Signal to noise ratio on headphone output

[mackemint]

First I'd like to say a job very well executed with the Circuit Sword!
I've used many different raspi handhelds / emulationstation configs as well as Android devices running various emulators and this device is by far my favourite! A hat off to you Kite!
I'm a chiptune artist and I use the device mainly for writing tunes in LGPT and LSDJ. One feature I've found is that the output amplifier distorts / compresses the sound in a good sounding way when pushing the output a little, so there's a little bit of free "mastering" available - an unexpected and very rewarding finding.

I have one little issue though, and that is that I find that the output volume at its loudest is still pretty weak, it's fine for use with headphones but when running it into an FX unit or into a mixer the volume level needs to be cranked very high, which in turn raises the noise floor and also produces some high-pitched noises that isn't filtered out in the circuit boards' filtering.
I've been looking for information on what headphone amp is used and what the options are (if any) for further increasing signal level, can you see a feasible solution to my situation?

Thanks!

[kite]

Hi thanks for the feedback!! All good to hear!

The sound chip is the "CM102S+" and you can find the datasheet online if you want to do some reading for any tweaks! The chip has an AB amp that is enabled so it should be able to drive outputs well (in theory, i'm not a sound guy so i've been using recommended layouts).

One thing you might be able to play with is the "VOLADJ" pin, which I have set in a voltage divider to a fixed voltage in order to get suitable sound levels (for me, my headphones are so sensitive that even on 3% volume they're really loud!).. so I did calibrate them for that

Here is the layout, with R46 and R48 being the divider and the lighter red pin/trace being the VOLADJ pin and it's path:

Screenshot from 2018-10-23 10-46-47.png (25.08 KiB) Viewed 4011 times

What would be best is to get a potentiometer, put the end leg to GND, the middle leg to the highlighted pin, and the other end leg to 3.3v. When you turn the pot it will then adjust this ratio. It is a little strange so have a play.. for example there is a position that will enable mute, and a position that will suddenly go full volume.. so the pot will make it easy for you to play with what works best for you! To undo, just remove the middle pin and it'll be back to stock!

Another thing that MIGHT help, is the big square caps to the left of the IC are rated at 100uF, you may have better luck increasing them to 200uF or so?

[mackemint]

Alright, thx!

The cap u mentioned, is that C33?
I read up on the IC, it is very well documented although I'm not sure what dB reference they're using. I e-mailed them about it anyway.
One follow-up question about the voltage division with the potentiometer, is the rail that goes from R48 at 3.3V?
Wouldn't that be a little risky without removing R48, as two voltage references would be added together on the VOLADJ pin? My Kirchoffs law is a bit rusty

Cheers
/M

[kite]

The cap i'm talking about are the two identical ones to the LEFT of the main IC which are C32 and C37.

Excuse the TERRIBLE drawing (gnome paint is really bad) but the following should help:

The two caps mentioned are circled in RED.
The Pot should be wired to the nearby 3.3v/gnd solder pads (as they are strongly supported and easy to solder to) and then the middle pin should be soldered to any of the highlighted pins previously! Or in the picture I just put it to one of them (they're slightly whiter in colour in the pic).

The voltage divider resistor values are quite high so there is no issue with doing this mod at all, the input pin is designed for 0-3.3v (vcc) operation so you're perfectly safe wiring it as i have drawn!

audpot.png (44.83 KiB) Viewed 3991 times