Audio preamplifier with asymetrical power supply, balanced and unbalanced input - Page 1
Forum section: electronics / projects

author: Nei
author_rank: Newbie
timestamp: July 24, 2025, 04:12:23 pm
content: Hello community !I'm working on a project of audio preamplifier, to be used on stage for RF in-ear monitoring.This is a part of a device connected to an AUX output of mixing table, that acquires balanced audio signal (from XLR) and amplifies it to a level that complies with ADC specifications. This ADC is (for now at least) the one on a Raspberry pico, so in range 0 to 3.3V.For simplicity purpose, I want the preamplifier to be powered by a single power supply, say a standard 5V USB.So here is the preliminary version of the schematics :Kicad simulations give good results. Before I design a PCB, I'd like to know if you think that :- things are missing (decoupling caps, ...)- things could be made in a better way (KISS style !)- components are well chosen- etcAs you cannot see on the schematics, I want to use NE5534 op-amp.So do you have any suggestion to improve my design ?I already built prototypes I want to improve (better input management).The next step is to use a real audio ADC (PCM1802), connected to raspberry pico with I2S interface (and I²C).For your curiosity, transmitter and receiver use NRF24L01+PA+LNA radio, audio format is 12bit@44100Hz, on 3 channels at 2Mbps, with samples interleaving (each packet carries 50% of samples from aqcuisition period n-2, and 50% of samples from aqcuisition period n-1).Audio amplification on receiver side is performed by MAX98357 audio amplifier, connected to raspberry pico with I2S interface.Thanks in advanceregardsNei

author: shabaz
author_rank: Frequent Contributor
timestamp: July 24, 2025, 04:36:36 pm
content: Hi,This doesn't seem very efficient use of ISM band, with three 2 Mbps channels consumed for the audio, however I don't know ISM band requirements at 2.4 GHz that you may need to double check are not being violated.Regarding some other points:Is there an I2S input interface on the Pi Pico? I think it would need to be done with PIO, and I don't believe there is any sample code for input (there is sample code for I2S output) but I could be wrong. I checked a few months ago, maybe I missed it. So, you may need to become comfortable with learning PIO if I2S input is indeed the plan. It might not be too difficult to implement, but it's something to be aware of that might not be available as off-the-shelf code to consume.Also, is this for basic speech, or high quality for music? And does latency matter? You'd use the answers to those to decide if you want to use compression, size of playout buffer, and whether you really need to sample at such a high rate or not.I'm also curious what's the benefit of this solution over existing wireless audio transmitters.

author: themadhippy
author_rank: Super Contributor
timestamp: July 24, 2025, 05:01:18 pm
content: Before you go too far ,as your converting to digital what sort of latency are you aiming for? even a small amount is enough to bugger up sensitive buskers.Also avoid multiple conversions,if the desks digital keep the signal digital until it comes out the receivers

author: Nei
author_rank: Newbie
timestamp: July 24, 2025, 05:47:40 pm
content: Hi,Some answers :- three 2Mbps channels are used, one at a time (I decided to use 3 of them, but it could be any number up to 64)- the NRF24L01 makes use of same channels as WiFi, plus a few extra channels out of ISM band (I cannot use these)- I plan to use PIO to read I2S. I already use it for I2S output, I will reuse and adjust my component.- This is for acceptable quality : I already tested it, but there is too much noise with my prototypes. I guess it's because of raspberry pico ADC quality (slightly less than 9 ENOB). Good quality would be 16bit@48kHz with decent analog circuit. It won't be this quality.- latency is about 4ms (theoretically) : good enoughNei

author: Simmed
author_rank: Frequent Contributor
timestamp: July 24, 2025, 06:29:42 pm
content: since you already have a protohow is the noise coming out of U3 ?

author: Nei
author_rank: Newbie
timestamp: July 24, 2025, 07:49:33 pm
content: Hi,The proto does not implement this schematics. It's made with :https://www.eevblog.com/forum/beginners/problems-with-a-simple-opamp-audio-preamplifier/msg5615173/#msg5615173Nei