48V phantom in the G9

[Emperor-TK]

How important is it to achieve a full 48V for phantom power? I've built two G9's so far. My first can get 48V on the XLR pins with trimming, but the second can only get up to 44V. Does this seem like it presents any problems? I've heard of some cheaper pres only putting out 24V at phantom, but I assume that is less than ideal.

Secondly, could this be a sign of a more serious problem in the phantom supply that might need fixing? When I first powered the unit up, I had the second step up transformer miswired. When I fixed this, I was popping fuses on power-up. I pulled all the power connections to he board and then hooked up each section one-by-one and now no more fuses are popping.

Any comments or suggestions?

Thanks,
Chris

 

[PRR]

> How important is it to achieve a full 48V for phantom power?

Not very. There is a +/-4V allowance in spec, and most (not all) mikes will take much wider variations without change in performance.

> My first can get 48V on the XLR pins with trimming, but the second can only get up to 44V. Does this seem like it presents any problems?

If two identical circuits are not working the same, one is sick. (And not necessarily the 44V one.)

What is the voltage going INTO the regulators?

 

[Emperor-TK]

PRR, I think I found the culprit. If I understand zenner diodes correctly, they act as simple voltage regulators, so they should read the rated voltage drop across them. By my measurements, it seems like I might have used a 39V zenner where I should have used a 56V one. Does this sound right?

In the schematic below, the red vaules are voltages for the new G9, the one with the low phantom output (actually 41.5V, not 44 like I thought). The blue values are for my older G9, which puts out the full 48V. It seems like the DC voltages going into the regulator section are pretty close, but the output voltages are very different. The first big discrepancy seems to be at the zenner's cathode. What I dont understand is why the voltages are about 4V above the zenner rating here (assuming I did indeed use a 39V zenner by mistake). Is it normal for the voltage to be higher than the zenner's rating here? Are these within tolerance specs?

Thanks,
Chris

 

[gyraf]

It's either the zener, or the resistor that sources the current through it. Could be a bad pass transistor, but unlikely. Try a known-right <51V Zener.

Jakob E.