conversion of single ended audio input to differential

I am working on a bluetooth module for streaming audio signals. Audio signals coming from audio jacks are single ended,but datasheet of module has mentioned that differential audio signals are recommended. is there any way to convert single ended audio signal to differential one. Through internet search, I found this schematic:

Is this method valid for conversion?

• If the bluetooth module has an input that accepts differential signals, isn't it the output you want to convert to differential? – Phil Frost Jan 8 '14 at 12:33
• While this thread hardly needs it, there's some great material in amazon.com/Systems-Design-Installation-Philip-Giddings/dp/…, chapter 9, all of which is available in the "take a look" amazon control – Scott Seidman Jan 8 '14 at 15:19

It is possible that I have misunderstood the requirement - but if not...

The vintage way of getting differential audio signals from a single-ended input consisted of merely using an audio frequency transformer, usually a 2 pin input 3 pin output little thing, with a 1:1 turns ratio.

The single-ended signal feeds the transformer input, and the output side is a differential signal pair you can bias to wherever you need it to be - connect the middle pin of the output to ground, and the differential signal is ground-referenced.

In addition, a turns ratio other than 1:1 allows impedance matching / voltage gain if desired.

This is an extended comment to address what I believe is an important distinction to make - the distinction between a balanced output and a differential output.

According to the link provided by Phil Frost, we have:

A good, accurate definition is "A balanced circuit is a two-conductor circuit in which both conductors and all circuits connected to them have the same impedance with respect to ground and to all other conductors...

Thus, this would be an example of a balanced output:

simulate this circuit – Schematic created using CircuitLab

Though the output is balanced according to the above definition, it is not differential.

For the output to be differential, we must have $$\V^+_{out} = -V^-_{out}\$$

But, for the circuit above, we have (for open circuit condition) $$\V^+_{out} = V_{oc} \$$, and $$\V^-_{out} = 0\$$

This is an example of a differential output:

simulate this circuit

Note that this is not a balanced output since the impedance to ground for the positive output node is $$\600\Omega\$$ while the impedance to ground for the negative output node is $$\0\Omega\$$.

However, for both circuits, the open circuit output voltage is $$\V^+_{out} - V^-_{out} = V_{oc}\$$ and the output impedance is $$\600\Omega\$$.

Finally, an example of a balanced and differential output:

simulate this circuit

As to the OP's question:

is there any way to convert single ended audio signal to differential one

The answer is yes, of course there is and certainly, as Anido Ghosh answers, a 1:1 audio transformer will give you both a balanced and differential output as well as isolation which may or may not be useful.

As other answers have pointed out, you can can also convert to differential only or balanced only.

Whether you actually need both, one or the other, or neither is a matter for experiment to decide.

• :-) Sometimes it helps to have enough grey hair to recall the days of the audio transformer with fondness... So simple, so elegant, so robust! I'm getting all nostalgic now. – Anindo Ghosh Jan 8 '14 at 14:45
• I've often wondered why true fully differential op amps like ti.com/lit/an/sloa054d/sloa054d.pdf never enter into such discussions – Scott Seidman Jan 8 '14 at 15:23
• Can you tell us why the OP would want a differential, but not balanced signal? – Phil Frost Jan 8 '14 at 16:21
• In fact, can you tell us how any differential signal can be not balanced? If the impedances are not equal, then you may drive each half with equal voltage, but not equal current, or with equal current, but not equal voltage. This doesn't sound very differential to me. – Phil Frost Jan 8 '14 at 16:24

Use an op-amp configured as a unity gain inverter. Feed its input from the audio jack. The output from the op-amp is an inverted version of the signal from the audio jack and suitable for devices requiring differential inputs. +input = audio jack signal, -input is o/p from op-amp inverter.

In all likelihood, you'll probably find that the Bluetooth module works just fine with a single-ended input.

