Oberheim DMX Type 1675 Voice Card Configuration

The type 1675, 1677 and 1678 voice cards, also known as the "Mark II", were designed to be even more flexible than the 1554 voice card. It can use one or two EPROMs, either 4K byte 2732s, or 8K byte 2764s. These can be configured several ways, as shown in Table 1.

EPROM U8 EPROM U9 Function
2732 - One 4K sound (same as 1554 kick, snare, etc.)
2732 - Two 2K sounds (same as 1554 percussion)
2764 - One 8K sound
2764 - Two 4K sounds
2732 2732 One 8K sound
2732 2732 Two 4K sounds
2764 2764 One 16K sound
2764 2764 Two 8K sounds

Table 1

The 1675 voice card uses a low pass CEM3320 voltage controlled filter (VCF) to smooth the digital output. (This is the same filter chip used in the Oberheim OB-Xa and OB-8 synthesizers). A few 1678A voice cards were manufactured, using a slightly different CEM3350 filter instead of the CEM3320. Otherwise, the 1678A is pretty much identical to the 1675C, and this document should apply to either, as far as configuration goes.

Specific jumper settings for type 1675C (720080) voice cards

The following voices are all assembled on the same type of printed circuit board, with variations in components and jumper positions used to set individual characteristics.

Generic card settings for type 1675C voice cards

Picture of 1675C Tom configuration

General information

The 1675 voice cards use one or two 2732 or 2764 EPROMs in various combinations. The EPROM sockets are pin compatible with 27128, 27256 and 27512 EPROMs, but only the top 4K or 8k bytes will be used - The 1675C card has EPROM address lines A15, A14 and A13 hardwired to a logic high state.

The correct way to install a 24 pin EPROM into a 28 pin socket

2732 EPROMs have 24 pins instead of 28 pins, and must be inserted into the lower part of the IC socket, as illustrated above.

Every voice card has three possible triggers: 1, 2 and 3. These correspond to the three pushbuttons for a voice on the DMX front panel, from top to bottom, in that order. When a voice card is triggered, a binary counter starts addressing the EPROM(s) on the voice card, and the data bits feed into a Digital to Analog Converter (DAC). This multiplying DAC also behaves as a Voltage Controlled Amplifier (VCA), and its output goes through a low-pass Voltage controlled filter (VCF) before leaving the voice card. Two simple envelope generators are also included which can control the VCA and/or the VCF.

The sample rate on most voice cards is approximately 24000 samples per second, adjustable using the trimmer T1, and also depending on the value of capacitor C1.

Jumper and component settings

There are several jumper settings that may be made to the generic voice card to change its playback configuration. In addition, there are some resistor and capacitor changes that will affect the sound.

With the exception of the A jumper block, the jumpers are numbered starting from 1 closest to the letter printed on the silk screen. The A jumper is the reverse of this, with pin 4 closest to the "A".

A and F jumper locations B, C, D and E jumper locations G jumper location

VCA envelope decay settings

The A jumper, capacitor C4, diode D5, and resistors R7, R8 and R9 control the VCA decay envelope. See Table 2 for jumper and resistor combinations. When the decay envelope is enabled, the sound starts out playing back at full volume and gradually becomes quieter throughout the sample playback. The decay time is set by the value of C4, typically 2.2uF to 6.8uF. If C4 is not installed, the decay time is nearly instantaneous.

The A jumper also controls the behavior of the VCF envelope decay. Pin 1 is common, and should connect one of the other three pins.

Out = Not installed, X = don't care.
Jumper A C4 D5 R7 R8 R9 Trigger 1 VCA Trigger 2 VCA Trigger 3 VCA
X Out X X 4.7K Out - - -
Connect 1 to 3 Installed 1N4148 4.7K Out 4.7K decay enabled - -
Connect 1 to 4 Installed 1N4148 4.7K Out 4.7K decay enabled decay enabled -
Connect 1 to 4 Installed 1N4148 4.7K Out Out decay enabled decay enabled decay enabled

Table 2

VCF notes

For minimal low pass filtering, a simple approach is to just put a jumper wire across capacitor C3 pads. This will set the filter in a wide open configuration, independent of any other settings described below.

