Pair of Presonus Eureka mono channel strips - Jensen Transformers Modified. Price is for the PAIR.
Both were modified by Jensen Transformers in 2017. These were some of the last units modified by Jensen, due to customers sending broken units and the unavailability of repair parts from Presonus. Both are in excellent condition, with some minor rack rash on the tops and bottoms of units. The mods we do include replacing the mic input transformer with our Jensen model, and replacing the incorrect transformer load resistor with the proper value. To do this, we also have to change the resistor values on the Impedance Selector switch.
Note that the 2500 Ohm and 1500 Ohm positions on the impedance switch will both become the transformer's actual 1240 Ohm input impedance with the correct 15k Ohm load resistor now in place. The other three switch positions return to the correct value listed on the switch after the resistor changes are made.
Finally, we change out some of the electrolytic capacitors that were undersized in the original design, and then bench test the preamp to be sure it's working as it should. These mods will make the Eureka sound much clearer.Not only is the Jensen better than the stock transformer, even the stock transformer was subject to overshoot & ringing because it had the wrong load resistor in the original design. An additional modification that can be done to the Eureka is to replace the 5534 opamps that follow the transformer at the front end of the mic preamp. These opamps are in sockets, and can be easily popped out & replaced with better opamps. The two popular opamp choices are the Texas Instruments/Burr-Brown OPA627A, and the Linear Technology LT1357. Both of my units have the LT1357's, Transformers, Resistors and Cap upgrades. The item "PAIR, PRESONUS EUREKA JENSEN FACTORY MODIFIED" is in sale since Tuesday, November 27, 2018. This item is in the category "Musical Instruments & Gear\Pro Audio Equipment\Preamps & Channel Strips". The seller is "brackenaudio" and is located in Las Vegas, Nevada. This item can be shipped to United States.