File:Precision vactrol drive (Soviet OEP-14, 200 Ohms to 3 kOhms usable range).png
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English: Precision vactrol drive for the variable-EQ tape playback amp, using Soviet OEP-14 dual-LDR devices. These are the only Soviet dual vactrols still easily available. Lowest possible LDR resistance is 120..200 Ohms; disbalance between the two LDRs may be substantial but it's not that difficult to select a well-matched one. Practical, well-controlled range of resistances extends from 200 Ohms to 3 kOhms. Memory effect error around 20% (ballpark figure - this one is hard to measure reliably).
The circuit, intended for battery-powered preamps with 4S lithium supply of 14...17V, is based on an earlier example from Silonex. Integrating capacitor increased to at least .47 uF (0.22 uF shown here may be unstable). Added precision current limiter at 16 mA. Long-term stability (tested at 0.5k and 1K target resistances over a week time, with a different current limiter arrangement and 5V reference but otherwise the same circuit) is on par with my multimeter's error. The LED in the power rail is merely for indication. The op amp may be LM258/LM358/LM2904 or anything with input/output voltages down to the negative supply rail. The lower quarter of the potentiometer's movement is wasted, as the LDR's resistance locks to the minumum value (150...200 Ohm). Ditto the upper 10%. Within the usable midband, if the potentiometer is of the linear type ("taper B"), resistance increases inversely proportional to the turn angle. In an RC filter, the cutoff frequency is proportional to the turn angle. Nice linear function. Русский: Схема прецизионного управления двойной резисторной оптопарой ОЭП-14 для цепи коррекции в усилителе воспроизведения катушечного магнитофона. Батарейное питание от четырех литиевых батарей (+14...17В). Один фоторезистор задаёт среднечастотную постоянную времени, другой - контрольный - включен в измерительный мост. Все оптроны имеют заметный эффект памяти, поэтому точная установка сопротивления возможна только в паре со следящей ОС. Практический диапазон контролируемой установки сопротивления - от 200 до 3000 Ом (выше - нестабильно). Разброс фоторезисторов в ОЭП-14 может быть значителен, но в моей практике всегда можно подобрать прибор с разбросом не более 5%. Схема испытана (c иным ограничителем тока и ИОН на 5В, но в остальном всё то же) на контрольных значениях 500 и 1000 Ом в течение недели: долговременный дрейф сопротивления сопоставим с погрешностью мультиметра (доли процента). Светодиод в шине питания - только для индикации, его можно исключить. Операционники LM258/LM358/LM2904 и т.п. (точность не важна, главное - чтоб диапазоны входных и выходных напряжений охватывали нижнюю шину питания). |
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| Source | Own work |
| Author | Retired electrician |
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
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| current | 03:39, 6 October 2021 | | 1,264 × 735 (95 KB) | Retired electrician (talk | contribs) | upd |
| 06:28, 5 October 2021 |  | 2,149 × 1,269 (212 KB) | Retired electrician (talk | contribs) | {{Information |Description={{en|Precision vactrol drive for the variable-EQ tape playback amp, using Soviet OEP-14 dual-LDR devices. These are the only Soviet dual vactrols still easily available. Lowest possible LDR resistance is 120..200 Ohms; disbalance between the two LDRs may be substantial but it's not that difficult to select a well-matched one. Practical, well-controlled range of resistances extends from 200 Ohms to 3 kOhms.<br>The circuit, intended for battery-powered preamps with 4S... |
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