Re: Rode NT1-A requires 3 minutes warm-up time
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Hi Heinz,
Troubleshooting electronics is a topic of interest to any mic builder.
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How to interpret your meter test result using Ohm's law:
Battery measured directly = 6.47V
Measured with .542M in series = 4.17V
(from here, I¡¯m going to round everything off to make it easy to calculate in my head}
Meter dropped by around 1/3
so 1/3 of the voltage is dropping across 1/2M
2/3 is dropping across the meter, ergo the meter is twice the resistance of the 1/2M resistor
meter resistance is 1M
I know that from having used and measured a lot of meters in the past. Common multimeters are either 1M or more expensive 10M ones. Even really cheap $10 ones are very accurate if they haven¡¯t been abused.
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Now as to your meter vs your NT1A, a 1M meter is going to present a heavy load to the capsule bias generator, so it¡¯s not going to read that voltage accurately. The filter circuit contains a 4.7M resistor which will drop 5/6 of the voltage, leaving 1/6 across the meter, but that¡¯s not the worst of it. The bias generator is designed for no load current at all, and ANY current draw will cause it to deliver less than the designed voltage. So just take it that your meter can¡¯t measure the voltage across the capsule with any accuracy.
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You¡¯ve determined that the capsule bias doesn¡¯t change when the mic starts working, so let¡¯s assume the bias circuit isn¡¯t at fault.
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Unfortunately also, since the NT1A circuit is a DC-coupled non-inverting op-amp, if any voltage is out of spec anywhere, none of the voltages will be normal anywhere.
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My instinct says the capsule is the most probable cause because:
1) it¡¯s edge terminated, and edge terminated capsules tend to leak more than center terminated.
2) Matt (mic-parts & Roswell) tells me edge terminated capsules tend to give trouble at more than 80V. They don¡¯t collapse until 120V or more, but they may become leaky over time, and the NT1A circuit is very sensitive to leaks, since the capsule is directly connected to the gate of the FET. Matt has seen a LOT more capsules than I have. And R?de runs their capsule at 100V.
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So let¡¯s measure a voltage someplace like the junction of D5 and D6 or anywhere else along that circuit line, the output of the op-amp. It should rest at 7.34V. Call it OK between 7V and 8V. My guess is, it will be way higher when the mic isn¡¯t working.
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Now unsolder the wire from the circuit at the capsule, so if the capsule is leaking it won¡¯t upset the electronics.
Did the op-amp voltage return to 7 - 7.5V? If so, the capsule is at fault. If not, the fault is on the circuit board.
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An alternate troubleshooting method is to swap the capsule with a known working one from another mic. That has the disadvantage of possible damage to TWO mics, and you may not have another mic to risk. OTOH, it tests both the NT! capsule in a good mic, and puts a good capsule in the R?de.
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