What should be the proper method to build a LINE-OUT signal from a Synthesizer that has only "Speaker" and "Headphones-out" outputs?

Question

I've always wanted to add some RCA Mono Line out to my humble CASIO MA-101 90's PCM synthesizer. It currently has 2 connectors: "9V DC in" and "headphones out".

On the internet I've seen some modders tapping out the wires of the output circuits, mainly getting signal from the mono speaker SIGNAL + GND combo. I've even seen some people adding a capacitor on one of the mono speaker wires, and then they derive this parallel tap onto a drilled mono RCA female output jack.

I still haven't decided about it (I've just added the RCA Mono output onto the plastic rear wall, and done nothing more). I can imagine tapping the mono speaker wires feels like an "emergency" way to get some mono output from them. So my question is: Without using a dedicated small amplifier pre made circuit, what should be "the proper" way to tap out a mono signal from the mono output sub-circuits (or even the speaker cables) of my synthesizer?.

I wouldn't like to use headphones output because when you connect something into it, (as the typical headphones jack it is), speaker output gets switched off.

BTW, I defend myself nicely using my soldering iron and adding components, so I'm not afraid of messing around with the current original circuit to add some passive component (R's C's, etc..) Note: The actual speaker inside my synthesizer is also CASIO branded one, with a 4 Ohm printed impedance.

Note 2: I've also taken some MOBO pics in case someone might peek and locate the proper output subcircuit. The sound generator chip is a typical one in the CASIO's from that era:  OKI M6387-06 I think I've located some sort of amplifier IC on the MOBO, but I don't have the actual specs of it... I suppose that some pin on this IC will probably be the amplified mono output I might get some juice from.

Whatever, TL;DR: I want to add proper Mono "Line OUT" connection capabilites to my CASIO synth and any nice idea about where to tap this output signal from will be more than welcome.

Thanks

EDIT: Added some pics (not amazing quality, but I'll try to add new ones taken under direct sunlight). Preliminar tracing can be done

EDIT2: I've started to study the tracks on the mobo, and the input signal for the top right amplifier (Sanyo LA4138) seems to get into the IC AFTER being attenuated (or not) by the main volume slider (consisting of silly value R's)... That probably means there's a "pure" unamplified synthesizer IC output signal that enters into the main volume slider, that could be the one I'm interested in... But then this signal is not amplified, as is the original synthesizer IC output... Maybe adding some small amplifier IC/components might be the final mod I need. Once added, that new circuit should get the" pure non volume slider processed" signal from there, amplify it and then drive it directly to my RCA Line out jack... (Just speculating)

Answer 1

Figure 1. Voltage vs. time of sine waves at reference and line levels, with V_RMS, V_PK, and V_PP marked for the +4 dBu line level. Source: Wikipedia Line level.

The most common nominal level for professional equipment is +4 dBu (by convention, decibel values are written with an explicit sign symbol). For consumer equipment it is −10 dBV, which is used to reduce manufacturing costs.

Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude (V_PK) of approximately 0.447 volts, or any general signal at 0.316 volts root mean square (V_RMS). A signal at +4 dBu is equivalent to a sine wave signal with a peak amplitude of approximately 1.736 volts, or any general signal at approximately 1.228 V_RMS.

Based on the article above I would do the following:

• Set the volume at about 75% volume (so that you have a some headroom, if required).
• Select an organ or flute tone that will give a continuous tone without too many harmonics.
• Play an A1 which should be 55 Hz and is close enough to mains frequency to give a reasonably accurate reading on a true RMS multimeter AC range. Measure the output voltage between one channel and ground.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. The attenuator "pad".

• Calculate the attenuation required to get the output down to 0.316 V_RMS.
• Use the attenuation value to calculate the values of R1 and R2.

You're generally going to be feeding high impedance equipment so R3 being 1k rather than 600 Ω will make little difference other than making the maths easier.

Note that it's not a true line out as the signal level will be dependent on volume control setting.

Answer 2

Without schematics (we're fanatic about schematics) we can only guess how your CASIO synth routes output to headphones/speakers.

Assumptions made:

• inserting a headphone jack breaks the signal path to CASIO internal 4 ohm speaker.

• headphone jack is at amplifier output rather than amplifier input.

• the added RCA jack will feed a high-impedance load.

• output to internal speaker and RCA jack simultaneously.

The most simple solution wires the RCA hot to the headphone jack hot. Of course, the RCA ground lug should be wired to circuit ground - best done at the headphone jack as well. The exact nature of the existing headphone jack is not stated. These are often stereo jacks with many more terminals than three shown below.
. Best to add a series resistor from headphone jack (tip) to RCA hot, so that if you accidentally short your RCA "line-out" to ground, the amplifier is protected. And whatever you connect to that RCA "line-out" is protected from over-current. A resistor value between 100 ohms to 1000 ohms would be appropriate.
This simple solution has a disadvantage: any volume control inside CASIO synth affects "line-out" amplitude on the RCA jack. In many cases, a "line-out" has amplitude that is independent of any amplitude controls.
Finding a tap-point in your synth that is amplitude-control-independent could be a difficult search. It likely involves messing with the motherboard - a risky venture. Such a tap-point could well be unavailable.