Using a Dynamic Mic with Electret system. Preamp Circuit questions - Page 1
Forum section: electronics / rf-microwave

author: TMCK001
author_rank: Newbie
timestamp: May 22, 2025, 08:00:48 am
content: Hey all,I’m building a mic preamp to interface a dynamic microphone with handheld radios and smartphone TRRS inputs. I’ve designed the following circuit to buffer the mic and improve audio quality while protecting from RF interference.Use Case:​- Works with dynamic mics- Designed for radio mic inputs that supply 2-5V bias through ~2k-4.7kΩ (like Yaesu, Icom HTs)- Also hoping it’ll work with smartphone TRRS jacks, which typically provide a similar bias voltageKey justifcation:2N5088 NPN transistor for buffering/amplification​- Ferrite beads (FB1, FB2) for RF filtering- Bypass and AC coupling capacitors (C1–C5)- No external power supply — powered only by bias voltage from device input- I am using a M-87 5Ω dynamic mic, outputs around 0.05–0.11 mV @ 103 dB SPLQuestions:​Will this work reliably with radio and phone inputs that provide ~5V through 2.2kΩ?Is the transistor biased correctly with this setup, or does it need tweaking?Is the signal level likely to be too low or too high for typical radio/smartphone mic inputs?Any improvements for RF robustness or audio performance?Can this safely be plugged into both radios and phones without harming either device?Planning to build it on a prototyping board. I’d appreciate any feedback, especially from people who’ve built something similar!Thanks in advance!

author: RoGeorge
author_rank: Super Contributor
timestamp: May 22, 2025, 09:04:53 pm
content: I remember from when the intercoms were a thing (where the same speaker was acting as a microphone while talking), the first stage was usually a common base transistor, because common base has the smallest input impedance (for maximum power transfer from the dynamic microphone to the transistor).Don't know what's the expected level of those transceivers, but I guess a single common emitter transistor won't be enough.

author: TMCK001
author_rank: Newbie
timestamp: May 22, 2025, 10:37:53 pm
content: The expect signal voltage is 200mV RMS. I’ll have a look at testing both types of configurations and see what works best.Thank you!

author: LM21
author_rank: Regular Contributor
timestamp: May 23, 2025, 09:41:15 am
content: Build one and check it. For that, use only audio stuff, so there is only a few of parts left. That will tell the audio levels. This is so simple circuit. I have not checked this kind of biasing myself, but I think current gain of your transistor matters. And every transistor has different current gain.I don't think it breaks anything.Common base base amplifier is an interesting way to handle the low impedance mic. But it needs a large input capacitor.

author: TMCK001
author_rank: Newbie
timestamp: May 23, 2025, 11:39:13 am
content: I look at putting it together this week after work. common base made the most sense in my head since the Vbias between devices can be slightly difference so emitter base seemed to be the most robust. I will also try common base and checking my results with my 90's Oscilloscope

author: A.Z.
author_rank: Super Contributor
timestamp: May 24, 2025, 05:37:58 pm
content: Check the datasheets and app notes for the SSM2166 and SSM2167 ICs they may fit; also see the MAX9814