deb611
Member
Hi there, long-time lurker.
I've been stalled on a project in which I created a multi-pattern stereo microphone. I've run into a significant frustration and I think I've found the issue. The schematic I used is a standard UCM multi-pattern Schoeps circuit, which uses a Hartley oscillator to ramp up the capsule polarization voltage, and also uses the oscillator to make a multi-pattern microphone. I doubled the circuit and laid it into a standard Chinese stereo mic body similar to the SP LSD-2.
The issue I'm running into is an injection of oscillation, but ONLY when both sides of the mic are on. If only one side of the mic is on, I don't get any oscillation whatsoever. Changing the pattern switch on either mic changes the pitch of the oscillation, but the oscillation pitch is shared on both microphones.
Schematic:
I found this from Zapnspark in my hunt for answers:
"It's a variation on a Hartley oscillator using 2 mutually coupled inductors (Instead of the usual center tapped inductor).
Oscillates at ~ 2 mHz. You can find this type of polarizing circuit in some older Schoeps microphones. (e.g. CMC-5)
The diodes and caps provide level shifting and rectification to create a voltage multiplier.
It's a simple and effective circuit. Very weak output current though so - it's not meant to drive ANY resistive load - just the capsule itself. Some el-cheapo China mics violate that rule in order to get a multi-pattern mic. Bad idea! That's another topic though."
I did as much research as I could on oscillators, but I'm very much a self-taught novice and I'm out of my depth here. I've spent a lot of time and effort getting my PCBs designed and I'd really prefer not to scrap or restart my project at this point. Any ideas on modifications I could make to prevent the oscillation injection? I've already added two 1000pF caps across R3 & R4 to roll off any potential injection above 20K, but the oscillation I'm hearing tends to be in the 3-5k range.
my next step is to try shielding in between the two boards to avoid injection, since the boards are back-to-back with less than a half-inch of separation, but I have no idea if that will work or not. In the meantime, I'm open to ideas and criticism of my complete and utter folly ;D
I've been stalled on a project in which I created a multi-pattern stereo microphone. I've run into a significant frustration and I think I've found the issue. The schematic I used is a standard UCM multi-pattern Schoeps circuit, which uses a Hartley oscillator to ramp up the capsule polarization voltage, and also uses the oscillator to make a multi-pattern microphone. I doubled the circuit and laid it into a standard Chinese stereo mic body similar to the SP LSD-2.
The issue I'm running into is an injection of oscillation, but ONLY when both sides of the mic are on. If only one side of the mic is on, I don't get any oscillation whatsoever. Changing the pattern switch on either mic changes the pitch of the oscillation, but the oscillation pitch is shared on both microphones.
Schematic:
I found this from Zapnspark in my hunt for answers:
"It's a variation on a Hartley oscillator using 2 mutually coupled inductors (Instead of the usual center tapped inductor).
Oscillates at ~ 2 mHz. You can find this type of polarizing circuit in some older Schoeps microphones. (e.g. CMC-5)
The diodes and caps provide level shifting and rectification to create a voltage multiplier.
It's a simple and effective circuit. Very weak output current though so - it's not meant to drive ANY resistive load - just the capsule itself. Some el-cheapo China mics violate that rule in order to get a multi-pattern mic. Bad idea! That's another topic though."
I did as much research as I could on oscillators, but I'm very much a self-taught novice and I'm out of my depth here. I've spent a lot of time and effort getting my PCBs designed and I'd really prefer not to scrap or restart my project at this point. Any ideas on modifications I could make to prevent the oscillation injection? I've already added two 1000pF caps across R3 & R4 to roll off any potential injection above 20K, but the oscillation I'm hearing tends to be in the 3-5k range.
my next step is to try shielding in between the two boards to avoid injection, since the boards are back-to-back with less than a half-inch of separation, but I have no idea if that will work or not. In the meantime, I'm open to ideas and criticism of my complete and utter folly ;D