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Sliding Blend Pot?


Dugz Ink
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I love my acoustic/electric bass, but I want to add an internal piezo pickup. (It already has an under-saddle transducer.) However, I hate A/B switches, and would like to use a blend pot, which would be wired between the pickups and the 9-volt pre-amp. I also don't want a big round knob mounted on the face of this guitar, and there isn't a "good" spot to mount one on the side.

However, I could mount a 60mm slide pot parallel to the top of the pre-amp controls.

So, I'm wondering if any of you electronics gurus have a source for a sliding blend-pot... like the ones that are in the "DJ" mixers. (They allow you to fade from one turntable to the other turntable.)

Anybody know where I can buy one of these? (I haven't found anything on Mouser, yet.)

Also, if you forsee any problems with this plan, please tell me.

D~s

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slide pots are hard to find, but you can source em from Mouser and a few other places. faders are even harder to find, but I'd be happy to hear if anyone has a source.

I love blend pots and wire them on my guitars instead of 3-way switches (on non-resto jobs). I find that normal knobs aren't so good because it's hard to tell where they're set, visually or even by feel- I used to end up cranking them back and forth to see where they were in their travel.

...until I started using chickenhead knobs- they're perfect for blend pots, you can see and feel exactly where they are, I can even remember fave settings. and they don't take up so much room visually as other knobs. I know, this isn't what you were asking for, but it's an idea...

telereznew2.jpg

btw, blend pots are internally wired like this:

blenderwork6.gif

Edited by x189player
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:D The problem I ran into was that all the good quality audio slide pots were low impedance (10K, occasionally 20K), and you'd have to put an active buffer between each pickup and the pan pot. If that's feasible, the ALPS RS60112A600N from Mouser will work as a 10K crossfader, but it doesn't have a center detent, and it's a true linear taper crossfader, not a blend pot, so you'll be turning one side down as you turn the other side up. The spec sheet is here.

I haven't been able to source a slide panpot or "blend" pot anywhere, and minimum order on custom sliders is usually well over $1K, so I didn't pursue the idea any further. You could do something tricky with voltage control and comparators, but that appeared way too complicated (and power-hungry) for onboard use, and I don't speak digital fluently, so that wasn't an option. If you do find a simple, cheap workaround, I'd really like to see it - it's a very cool idea! :D

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... good quality audio slide pots were low impedance (10K, occasionally 20K), and you'd have to put an active buffer between each pickup and the pan pot.

I've been looking around, and haven't found anything, either.

Another dumb question; if I use a 500k blend-pot, do I still need to buffer each piezo?

D~s

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I'm not sure, Dugz - I know that LR Baggs uses 5 Megohm pots with their X-Bridge pickups, so I had always assumed that piezos were extremely high impedance, but after thinking about it, I realize that I don't know for sure. :D Since piezos seem to respond better at higher frequencies, losing some high end might not be a bad thing, especially on bass (unless you like finger noise). :D

What pickup are you planning to add, and what's in there now?

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The impedance on piezo elements are in fact, very high impedance, just like most magnetic pickups. Adding up the non-ideal features of both, and drawing out a circuit of those, they also act the same way. They both are linear (ideally again) until a certain frequency, have a narrow spike (this can vary, it's called the 'Q') then go downhill in response in the frequencies higher than this peak. The lower the pot value, the less of a peak you''ll have, as it's only limiting the current capabilities, not the voltage, the rest should be unaffected. But, since the frequency of a piezo is usually tuned well above audible levels, you should be unaffected in volume pretty much, I could be wrong though. With 500K you should be fine running it passive, especially considering it's A. Acoustic and B. a Bass. Piezo elements can be used to generate some pretty high voltages, with low current, the ones they use in grill igniters makes about a one inch arc, and they're fun for zapping people with! LOL :-P

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What pickup are you planning to add, and what's in there now?

The AB-20 (in my avatar) comes with whatever under-sadlle transducer that Washburn bought and mounted. Finding the details is very difficult. (The "schematic" looks like it was drawn by a five-year-old!) So I have no idea about the specifics.

Meanwhile, I ordered a couple of K&K "Big Shot" piezos. I figured I would try one in this bass, and have another one around for experimenting. I'm not sure if these are better/worse than the Fishman stuff, but the folks at K&K were a lot more helpful (via email) than the folks at Fishman... and I tend to spend my money with people who really try to help me.

Kyle...

That was a well articulated post. Thank you for the insights.

D~s

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After looking at the K & K Big Shot "manual", I'd be tempted to try a 500K pot - since the transducer isn't inductive, like a magnetic pickup, it shouldn't roll off the high end terribly. Kyle makes a good point about the resonance of the piezos likely being well above the audible range. I'd say ask the folks at K & K if they foresee any serious problems with that setup, and unless they do, go for it. Then all you have to do is find a decent quality 500K slide pot.

Keep us posted - there's precious little info on using piezos for bass anywhere, and most of it is of questionable value to anybody who doesn't play an upright, so you're blazing a trail for the rest of us to follow.

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Newark has a listing for dual-gang slide pots that go up to 500k but the travel length is only 30mm. I don't think that would be very easy to use. You also have to contact their sales depy. for pricing. Not a good sign.

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It's beginning to look like I just need to experiment with a less expensive rotary blend-pot... just run the wires out the sound-hole and tape everything in place while I test the idea.

If it sounds good, then I'll call Newark for a good laugh. ("You want HOW much?")

D~s

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