Audio path I2S CS4344 stereo -> mono with volume control on seperate pots  - Page 1
Forum section: electronics / projects

author: jldesigns.eu
author_rank: Contributor
timestamp: May 18, 2018, 08:17:10 am
content: Hello EEI've created a project using STM32F4 and a CS4344 chip for wav audio playback from SD card.The system plays the melody on the left channel and the random jingle (short sounds) on the right channel when pressing the button. I need to have the volume control for jingle and musics on two different pots, then it is combined into a signal that goes to the amplifier module.questions:1. What values ​​should have resistors R11 and R12 (if both are necessary, I've read that i should short them out) and pots R6 and R9. I would like the signal to be "the most powerful" because in the same system I have a GSM modem and I would not like to hear these characteristic distortion sounds.2. Currently changing volume in the whole range for one of the pots causes slight changes in the volume on the other, it is somehow straightforward to eliminate?3. I tried logarithmic pots in this circuit but it was quite strange when it comes to volume level. Now I think that they probably have "sides" and maybe I mounted them "the other way round", how to get to know it? Unless my resistor used to covert stero -> mono on resistors affects them. Now I have a 10k linear pots, but the low volume control is really short.I have little to no experience with analog circuits and audio circuits so please let me know how i should improve volume control and summing the signal.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 18, 2018, 02:49:37 pm
content: You should add low-pass RC filters at DAC outputs after the high-pass filters as recommended in the datasheet, otherwise you'll get high-freq components. Now if this goes to an amplifier that already does low-pass filtering, you could do without them, but depending on the audio amp and speakers, that may not sound that good.R11 and R12 are there in case the jumper is closed (for mono output as I gather). This forms some kind of mixer. Don't short them or you'll short the DAC outputs in "mono" mode. The DAC outputs have 100 ohm output impedance but the datasheet recommends a load of 3 kohm min so if shorted, you would get bad distortion (and additionally increased power draw).The volume part is wrong. Last I've seen this kind of pot arrangement was on passive electric guitars. The pots cursors should not be connected to the DAC outputs. This will reduce the load impedance as you reduce volume, again leading to the issues mentioned above.See this for audio mixing:http://sound.whsites.net/articles/audio-mixing.htmBasically swap the cursor and one end connection of each pot and you should be fine. Adding an output opamp close to the DAC may help with the risk of picking up noise, especially if the connections between the DAC and the audio amp are long, but you can try without it to begin with.For audio volume, log pots should be used. They do have a direction for wiring, since they are not linear. Just measure the resistance between one side and the cursor when the pot is like 1/4 turn and you can figure out how to wire it.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 18, 2018, 03:24:12 pm
content: You're basically making a L/R balance adjustment, right? So why not put the two channels into either of the fixed sides of a single linear pot, then using the center tap for it as your output? Then if you also want volume adjust, just have a second logarithmic pot after that. You'd probably want some sort of fixed resistor before either of the channel inputs into your balance pot to keep the impedance the driver sees from getting too low at the ends of the pots, but it probably doesn't have to be much.

author: cvanc
author_rank: Frequent Contributor
timestamp: May 18, 2018, 03:37:06 pm
content: Quote from: DaJMasta on May 18, 2018, 03:24:12 pmSounds like an elegant solution, good idea.