GP-100 external switch

Hey, if you ever wanted to hookup your GP-100 to an external preamp and needed the GP's external jacks to switch the channels but couldn't, try this.

Of course you know that the warranty it's gone once you modify the GP, right? You can also cause permanent damage to your unit if you're not careful. AND.....listen good....I'm not responsible for any damage this may cause to you or you're equipment.

Also, this is my first try with the design program so the circuit looks kinda ugly...sorry.

I - The Circuit
II - The Parts

Item Count Label-Value Attributes Designation Notes
1 2 10K Axial 0.4 R1, R5
2 4 1K Axial 0.4 R2,R4,R6,R8
3 2 100K Axial 0.4 R3,R7
4 2 5VSPST DIP4 Rly1,Rly3 DSS41A05, single inline reed relay
5 1 LM358 DIP8 U1 Almost any opamp will do
6 1 n/a DIP8 n/a IC socket
7 1Shown belown/an/aPC board. Radio Shack #276-159
86Budweiser Cold BeerSubstitute for your favorite brand.

III - The theory

The GP-100 has two jacks in the back to control external equipment. They are supposed to act like foot switches but because they have approx. 600 ohms between their leads when closed, some outboard gear doesn't work.

This project solves this problem by using relays to close the jacks and achieve theoretical 0 ohms ( = good). It's quite possible that by just bypassing the two resistors in the jacks, you will be able to achieve similar effects but I didn't want to risk any GP internal components by bypassing resistors.

I selected the LM358 because it's a low cost, high output, dual operational amplifier and it was the first one I saw at the store :-) Many other OpAmps will do as well. I did look a bit more for the right relay. I needed a couple that would activate with 5V and a few milliamps, had 4 pins and would be "normally open". The DSS41A05 was perfect but others might do too.

The idea is that the GP will send the signal to "close" the jack, the OpAmp picks that up on the inverting pin (2 & 6) and sends a high (6 V) on its output pin (1 & 7) activating the relay which would then close the contacts. Very basic.

IV - The Procedure

1 - Drink all the beer first

2 - Cut the copper line on pin 4, like this:

3 - Solder the IC socket into the first 4 holes with Pin 1 towards the board's part number (276-159B). Blue outline.

4 - Solder the relays on the outer column of the row of three, starting at IC pins 4 and 5. Red outline. There's no polarity on the relays so orient them any way you want.

5 - Solder the resistors according to the circuit in part I. On resistors R2 and R6, solder one leg and leave the other hanging for now.

6 - Tie all grounds together.

7 - Solder 4 wires to the board: V+ to pin 8, Ground to pin 4, input A to pin 2 and input B to pin 6

8 - Solder one more small cable from ground so we can tie it to the GP's chassis by using the internal screw that holds the jack board. See last picture.

9 - Insert the LM358 OpAmp into the socket and apply power to the board using any old wall-wart power supply with around 12V DC.

10 - Test the circuit by tying the inputs to ground and checking for continuity across the relay's contacts.

11 - If you're reading this, then you didn't get electrocuted. Good.

12 - On the GP, take the cover off and remove the external jack board.

13 - Carefully remove the two jumpers L1 and L2.

14 - Solder one pin of the relay contacts (not the coil) to the jack output part of L1 (and then do L2) and the other contact to ground.

15 - Solder R2 to the resistor part of L1 and R6 to the resistor part of L2.

16 - Get that?

17 - Run a wire from power (pin 8) to the output pin of voltage regulator IC40 on the GP. The output pin is the one on the right if you look at it from the side where you plug your guitar into. Be very careful not to cross any leads with solder and do not overheat the board.

18 - Use the internal screw to tighten the board down and ground it.

19 - Double check all connections before you power up.

This is how it looks on my GP:

If you get a bad ground loop noise (I did) when connecting to your external preamp, simply remove the ground wire from the cable between the GP external jack and your preamp.