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preview comment
pic pic
  • frontpanel made by Frontpanelexpress.com
  • 4x20 character dot matrix display (HD44780 compatible)
  • input buttons: input 1 to 6, mute, menu
  • control buttons: up, down, ok, cancel
  • infra-red receiver, Alps rotary encoder
  • all functions accessable through frontpanel controls and IR remote control (Philips RC5 protocol)
  • rearpanel made by Frontpanelexpress.com
  • Neutrik NYS-367 RCA jacks
  • 6 line inputs
  • outputs: direct, attenuated, 2x attenuated+buffered
  • test points to level input volumes
  • Atmel AVR atmega32 microcontroller
  • relay based input selector (Takamisawa RY5W), mounted directly to input jacks (only 2mm of wire) to make signal traces as short as possible. upper and lower side of the boards got large ground planes for maximum shielding. inactive inputs are shunted to ground via 100kΩ resistors.
  • attenuation / gain is provided by a Burr Brown PGA2310
pic pic
  • β22 PSU
  • jisbos buffers for buffered line out (biased at 20mA)
  • jisbos buffers for headphones (biased at 30mA)
pic pic pic The software:
1. ghost mode
activate / deactivate ghost mode. when running in ghost mode the input buttons to rca jacks mapping is active, hence one don't know which input is active in fact. feature for blind A/B listening tests.
2. volume offsets
set volume offset for each input in 0.5dB steps to set all inputs to the same volume level
3. mix input ports
mix up the mapping of input buttons to rca input jacks
4. reveal mapping
show the mapping of input buttons to rca input jacks
5. volume step size
configure the volume change for one click (min 0.5dB, max 3.0dB)
6. zero crossing
enable/disable the zero crossing detection feature of PGA2310
7. edit input names
edit input names displayed with the input's port number, e.g. CDP, DAC, ... the names are only displayed when not running in ghost mode, of course.
built Mar 2008


Input/Output Selector

preview comment
pic pic Frontpanel made by Frontpanelexpress.com
2x16 character dot matrix display (HD44780 compatible)
rugged pushbuttons for a solid and precious look
pic Rearpanel made by Frontpanelexpress.com
the inside of the panel is chrome plated, carries signal ground and acts as a large ground plane
Neutrik NYS-367 RCA jacks

input and output section are bridged outside the case and can be split. A volume potentiometer can be inserted to turn the switchbox into a "passive preamp", an active preamp can be inserted to turn it into a full linestage, an equalizer ...
pic the boards carrying the relays (Takamisawa RY5W) are mounted directly to the input jacks (only 2mm of wire) to make signal traces as short as possible

upper and lower side of the boards got large ground planes for maximum shielding

inactive inputs are shunted to ground via 47kΩ resistors, inactive outputs are shunted directly to ground
pic pic the brain of the switchbox: an Atmel ATmega8 microcontroller controls inputs (pushbuttons), outputs (relays) and drives the display
the main board carries the power supply, the microcontroller and some parts to create the voltages for the display (LED backlight, display contrast)

the second board is for i/o-expansion:
2x 74HC147 (10 to 4 line priority encoder) are compressing 2x8 buttons to 2x4 bits
2x 74HC595 (8 bit shift register) are expanding output pins from 3 bit to 8 bit and feeding ULN2803 Darlington Arrays which are driving the relays and LEDs

wiring the frontpanel was a pain, I guess next time I'm going to pick pcb mounted buttons and build an input board instead of using panel mounted parts
pic Keeping the mute-button pressed during powerup enters the setup mode. In this mode the display names of the outputs can be edited via the output buttons, so the names can be changed without having to reprogram the microcontroller

software and i/o-expansion-board are capable of controling 8 inputs and 8 outputs, I'm only using 6 of each with this build
built Dec 2007


input/output switch-box

view preview size comments
front pic 800x600
127 kB
frontpanel pic 800x600
84 kB
by Frontpanelexpress.com
rearpanel pic 800x600
121 kB
by Frontpanelexpress.com


pic205kB dummyload 6.3mm
pic118kB ad744 bypassed output stage
pic20kB passive opamp
pic137kB y-splitter


Cable Inventory:

  • WBT: 2020 CCS
  • van den Hul: The Source
  • Sommer: Aqua blue
  • Sommer: The Goblin


  • download hpgl.zip [43 kB] contains various icons and signs in hpgl-format, for use with FPE (FrontPanelExpress)