FWIW, a higher impedance load draws less power. Probably what happens is that your device has a relatively high output impedance and when discharged into a high impedance load, the voltage exceeds some maximum and clips. A low impedance load creates a voltage divider which prevents something from clipping.
No idea whats going on that would cause this (maybe a failing transistor somewhere in the output stage???)
but if putting a load prevents clipping you can fix that by getting a headphone splitter and connecting a resistor on one of the outputs (or even a pair of headphones).
AFAIR, we found, predictably, that the headphones whose specifications we could find had a considerably lower impedance than my monitor speakers, and I presume the same would apply to the other speakers and soundcards on which I tested the synth and observed the same problem.
So its plausible that having a load could help you reduce clipping. I would get a splitter and put a load on it and see if the other output into line in is ok.
Since it happens to both stereo channels, I guess it would not be a transistor in the amp itself (since that would only clip one channel). Probably its either made like this for some reason (I know nothing about synths...), or possibly the power supply voltage is off meaning that the full dynamic range of circuit cannot be used. Maybe someone else here has more experience.
Like saratoga mentioned, check your power supply for proper voltages.
If it turns out it's working as intended, and just can't handle a high impedance load, it wouldn't be hard to stick a resistor and opamp at the output to get a more useful output.
Tantalum caps can fail too, but they tend to short-circuit and/or explode, so you'd probably know if one died. Electrolytic caps die in all sorts of ways, I had one in the power supply for a headphone amp leak out the top vent a tiny bit and fail open-circuit with virtually no capacitance remaining. Other times they can keep their capacitance but have their ESR shoot way up, making it even harder to tell what the problem is unless you have an ESR meter.
Is it actually a stereo device, or just mono split into two channels for output?
[to pdq:] Am I reading you correctly that it’s only the electrolytic ones that will be responsible, if any capacitors are? And only the ones near the jack, not earlier in the circuit?
Also, what would be the likely cause of the failure? It seems highly coincidental that both outputs exhibit the same problem, to a perceptually equivalent degree, and it seems unlikely that they would have degraded at exactly the same rate with age, so my money would be on a previous voltage surge that went through both preamps. Either that or a really terrible batch, maybe?
Film capacitors are not likely to be the cause, both because they very rarely fail, and they would not likely cause your symptoms.
I probably should have asked you first to do some testing. If you have access to a voltmeter, measure the DC voltage at the outputs, both with and without the headphones connected. If the DC voltage is near zero (i.e. hundredths of a volt or less) then this is working correctly and my original guess was wrong.
There is some possibility that the output stage is bipolar (with both positive and negative supplies) and there is no blocking capacitor. A problem with one of the supplies would then cause a DC offset and the kind of symptoms you are seeing.
http://www.google.com/patents/US5179293[/url] ]When the output stage is an analog amplifier, in active mode it amplifies its input signal. In either arrangement [digital or analogue], in the "inhibit mode", the output stage transistors are turned off by reverse-biasing their base-emitter junctions relative to the output node of the circuit, to provide a floating high-impedance output node. In the inhibit mode, additional means are provided for substantially cancelling leakage current at the output node.
I don’t suppose it’s as simple as shoving a large resistor into the circuit somewhere.