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Hello fellow tinkerers. I'm a guitarist working on an overdrive circuit that will require transistor switching to shunt the signal to ground at various places. I've found a lot of good info online but haven't been able to see the whole picture yet. I need to accomplish the switching with the fewest possible components, as my circuit board is running out of real estate, and with the lowest possible current draw. A lot of the info I've seen about BJT switches suggests that typical current draw is 100 ma, which is far too much current, considering that a lot of effects pedal power supplies are only rated for twice that. I've gotten to the point where my brain is full for the evening and I'm hoping someone can just gift wrap a solution for me. Basically, what I have available is about 8.5v after power supply filtering and one pole of a TPDT stomp switch to alternate power to two pairs of transistors, one pair shunting a gain and volume control to ground as the other pair breaks a shunt to ground for another gain and volume control. I'm not opposed to using SOT transistors if that's what it takes to keep the current draw down. Any takers?
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Audio switching to ground with Bipolars does not take that much current.
Maybe they talked about using "100mA" capable transistors, meaning small (TO92 or SOT) ones.
FYI, I switch my distortion circuits ON by grounding clipping diodes+tone/volume pots as needed with a humble TO92 BC546 NPN transistor, with its base connected to +15V through 2 x 10K resistors in series, whose centerpoint gets grounded or not.
Base (switching) current is: 15000mV/20000 ohms= 0.75 mA.
Resize resistors depending on your available PSU voltage.Juan Manuel Fahey -
So you're connecting a positive voltage to the base? I knew it had to be easier than the examples I was finding. All the ones I saw were connecting power to the emitter of a PNP, and it was really only good for energizing relays. So, since I have a ton of 2N5088's, how do I calculate the resistor values for that transistor and 8.5v? Also, I'm assuming your power on/off is affecting conduction from the emitter to the collector, so that the collector is connected to ground and the circuit fragment you want grounded is connected to the emitter. Is that correct?Comment
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Almost. 2N5088 are fine.
Emitter to ground, collector to whatever you want to ground, base to +8.5V through a 10K to 22K resistor turns it on .
Remember you can not have DC, only audio signal, or it will pop horribly.
If needed, add a series DC blocking cap (1uF to 10uF) and a high value resistor (100K to 1 M) from it to ground to discharge any small voltage which can leak through.Juan Manuel FaheyComment
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Yes. Sorry, I got my emitter and collector mixed up. I'm used to working with op amps and jfets. SO the blocking cap is in series with the audio signal being grounded. Where's the high value resistor going? Is it parallel to the capacitor?
What you described in your first post sounds a little different. It sounded like you were grounding the center of a voltage divider at the base rather than switching power on/off to get collector-emitter conduction. Is there an advantage to that?Comment
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1) the high value resistor is in parallel with emitter/collector, so even when OFF there is a path to ground for any DC buildup.
2) I described what I use, as an example.
In my case the transistor gets switched ON-OFF thanks to a footswitch which grounds a contact.
You mentioned using some kind of switch which supplies +8.5V to the transistor, practically the opposit arrangement.
Both are valid, choose one.
If you want, post a rough schematic of what you are trying to do.Juan Manuel FaheyComment
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Hopefully the image shows up ok. As you can see, it's just two sets of tone and volume controls with a switch providing power to one pair or the other. Does this look like I'm on the right track?Comment
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I meant volume and gain, not tone.Comment
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Yes, that's basically it.
You will ground either both pots on the left or both on the right.
Use 10K to 22K resistors in series with each base and I suggest you add from each base to ground a .1 to 1uF cap, to damp any switching click.Juan Manuel FaheyComment
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Juan, thank you so much for all of your help. Great information!
-TimComment
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Most welcome.Comment
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^^^^^^^^^^^ Hey !!! , now *I*'m being cloned?
You will *need* at least to grow the same beer belly , the same greying beard , semi balding head and other distinguishing features to be at least minimally close
Juan Manuel FaheyComment
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