Update March 1, 2018:There is now a build thread for the MTC-IGFO located here: https://www.proaudiodesignforum.com/for ... ?f=7&t=940
Mastering Transfer Console MTC-IGFO product page: https://www.ka-electronics.com/kaelectr ... C-IGFO.htm
The Mastering Transfer Console Input PC board has the following features:
- Balanced Input
- Left Mute, Right Mute, Stereo Mute
- 0 or +5 dB Gain
- Adjustable Gain from -5.5 to +5.5 dB in 0.5dB steps
- Four 12 dB/octave filter stages per channel with relay-selected tuning
- Balanced Output to drive an Insert Switcher.
The MTC's high and low pass filters use banks of Sallen-Key stages instead of switching a single filter's tuning components.
A "two pole" 12 dB per octave filter requires a two pole switch.
By duplicating filters and switching only their outputs, the need for expensive multi-pole switches is eliminated.
Relay-switched filter outputs allow multiple channels to be controlled by a single-pole switch.
FET or transmission gate switching tends to be frowned upon in transfer consoles.
There are only electromechanical solutions.
A four channel modulation plus preview transfer console for example would require an incredibly expensive 8 pole switch.
Duplicated filters with switched outputs require lots more components and PC board area but is far less expensive than a big, long, hard-wired switch.
It only takes two relay poles to select a stereo filter...
Four filter frequencies for Left and Right are available per board.
For applications requiring more than four filters, partially-assembled boards can be cascaded.
A signal path with 4 high pass filter frequencies and 4 low pass filter frequencies requires two boards.
One board has the balanced input, gain stage and four HP filters.
The second board has four LP filters and the balanced output.
Sheet 4 of the schematics provides examples of filter configurations.
The example filter frequencies are High Pass at 20, 30, 40 and 50 Hz and Low Pass at 12, 14, 16 and 18 kHz.
Mastering Transfer Console Input, Filter, Output Stages.
Large format schematic: https://www.ka-electronics.com/images/p ... heet_1.png
Bill of Materials:
There are three bill-of-materials. One is for the base IGFO board, the second for the gain switch, the third for the filter tuning resistors.
When ordering filter components for multiple boards make sure the number tuning capacitors are correct.
This is for the MTC-IFGO board configured for 2 HP and 2 LP filters.
It includes THAT ICs and capacitors for filters.
It does not include resistors to tune the filters or resistors for the gain switch.
Mouser IGFO base components link: https://www.mouser.com/ProjectManager/P ... 05a212c717
These are the 23 resistors needed for the Gain Switch.
Only 1 resistor of each value are needed per channel.
The BOM shows 10 of each resistor because its almost as cheap to buy 10 each as it is 1.
Some of the values are not available in Xicon (271-value) and they are shown as KOA.
Occasionally Xicons will be backordered and KOA values can be substituted.
Mouser Gain Resistor link https://www.mouser.com/ProjectManager/P ... b6d456c09d
This list is for the the filter tuning resistors.
Each build is a unique. This list has the values for 20, 30, 40, 50 Hz HPF and 12, 14, 16, 18, 22 and 27 kHz.
There are 20 each in the BOM which are more than needed for most builds.
Please refer to the schematics for the proper values.
Mouser Filter Tuning Resistors: https://www.mouser.com/ProjectManager/P ... 2c2462aa47
Balanced Input and Gain
A THAT1246 provides balanced inputs.
Relays RY1 and RY2 provide individual Mutes for the Left and Right Inputs.
IC3A and IC4A are a variable or fixed gain stage.
RY3 increases the Left and Right Gain by +5 dB.
When J3 is linked IC3A has a gain of -1. (External gain open.)
An external variable resistance, 0-10KΩ, varies the gain of IC3A from -5.5 dB to +5.5 dB.
VR1 calibrates the lowest gain step to exactly -5.5 dB.
J1 bypasses the THAT1246 input for board cascading or unbalanced input.
J5 bypasses IC3A.
J1, the Mute relay NC contacts and J5 link the input connector directly to the filter insert switcher for board cascading.
Filter Insert Switcher
The filter inputs are driven in parallel by IC3A/IC4A with the option of the last two filters being cascaded from the output of the first two.
When J7 is in the A position all four filters are fed in parallel.
Placing J7 in the B position allows cascading the second two filters to the output of the first two.
An example of cascading would be to have the first pair configured as HP filters and the second pair as LP filters.
This would provide HP filters of 20 and 40 Hz and/or LP filters of 14 or 18 kHz.
The A position is used when two boards are cascaded and more than four filters are required.
The B position is used for a single board providing four filters.
RY4-RY7 are an insert switcher that select which filter output appears in the chain.
When no filters are selected the insert provides hard wire bypass.
IC3B/IC4B are a unity gain inverting stage to supply a "0Ω" source impedance to the THAT1646 balanced output.
IC3B/IC4B also correct for the polarity inversion of IC3A/IC4A.
J9 in the A position allows the filter insert switcher output to cascade directly into a second board.
J9B lets IC3B/IC4B drive the output directly.
When J9 is used the THAT1646 should not be installed.
A THAT1646 on the final board provides a balanced output to drive the transfer system's insert chain.
The MTC filters are standard unity gain Sallen-Key filter blocks.
Because the filter blocks can be either high or low pass, impedances (Z1-Z32) are used to represent resistor and capacitor component locations.
The filters are configured as high pass or low pass depending on where the PC board's components are loaded.
Tuning component locations on the PC board accept 5mm Wima film caps or 1/4W resistors to allow a common layout to be a low or high pass filter.
Schematic sheet 4 illustrates the transformation from high to low pass by swapping the locations of Rs and Cs.
Resistors R20-R23 provide an op amp input bias current path for low pass filter configurations during relay switching.
Relay, LED and Power Distribution
Mastering Transfer Console Relay, LED and Power Distribution.
Large format schematic: https://www.ka-electronics.com/images/p ... heet_2.png
The relay coil common is isolated from audio ground and has its own optional regulator.
RY1/RY2 are the individual Left and Right Mute relays.
D8 and D9 allow Stereo Mute capability with a single contact closure.
RY3 is the +5 dB Gain relay.
RY1-RY3 also have current-limited LED tally outputs for switch illumination.
RY4-RY7 are the filter insert relays.
Diodes D10-D13 provide a current-limited LED output when any filter is engaged.
Mastering Transfer Console Gain Switch Detail.
Large format schematic: https://www.ka-electronics.com/images/p ... heet_3.png
The gain switch is designed around the 24 position Elma Type 04 15° shorting rotary switch.
Note that the cable shield to the switch is driven by IC3A/IC4A's output.
The driven shield makes the shield-to-center-conductor capacitance of the cable beneficial to the circuit because it is in parallel with the feedback resistance.
Mastering Transfer Console Configurable Filter Examples.
Large format schematic: https://www.ka-electronics.com/images/p ... heet_4.png
More to come...