Nice article on electrolytic capacitors and their use in AC coupling:
It’s actually the second in a series of articles on AC and DC coupling.
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Here’s the third in the series:
and Skala’s timing is impeccable.
I have a PCB I just assembled and tested and… it didn’t work. Partly because half my INPUT net labels were globals and half were heirarchicals 
and partly because, well, really dumb design mistake. So I figured I could fix the mistake by squeezing in another TL074 to buffer the signals after the AC coupling capacitors and amended the design accordingly. Then I thought about going ahead and uploading it for fabrication because “How can it not work?” but that was the same attitude that caused me to miss the mistake in the first place, so I tested it and… it didn’t work. Two days ago I would’ve been saying “WTF?”. Today I was saying “Oh yeah, that’s the bias current thing Skala was talking about.”
Can I squeeze in four more resistors? Gonna have to.
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Remember that in analogue synths, the signals are typically much higher than in ‘normal’ audio circuits, and so it can be easier for a signal to reverse-bias an electrolytic and damage it.
Another thing to bear in mind about aluminium electrolytics is that heat kills them. You will often see a manufacturer quote a maximum operating temperature, and lifetime of the capacitor at the rated temperature. It can be worth paying a bit more for a higher-specced device (higher temperature/ hours life).
When I use large electrolytics in power supplies, I try and site them away from the regulator/heatsink, in the hope that they will not be exposed to such high temperatures for long periods. Also, thermal cycling (repeated heating/cooling) will eventually kill any electrical component, so again I try to reduce the exposure to large temperature changes. Cooling a PSU with a fan may not be an option in a synth, as fans are acoustically noisy.
When electrolytics fail, it is noisy and smoky and messy, and can burn the PCB.
One argument against using electrolytics in a signal path is their high tolerances, making frequency response variable from circuit to circuit.
Tantalums are not popular in audio, but they are compact and I regard them as OK for local power supply decoupling if a film capacitor is not beefy enough for the job.
An anecdote regarding tantalum electrolytics. When they blow up, they have a distinctive smell.
I used to work for a company where we would soak test new circuit boards overnight, and sometimes come in to the smell in the morning, and hunt out the charred remains of the failed capacitor.
The smell is very similar to that of TCP mouthwash. A colleague with a cold came in one day after using TCP and caused a bit of a panic in the test department…
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Interesting. Is electrolytics small resistance to reverse biasing the reason they’re used a lot in AC coupling of line level signals?
See the article. It depends on the impedance of the circuit you’re going into. If that is small then you need a large capacitance to get a low enough cutoff frequency, and for capacitances above about 10 µF electrolytics are pretty much the only game in town, at least if you want reasonably small size and low cost.
The Wikipedia article “Capacitor Types” is a very good brief reference if you want to learn about different capacitors, and how to use them effectively (and in some cases, safely).
I might add to this the MFOS page on capacitors, which goes over the basics in terms of synth building.
http://musicfromouterspace.com/analogsynth_new/ELECTRONICS/capacitors.html
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