HeadPho

Footprint for NJM4556AV-TE1 can be found here: https://componentsearchengine.com/part-view/NJM4556AV-TE1/New%20Japan%20Radio

Todo

TODO: take power footprint out of stereo pot footprint (this overlap isn't good!) DONE: double check schematic against original TODO: Convert panel holes to NPTH to avoid maximum plated hole size warning (although we won't sent these to fab, so doesn't really matter)

Inspiration

https://llllllll.co/t/exits-a-4hp-mixer-line-headphone-output-module/52221 https://github.com/forestcaver/Analog-Voice/tree/master/AJH_Headphone_Amp - not kicad https://www.ericasynths.lv/shop/diy-kits-1/edu-diy-output/ https://busycircuits.com/alm019/ https://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html https://github.com/russellmcc/eurorack_headphones - kicad https://github.com/ramphands/Exits - kicad, SMT, op amp available on ali but not common https://github.com/wntrblm/Speak_to_Me - kicad, headphone amp with inbuilt speaker https://github.com/tkilla64/eurorack/tree/main/output

Features

• left and right mono jack 3.5mm inputs - mono sums over when only left plugged
• stereo volume pot
• L and R mono jack 3.5mm outputs (line level)
• stereo 3.5mm jack output

Circuit explanation

• Audio comes in on 3.5mm mono jacks (L and R)
• The switched input of the R jack takes audio from the L jack, so that when R is not plugged in we get a copy of the L signal in the R output
• Audio level is dropped to line level for L and R channels by two LME49720 audio op amps
• Audio goes out to L and R 3.5mm line level jacks
• A stereo potentiometer attentuates the line level signal for both channels equally
• Two NJM4556 chips drive the headphones
• Audio op amps are needed (LME49720) and an amplifier chip capable of driving (low impedence) headphones

How It Will Be Used in Our Rack

• two of these will be installed in our rack
• the buff mult can duplicate the final output signal
• we can send this independently to two separate headphone amps

Notes

• The large amount of 100n decoupling caps is because we need 1 pair per chip, which on the PCB should be placed as close to chip power pins as possible
• I removed the ADP0204 dip switch in the original circuit
  • when switched on, engages an extra 100K resistor between input and output of the opamp on each channel
  • I guess to further attentuate the input signal perhaps to allow for line-level input
  • I will just experiment with different resistor values here instead
• is there any THT alternative to NJM4556AV-TE1? Yes NJM4556AD DIP with same pinout
• symbol pins 7 and 8 on dual pot are just mount points

Switched plugs explained simply

• Switched plugs have a switch contact/pin
• When there is no plug inserted into the jack the tip contact is touching the switch contact
• This creates a closed circuit between the input signal and the output path through the switch contact
• When a plug is inserted into the jack the signal now flows through the plug instead of the switch contact
• In the case of my headphone amp, I want L input to connect (normal over) to the R when no plug is inserted in the R
• This means that the tip output of the L jack is connected to the switch contact of the R jack