Introduction I started off this project when I realised that the official Discovery BNC adapter board has too high an input capacitance to be used with generic scope probes, which usually have a compensation range of 5-35pF ish. Using scope probes is really necessary in some cases where low loading is desireable, since the quoted 1MΩ || 24pF input impedance of the Analog Discovery 3 loads down sensitive analog circuits quite significantly....
Before reading on, please refer to the previous part/MK1 of this project, otherwise some of the stuff here might seem a little odd. Schematic <?xml version="1.0" encoding="UTF-8" standalone="no"?> SVG Image created as AD3 Front End Amplifier-CH1.svg date 2023/12/18 09:48:42 Image generated by Eeschema-SVG C207 C207 1u 1u -2V5 -2V5 1 1 2 2 SW201 SW201 AC/DC AC/DC C203 C203 1u 1u V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 U202 U202 C201 C201 10n 10n -2V5 -2V5 +2V5 +2V5 C208 C208 1u 1u R207 R207 402 402 R201 R201 1k 1k R208 R208 22 22 V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 U201 U201 D201 D201 R202 R202 1M 1M C205 C205 1u 1u C204 C204 1u 1u R203 R203 100 100 R204 R204 402 402 R206 R206 402 402 +2V5 +2V5 C206 C206 1u 1u R205 R205 22 22 C202 C202 10n 10n 1 1 2 2 J201 J201 CH1 CH1 COM COM 1 1 NO NO 2 2 V- V- 3 3 IN IN 4 4 V+ V+ 5 5 IC201 IC201 TS12A4516DBVR TS12A4516DBVR OUT+ OUT+ OUT- OUT- V+ V+ V+ V+ V- V- U202 U202 V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 -2V5 -2V5 SW201 SW201 AC/DC AC/DC 1 1 2 2 -2V5 -2V5 U201 U201 V- V- 2 2 V+ V+ 5 5 1 1 + + 3 3 - - 4 4 C206 C206 1u 1u IC201 IC201 TS12A4516DBVR TS12A4516DBVR COM COM 1 1 NO NO 2 2 V- V- 3 3 IN IN 4 4 V+ V+ 5 5 D201 D201 Check out the design files here Compared to the MK1 design, the gain stage is essentially identical....
I’ll say it up front, in case you missed it like I did, the power supplies on the Analog Discovery 3 cannot do current limitng, which is a huge bummer. No constant current limiting for you sir! Also, just go ahead and set the power limit to 5.5W please…
Failed Calibration Coefficients For some reason, going through the calibration wizard in Waveforms (as of June 2023) introduced offsets and gain errors which were different for both scope channels. This doesn’t make sense, as both scope channels are calibrated with the waveform generator channel 1, after that has itself been calibrated, of course. My failed calibration attempts were done with a calibrated Keithley DMM7510, so basically with the best multimeter in the world save for the insane 8....
When I got my Analog Discovery 3, I made the decision to also purchase the BNC adapter board. Trying it out with some OWON scope probes I got from Amazon, it unfortunately didn’t cut the mustard. This was the most square I could get by twiddling with the compensation adjustment. To prove that I haven’t goofed it somehow, this is at the opposite end of the compensation range: This waveform ain’t even square at this point....
The Analog Discovery 3 is a great debugging tool, but as I discovered here in a note on using it with 10x scope probes, the input capacitance was too high for the probes to be compensated correctly. Custom LNA design To combat this, I decided to design an amplifier that would have a low input capacitance so that we fall within the 5-35pF compensation which is typical of these 10x scope probes....
Why I bought it in the first place The Analog Discovery 3 seemed like a band-aid solution to an ongoing problem I have been facing here at Cambridge: how do I get my own oscilloscope so that I can get back in to doing electronics without having to rely on the few shared scopes at the Dyson Center (the Dyson Center is sort of like a makerspace for engineering students, it isn’t great though…)...
This is an attempt to try to get more signal out of dynamic microphones, as I literally have to max out the gain on my Presonus AudioBox iTwo to barely get a usable signal level, and that is with me speaking right on top of the microphone. This design is based on Project 66 by Eliott Sound Products , with some minor modifications, mostly me just playing around to see what are the effects of changing stuff around....
So here’s the situation, I accidentally ordered the parts for the Inline Microphone Preamp Project … to Malaysia. One month and a trip to Portugal later, I finally got the chance to use the electronics lab here in Cambridge for the first time to asesemble this thing (thanks Elena for bringing the stuff over for me!), and in good old DIY analog electronics fashion, it didn’t work first time. Turns out, the SOT-89 footprint in KiCad has it’s pins in the opposite order from those in the NXP datasheet for the PBSS4540X and PBSS5540X transistors I was using....
I recently picked up a Behringer BA85A (yes, yet another Behringer clone…) dynamic microphone, and have thoroughly enjoyed the sound that comes out of it. However, it does require me to crank the gain on my PreSonus AudioBox iTwo audio interface all the way to the maximum, and even then, I either have to speak quite loudly or be right on top of the microphone to get a signal that I don’t have to apply any post amplification, which is kinda annoying because I use it quite a lot for video calls, and sometimes adjusting the levels for ASIO devices can be tricky....