pstamler
Well-known member
For a Zobel I'd go with Yamaha's recommendation; they generally knew what they were doing. The 1.6uF cap means that over most of the audio spectrum (everything above 62Hz) the transformer's load is 1.6k || 20k, or 1481 ohms. That means your mics are seeing an input Z of about 300 ohms, way too low for most modern mics. Really, go back to the Yamaha Zobel.
About opamps: the LME49710 has an ideal source impedance of 1562 ohms; the LME49990 has an ideal source impedance of 338 ohms. Since, as I recall, your transformer's secondary is 750 ohms it's about midway between those two ideals, and should give fairly quiet results; however the LT1128; ideal source impedance is 850 ohms which is closer than either of the others. Forget FET imput opamps like the OPA604 and OPA627; they're only quiet with high-turns-ratio transformers, which yours aren't.
Figure out the input Z of your recorder's line input; unless it's 600 ohms the opamp should be happy driving it without the booster transistors; without them you'll decrease both current draw and stability problems.
The sparks, however, don't sound like oscillation to me; they sound like two things that are too close to each other.
Peace,
Paul
About opamps: the LME49710 has an ideal source impedance of 1562 ohms; the LME49990 has an ideal source impedance of 338 ohms. Since, as I recall, your transformer's secondary is 750 ohms it's about midway between those two ideals, and should give fairly quiet results; however the LT1128; ideal source impedance is 850 ohms which is closer than either of the others. Forget FET imput opamps like the OPA604 and OPA627; they're only quiet with high-turns-ratio transformers, which yours aren't.
Figure out the input Z of your recorder's line input; unless it's 600 ohms the opamp should be happy driving it without the booster transistors; without them you'll decrease both current draw and stability problems.
The sparks, however, don't sound like oscillation to me; they sound like two things that are too close to each other.
Peace,
Paul