TX-429D

Information and comment about the TX-429D used by some Korg peddles.

• korgpeq1 - Korg Parametric EQ (Devices IC1-3 with the unique circuit symbol, TX-429D, are Dual Gate MOSFET's, similar to the MPF131.)
• korgphs1 - Korg Phaser (Devices IC1-4 with the unique circuit symbol, TX-429D, are Dual Gate MOSFET's, similar to the MPF131.)

Dateline: 2017/12/21

TX-429D devices as used in the KORG phasers etc.

Korg were trying to make a voltage controlled resistor, i Jfets have a problem due to the internal diode between gate and D,S, and mosfets have problem with a diode between D and S, if you can tie off the substrate somewhere then MOSFETs might be useful.

I don't think the device is similar to the typical dual gate RF MOSFET. In the circuit applications the drain and source can reverse polarity i and MOSFETS can't do that because of the body diode connected to the substrate. Also the TX-429D device is a P-channel device. It looks like it operates similar to a "transfer gate" in 4000 series CMOS or the switches in a CD4016.

For the TX-429D

• pin 2 is the substrate and connects to +
• pins 1 and 3 are the in/out terminals of an analog switch
• pins 4 and 6 seem to be a distributed gate

You could probably replace it with a pair of P channel MOSFETS with

• the sources are tied together and become pin 2
• the drains are pins 1 and 3
• the gates are 4 and 6

This is a reasonably common way to make an AC switch.

Compare with a CD4007

TX-429D	CD4007
1	1
2	14
3	2
4	3
5	3

You could also make it more symmetric by using other devices in the package.

Source: BobT

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