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hello everyone!

first of all... please, excuse my poor english :icon_confused:

Hope you can help me... I´ve googled but, find no answer.

I'm trying to trace and mod an acoustic guitar preamp that uses gyrator eq. The mid's section originally uses a 2n5485 (Right section on image below). Since they are rare to find on my country, i would like to change it by using a MPSA18 (Left section conencted to the potentiomter)... by following the proposed by RGkeen here
(Values arround the MPSA18 obtained with the online calculator by Jack Orman... )

Well...i've simulated it on multisim... But I can´t figure out how can i match the gain (Boost/cut db) delivered by using the 2n5485. Red line: MPSA18, Gray line: 2n5485

How can i achieve an answer as close as the original by using the MPSA18?

Hope you can help me!... i really would like to learn the theory behind this.

Best regards from Chile!

I seriously doubt the gains in your multisim simulation.
The gyrator circuit needs a 1-time amplifier. This can be a n opamp in buffer config, as well as the (budget) solution with a transistor as emitterfollower. In both cases no gain is added. I seriously doubt that changing the transistor will give any benefit, let alone audible...

lukatosh

Did you tried to increase R1 a little? I suggest R1 100k and reduce C1 to 220pf.

Well... This is about the LR Baggs Para Acoustic DI. It's the mids control

Still Playing with this... Today i've tried replacing the transistor, use more common values and the response looks similar to the original (400 Hz to 1.6kHz App.) There is a slight thifference on the "Q"

RED: Original/2n5485 - LIGHT BLUE: Proposal/MPSA18
RIGHT: Original/2n5485 - LEFT: Proposal/MPSA18
After this, what's behind the design criteria in the use of 2N5485 or 2n5088/MPSA18? Are there benefits between each one?

I'm thinking to switch between different capacitors using a DPDT to add a "mid shift" switch and offer a "low mids and high mids" option.