Each side of the input signal is going to see a low pass filter formed by the input capacitor and the input resistors in parallel. This directly contradicts Alex Rice’s assertion that the frequency response is down by only 3dB at 20Hz. The above circuit simulates with a lower -3dB point at 2.2kHz and the output is down by 41dB at 20Hz. ac dec 20 1 20Meg to give a small signal response from 1Hz to 20MHz. Note: Please read Phantom Piezo Preamp revisited for updates and corrections. We take our output from the differential voltage OUT+~OUT- which is approximately what the high-impedance differential-input-stage of the mixer-preamp will see. We’ve left the Zobel network out because we want to see how the circuit simulates without it. Q3 is replaced by a 1mA constant current source The input is an ac voltage source R5, R6 and V2 represent the phantom powering circuitry in the mixer and C3, C4 and R7 represent the early part of the input stage of the mixer. I simulated the design in LTspice like this: It seems obvious that the value of C2/C3 is really rather low and therefore the low-frequency response of the above circuit is not going to be very good. Now I’m no expert at analog simulation, but I have played fairly extensively with LTspice and the above circuit does not appear to work. Alex Rice claims that the final design evolved as a result of “ a lot of fiddling around with circuit simulators and breadboarded prototypes.” R5 and R6 provide a little negative feedback and R4/C1 forms a Zobel network to reduce high-frequency ringing (oscillation) in the cable.
R5/R1 and R6/R2 bias their respective JFETs. Q1 and Q2 are cited by Alex as a matched pair of N-channel JFETs with Idss 1-2mA and that Q3 should sink 1mA, giving a 0.5mA bias to each of Q1 and Q2. Here’s the schematic of the original design as reported by Zach Poff: Update: Phantom Piezo Preamp modules are available for purchase in the shop here.Ī few years ago, Alex Rice put a design up on the internet for a phantom-powered JFET piezo preamp. Update: Please see also Phantom Piezo Preamp revisited
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