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Old 3rd January 2011, 03:13 AM   #1
nezbleu is offline nezbleu  Canada
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Default Input selector (like Quad)

Hi:

I am building a new preamp based on a Mesmerize board (Pass Labs B1 buffer with relay input selection). The input switching is already there, I just need to complete a circuit for the selected input to close the associated relay. The usual way of doing this is with a rotary switch. However, I always liked the Quad 44 preamp's input selectors with momentary-contact push buttons.

Luckily, many years ago I had to repair one of those; I had to replace a (dual? quad?) D-type flip-flop chip. I vaguely remember the circuit: each selector button asserted a common clock line for all the flip-flops, and also the data line for one flip-flop. All the other data lines were low. The flip-flop IC outputs drove an analog switch IC. I also seem to remember a bunch of small transistors, one per input, which may have either been between the flip-flop and the switch, or they may have been used to turn on the associated indicator LED. It's been a very long time...

In this case I want to use the logic to fire a relay, not an analog switch chip, but the principal remains the same. The good news is I remember enough of the circuit to be able to sketch something similar. The bad news is I don't remember the details, and even if I did, the Quad 44 input selector was notoriously unreliable (I was repairing one, after all). And I lack the expertise to put the circuit together and be sure I am doing it right.

I can see some issues with what I remember of the circuit: First, a flip-flop needs some small set-up time, that is, the data line needs to be stable before the clock line low->high transition. So there should be some small delay on the clock line (we're talking nanoseconds) when the switch is depressed. Second, how much current does the relay need? The indicator LED? What is the best way to use the output of the flip-flop to drive the relay? In this case the relay coils all have +12V on them, the switching circuit needs to close the circuit to ground for the selected relay. Should I use a small-signal transistor to handle that, switched by the flip-flop output? Or some IC (maybe even an analog switch IC!)? Should the LED be in series with the relay coil, or parallel with it's own current-limiting resistor?

In short, has anyone got a nice little circuit like this? Any suggestions?

(Oh, I also looked up a Quad 44 schematic on line, but it doesn't look like what I remember -- which might say more about my memory than anything else.)
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Old 3rd January 2011, 04:58 PM   #2
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Is this attachment what you are looking for? It is a selector for 4 signals. The cap across the off switch makes sure the off is on (ouch!) on power-up. The circuit was used in a custom-commercial install years ago. The circuit is easily expanded to multiplr of 4s.
Cheers, E
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File Type: pdf 4posswitch.pdf (26.5 KB, 257 views)
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Old 7th January 2011, 02:29 AM   #3
nezbleu is offline nezbleu  Canada
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Quote:
Originally Posted by mickeymoose View Post
Is this attachment what you are looking for? It is a selector for 4 signals. The cap across the off switch makes sure the off is on (ouch!) on power-up. The circuit was used in a custom-commercial install years ago. The circuit is easily expanded to multiplr of 4s.
Cheers, E
Thanks for that, very nice. Pretty much exactly what I was looking for. However, this being DIY, I'm still trying to put together something on my own.

Oh, and the Quad pre I repaired many years ago was a 34, not a 44. I finally found a 34 schematic on line. The good news: the D-type flip-flop IC's are there like I remembered. The bad part: they aren't where I remembered! They used them as toggles. I still think I can make them work as selectors. There's no good reason to do so, but I never let that stop me before.

Thanks again!
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