VCF envelope decay settings

There are several settings that affect the VCF mode of operation. Jumper A, capacitor C3 and resistors R12 and R13 control the filter frequency as shown in Table 3. The decay time is controlled by the value of C3, typically around 10uF.

Out = Not installed.
Jumper A C3 R12 R13 Trigger 1 VCF Trigger 2 VCF Trigger 3 VCF
Connect 1 to 2 Out 4.7K Out Wide open Wide open Wide open
Connect 1 to 3 Installed Out 10K Decay envelope Wide open Wide open
Connect 1 to 4 Out 4.7K Out Track sample rate Track sample rate Track sample rate
Connect 1 to 4 Installed 4.7K Out Decay/track combo Decay/track combo Decay/track combo
Connect 1 to 4 Installed Out 10K Decay envelope Decay envelope Decay envelope

Table 3

EPROM configuration settings

Pitch and amplitude control settings

The arrangement of diodes D1, D2, D3 and D4, and resistors R4 and R5 determine the control of the triggers on sound pitch or amplitude. See Table 4 for the allowable configurations. The amount of any changes are controlled by the setting the values of R4 (trigger 1) and R5 (triggers 1 and 2).

Out = Not installed, X = don't care.
D1 D2 D3 D4 R4 R5 Trigger 1 Trigger 2 Trigger 3
1N4148 Out 1N4148 Out Installed Installed Raises pitch (R4 & R5) Raises pitch (R4) -
1N4148 Out Out 1N4148 Installed Installed Raises pitch (R4) and lowers volume (R5) Raises pitch (R4) -
Out 1N4148 1N4148 Out Installed Installed Lowers volume (R4) and raises pitch (R5) Lowers volume (R4) -
Out 1N4148 Out 1N4148 Installed Installed Lowers volume (R4 & R5) Lowers volume (R4) -
X X 1N4148 Out Out Installed Raises pitch (R5) Raises pitch (R5) -
X X Out 1N4148 Out Installed Lowers volume (R5) Lowers volume (R5) -
1N4148 Out X X Installed Out Raises pitch (R4) - -
Out 1N4148 X X Installed Out Lowers volume (R4) - -
X X X X Out Out - - -

Table 4

Control voltage settings

Jumper wire G controls the path of the external control voltage to control either playback pitch (connect pins 2 and 3) or volume (connect pins 2 and 1). The external control voltage comes into the DMX through the Molex jack on the back of the unit.

Other settings

Resistor R11 sets the default loudness of the sound.

Capacitor C1 may be used to set the nominal sample rate. Typical values range from 0.0022uF to 0.0047uF.

Triple Voice Card

You can set up this voice card to have three separate samples, one for each trigger. The settings are as follows, assuming any standard configuration as the starting point:

U8 should be a 2764, containing two 4K samples. U9 should be a 2732, containing one 4K sample.
The sound for the top trigger button on on the 2764 at addresses 1000h - 1FFFFh, the sound for the middle trigger button go on the 2764 at addresses 0000h - 0FFFh. The sound for the lower trigger button goes on the 2732.

Jumpers A, D: connect A1 to A4. Connect a wire from jumper position A2 (on the back of the board) to jumper position D2. No other connections to the D jumper group.

Jumper F: Connect F3 to F4.

Jumpers B, C: Remove any existing jumpers from the B and C groups. Connect a wire from jumper B1 to jumper C4. Connect a wire from jumper C3 to jumper B3.

Jumper E: Connect E2 to E3.

Jumper G: Connect G2 to G3

Jumper across C3.

Cut out or otherwise remove the following parts: C3, C4, R4, D1, D2, R5, D3, D4, R9, R12, R13.

If you want to vary the sample rates between the three sounds, re-install D1 and D3, and experiment with different resistor values for R4 and R5.
Click the thumbnail images to see photos of a triple voice card modification:

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