I'm designing an analog audio circuit, but I'm not sure how to wire the audio plug from my mp3 player into my circuit. It's a standard TRS plug. So far what I've learned about TRS connectors can be summarized in this table:

             Mono Jack             Stereo Jack               Stereo Jack
           (Mono signal)     (Balanced Mono Signal)        (Stereo Signal)

 Tip             +ve                    +ve                       Left
 Ring            -ve                    -ve                       Right
 Sleeve         Shield                 Shield                     Shield

I want to be able to handle all 3 cases (Mono, Balanced Mono and Stereo). I also want to treat the tip and ring signals separately (in other words, I'll have a separate circuit for each signal). I'll be applying some active filters to these signals (simple - something I've learned in school), but right now I'm stuck because I don't know how to prepare these signals for inputting into op amps.

 

Were you planning to use different input jacks for the cases (by the way, straight mono is signal to the tip, and ring & sleeve are ground) you present? About the only time you see balanced signals is with microphone inputs, which get handled quite differently from line-level audio.

Of course, the more information we get translates to better advice. We have no idea if you are planning on all line-level inputs, or if that balanced input is for a mike. Also, why is it difficult to buffer the inputs before presenting them to the filters?

You start off as if it's just the MP3 player you are going to use. If that is the case, it's only the stereo arrangement that you need to accommodate. You say "I don't know how to prepare these signals for inputting into op amps." Could you expand on that?

 

Thanks for the reply. I only want to use 1 jack, and I'm expecting a speaker-level signal. Since the mp3 player is the input, I will get stereo output, and perhaps mono output if the audio file was mono?

As you said, I would definitely like to buffer the inputs, but this is what I don't know how to do. I'm not exactly sure what the tip and ring voltages are measured with respect to (I'm guessing/hoping it's the sleeve - please tell me), nor how to hook them up to op amps so that I can get a left and right buffered signal.

 

There is an immediate problem. "I'm expecting a speaker-level signal. Since the mp3 player is the input" means that the MP3 input will be overdriven. An audio input should be at 0 VdBm, around 1.25 VRMS. Speaker level outputs can be much higher and cause damage to the MP3 input.

The difference between mono and stereo as far as your audio plug goes, is weather there is any difference between the right and left channels. The shiled is the ground reference for measurement.

Always couple audio signals in through capacitors to eliminate any DC bias or offset. Use good quality audio op amps like OPA134 (or 2134 for a dual). Use a follower with gain configuration - http://www.allaboutcircuits.com/vol_3/chpt_8/5.html. How much gain you need depends on your so far unknown signal processing.

 

Thanks for that clarification on the role of the shield. Also, knowing that I should expect 1.25Volts RMS is going to be very handy in planning how much amplification I will be applying later on in the circuit. Can you link me to some useful information on DC bias/offset?

 

Are you going to run your circuits with one or two voltage supplies? Having a plus and minus supply tends to eliminate most of that problem.

So you have filters covered, but not active devices?

 

I will be using a plus/minus supply for my op amps. I am familiar with basic op amp operation (inverting/non-inverting, voltage following, active filtering) through school and have been doing some more reading myself.

Do I need to implement some sort of differential amplification to my audio signal? And can I buffer the signal with a voltage follower before doing the differential amplification?

 

You can always buffer a signal. As far as the need for differential amplification, it depends on what you are trying to accomplish.

 

What's really confusing me right now is the fact that I have two grounds. Let me explain:

The mp3 player has a signal T, measured relative to its ground, S. The op-amps will be powered from a wall-adapter that outputs a DC voltage Vwall relative to a different ground GND.

I've drawn a sketch of a differential amplifier. What I'd like to know is, which ground(s) are my voltages being measured with respect to?

PS. I realize that with a bi-polar input S, the differential op-amp will clip the waveform when it goes negative, but that's not a problem in my application.

 

Everything will be relative to S. The only problem I see with the setup is that if any component of the audio signal in goes below ground (S), the op amp will clip. It can't swing below the pin 4 level.

 

Great, thanks for all the help

I'm not worried about clipping in this application. Of course, if I wanted to prevent clipping, I could add a voltage regulator or inverter to produce -Vwall.

As I understand it, I still need to buffer T prior to the differential amp so that I don't load the mp3 player's output. I would not need to do the same for S, would I?

 

If the input resistors are large enough, you won't have to buffer the signal. Most line-level audio should be ok if you use 47K (49.9K in 1% decade).

Go to the TI site - http://focus.ti.com/analog/docs/ana...57&familyId=57&documentCategoryId=1&Input3=Go - and get this document -

Single-supply op amp design (slyt189.pdf, 244 KB.

You might find it useful. You may also find a number of other application notes of interest.