Here is a diagram from the design of high-performance balanced audio interfaces by Bill Whitlock & Rod Elliott - it shows several methods:

Note the top left circuit. What I'm proposing is a cut-down version of this. The non-inverting stage I'm proposing can be removed leaving only the inverting op-amp stage. The diagram does show RS1 and RS2 for when impedance balancing is required and I'm not saying you don't need these components but in many circumstances they are not needed, particularly in the application in the question.

• Whoever downvoted, it would be nice to also post a comment stating why the downvote. Thanks. – Anindo Ghosh Jan 8 '14 at 12:12
• This does nothing to balance the signal, which is the important part of differential signals. Inverting the "-input" is not required. – Phil Frost Jan 8 '14 at 12:12
• Normally I would expect the use of dedicated single-ended to differential converters. Does this trick with the inverted opamp mode really work? – alexan_e Jan 8 '14 at 12:13
• @Andyaka but you haven't matched impedances. You've just inverted the signal, and hoped that the output impedance of your inverting op-amp is equal to whatever the output impedance of the inverted signal happened to be. It almost surely isn't balanced to any useful degree. – Phil Frost Jan 8 '14 at 12:21
• @PhilFrost I've not mentioned balancing impedances for sure but in many cases they are not required (audio, interfacing over short distances etc..) – Andy aka Jan 8 '14 at 12:47

The schematic in your question does not convert to a differential signal. It is merely a circuit to bias two microphones (likely, electret microphones).

The advantage to a differential signal is increased noise immunity, especially over long runs. You may not require any additional noise immunity, and unless you can make the single-ended runs shorter by converting to differential somewhere, you aren't going to get it anyway.

To connect your single-ended audio signal to the differential input is easy: connect the inverting input of the differential input to ground, and the ground-referenced audio signal to the non-inverting input.

To make a differential signal, you need two signal lines with equal impedances that carry the signal in their difference. What you don't need is an inverted copy of your signal. There are a lot of ways to accomplish that, but with modern electronics, at audio frequencies, a common approach which works well enough while remaining quite simple is this:

This is from an article, Design of High-Performance Balanced Audio Interfaces, by Bill Whitlock & Rod Elliott. Notice there's no inverting of the signal. R2, R3, R4, and C1 are the balancing. It's important that R2 and R3 are exactly the same value: use 0.1% tolerance resistors.

• Phil, this may not work as you expect - the signal on pin 2 will be higher than the signal on pin 3 - it has to be or they would be exactly the same signal. How can this work as a balanced driver for a differential pair? – Andy aka Jan 8 '14 at 12:36
• @Andyaka there is essentially no signal on pin 3. There doesn't need to be. The circuit even works, with reduced tolerance to component variation and non-idealities, with R4 and C1 left open. The signal is still in the difference. The output impedances of pin 2 and pin 3 are equal, thus common-mode noise will be rejected. What's the problem? – Phil Frost Jan 8 '14 at 12:38
• This may be 'balanced' but it certainly is not a single ended to differential conversion. For open circuit operation, pin 2 has the full audio signal and pin 3 has 0 volts. Thus, the output has a common mode component equal to 1/2 the audio signal. If this were a true differential output, there would be no common mode output component. – Alfred Centauri Jan 8 '14 at 13:15
• @PhilFrost, you've missed the point entirely. If there is a common mode component, the output is not a differential output - full stop. Your answer is written as if you believe this circuit provides a differential output - it does not. The OP asks for a single ended to differential conversion. – Alfred Centauri Jan 8 '14 at 13:23
• The OP seems to be asking for a way to make a single-ended audio output compatible with a device that has a balanced input, regardless of the language it was couched in. Every answer in this thread will do this, some better than others, as will just about every \$20 DI box you can buy in Guitar Center. While spending too much effort to match impedances on kHz signals seems a bit misguided to me, the audio engineers, IMO, have one-upped us all. The best book I've ever read on grounding comes from audio engineers, who put up complicated systems with no hum! – Scott Seidman Jan 8 '14 at 13:37