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I've recently become interested in analog synthesizers and have successfully built this VCO from MFOS.

The VCO outputs a signal that varies from 0 to about 10 or 12V DC at 28mA. I want to bring this down to (roughly) line level so I can record it without blowing up all my stuff.

  1. Using Ohm's law I figure that a 20Ω resistor should bring the voltage down to about 0.5V. Does that sound right?

  2. I'll need to use a capacitor to filter out the DC bias of the signal (right?) How do I calculate what capacitance value I need?

Edit:

Here's a link to the schematic for the main part of the oscillator. The output I want to tap (at least at first) is the sine wave output next to R49 (near the bottom right.)

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  • \$\begingroup\$ Maybe you can point out on the circuit you linked to where you are taking the output from that is too large? \$\endgroup\$ – Andy aka Sep 22 '13 at 20:25
  • \$\begingroup\$ @Andy aka, I updated the post with a link to the schematic showing where I want to take the output from. \$\endgroup\$ – friedo Sep 22 '13 at 22:04
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For a DC blocking cap, the impedance of the cap should be small relative to other impedances over the intended frequency range. For example, hi-fi audio goes down to 20 Hz. For some headroom, it's a good idea to design your circuits with high pass filter rolloffs at 10 Hz or so.

The impedance magnitude of a capacitor is 1 / 2πfC, with f in Hz, C in Farads, and the result in Ohms. When this roughly equals whatever impedance is in series with the cap at 10 Hz, then it will pass 20 Hz and up well enough to be considered hi-fi.

For example, if 1 kΩ will be in series with the cap, then it needs to be about 16 µF.

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  • \$\begingroup\$ Why does the impedance of the capacitor needs to be roughly equal to the impedance that is in series with that capacitor? \$\endgroup\$ – m.Alin Sep 23 '13 at 8:55
  • \$\begingroup\$ It doesn't... I think he made a typo. Probably meant 160uF, which would be 10x smaller than the 1k resistor. You want the cap here only for its DC blocking; the lower resistance it has (at all frequencies of interest) the better. \$\endgroup\$ – Paul L May 24 '15 at 5:14
  • \$\begingroup\$ @Paul: No typo. 16 uF followed by 1 kOhm to ground form a high pass filter with a rolloff of 10 Hz. That's fine for "HiFi" audio, which is usually allowed -3 dB at 20 Hz. I set this filter to -3 dB at 10 Hz to leave some room for other things to attenuate at low frequencies too. At higher frequencies, the cap will be less and less of the overall impedance and the gain of the filter approaches 1. \$\endgroup\$ – Olin Lathrop May 24 '15 at 12:23

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