eBay microphone preamp mistake or genius? - Page 1
Forum section: electronics / projects

author: A.Huggy
author_rank: Contributor
timestamp: October 11, 2025, 05:36:02 pm
content: I recently purchased an in-line microphone preamp from eBay. This preamp is meant to allow the use of a dynamic microphone with systems designed for electret microphones. It doesn't require any external power, it's powered by the bias voltage that is normally applied to an electret mic. I think these are used in motorsports and aviation applications like racecar radios or helicopter intercom systems. I'm using it for a similar application, but I plan to integrate it with another board that mixes the mic audio with other sources to be fed into an audio recorder. Anyway, my use is not the subject of this post. I want to share the circuit because it is puzzling to me:We have a BC337 NPN, a bias resistor and a DC blocking cap for the mic input. This is a pretty textbook barebones amplifier that I'm sure most of us have seen many times, but it looks like the NPN is populated "backwards" with the collector grounded in this case. I looked up several versions of the BC337 datasheet and probed the NPN with my DMM to make sure I didn't have a version of this part with E and C swapped. This really is how it is connected. It somehow works though. I connected this circuit to a dynamic mic and the mic-in jack of my PC. I can record audio from the mic.I also simulated this circuit side by side with another iteration with the transistor oriented as I would have expected: with the emitter grounded.Both configurations seem to work, but the emitter ground version seems to have a little more gain. This screenshot shows both circuits and their output while being fed a 20khz 8mV sine wave (signal from dynamic mic). The waves look about the same from 800hz to 20khz. The emitter ground version of the circuit always has a little more gain and less DC offset.Has anyone seen an amplifier configured this way before? I am struggling to see the benefit of the collector ground configuration. I am also struggling to find a datasheet for the BC337 that describes its behavior when Vce is negative. I feel like I'm missing something obvious and I'm ready to be humbled.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2025, 09:30:38 pm
content: The emitter grounded version is the correct one.BJTs will work in both directions, but the hFEwill be lower, when the transistor is reversed. This will reduce the base impedance, causing more loading on the dynamic microphone, which effectively reduces the voltage gain.

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2025, 10:19:00 pm
content: OP your schematic is wrong, please check again. It won't work even if the transistor is correct NPN? because there is no base bias resistor. It will sit there, off.I had a Aliexpress BM800 mic that was really noisy and a bit distorted. Digging in I found they'd put in the wrong transistor which made a mess. I drew that schematic posted somewhere here.edit: I'll just repost it here.

author: A.Huggy
author_rank: Contributor
timestamp: October 12, 2025, 12:59:44 am
content: I just checked again and redrew the circuit. I think this schematic is electrically equivalent but just easier to look at.Is R2 the bias resistor you're looking for?I also found this circuit fromthis pagethat is intended to do exactly what the eBay amp claims to do.It's equivalent to the circuit I have but with the emitter grounded as expected.Thank you for the replies. After reading your replies it seems obvious the transistor on the ebay amp is flipped. It worked so well on the bench that I was really wondering if there was a common collector trick I was now aware of (there probably is but this isn't it lol).

author: floobydust
author_rank: Super Contributor
timestamp: October 12, 2025, 01:27:57 am
content: As you mentionedBC337pinout is CBE, not EBC. That would explain things- it could have the wrong transistor in there. PCB designed for an EBC part? Or a JITA ECB part.The base bias resistor at 2.2k seems way low.Dynamic mics need some load resistance or they are honky and peaky sounding. It might be for that purpose.When the board is connected, measuring the transistor voltages or hearing distortion will tell you if it's working ok.

author: Martian Tech
author_rank: Regular Contributor
timestamp: October 12, 2025, 01:46:59 am
content: Is it possible that the bias voltage is negative?