A Mastering Transfer Console Input and Filter Board

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A Mastering Transfer Console Input and Filter Board

Unread postby Wayne Kirkwood » Thu Jan 04, 2018 3:25 pm

Hi Trolls. This thread describes a new Mastering Transfer Console input, filter and balanced output PC board that I've developed for simple vinyl transfer. It's basically the front-end of an MTC-1 without source selection.

I've supplied several of these and will be ramping up production quantities. PM or email me for availability.

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 board does not provide input source selection. This decision was made to maintain simplicity and a less than 21 square inch footprint.

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.

A four channel modulation plus preview transfer console for example would require an incredibly expensive 8 pole switch.
Relay-switched filter outputs allow multiple channels to be controlled by a single-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.

Schematics

Image
Mastering Transfer Console Input, Filter, Output Stages.

Large format schematic: http://www.ka-electronics.com/images/png/MTC_Input_Filter_Output_Sheet_1.png

Circuit Description

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.

Configurable Filters

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

Image
Mastering Transfer Console Relay, LED and Power Distribution.

Large format schematic: http://www.ka-electronics.com/images/png/MTC_Relay_LED_Power_Sheet_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.

Gain Switch

Image
Mastering Transfer Console Gain Switch Detail.

Large format schematic: http://www.ka-electronics.com/images/png/MTC_Gain_Switch_Sheet_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.

Configurable Filters

Image
Mastering Transfer Console Configurable Filter Examples.

Large format schematic: http://www.ka-electronics.com/images/png/MTC_Filter_Examples_Sheet_4.png

Image
An Input, Gain, Filter, Output board for mastering transfer.

The above was constructed with two 12 dB/octave high pass filters at 20 and 30 Hz.
The second pair of filters are low pass at 22 and 27 kHz.
Capacitor/resistor placement on the PC board determines whether the filter is high or low pass.
The boards can be cascaded to provide more than four filter frequencies.

This is the filter response for the IGFO board built for 20, 30 Hz HPF and 22, 27 kHz LPF.
The DAC used to plot this has 22 kHz output filters (despite its 96 kHz sample rate) so above 22 K the steep roll off seen with the filters bypassed is not real.

Image

Image

Image

Some pics of an 8 filter 2 board build I did for tape-to-vinyl.

The first board has the input with stepped gain, high pass filters at 20, 30, 40 and 50 Hz. The second board has 12, 14, 16, 18 kHz low pass filters and the final output. A balanced insert is available between boards.

Image
Mastering Transfer Console Input and High Pass Filter PC Board

Image
Mastering Transfer Console Low Pass Filter with Balanced Output PC Board

The final system has preview which requires four boards. Switching all four filter channels only requires two single-pole rotary switches: One for the high pass and one for the low pass.

The filter responses:

Image
Mastering Transfer Console Filter Responses

The total BOM for the two boards, excluding the bare PC boards, is about $220 US which is far less expensive than the two 8 pole switches needed to tune 4 filters.
User avatar
Wayne Kirkwood
 
Posts: 38
Joined: Tue Jul 21, 2015 12:43 pm
Location: Dallas, Texas


Re: A Mastering Transfer Console Input and Filter Board

Unread postby Wayne Kirkwood » Sun Jan 07, 2018 4:05 pm

Wanted to let everyone know that boards are on order and that production quantities of the MTC-IGFO Input/Gain/Filter/Output board will be available for shipment around Thursday January 18th.
User avatar
Wayne Kirkwood
 
Posts: 38
Joined: Tue Jul 21, 2015 12:43 pm
Location: Dallas, Texas


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