Do you see any problems with my microphone preamp schematic? - Page 1
Forum section: electronics / beginners

author: availeon
author_rank: Newbie
timestamp: July 17, 2022, 04:55:46 pm
content: I designed a very simple amplifier based around a TI INA849. I think the amplification part is alright but I'm not sure if I tapped off the phantom power correctly. The data sheet for this zener diode (1SMA4740AG) is pretty sparse but I found a post here (https://electronics.stackexchange.com/questions/80145/how-to-calculate-minimum-zener-current-if-it-is-not-explicitly-specified-in-the) saying that I should target half the maximum zener current of (1 watt / 10 V = 10 mA) or 5 mA. Is that correct? Any other oversights in my design?

author: Martian Tech
author_rank: Regular Contributor
timestamp: July 18, 2022, 02:38:08 am
content: R9 and R10 already exist at the supply end, and vary depending on the actual voltage being supplied (not all phantom power is +48V - sometimes you only get +12V).With R9 and R10 as specified, you can only draw about 6mA because they're in series with 6.8K resistors (assuming +48V). And that is total current, for the zener, the virtual ground splitter and the instrumentation amp.

author: Gyro
author_rank: Super Contributor
timestamp: July 18, 2022, 10:10:18 am
content: I don't understand why the Schottky diode clamps on the input need to be 10A rated. Even if mains was to be accidentally applied differentially between pins 2 and 3 of the XLR jack, the reactance of the two 16uF capacitors (voltage rating?) in series (~400R) would limit the current to less than 1A. If mains was applied between either pin and ground / pin 1, the current would be a little over 1A, but the current would appear on 10V rail, which is only clamped by a 1W zener (D5). This would dissipate over 10W under these conditions.I'm not sure what the actual fault conditions the clamping diodes are required to protect against, and your calculations, but if they need to be 10A rated, the input 16uF capacitors would need to be rated for high voltage and D5 would need to be massive. Larger Schottky diodes have greater leakage current, which sounds undesirable.Maybe I'm missing something.P.S. I don't see any equivalent protection on the output XLR, which is presumably subject to the same risk of misuse.

author: availeon
author_rank: Newbie
timestamp: July 18, 2022, 03:16:56 pm
content: Thanks for the tips!Regarding the diode clamps, I originally planned to have 48 V phantom power on the input side as well so the high current rating was in case the two blocking capacitors charged up to 48V were inadvertently shorted. I don't need that now so I'll replace them with some smaller ESD protection clamps on the input and output.

author: Gyro
author_rank: Super Contributor
timestamp: July 18, 2022, 04:44:10 pm
content: Ah, ok. That makes sense. The best weapon for ESD is some series resistance to limit the peak current into protection diodes.Looking at the INA849 datasheet, I notice that the absolute maximum input voltages are supply rails +/- 0.5V (a typical value for on-chip protection diodes) with an absolute maximum current of 10mA. the 0.5V isn't dissimilar from the forward voltage drop of the external diodes. Including some series resistance between the two will ensure that the IC input protection diodes don't exceed maximum limits. Aiming for 5mA max is a good target - it doesn't take much series resistance for you to be able to use standard (non leaky) Silicon diodes rather than Schottkys. A small amout of resistance before the external diodes will again allow them to withstand high peak corrents. From a quick look at the IC datasheet, I can't see any reason why some distributed series resistance on the inputs would affect performance.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 18, 2022, 06:03:42 pm
content: The output stage is meh. I would personally either provide a single-ended output or add a proper differential output instead of this, but that's just me.