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Old 4th October 2011, 10:04 PM   #21
udailey is offline udailey  United States
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Charlie
I will try your example tonight on the bench. Right now heading to store to get some birthday gifts.
As to no current=no signal, not so. Imagine an opamp with multimegohm impedance.
Uriah
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Old 4th October 2011, 10:44 PM   #22
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Quote:
Originally Posted by udailey View Post
Charlie
I will try your example tonight on the bench. Right now heading to store to get some birthday gifts.
As to no current=no signal, not so. Imagine an opamp with multimegohm impedance.
Uriah
Yes, even with Megaohm input impedance, there still must be current flowing in to the input terminals. It is just so small, that it can be assumed to be essentially zero when compared with other currents in the circuit. For instance, a JFet input stage only draws some tens of pA of current, but it's there.

Anyway, I just came back from lunch, where I though about my circuit proposal. You are correct - it won't work! Here's why: both the LDR to ground and the series LDR are essentially just connected to the input (source) in parallel. In fact my little diagram should have tipped me off to that point. So it's really just like an LDR in series, which we both agree is not going to act as a switch. So I was wrong.

Anyway, your idea is a good one, because it combines a voltage divider and an additional isolation impedance. I believe that you could replace the relay with an LDR, but that would just cost more! And since that is basically out of the circuit (relay open) when the input is selected and in the circuit (relay closed) only when the two other LDRs are at 25M ohms, it should not inject noise, etc. Nice.

-Charlie
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Old 4th October 2011, 11:46 PM   #23
udailey is offline udailey  United States
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Thanks Charlie, I do like the idea and it works very nicely. My concern building it and testing it was that the LDRs take a moment to turn full on/full off but I have no noticed this at all. Its instant, just like a switch would be but the sound through it is fantastic. Still will hook it up to an AP for you guys and show a video of the measuring.
Uriah
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Old 5th October 2011, 03:28 AM   #24
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This will work also:

LDR SWITCH.JPG

Mike
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Old 5th October 2011, 03:55 AM   #25
udailey is offline udailey  United States
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Mike yes that's a good circuit also. I had two hesitations..hmm well three with that style. The ldr to ground is expensive, can't go real low in resistance ( with 25M on either side probably it'd be fine) and with associated circuitry it takes up lots of room on the board. But, yes that's a good option and I know it would sound great.
Have you built it?
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Old 5th October 2011, 11:03 PM   #26
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Yeah, I used that design in a preamp I built about 15 years ago using home-brew LED/LDR units and a couple of CD4049 hex inverting buffers. I wanted a way to control 6 stereo inputs without routing all those signals up to a front panel switch and back to to the PCB. It worked really well and sounded fine. The attenuation was in excess of 90 dB for the "off" input signals, and a fraction of a dB for the "on" signals if I remember correctly. It wasn't any more expensive than using a premium selector switch, but did take up some extra room in the case for the additional circuitry.

Mike
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