A local artist complained that the “Instrument” connection on the front and “Microphone Input” connection on the back of his PreSonus Eureka PreAmp no longer functioned properly.

After running a few test audio signals through the PreAmp, we determined that the input J-Fet (first stage amplifier before the Op-Amps) had been damaged.  Based on the failure mode, we contacted the artist inquiring if the PreAmp stopped functioning after he had connected an external guitar effect pedal to the “Instrument” connection on the front of the device.  The artist confirmed that this was the case.  The front input is connected directly (not AC coupled) to the gate of one of the J-Fets.  Thus, a large enough DC bias from the output stage of an effect pedal could possibly cause the damage that was observed.

A rough hand drawn schematic of the input stage can be seen below.  Note: We were seeing +14V on pin 3 of the J-Fet (and consequently on the tip connection of the TR “Instrument” input) because the gate oxide in the J-Fet had been punctured.  Normally that is a high impedance input and should be at 0V (Gnd) because of the pulldown resistor.

Chad Kelly was really helpful and sent a replacement input J-Fet free of charge.  Interestingly, the original part in the Eureka was a U/ST441.  From what I found searching around for this part, Vishay/Siliconix no longer manufactures this particular part.  Linear Systems makes a direct replacement for the U441.  But I received a Linear Systems LS844 from Chad.  It has a higher breakdown voltage and works like a champ! Check out the classic VLSI layout in the datasheet!

To prevent this problem from happening again in the future, we instructed the artist to use “Insert” “Send” and “Return” connections on the back of the unit if he wants to use the PreAmp with his guitar.  An alternate option would be to connect his guitar to his pedals and then connect the output of the pedals to the “Line” input on the back of the unit.  This bypasses the PreAmp un the Eureka, but still lets you use the compressor and equalizer in the unit.  I have mirrored the Eureka owners manual here to help anyone that is confused by what was state above figure out how to connect things properly.