Open frame jacks question

All frequent questions on switching: true or not true bypass, transistor-based or mechanical.
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boblob
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Post by boblob »

I use those open frame Neutrik jacks and have a question...

With the power switching 3-pole jack wired in the usual way (ring to battery -ve), if I plug a jack into my guitar, then halfway into the pedal so that the plug tip is touching the ring the circuit will complete and current will flow. I'm a little concerned as this DC current is going through my guitar's pot and pickup to ground.

With a fuzz pedal drawing < 10ma, it's probably not going to do any harm, however with a larger current draw I would maybe be a bit concerned.

Is there a way round this?

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Post by Nocentelli »

boblob wrote:I use those open frame Neutrik jacks and have a question...

With the power switching 3-pole jack wired in the usual way (ring to battery -ve), if I plug a jack into my guitar, then halfway into the pedal so that the plug tip is touching the ring the circuit will complete and current will flow. I'm a little concerned as this DC current is going through my guitar's pot and pickup to ground.
What is the resistance of your pickup(s)? Maybe 5-10k? If you are worried you could try a 5k1 resistor across the sleeve and ring and measure the current flowing. I have little expertise and perhaps this is oversimplifying it, but if it was a problem I would have thought this method of power control would not be standard throughout the industry.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by induction »

It seems to me that the current draw of the pedal is not the controlling factor here. The DC resistance of the pickup is going to limit the current all by itself.

Assume 4k (very low) DC pickup resistance: I = V/R = 9/4000 = 2.25 mA

Higher DC resistances allow even less current.

I don't think it's a problem.

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Post by boblob »

It's not so much the pickup - more the volume pot that concerns me. With the volume all the way down there is no limiting resistance, so you have whatever the current draw is going through the pot wiper to ground.

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Post by astrobass »

What's the concern, though? What about that scenario bothers you?

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Post by Nocentelli »

I don't think it's going direct to ground through the pot:

http://www.flyguitars.com/graphics/G3schematic.gif

With the volume full up, i.e.zero resistance between pot lug 2 (wiper, briefly connected to power ground) and lug3, the paths to ground go through either a 300k pot (in that diagram) or a 5k-ish pick-up. It seems like it would be a parallel resistance of just under the pot value - Maybe not 5k, but enough to limit the current enough. I've burnt out pots on the breadboard in the past, and you need to apply 9 volts directly across two shorted lugs for several seconds. A safety resistor of 1k or so is usually sufficient to protect the "starve" pots incorporated in loads of noise fuzz circuits.
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Post by boblob »

The current draw of the pedal gets shorted to ground through the pot when the volume is at zero - that would be my concern. I know it takes a fair bit of current to burn out a pot, but they aren't actually rated to handle very much DC current. As a rule of thumb, to calculate the max current:

P = I*IR

So, plugging in the numbers for that 300K pot and assuming a rating of 0.25W, that would be rated at just ~ 900uA. I don't think a 3mA fuzz pedal is going to be much of a problem, but how about a delay or similar that draws upwards of 100mA?

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Post by Nocentelli »

Sorry, I didn't read your previous posts properly: You did clearly state you were concerned about when the volume was fully rolled down.

I suppose it's a fairly specific set of circumstances that could lead to a problem: High current draw pedal first in chain, already plugged into power, guitar gets plugged in with volume rolled right down. We are still talking about a few milliseconds of contact even so.

I recall that some high current draw delay/sampler pedals by BOSS (DD20, RC2) have the power switched by the output instead of input: This is ostensibly so you can record a loop and have it continue playing whilst you unplug the instrument, but it would also avoid any potential problems if you're worried.
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Post by boblob »

Switching on output is better, but if the pedal is true bypass it's still possible to dump the pedal's current draw through your volume pot if you were to connect the input first, then the output with the pedal in bypass. Those Neutrik jacks short the ring and tip poles together via the jack tip when the jack is 1/2 inserted. An edge case, for sure, but still possible.

I tried this with my fairly recent Phase 90 which has enclosed plastic jacks. With the jack half way in it doesn't power up, so there must be a proper way to do this.

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Post by Nocentelli »

Maybe just use those enclosed jacks instead of the neutriks? I think that any electrolytic caps in the circuit would have to charge before DC can flow, which would buy you some more time to fully insert that jack.
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Post by boblob »

I have some plastic enclosed jacks (they're Neutriks too) - it is still possible to make that connection with those. Might have to crack open my Phase 90 and see what they use. Perhaps some sort of switched contacts?

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Post by boblob »

My Phase 90 has two 3-pole plastic jacks with a NC switch on the sleeve and tip and an NO on the ring. In this arrangement the battery is not connected until the plug is fully inserted (the NO switch on the ring closes and power goes on).

I kind of like this way of power switching - anyone know if I can buy something like this? Had a quick look on Small Bear, but they don't seem to have 'em.

Incidentally, my Boss TU-3 seems to have something similar. It has a pretty hefty current draw, so really makes sense there.

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Post by boblob »

I stand corrected. My TU-3 does sink 20mA or so through my volume pot with the input plug half inserted. Not great.

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Post by Nocentelli »

boblob wrote:My Phase 90 has two 3-pole plastic jacks with a NC switch on the sleeve and tip and an NO on the ring. In this arrangement the battery is not connected until the plug is fully inserted (the NO switch on the ring closes and power goes on).

I kind of like this way of power switching - anyone know if I can buy something like this?
Nice to know how it's done: I must admit I was sceptical about it being a problem, but I seem to recall that BOSS pedal instructions advise plugging in guitar first, then power on pedals, then turn on amp with volume rolled down. I always assumed this was to avoid pops through the speakers, but maybe it also helps avoid this problem.

It's good to know, but it's not something that would affect me: I long ago discovered never to roll the volume off before putting a guitar down - The number of hours in the past I have wasted debugging a new circuit or trying to figure out why my pedal board "wasn't working", only to discover the volume was off are numerous.
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Post by boblob »

To avoid this you'd plug your pedals in first, then guitar. I think that advice is mainly aimed at avoiding transient pops as things power up - amp manuals tend to tell you the same thing.

I guess I'm making a bit much of this. Can't imagine Boss having sold 50 squidillion units haven't thought about this. Then again, MXR have...

Cliff do those jacks, btw - the S4/BMB, but looks like you can only get them made to order. Strange as exact MXR replacement parts are usually pretty easy to get.

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