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Author Topic: Solar cell resistor question Ed Inman Expert Film Handler Posts: 103 From: Jackson, Mississippi USA Registered: Jul 2004 posted 05-24-2013 12:42 PM                        Lacking a proper sound processor, I am currently bridging my solar cell with a 680 ohm, 1/2 watt resistor to plug into a conventional amplifier microphone jack for my home theater. It seems to work well and sound OK, but I've seen conflicting opinions on what resistor should be used--ranging from about 200 ohms to 1000 ohms. Is there a "best" resistor to use for this purpose? Do different resistors change the EQ? Also, is it best to connect the resistor at the cell terminal on the projector or closer to the mic jack (or does it matter)? Thanks for any suggestions.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-24-2013 03:14 PM                     Solar Cells are a current device and above a few milliohms their distortion goes up quickly as does the loss of bass response as such as Bang and Olfsen determined long ago the ideal preamp was a opamp running as a current sink to convert the change in current to a change in voltage and then a preamp after that The first cinema system to do that i believe was the original Kintek preamp boxes On the reverse readers the cell is fed into the negative input of a opamp so it feeds a virtual ground All that said the average mono system used 250-600 ohm load either by a resistor or by (in a better world) a matching transformer  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 05-24-2013 03:36 PM                        Used hundreds of Shure mike mixers (150 ohm) with transformer input in drive ins and cheap mono indoor theatres. Louis  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-24-2013 03:51 PM                     that was a popular choice for radio sound for am driveins  |  IP: Logged Ed Inman Expert Film Handler Posts: 103 From: Jackson, Mississippi USA Registered: Jul 2004 posted 05-24-2013 03:52 PM                        Yes this is a mono cell. So, what I gather is that the 680 ohm resistor is probably on the high side and I may do better with around a 400 ohm resistor?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-24-2013 04:04 PM                     trial and error it is a sawoff of bass response distortion and gain  |  IP: Logged Rob J. Buskop Expert Film Handler Posts: 111 From: Rotterdam, the Netherlands Registered: Aug 2005 posted 06-08-2013 03:12 PM                     As Gordon replied very correctly, the solar cell is basically a current source, and not a voltage source. Kintek indeed designed their preamps as current amplfiers that were of course grounded on one side at the input. As to the best resistor value, you have to understand that the introduction of a resistor in a current circuit generates a voltage differential over the resistor. That voltage swing is what you are amplifying. So far so good, right? Now, the value of the resistor determines how much (milli-)voltage you generate. The higher the value, the more voltage. So if you still have plenty of gain with a slightly lower value of you present resistor, you're better off. 560 Ohm was a fairly common value that worked well.  |  IP: Logged Ed Inman Expert Film Handler Posts: 103 From: Jackson, Mississippi USA Registered: Jul 2004 posted 06-08-2013 05:35 PM                        Thanks Rob, I'll try replacing the 680 ohm resistor with a 560 & see if I can tell any difference.  |  IP: Logged Levent Caliskan Film Handler Posts: 1 From: toronto , on, canada Registered: Jun 2013 posted 07-02-2013 12:42 PM                     The best way to find a suitable resistance for your circuit is to use a variable resistance. Start tuning from 1000ohm, backwards till you get the best result. Since the basic parameter you’re tuning is a few miliamps of current, make sure your variable resistance has that has a suitable resolution according to your given range.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 07-02-2013 06:30 PM                        Note as listed above it is a LOAD resistor, wired across the solar cell. As Gordon said, best to use a low impedance mike transformer.  |  IP: Logged Rob J. Buskop Expert Film Handler Posts: 111 From: Rotterdam, the Netherlands Registered: Aug 2005 posted 07-23-2013 02:23 PM                     A low impedance transformer will certainly work very well, as it did in the earlier Dolby-, and other, processors. Having said that, it still does not represent the ultimate medium for a current amplification or rather current to AC transference. A transformer here is just a way to get an acceptable impedance match and it does that quite well. But it is essentially not a current (mA) to/AC converter. There are better ways to emulate that.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 07-23-2013 03:40 PM                       While transformers are not the absolutely best solution to coupling a solar cell to the electronics of a processor, in real applications, they perform better. Dolby used as balanced, transformerless preamp design in the CP65 and it was very well done, but electronics noise is always higher than even a good transformer. Also, there are common wiring faults that will keep an active balanced design from working at all while the inherent isolation the transformer offers keeps the audio going. In the end, it's about cost as a chip and a few parts are cheaper than a good transformer.  |  IP: Logged

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