Cold (-) goes to XLR pin 3 or TRS "Ring"


For both units, power inputs must be connected to positive and negative voltage rails and the power supply ground. The power rails must be a DC voltage within the range of +/-4 to +/-18vDC. Your power supply must be a dual supply that generates both positive and negative voltage rails; these will typically be specified with a +/- as in "+/-12vDC" instead of simply "12vDC."

Each PCB can be expected to draw between 2-20mA of supply current. If you are in doubt regarding power supply voltage or current, check the datasheets for your chosen ICs. The proper connections are:

  • + goes to the positive power supply output
  • 0v goes to the power supply ground
  • - goes to the negative power supply output


Each PCB includes three .125" (3.2mm) mounting holes, designed to work with 4-40 or M3 screws and standoffs. For easy mounting, adhesive-backed, nylon standoffs may be used.