Line out for an audio amp with VDD/2-referenced output - Page 1
Forum section: electronics / beginners

author: ratatax
author_rank: Regular Contributor
timestamp: December 04, 2020, 12:53:28 pm
content: Hi,I'm using an FM8002A audio amp chip to drive a speaker (see first attached pic), it's single rail supply and doesn't need a big DC blocking capacitor thanks to its output using VDD/2 as reference.The problem is when I try to add a line output jack that should mute the speaker when inserted.It's usually easy to do just by connecting the jack socket in series with the speaker, but since the negative output of the amp isn't ground but VDD/2, I think it cannot work (VDD/2 will be forced to ground once the jack in inserted)Is there some way to solve that problem? Some amp chips have separated grounds for the speaked and line output, and a dedicated "sense" pin that will mute the speaker (2nd attachment) but they're harder to find.Any other solution ? Besides going with a standard amp chip that uses ground reference as output (and needs the big DC blocking caps)

author: fcb
author_rank: Super Contributor
timestamp: December 04, 2020, 01:07:00 pm
content: Just get a lineout jack (3.5mm? 6.35mm?) that has all the contacts switched - easy to find. Wire the switched GND to the bias circuit as shown on the first drawing.What am I missing?

author: Ian.M
author_rank: Super Contributor
timestamp: December 04, 2020, 01:09:33 pm
content: Use a cheap OPAMP to drive 'line out', and use the FM8002A shutdown pin to mute the speaker when the plug is in the jack.

author: ratatax
author_rank: Regular Contributor
timestamp: December 07, 2020, 03:05:17 pm
content: Hi,Jack with switched GND aren't the most common, and I use small 3.5mm smd jack sockets. I drew this schematic can I get your opinions on it?It uses RC4580 opamp as a buffer to drive the line/headphones out, then mix the stereo signal into mono with two resistors, the goes into the amplifier IC for the speaker.However I'm not sure about the impedance of the FM8002A input, if you look on the first pic I posted you see the FM8002A expects a 0.39uf cap and 20Kohm resistor in series with its audio input.Here it's placed after the big 220uF DC blocking caps, and there are the two 10k resistors to mix L/R channels. Will the amplifier see the input impedance as 5K ? (two 10k in parallel) Is the capacitor size a problem ?

author: fcb
author_rank: Super Contributor
timestamp: December 07, 2020, 04:57:25 pm
content: Should work, although you do risk a bit of popping when you plug/unplug (depending on the termination of the 'thing' connected to lineout).The gain of your DAC amplifiers (RC4580) is +2, so if your peak audio level is +3.3V you'll be well into clipping. RC4580 isn't a rail-rail device and the outputdriver of the RC4580 will at best go within a Vbe of each rail.e.g. 5V-0.65=4.35V, divide by g=2 means 2.175V as best case maximum input from the DAC. Assuming 1.65V is DAC zero, then you'll be able to swing (at best) +/- 0.525V (about a third of your stated DAC output) without clipping... at best.

author: ratatax
author_rank: Regular Contributor
timestamp: December 07, 2020, 07:10:56 pm
content: You're right I made an error in my schematic i wanted no gain at all, I will use the RC4580 in inverting mode to be able to have g=1 (the total margin to avoid clipping will be 5-3.3 = 1.7V)Ian.M suggested to use the shutdown pin of the FM8002A to switch the speaker on and off, but with this type of non-isolated jack contacts I can't, right ? Since the contacts carry the signal.

author: ratatax
author_rank: Regular Contributor
timestamp: December 07, 2020, 08:18:06 pm
content: Oh wait I forgot that a non rail to rail opamp also can't get to GND.So basically I need to offset the input a bit, let's say instead of having a GND...3.3V swing, having a 0.7...4V range ?

author: ratatax
author_rank: Regular Contributor
timestamp: December 07, 2020, 09:27:10 pm
content: Well I might just go with this chip instead:https://datasheet.lcsc.com/szlcsc/STMicroelectronics-TS482IST_C169775.pdfRail to rail and quite cheap.