Electronics > Projects, Designs, and Technical Stuff
Tuning Microphones with an Oscilloscope
KungFuJosh:
Hi,
I build different microphone projects, and using a scope on them is more straight forward on some circuits than others.
My latest build is an M49 tube mic (schematic attached). The build went great, and there aren't any issues with the circuit, but I'd like to be able to get useful information out of my oscilloscope. I thought I would be able to see some differences while adjusting the value of R7 (my build has a 5k pot instead of the 2.2k resistor in the schematic). However, inserting my scope around C5, or the input or output of the transformer all gave a consistent sine wave without any change while adjusting R7.
My guess is I'm measuring the wrong thing, or wrong area, or just wasting my time. :-//
Anybody experienced with using a scope on a microphone have any tips? Even general info when scoping mic circuits would be helpful.
Thanks,
Josh
mkschreder:
You need to do a frequency sweep. Basically get a signal generator, set it to sweep 1-40000hz range sine, connect your scope in fft mode and behold the graph that will change as you adjust things.
Sent from my SM-G960F using Tapatalk
terminus:
Hi,
By adjusting R7 you set the plate current. Also check the cathode and plate voltage. The cathode voltage has to be higher than the signal from the mic peak-to-peak. The plate voltage has to be around indicated value to allow full voltage swing without clipping on the output.
The voltages can indicate that the tube DC operation is in healthy state. High deviations from the shown values can suggest that the tube is faulty.
So you can use your scope to check the voltages and make sure the signal doesn't clip.
mkschreder:
--- Quote from: terminus on June 13, 2019, 07:18:44 am ---Hi,
By adjusting R7 you set the plate current. Also check the cathode and plate voltage. The cathode voltage has to be higher than the signal from the mic peak-to-peak. The plate voltage has to be around indicated value to allow full voltage swing without clipping on the output.
The voltages can indicate that the tube DC operation is in healthy state. High deviations from the shown values can suggest that the tube is faulty.
So you can use your scope to check the voltages and make sure the signal doesn't clip.
--- End quote ---
Good input. I interpreted it as the op wanted to check dynamic range of the mic but your reply seems to be closer to what op was probably trying to measure.
Sent from my SM-G960F using Tapatalk
KungFuJosh:
Thank you both! I like both answers, the first one finally gives me an excuse to buy a nicer signal generator. :-DD
In the attached screenshot my cathode voltage is on the target voltage of 1.6V. You're saying I should be around 2V to cover the full pk-pk swing? I think there's an issue with that, because I guess I'm not sure what my signal generators amplitude should be set at. If I lower the amplitude, I can make 1.6V work as the pk-pk value. I'm using a very basic Tenma signal generator. It doesn't have an LCD, so I don't actually have any values beyond the knob to select the freq.
Thanks,
Josh
EDIT: I added a second screenshot with the amplitude adjusted to give me exactly 1.6V pk-pk. (Cathode currently reading at 1.65V)
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