Seeking Advice: Best Preamp Design for Low-Noise, Balanced Microphones

[ccaudle]

Perhaps a job for a little ESP32 Arduino project.

Yes, perhaps OP can explain the archaic seeming requirement of setting gain using a DC voltage.

 

[DaveDC]

For a single preamp, an ESP32 controlling a PGA2500 or THAT 1580/5171 is a very good solution. The ESP32 would create a WiFi access point serving a web page that has controls for the preamp gain, pad, and phantom. Lots of example code on GitHub in both Arduino and MicroPython. No need to develop any code for the phone/tablet. Any browser would work. And the ESP32 is likely cheaper than the wire and connectors for the original control cable! The ESP32 could also handle a long-haul RS485/RS422 serial link if a wired solution is mandatory, with another ESP32 at the controller end.

If you wanted to have multiple preamps, a good way to control them all would be to use a Raspberry Pi running MQTT. This is a common multi-device protocol for home and factory automation. Each preamp would communicate to the MQTT server and hence would be individually controllable. A front panel dashboard for the whole network could be written in NodeRED, then accessed via any web browser. The Tasmota RTOS for ESP32 would be a very convenient and easy to use platform for this. It has a built-in MQTT support and scripting language called BERRY with which a custom driver for the PGA2500/THAT5171 could be written. Attached is a BERRY driver for an Austria Microsystems AS5043 Hall Effect angle sensor. It uses SPI, so it could be pretty easily adapted into a PGA2500 or THAT5171 driver.

 

[ccaudle]

controlling a PGA2500

PGA2500 runs from +/- 5V power supplies, it is made to connect directly to an ADC. If you want traditional pro-audio levels out of it (20 dBu or higher) you would have to add substantial amplification after the PGA2500. The That devices have a big advantage in that regard for analog output designs.

 

[bigben83]

I did not spend two minutes to find 10k linear motor pots on the Bay.
However, these days a radio/BT/Wifi may be preferable, with a phone app for those two channels. Perhaps a job for a little ESP32 Arduino project.

Yes an ESP32 would be perfect for the controls. even with RS485, I have been looking into this as an option.

 

[JF]

@bigben83 Does it NEED to be completely DIY? How many channels do you need/build in the long run?

 

[bigben83]

Attached is my first version of the schematic.

 

[exlabs]

Looks like a very well drawn schematic at first glance, good job. Are you paying for Altium?!

I was working in a mic preamp a few months ago too.

Can you explain the input? Is that a common mode choke for RF filtering.

 

[Voyager10]

The NJM2172 data sheet (https://www.nisshinbo-microdevices.co.jp/en/pdf/datasheet/NJM2172_E.pdf) seems to show that the 'EVR' block only offers attenuation, and can't add any gain to what the SSM2019 provides. Also, its maximum output level is considerably lower than the SSM2019's.

In other words, all this is doing is reducing the overall dynamic range of the circuit. If you really need adjustable gain, how about a relay to switch the SSM's gain resistor?

 

[pvision]

Has anyone considered the OP may be a bot driven by ChatGPT? Or maybe a bot teaching ChatGPT to learn circuit design?

Maybe we should ask ChatGPT what the OP really needs?

 

[bigben83]

Ha ha, no I'm not a bot but a acoustic engineer, with no background in electrical circuit design.

 

[MidnightArrakis]

Attached is my first version of the schematic.

[Attached is my first version of the schematic] -- GREAT JOB!!! I would say that your schematic receives the nomination of "being one of the best GroupDIY-forum submitted schematics" that I have seen on here.....(ahem).....other than my own!!! I can tell and also know that it took you a considerable amount of time in order to get all of this drawn up and neatly-placed as it is. I know that creating a clearly drawn and well-placed schematic like this is no small feat. I know this myself all too well.

https://groupdiy.com/threads/vintage-russian-4-bands-equalizer-full-project.90005/post-1192808

The only main item I see that I would do differently would be.....in placing CON1 and CON3 over in the upper-left hand corner to where -- "mic one pos/neg" and "mic two pos/neg" -- are located, in order to maintain your orderly LEFT-to-RIGHT reading flow.

In other words.....upon initially looking at your schematic for the very first time, someone may wonder "Is this the main input coming into the circuit and what type of connectors are being used"? Then, after spending some time reviewing the entire schematic, they then discover that the -- MAIN INPUT -- is over on the complete opposite side of the drawing and the connectors are XLR's, I am more than certain that the readers of your schematic will wonder, "Why in the heck isn't this part of the schematic located where the main circuit inputs are"? Just sayin'.....

Now.....I already know that there are members on this forum who will say, "Who the **** cares where the input connectors are placed on this schematic? Just as long as they're there and connected correctly.....who cares"??? However.....2 things.....1) As I just mentioned a moment ago, there is basically a "universal" understanding that schematics are "read" from LEFT-to-RIGHT and that flow needs to be maintained and, 2) As one of my engineering bosses once said to me....."If -- YOU -- don't pay attention to the details.....who will"?

The only other comment and/or piece of advice that I can pass onto you is.....when creating either schematics or routing PCBs.....you ALWAYS CONNECT TO THE CAPACITORS FIRST!!! As I had mentioned to some other member on this forum recently who was working on a project similar to yours here, this innocent and little factual idiom had been drilled into me by countless numbers of "RF" engineers I have worked with at the various "RF/microwave" companies that are sprinkled around the region here where I live (DC). These engineers showed me on their oscilloscopes and other pieces of test equipment just how much of a difference in the overall circuit performance there is between placing a capacitor in the circuit first and placing the same capacitor later on in the circuit.

While the difference may be rather minor or even nearly imperceptible in a single instance, the combined effect to the circuit when you include all of the instances together, does create a measurable circuit performance improvement overall. Now, while the "RF" environment I once worked within was involved with designing communications equipment for "covert intelligence-gathering, surveillance and/or tracking operations" projects used by all manner of "secret and unknown" U.S. Government agencies.....the fundamental basics of what I am detailing here to you is still applicable to "audio microphone input" circuits as well. Just continue to remember this little ditty as you place the components on your board and route your PCB. The electrons going through your circuit will "THANK YOU"!!!

GREAT JOB!!!

-- Someone who has created schematics and has designed PCBs for the CIA, DHS, FBI and "other publicly unknown about covert U.S. Government agencies". (YES, Virginia.....there are actual "spooky" U.S. Government agencies "that don't exist")!!!

https://groupdiy.com/threads/vintage-russian-4-bands-equalizer-full-project.90005/post-1192950

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