This pre-amp on my car's mic - Page 1
Forum section: electronics / beginners

author: Charlie Whiskey
author_rank: Newbie
timestamp: May 20, 2024, 04:15:10 pm
content: Hi all, I'm trying to wire the stock mic on the overhead console (which sounds really good) into my new aftermarket head unit.I found out the reason why it sounds good is because there is a preamp attached to the mic (pictured). I want to keep this set up for my new head unit but I need to confirmed a few things...1) The op-amp there is a JRC 2732 and the datasheet is this one I believe:https://pdf1.alldatasheet.com/datasheet-pdf/view/85612/NJRC/NJM2732.html--> It says max 6V input - I got little idea how this circuit board works, is my suspicion correct that my old head unit was supplying this amp with <6V and not the car's usual 12V?2) My new unit's included external mic (known to be not-so-good) doesn't have such a preamp so I presume if I were to keep this preamp I've to attach an attenuator just before the head unit. How do I work out how much dB loss I would need?

author: Buriedcode
author_rank: Super Contributor
timestamp: May 20, 2024, 07:20:28 pm
content: Looks like the SOT23-5 (the 5 pin small IC bottom right) is a regulator as its connected to red wire via a ferrite bead - I'm assuming that red wire is the 12V from the cars system.In order to know the gain of the preamp, you would have to measure the value of the resistors conencted to it.If you can post a pic of the other side of the board, I, or someone else, could have a go - or at least guide you - at making a rough schematic so you know what goes where, and can work out the gain, and any possible way of adjusting it - or at least knowing it so you can work out values for attenuation.

author: themadhippy
author_rank: Super Contributor
timestamp: May 20, 2024, 08:04:39 pm
content: Quotewire a pot between the pre and head amp and twiddle it till it sounds right, replace pot with 2 resistors,or just leave it.

author: Charlie Whiskey
author_rank: Newbie
timestamp: May 21, 2024, 05:31:20 pm
content: Quote from: Buriedcode on May 20, 2024, 07:20:28 pmThanks mate this is the reverse side of the board - just two wires going to the mic capsule.I'm thinking about doing something like this:on the head unit side:https://www.instructables.com/Homemade-RCA-Attenuator/but I need to figure out the resistor values.Alternative, would any of the following work?https://www.amazon.com.au/Extension-Headphone-Speaker-Computers-Headphones/dp/B0BTNMV5G1/?_encoding=UTF8&pd_rd_w=GxK39&content-id=amzn1.sym.663dc634-0c9f-4c89-9887-a8c4cbe77563%3Aamzn1.symc.d4144268-5a64-40b3-8287-98ceee0c3582&pf_rd_p=663dc634-0c9f-4c89-9887-a8c4cbe77563&pf_rd_r=M1TGK7ZHDRD4E08ZXS0F&pd_rd_wg=eHcEp&pd_rd_r=6e0b0c66-f991-4cc2-8181-b62afff2ed5c&ref_=pd_hp_d_atf_ci_mcx_mr_hp_atf_mhttps://www.amazon.com.au/Gyorwst-Passive-Attenuator-Microphone-Converter/dp/B0D3342R7P/ref=sr_1_98_sspa?crid=3JLG8SM7OJTH1&dib=eyJ2IjoiMSJ9.CvvyCyh9ReJD6RU-RbyJJ1dZfXt7U98TvAXfwzLCnAxhd4TVuI2_zrbjuhoWf_Lr6B2Kl6zZ0pBJQEb62MTArlYEg3ZsmJ9ctgyn3fvbkxT3ZfPagzhi9xZzkuX8eir237cy4UdFt_QrzesYR2d8nmbL0Ds4sO0HS5KWFytetWzv0EUdjV7svTuDu2UI5qyZsik3BcbkxFxKFSnQ1RH1gE9oKiLIpbywERGuO_xBiji32jI9MXlmn8zuIee7z63SfVqFpx24nOLUgFPOcUx1CJhyFo28EFwbshgsu_iRXV0.1WFqpCXUgcbU7t3a13Bb4mi6s9YOY548ZfgTljHBIwA&dib_tag=se&keywords=audio+attenuator&qid=1716313201&sprefix=audio+attenu%2Caps%2C320&sr=8-98-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGZfbmV4dA&psc=1

author: Buriedcode
author_rank: Super Contributor
timestamp: May 25, 2024, 03:57:54 pm
content: Righty, sorry for the late reply.I had a quick look and whilst I can't be certain - I can't see under the IC, it looks to be a two stage amplifier. With "R1", "R2" and "R3" being the input resistor to an inverting opamp.Changing either the input resistor, or the feedback resistor in an inverting opamp circuit will change the gain, measuring the ratio of feedback to input resistances will give you the actual voltage gain of thatopamp. The fact that the only real silkscreen on that are the labels R1-3, tells me thats the gain setting resistor - with non populated footprints for placing different value resistors.On the right of the dual opamp is the resgulator, and a few passives which, to me, look like a bias network - creates a mid-rail voltage (could be VCC/2, but not always) - that allows the opamp to work with a single supply. Above that is the mic bias supply, that provides a DC bias to the electret mic element, looks like that has a 6.8V Zener to cap it at that voltage, and to protect it from high voltage transients.It's hard to see if that second opamp is inverting or not, since I can't see if the input to that opamp is conencted to the inverting, or non inverting input.I've labelled up some pointers on your image. Take tihs with a pinch of salt - I didn't spend too long looking at it so I could be wrong, but I woudl start by measuring those feedback and input resistors on the image. The unpopulated footprints could have wires soldered to them, with a resisotr wolders to it, which puts a resistor in parallel with the input resistors, reducing its value - and increasing the gain.If you want to reduce the gain, then themadhippy's suggestion of a potentiometer on the output - just like a volume control - woudl be easiest

author: Charlie Whiskey
author_rank: Newbie
timestamp: June 05, 2024, 05:14:18 pm
content: Thanks for the analysis!I ended up going the simpler route by bypassing that preamp and wiring the the mic capsule straight to the head unit via a shielded mic cable. The resulting volume is slightly softer than I like but useable for phone calls.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 05, 2024, 07:48:20 pm
content: I have used a very simple single transistor "preamp" for a desk mic that worked a treat. Of course, the downside is you need some kind of power for it, and as its a preamp is generally best placed close to the mic - with any long cable running being after the preamp rather than before it. But if you end up wanting some gain/sensitivity, then a few resistors, two caps and an NPN will do, rather than a full blown opamp circuit.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2024, 08:04:45 am
content: Quote from: Buriedcode on June 05, 2024, 07:48:20 pmI've done a similar thing before, but I included AGC and designed it so the pre-amplifier could be at one end of a long cable and the signal and power shared the same wire.