Audio Amplifiers - Differential Speaker to TRRS Mono  - Page 1
Forum section: electronics / beginners

author: tendercowboy
author_rank: Newbie
timestamp: September 21, 2022, 01:50:59 pm
content: Hi All,I'm student and a complete rookie when it comes to anything audio circuitry. I'm currently trying put together an amplifier for the FM tuner chip Si4825-A10. The datasheet for this chip says that the output voltage for FM audio is 77mVrms.Having no prior experience with audio circuits I'm lost as to what kind of amplification is required for this signal. From some googling, it seems that standard audio signals are approximately a 2Vrms? I came across this mono audio amplifier IC: MAX98300ETA+T but am unsure if this a good choice for my needs as I am only looking to listen using a pair of earbuds and this is a speaker driver. As well as the problem of power levels, I'm unsure about how to convert the differential input and outputs of this amp to single ended for my needs.Any help would be greatly appreciated.

author: magic
author_rank: Super Contributor
timestamp: September 21, 2022, 02:16:31 pm
content: Quote from: tendercowboy on September 21, 2022, 01:50:59 pmThat may be enough voltage, or almost enough. You will probably find that 10x gain is already too much. However, some sort of buffer will be necessary to drive the current-hungry 16Ω or 32Ω load presented by typical small headphones. The receiver is specified for 10kΩ load, which is not even close.You basically need a headphone amplifier. Such things are available as standalone boxes, there are also ICs dedicated to this job, or opamps can be used. (Some of those dedicated ICs are rebranded "industrial" opamps or even more bizarre chips like DSL line drivers). A fairly simple DIY HP amp is so-called CMoy, which consists of OPA2134 dual opamp with a few passives. Other opamps could work too, though not all of them are stable with capacitive load (a headphone cable may be a few hundred pF).What sort of power supply is available?

author: Terry Bites
author_rank: Super Contributor
timestamp: September 21, 2022, 06:33:26 pm
content: The (-) output is not meant to be connected to ground. The amplfier has a (+) and a (-) output to give double the output voltage across speaker than a single ended amplifier can.This is done because creates 4 times the power. P=V2/Rload A bridge tied load.They dont specify the single ended performance of this amp so it will be a bit of an experiment. I'd try leaving the (-) output unconnected and putting a capacitor in series with the jack. A 100-220uF electrolytic, say 6.3V will be about right. Note the +Ve side of the capacitor connects to the amplifier.The input impedance of this amp is about 10k which suits your radio ic. Tie the unused input to ground via cap, tha same value as the input cap. This prevents internal bias problems inside the amp.There's nothing to stop you using another amp. It will be hard to find an opamp that will run on 3.3V and put up with a low load resistance like earphones.

author: tendercowboy
author_rank: Newbie
timestamp: September 21, 2022, 11:08:11 pm
content: Thanks for your replies! The power supply is fairly flexible, I'm currently using a 9V supply from a DC barrel jack and was considering using an inverting charge pump if I need to get a -9V rail. I think I'll keep having a poke around at some headphone amplifier ICs as I'm partial to something with less discrete components.I may also order that same speaker IC and a make a breakout board to experiment with it.Thanks again for your help.