Differential audio output to single ended output - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2018, 06:49:05 pm
content: So I got a bluetooth module and apparently they choose to implement the audio outputs as differential outputs for speakers.From a quick google (as I haven't fired it up yet), it seems they are differential and that connecting the left and right minus outputs together shuts the board down in protect.Two questions,Other than shorting the two minuses together and forcing abnormal shutdown or damage to the BT board, is there an easy way to test for differential outputs? Is it as simple as scoping both + and - relative to ground? If neither are at ground potential they are differential?To change these to be single ended and thus compatible with 99% of the rest of domestic audio gear, do I just put the + and - into a differential opamp and take the single output relative to ground as my signal?

author: Audioguru
author_rank: Super Contributor
timestamp: April 03, 2018, 07:10:13 pm
content: The "differential speaker output" is really a bridged speaker output that drives both speaker wires so that the voltage swing is doubled then the current is also doubled resulting in almost 4 times the power of a single-ended amplifier driving the same speaker with the same supply voltage.The + or the - output can have a series coupling capacitor to feed a speaker that has its other wire grounded. The output power will be 1/4 of the bridged output.The amplifier shut down because you shorted the amplifier stereo outputs together. Dont doo dat. Keep the two stereo channels separate or mix their inputs into mono.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2018, 07:15:31 pm
content: Quote from: Audioguru on April 03, 2018, 07:10:13 pmThanks. It wasn't me. I googled the problem and found a query about a similar module. The user connected the two - signals together. The board shut down. This was his confirmation to the question, "Are they really differential?"I suppose I could just put a stereo signal in and check the potential across the two - outputs.I don't want to mix them down to mono. I want to keep them stereo.Also... when I rent a disco rig, the usually disable the stereo outputs and only bridged mono works. Not sure if this is the same thing. I think they do that to stop people running the amp with only one speaker and torturing it.I believe there simpler circuits with capacitors, but is it worth it, or should I just put another opamp on each output and convert it to two mono single ended outputs?EDIT: If I understand a differential signal, at full amplitude the minus will be near the amps lower rail, say -5V, the positive will be near it's higher rail, say +5V. Giving a total voltage of +10V on the speaker. At it's lowest amplitude the + output actually goes negative and the - output goes positive, giving -10V across the speaker.So AC coupling the two outputs and combining would not work. I need to take the difference between the two and reference that to 0V / Ground.

author: Buriedcode
author_rank: Super Contributor
timestamp: April 03, 2018, 08:39:49 pm
content: It depends on what you wish to connect the module to. I have several bluetooth modules (two DIY headphones, two BT speakers) and only one of headphones have differential to single ended conversion because it also has an AUX in jack (common ground for both audio channels, so L and R speakers have common ground).Many headphone amps, or chip speaker amps have differential input, which makes life easier as you just require ~1uF ceramic caps. It is also preferable to keep the signal fully differential if you can as the common mode noise on these bluetooth SOC's is pretty horrendous - which is partly why they have differential outputs, the other reason being they have built-in power amps for 32ohm earphones. Difference amps require fairly good resistor matching, but if the common mode noise isn't that great you could get away with 1% (try it and hear for yourself!).You just need a difference amp (one op amp per channel) and you can size the resistors for a gain of 1 (which would actually be double peak-peak amplitude of each individual output).You can of course have a single supply difference amplifier as you can bias the inputs at VCC/2 with capacitive coupling both input and output, no need for a negative supply.I am unaware of any non-active ways of converting differential to single ended, save an audio transformer.

author: slurry
author_rank: Regular Contributor
timestamp: April 03, 2018, 08:43:38 pm
content: A pair of Lundahl 1540 would do the trick but it is kind of a waste on a bluetooth audio connection

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2018, 08:47:41 pm
content: I want the output to be a standard line level left and right audio signal.I do have some flexibility on how I treat the BT input individually, but will mix down with other line level signals into a mixer amp.The others go via a baxandall pre-amp section, so if I could devise a baxandall pre-amp which incorporates the differential aspect and produces single ended audio this would be ideal. I'll see if I can find anything.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2018, 08:49:59 pm
content: Quote from: slurry on April 03, 2018, 08:43:38 pmYes, avoiding transformers is a plus, unless I can't avoid it. It seems to me that the old school discrete analogue solution, while correct is not always the modern solution.

author: paulca
author_rank: Super Contributor
timestamp: April 03, 2018, 09:23:49 pm
content: Quote from: BrianHG on April 03, 2018, 08:58:09 pmYep. I have the wooden spoon out today it seems, stirring..... stirring.

author: BrianHG
author_rank: Super Contributor
timestamp: April 03, 2018, 09:34:17 pm
content: You do realize if you read the high end DAC data sheets I recommended to you that they are differential output with DC offset & they tell you how to wireup an opamp to convert that signal to single ended... You just need to change the series resistors and parallel load resistor.

author: xani
author_rank: Frequent Contributor
timestamp: April 04, 2018, 04:56:12 am
content: Quote from: paulca on April 03, 2018, 06:49:05 pmScope both, they should be a mirror of eachother. You can use your scope's A-B math function to get "true differential" resultQuoteThe proper solution would be opamp with few resistors, that would also ensure any common-mode noise (pulse supply etc) gets filtered. The ghetto one would be just ignoring minus output and using ground and plus (with a cap for zero DC offset) as single ended output