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Q: Switch btw jack + speaker / shared volume LDR
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darematheson



Joined: Nov 28, 2012
Posts: 12
Location: Rhode Island, USA

PostPosted: Mon Mar 04, 2013 7:01 am    Post subject: Q: Switch btw jack + speaker / shared volume LDR Reply with quote  Mark this post and the followings unread

Hi all -
I have a straight-forward Nic Collins style NAND synth, with a speaker, as well as an output 1/4" jack. Its a switching jack, so it will automatically sever the speaker circuit when plugged in.

However, I'm also using a photocell as a (sort of theremin-style) volume control. I'm having a tough time getting that control shared between the jack circuit, and the powered amp circuit.

Please take a look at the image.
The original circuit on top works fine as is (but doesn't incorporate the speaker, amp, and switch). The circuit at the bottom attempts to keep the volume LDR right before the switch (point 'A'), but then it doesn't interact well with the LM386. I also tried moving the volume to point 'B' where the 10K gain pot is meant to go on the LM386 input, but it is not acting as I'd expect (attenuates for a portion, but not all the way).

Here are my questions:
1) Where and how should I wire the photocell and switching jack to get the functionality I want?
2) If I wire the switch after the amp, is there a concern of overpowering, say, headphones plugged into the jack (the speaker would be severed, but not the amp)?

Any help will be much appreciated!
Thanks all
DM


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darematheson



Joined: Nov 28, 2012
Posts: 12
Location: Rhode Island, USA

PostPosted: Mon Mar 04, 2013 4:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Please let me know if I've posted this in the wrong area - thanks.
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analog_backlash



Joined: Sep 04, 2012
Posts: 391
Location: Aldershot, UK
Audio files: 21

PostPosted: Mon Mar 04, 2013 4:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

What I would try, is to make the LDR the top half of a voltage divider, with a fixed value resistor as the bottom half, see:

http://www.doctronics.co.uk/voltage.htm

The fixed resistor needs to be a small value, compared to the dark resistance of the LDR. The 0.01uF capacitor can be wired in parallel with the fixed resistor and the jack socket switch left in the same position relative to the LDR circuit.

The main problem with this, is that with a fixed resistor at the bottom of the voltage divider, there is always going to be a finite amount of signal getting through. However, if the value is sufficiently small compared with the LDR dark resistance, hopefully, the signal will be inaudibly low. You'd have to experiment a bit, as LDRs vary quite a bit in their resistance ranges.

I hope that this works...

Gary

P.S.

darematheson wrote:
Please let me know if I've posted this in the wrong area - thanks.


It should really be in the Lunetta Forum, I suppose, but I wouldn't worry too much!
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darematheson



Joined: Nov 28, 2012
Posts: 12
Location: Rhode Island, USA

PostPosted: Mon Mar 04, 2013 5:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great - thanks, Gary.

I'll play around with this. Is there any reason I should avoid wiring the LDR voltage divider right at the LM386 input (point B in my schematic)... and placing the switching jack at the same point (switching the flow out the jack right before it engages with the LM386)?

Thanks - I'll report back with results.
DM
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darematheson



Joined: Nov 28, 2012
Posts: 12
Location: Rhode Island, USA

PostPosted: Mon Mar 04, 2013 7:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah - I see you covered the switch location... misread that the first time.

Trying now - thanks!
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darematheson



Joined: Nov 28, 2012
Posts: 12
Location: Rhode Island, USA

PostPosted: Mon Mar 04, 2013 9:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Worked perfectly - thanks for the tip, A.B!
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analog_backlash



Joined: Sep 04, 2012
Posts: 391
Location: Aldershot, UK
Audio files: 21

PostPosted: Tue Mar 05, 2013 2:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Glad that it worked - I thought that it should, provided you chose the right value resistor. That Doctronics site has a quite a lot of useful information on it, that isn't too complicated. I've found it useful myself.

Cheers,

Gary
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darematheson



Joined: Nov 28, 2012
Posts: 12
Location: Rhode Island, USA

PostPosted: Tue Mar 05, 2013 5:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah - only issue now is that the speaker still sounds somewhat when the headphones are plugged. This happens even if I completely detach from the input pin of the amp chip. Must be electrical bleed somehow - only way I can find to get rid of it is severing the LM386 power.

I tried a decoupling cap directly btwn power and ground pins on the chip, didn't help.

Any basic ideas for isolating the chip, so it doesn't drive the speaker?
Thanks
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analog_backlash



Joined: Sep 04, 2012
Posts: 391
Location: Aldershot, UK
Audio files: 21

PostPosted: Tue Mar 05, 2013 9:06 am    Post subject: Reply with quote  Mark this post and the followings unread

It sounds like the LM386 is still getting some signal through the power supply connections. Have you tried connecting a capacitor between pin 7 and 0V? A 4.7uF electrolytic, or thereabouts (with the positive lead connected to pin 7) should help to isolate the chip from power line signals. I'm not certain about this, as I haven't tried it myself, but it's what the pin is there for (but often, it is not used).

Let me know if this works or not...

Gary
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