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| transistor replacement for VCO? |
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| FrankBuss:
--- Quote from: RandallMcRee on July 21, 2017, 01:36:03 am ---Digikey says 3,000+ film capacitors surface mount. --- End quote --- Right, I guess I clicked a wrong search filter. Which capacitor would be good for the integration capacitor? I guess the main problem with ceramic capacitors is the non-linearity, because they change capacity with voltage, and with temperature. I've read that polyester is much better. Would this capacitor be a good choice: https://www.digikey.com/short/3vmq7t ? |
| Zero999:
Nice circuit. The problem I found was the comparator's response time limited the high frequency performance and could be responsible for the distortion at 30kHz. The sine wave isn't great though. Have you tried a different wave shaping circuit? Here's one I've used in the past. Unfortunately I can't find the original source of the schematic. |
| Kleinstein:
Usually the saturated differential amplifier gives a reasonable good sine, if the amplitude is adjusted correct. This might need to have a temperature dependent resistor. Like with the other sine shapers it needs a reasonable good adjustment. For this relatively simple circuit about any film capacitor is OK. So in this case polyester type is good enough, though polypropylene (PP) and polystyrene (PS) caps would be high quality. But one would not notice the difference. There are also SMD film caps, but they are somewhat sensitive to temperature during soldering and thus a special relatively temperature stable type. So they are OK though somewhat expensive and more difficult to solder. For the smaller caps NP0 ceramics is a good alternative in SMD. Modern MLCC ceramics of NP0 type are even available in the 100 nF range. However large form factor ceramics are somewhat sensitive to board bending. Als an example : https://www.digikey.com/product-detail/en/murata-electronics-north-america/GRM31C5C1E104JA01L/490-1767-2-ND/586653 So if would be only the µF range where you would really need a THT film cap. |
| FrankBuss:
--- Quote from: floobydust on June 21, 2017, 05:35:50 pm ---Curtis CEM3340 VCO is back in production, popular for analog synths since the 1980's. --- End quote --- I tried to build an oscillator with the CEM3340, but there is a problem with the sawtooth output: after half of the output wave, there is a high frequency wave on top of it, about 3.6 MHz, see this log entry: https://hackaday.io/project/160538-cem3340-module/log/151718-first-revision-tested Today I soldered the second version, with all the changes I described at the end of the log and what you can see in the circuit diagram, and it has the same problem, so I guess it is not related to the layout. I even unsoldered the 1 nF Mica cap and connected it directly to pin 11 and 12, but same problem. Maybe there is a problem with the trimmers, or do I need to change some part values? Circuit diagram: https://cdn.hackaday.io/images/1641441535289908627.png What could cause this strange high frequency oscillation? |
| ogden:
--- Quote from: FrankBuss on September 30, 2018, 07:41:25 am ---What could cause this strange high frequency oscillation? --- End quote --- LF356 wave-shaper circuit: amplitude POT with wires may convert intended gain stage to generator. CEM3340 circuit may also have feedback somewhere. To suffer all you need is fast opamp on breadbord and some wire ;) Solution: replace POTs with SMD resistors placed close as possible to opamps and see if it helps. |
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