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I'm studying a circuit that is based on a clone of the Apple Mockingboard from way back when. Being new to EE, the first thing that struck me as odd is why the designer put all three output pins for the audio IC through two capacitors that are arranged back to front.

For example, looking at the image:

Mockingboard Clone

...notice how Ch.A, Ch.B, Ch.C all pass through two capacitors where negative ends appear to be tied together ("Left Channel Combo" section). Am I reading that right or is it just a goofy way the image was drawn?

Does that just mean the negative ends should really be to GND?

Any help in understanding this layout is appreciated.

Oh, the original URL for the image is here:

http://www.downloads.reactivemicro.com/Public/Apple%20II%20Items/Hardware/Mockingboard_A/Technical/Mockingboard%20Schematic.gif

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The components used are electrolytic capacitors. These components are unipolar or polarised, meaning that the voltage over them can't simply be reversed without the risk of destruction and even explosion if the source can supply enough current.

Sound is AC. So if you want to use an electrolytic capacitor to decouple successive amplifier stages you must bias it in the correct way with a DC source to prevent the AC from reversing the polarity, like is done here.

enter image description here

If biasing isn't an option an alternative is to put 2 capacitors with the opposite polarity in series. That creates a bipolar capacitor. The downside is that the capacitors need to have twice the capacity to produce the same value a single polarised device.

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  • \$\begingroup\$ +1 That is a wonderful explanation. But I'm a little confused with the "to decouple successive amplifier stages". I know I need to learn more about capacitors and AC. lol. Would you mind talking about that a little more? The "decouple successive amplifier stages" that is. Thanks!! \$\endgroup\$ – cbmeeks Jan 23 '15 at 14:32
  • \$\begingroup\$ The biasing is also required to set the operation point of the transistor. If you cascade amplifier stages without decoupling or if is the source also has a DC resistance, you will shift the operation point, often so much that the amplifier does no longer work properly. You can in fact design DC-coupled amplifier stages, but it just harder to design. It has some advantages though, no bulky caps, and a frequency response that starts from 0 Hz, instead of the usual 20Hz. \$\endgroup\$ – Ambiorix Jan 23 '15 at 22:15

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