Thoughts on this Dynamic/Condenser Microphone Differential Preamp Design - Page 1
Forum section: electronics / beginners

author: R-802
author_rank: Contributor
timestamp: July 09, 2025, 02:02:21 am
content: I am designing a single supply microphone preamp for an audio effects processor, I'm quite inexperienced with this so I appreciate any feedback.The output of the preamp is used to drive a CODEC (TAC5112) with 2Vrms full scale differential input, single ended input is possible but complicates firmware development, input impedance of the ADC is 5k.The opamps I plan to use for this design are all RRIO low noise OPA2365 (https://www.ti.com/lit/ds/symlink/opa2365.pdf) single supply 5V, biased to 2.5V. The singular opamp on the bias supply is left over from another circuit so it's not a problem.Transient analysis looks fine to me but THD of the output is horrid (9890352.109310%) gotta be a mistake somewhere.Ideally, this is something low cost but effective and clean. Can't really find much information on this stuff, datasheets/reference applications seems to cover alot of electret or MEMS mics, dedicated preamp IC's like INA217, THAT1580, THAT1510/1512 etc are great but far too expensive and require negative supplies greater than 10V to 5V, theres discrete transitor approaches which are interesting but haven't found any reference design's not using a negative supply. I know DC-DC converters like Murata NKA series exist (https://www.mouser.com/datasheet/2/281/1/kdc_nka-2940963.pdf) but they're just too expensive.EDIT: little mistake in schematic, XLR_2_IN- should be XLR_3_IN- for pin three of an XLR connector. V8 simulates common mode noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 09, 2025, 04:26:51 am
content: One can do the gain already with the differential stage. So no need for the U4 part. There is also no need for the extra AC coupling in the intermediate stage around U4 unless one really needs the high pass character as a defined high pass. As shown the DC path for U1 is missing anyway.Dynamik microphones want a rather low noise amplification. Usually condensor mic's include a pre-amplifier and are than less critical. Electrete types often want some supply, though I don't know about XLR types (may include a battery). One may still need a switchable / adjustable gain depending on the source type.The OPA2365 is not so low noise in the audio range with lots of 1/f noise and the x 1 differential amplifier configuration adds extra resistor noise.I would consider a fully differential amplifier, like maybe OPA1632 or OPA1637 (still open with the type).

author: R-802
author_rank: Contributor
timestamp: July 09, 2025, 05:07:18 am
content: I have considered differential op-amps but getting it setup in a single supply configuration is confusing and I haven't really seen any implementations of single 5V supply Dynamic/Condenser microphone preamplifiers using FDA like OPA1632 or OPA1637. I have seen filters using FDAs but not preamps, except for THAT1583 but at $10NZD an IC it's just not worth. It's suprising that there aren't many single supply differential op-amp preamp IC's considering the prevalence of usb audio interfaces, really wish I knew how they achieved this.Really only concerned with Dynamic and Condenser microphones for professional audio recording. Not electret types.EDIT:Quote from: Kleinstein on July 09, 2025, 04:26:51 amHow do you know this, surely the noise won't be that bad provided we use precision components and adequate filtering?

author: MariuszD
author_rank: Regular Contributor
timestamp: July 09, 2025, 05:09:44 am
content: The signal is 2kHz and the command ".four" expects 1kHz.

author: R-802
author_rank: Contributor
timestamp: July 09, 2025, 05:12:23 am
content: Ah thanks, THD=0.058439%. That makes more sense

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2025, 06:20:29 am
content: Missing resistor for U1 (+) input to +2.5V

author: Kleinstein
author_rank: Super Contributor
timestamp: July 10, 2025, 07:57:25 am
content: Quote from: R-802 on July 09, 2025, 05:07:18 amPrecision components don't help with the noise. They may help with the common mode rejection. Filtering does not work for the noise in the frequency range of interest. The OPA2365 datasheet gives low noise for 100 kHz, but that is outside the audio range. At 1 kHz the noise is no longer good (~ 12 nV/sqrt(Hz). This could still be OK for a condensor microphone, but would be limiting for a dynamik microphone with a usually lower signal level.The amplifier does not have to be single supply or Rail to Rail for the input. It can however help to have a rain to rain output. The OPA1637 has this, though the BW is a bit limited (could still be acceptable) for the use with a high gain (like 20 x as shown). There are other types, some also made for low voltage (like 5 V) operation. The question would be if one needs high input resistance and thus an extra 2 amplifier stage in front, similar to the 3 OP-amp INA circuit.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 10, 2025, 04:23:21 pm
content: Quote from: R-802 on July 09, 2025, 05:07:18 amCurious what you find confusing about it? There's a few different methods to get a +/- supply rail from only a positive voltage source. You could use a charge pump(here's a Falstad circuit)to generate a negative voltage (ex. theLMC7660has an input voltage range of 1.5v-10v) and then add a lot of filtering on the output (can still be a bit noisy), using an op-amp to generate a negative voltage rail (will create a much cleaner negative voltage rail but limited in current output), using some TL431's(here's another simple Falstad circuit), or virtual grounds, which EEVBlog Dave has talked about quite a few times.https://youtu.be/7FYHt5XviKc?feature=shared&t=1041You'll need to see whether any of these various negative voltage techniques will work with your circuit based on the noise/current/idle power consumption limitations of each design, but generating a clean negative voltage is relatively easy?

author: themadhippy
author_rank: Super Contributor
timestamp: July 10, 2025, 04:38:08 pm
content: QuoteIve yet to find a condenser mic that dont need some form of phantom power shoved up it,unless your using something with a built in battery like the sm94 or an external power supply your condensers are gonna be very quiet.