Randaleem
Joined: May 17, 2007
Posts: 456
Location: Northern CA, USA
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 Posted: Mon Jul 02, 2007 2:41 am Post subject:
Re: AC DC-switch on Soundlab |
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| Pascal wrote: | The first VCO of the SL has a switch Coupling AC DC.
What does that mean? |
Hi Pascal,
That switch lets you choose to put a capacitor (often referred to as simply a "cap") in the path of the circuit or not. A capacitor will allow an AC (Alternating Current) waveform to pass, but will block DC (Direct Current) current. This is one of its important electrical characteristics.
So what this does for you is allow a "musical" changing signal (AC) to get through this part and move on to the next part of the circuit. BUT the cap blocks any DC voltage which the signal may be "riding on". We want to do this because the DC part of the signal will usually cause our amplifiers and speakers to thump and pop. (This is the same thing that happens if you connect a battery directly to a speaker. The speaker coil moves VERY fast and very far.) This is not good for a speaker, and it doesn't usually sound sound good to our ears either!
Let me say this another way. If an AC signal is centered around zero volts, you will not need a capacitor, because there isn't any DC voltage to block!. Thinking of the speaker again, the voice coil will move smoothly in and out, because it too is centered around its zero (no volts) position. This is the same position it has at rest; so there is no added stress and everything works as it should.
But if there is a signal that has a DC "component", now the speaker will be "biased" OUT of its comfortable resting (centered) position (in the direction represented by the polarity of the DC.) And because of this bias, the alternating signal itself may no longer have enough "room" to move the speaker coil and cone the full distance that it could if it was centered as in the previous paragraph. The result may be distortion as the speaker cone and coil run into their physical limits, and even worse, permanent damage may occur.
(Note that circuits and signals aren't perfect, so there usually IS at least some DC component to nearly all AC signals.)
With the capacitor in place (AC setting), the dc is blocked and the speaker can once again work smoothly around its relaxed center position!
So why not leave the capacitor in place ALL the time?
Because sometimes we WANT the DC to pass on to the next part of the circuit. And sometimes there is NO AC component to the "signal" we are moving through the circuitry. (Again, note that since perfection is rare: most DC has at least some AC--called noise--carried within. )
Synthesizers are versatile machines. They do not easily (or always) distinguish between what might be called a signal (AC) and what might be called a control voltage(DC). If we have a control voltage which is moving quickly up and down is it AC? And now should we call it a signal? If we have an up-down waveform from an oscillator which is moving slowly; doesn't it look a lot like a changing DC control voltage?
The answers will depend upon how you have connected the parts of the synthesizer circuitry together.
Here's the simple answer to your question!
The result of this versatility inherent in the synthesizer is that you may sometimes want to block the DC voltage and sometimes you may want to let it pass. The AC/DC switch allows you to make that choice.
The rest just explains WHY you might want or need to choose.
One last thing: The speaker in the examples above was used because I think most of us have direct experience with them. Often including having put a battery across the terminals of one and hearing the result!
BUT the speaker is merely a stand-in for the example of any other type of electronic device, or circuitry portion in a synthesizer. All the things I said in reference to it being "relaxed" and "centered", or "biased" and "distorted" will apply ALSO to other types of circuits that may be far more complex.
Matching up all the different parts of a Synthesizer may require us to use little helpers along the way so the circuits work well together. The AC/DC switch is a good example of that practice.
So if you want to pass AC and block DC (usually to prevent thumps, pops and overdriven following circuits); you include the capacitor.
And when you want to process the "DC" control voltages alone you will leave it out so they can pass on to the next part of the circuit unaltered.
Hope this helps you to understand why the switch is there.
Randal
BTW, the capacitor has other properties which can cause it to selectively block only SOME of the AC or DC signal. This is not really needed to understand the answer to your question, and can become very technical very quickly. So I'll only mention it here at the end in case you want to explore further.
IMO It's worth discovering the science behind the music. These very simple parts have incredible variety, nuances and details that rival the music being produced. Knowing how and why they do what they do may also help you to understand how to better go about making your music. |
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