Tuning Microphones with an Oscilloscope - Page 1
Forum section: electronics / projects

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 13, 2019, 01:05:29 am
content: 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

author: mkschreder
author_rank: Contributor
timestamp: June 13, 2019, 02:35:00 am
content: 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

author: terminus
author_rank: Contributor
timestamp: June 13, 2019, 07:18:44 am
content: 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.

author: mkschreder
author_rank: Contributor
timestamp: June 13, 2019, 07:22:20 am
content: Quote from: terminus on June 13, 2019, 07:18:44 amGood 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

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 13, 2019, 11:45:15 am
content: Thank you both! I like both answers, the first one finally gives me an excuse to buy a nicer signal generator.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,JoshEDIT: I added a second screenshot with the amplitude adjusted to give me exactly 1.6V pk-pk. (Cathode currently reading at 1.65V)

author: terminus
author_rank: Contributor
timestamp: June 13, 2019, 04:31:26 pm
content: The recommended cathode bias on the schematics is 1.6v, so your signal generator should be below it. No need for LCD. Just measure the output of it with with the scope. Most interesting is what you get at the output. DC (43V) and AC with no clipping.

author: Audioguru
author_rank: Super Contributor
timestamp: June 13, 2019, 05:06:30 pm
content: Why are you using an old vacuum tube that will have a short life?

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 14, 2019, 12:38:28 am
content: Quote from: Audioguru on June 13, 2019, 05:06:30 pmIn my build is a military spec 5840W. I tested it on my tube tracer, it's perfect and will last many years...and if it does eventually die, I have 4 more.But to answer your question, because they sound great.Audio = Tubes > Transistors

author: floobydust
author_rank: Super Contributor
timestamp: June 14, 2019, 12:48:06 am
content: Neumann condenser microphones are world renowned for their sound, behind the best vocals. They use a custom triode for a few reasons. It lasts decades as mic's aren't always powered on.U47was eventually converted to JFET in the 1970's but it didn't sound good beyond kick drum.

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 14, 2019, 01:08:32 am
content: Quote from: floobydust on June 14, 2019, 12:48:06 amFunny that you mention that. I'm building a U47 FET next.I listened to vocal tracks in a double blind taste test, and I consistently liked the jfet version better in that case. But don't tell the other guy.

author: floobydust
author_rank: Super Contributor
timestamp: June 14, 2019, 02:27:51 am
content: R7 sets the Q-point for the tube, you would choose the sweet spot or best linearity based on the curves for the tube you are using. A scope will not tell you much aside from clipping or cramped swing.When the same mic capsule from going from triode to JFET gets abandoned, you have to wonder why. I haven't compared transconductance between the two parts, but know an output transformer is part of the recipe.

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 14, 2019, 10:54:50 am
content: Quote from: floobydust on June 14, 2019, 02:27:51 amThe tube they used to use on that build became exceptionally rare, and too expensive. If you try to get a good one now, you can easily spend $1500 on an AC701K. That's why a lot of us use the 5840W converted to triode. The capsules used in that mic, were/are used in at least a few different models.

author: KK6IL
author_rank: Regular Contributor
timestamp: June 17, 2019, 05:51:22 am
content: Looks to me as if R3 is providing negative feedback, which probably sets the gain regardless of the plate current.Are you driving the signal generator directly into the preamp? driving a speaker, and watching the microphone output might be more meaningful.John

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 17, 2019, 11:36:37 am
content: Quote from: KK6IL on June 17, 2019, 05:51:22 amI've actually removed R3, and dropped C4 to 2pF. R3 contributes oscillations, and either needs to be raised or removed to get rid of them.The signal is inserted where C4, R5, and the Capsule connect to the tube grid in the mic.Josh