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PostPosted: 21 Nov 2017, 11:18 

Joined: 21 Nov 2017, 11:01
Posts: 1
Working on small project and currently at the final hurdle of the circuit board design which is audio. I'm using the RPI Compute Module 3 which is housed on a custom board I'm creating.

What I want to achieve it to convert the stereo channels PWM0 & PWM1 on GPIO pins 40 & 45 on the CM3 into mono using an differential op amp circuit and then play this audio through 3.5mm jack if a plug is in the socket else it will reroute the signal to speaker. I managed to bash out a basic design using information from the RPI forums and my electronics buddy who I get to talk to roughly once or twice a week (see below). However I'm not exactly clued up on the final design. As when questioning people I found out the amp op I was using only outputted 10MA which wasn't enough to power the 3.5mm jack or speaker and was reccomended I try adding a power amp or going with a power op amp circuit.

Wondering if anyone could provide their input on what would be the best approach to take, how I can improve the design and even the values of parts that would be required i.e. 10kohm 0.001uf etc.
The audio itself that want is that of what you would find in say a mp3 player or an radio with as minimal sound as possible.

The board I'm making with all components runs at max on 5v.


PostPosted: 30 Jan 2018, 00:31 

Joined: 09 Oct 2012, 19:43
Posts: 363
Location: Vancouver Canada
The amp you suggest is not powerful enough to drive speakers. The amp below is designed for your voltage range of
handheld tablets and cells etc. There is a whole family of these types of amps in the market place. This amp sells for
$1.12 Can so very reasonable. Scroll threw the full data sheet and you will usually find an sch diagram with res and
caps etc. to get you started. It is very important however you note: ---neither terminal of the speaker is connected to
gnd. This type of drive circuit is becoming more and more popular. Main reason is the low voltage on the supply rails
does not allow for a high enough voltage to be produced across the spkr. So 2 smaller amps one driving each
speaker leg produces 2 times the power to the spkr using a much lower voltage. BUT, both spkr leeds are conn'd back
only to the amps pins and never touch ground.
There are of coarse other amps which do use ground as the speakers return leg. (TDA2030 and it's family) But they
usually require higher voltages and as their power raiting goes up so does their demand for greater amounts of power
from battery or whatever power supply is being used.. Have a look around and get a feel for it all. ... 003243.pdf ... DA2030.pdf

PostPosted: 31 Jan 2018, 20:10 

Joined: 23 Feb 2017, 02:02
Posts: 677
The only way you can have a power amp and have some decent output voltage is to get a class d power amp that operates on 5v. I don't think

You can get about 8-10v pk-pk swing by making a low dropout bridged amp that runs on 5v. Connecting one side of speaker to 2.5v and 2.5v of a equivlant amp but the output is inverted. Effectly 0v accross the speaker and can get 8-10v pk-pk. I don't think there are any schematics online like this. ... gJspfD_BwE
This one seems to do 5v. This may be what your loking into

You can directly input your phones output into this amplifier, I don't think much mroe is required

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