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Post by reddeluxe on Jan 7, 2020 0:47:38 GMT -5
First new member post (former member from the old FDP)….good luck and best wishes to Moe's Tavern! Have an electronics question for you gurus. Want to add a separate volume pot for a passive (no active circuitry of any kind, or powered preamp) magnetic, humbucking, sound hole mounted, acoustic guitar pickup. Specifically a DeArmond Tone Boss. The factory pickup has a thumbwheel volume control incorporated into its one piece assembly and works perfectly and quietly...but it is located in the worst location ergonomically possible; bottom of the pickup about 1 1/2 inches below the sound hole (actually inside of the guitar), rendering it useless to make adjustments in a live playing situation. Adding a standard audio taper 250K volume pot between the pickup and output jack, it works as normal through the volume range and is quiet from about 8 through 10 on the knob range, but as you gradually turn down below that setting, it is introducing noise (hum) into the output signal, as though it is loading the pickup. As it reaches the end of its travel 3 through 0 (off) the hum is again reduced, similarly to the high end of the range until the pickup is completely off. Is adding this second volume pot loading the circuit and causing this problem? Do I need a different value pot to eliminate this problem? Thanks for any info.
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Post by Peegoo 🏁 on Jan 7, 2020 15:00:45 GMT -5
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Post by reddeluxe on Jan 7, 2020 16:10:56 GMT -5
Thanks Peegoo, but wanting to mount the volume control into the guitar in the traditional easily accessible position on the bass side, near the neck join. I can wire, drill and install the pot, just need to know the proper ohm value, to mitigate the loading problem. Can't disassemble the pickup without damage, as it has a one piece modular housing that appears to be epoxied together. I know active pickups/preamps use very low value pots (10k-25k ohm), bit I'm guessing this pickup will be much higher. Suggestions?
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Post by Peegoo 🏁 on Jan 7, 2020 17:49:15 GMT -5
I don't know why you're getting noise in a narrow range of the 250K pot. That's strange.
I looked online and cannot find the specs/construction of the Dearmond Tone Boss. It's touted as a humbucker, so it's either a stacked coil or a twin-coil with three poles per coil, similar to the original P bass pickup and the G&L Z-coil design.
Being a humbucker, it probably would work best with a 500K audio/log pot. Give that a try; make sure the pickup's onboard vol control is maxed out.
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Post by reddeluxe on Jan 7, 2020 22:48:11 GMT -5
Peegoo, all above is correct.....humbucker, stacked coils, adjustable allen head pole pieces...a very accurate knock off of the L. R. Baggs version. The on board volume is and stays maxed out. I'm guessing maybe a 1 meg ohm value pot? Will post an update when I get it figured out.
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Post by Peegoo 🏁 on Jan 7, 2020 23:04:42 GMT -5
Yeah, a 1M pot will work too. Be sure to get an audio taper pot--not a linear taper.
Many makers stamp their audio pots with an A (e.g., A250, A500, A1M), and their linear pots with a B (B250, B500, etc.).
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Post by reddeluxe on Jan 9, 2020 18:58:25 GMT -5
Still no joy....replaced volume pot with 1 meg ohm value....still has same issue as volume is rolled down. From completely open (10) down to around 7-7 1/2 control pot works properly, then begins to hum progressively louder as pot is rolled down to around 2. The pot is functioning correctly as to the volume rolling off, but don't understand the added noise. Perhaps need a shielded hook up wire for the circuit? Your thoughts?
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Post by Peegoo 🏁 on Jan 10, 2020 0:00:55 GMT -5
That is totally possible. I have a Gretsch Rancher acoustic with a piezo under the bridge saddle that had a buzz at all volume positions. I solved the problem as follows: 1. Unstring the guitar. 2. Cut a strip of adhesive copper foil shielding tape 3" long and 1" wide. 3. On one end of the copper tape, solder a 16" length of 20-ga insulated wire. 4. Reach into the guitar sound hole and carefully clean off the bridge plate with your fingertips to remove any stray splinters that may be there. I mention "carefully" because I've seen some guitars (even big-names) that are pretty ragged, and the maple splinters can be like a cactus. Vacuum out all the stuff you knock off. 5. Peel the paper backing from the tape and install it across the six string holes. It doesn't have to be perfectly straight; all that's required is that all six holes are covered. Press the tape firmly into place all around, paying attention to each hole so you can feel a little crater in the tape where each hole is. 6. Reach into the sound hole with a wooden golf tee and poke the point through each crater in the tape. Gently push the tee upwards until it stops. A golf tee works better than a little screwdriver or other pointed tool because it has a taper (like a trumpet), which neatly forms the tape around the inside of each string hole. This step is critical because if you poke through the tape from the outside of the guitar, it pushes the tape away from the plate. 7. Solder the other end of the wire to the pickup system's ground side, and restring the guitar. What this does is create electrical continuity with each string (like a string ground on an electric guitar) and that helps quiet things down.
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Post by reddeluxe on Jan 11, 2020 3:42:39 GMT -5
Thanks for the grounding idea. Peegoo. Going to try a shielded cable, then the copper foil tape trick.
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pdf64
Wholenote
Posts: 553
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Post by pdf64 on Jan 11, 2020 9:48:06 GMT -5
Turning the pot to its middle range increases the signal impedance after the pot, thereby making it more liable to pick up hum, compared to very high or low pot settings. So the entire cable run between pot output and amp input needs to be a high quality, well screened INSTRUMENT (not mic) cable, ie for high impedance signals. The pot body metal shell should be connected to the screen / signal common. Also the bridge / strings, if they’re not already.
The lower the value of the pot, the better, as its middle range output impedance will be likewise lower. What are the pickup specs? Is it active? Link?
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Post by reddeluxe on Jan 13, 2020 20:19:19 GMT -5
Quarternote, thanks for suggestions. Have utilized high quality shielded two conductor (plus sheathed shield) wire to run the signal from the volume pot to output jack. Tried 250K, 500K, and now 1meg ohm audio taper pots. Shielding sheath from original pickup output wire is grounded to pot shell, as well as extended wire from pot to output jack ground, so complete continuity of ground wire plus shield sheath. Still getting electronic buzz in middle range of pot's travel, but not on the extreme of either high or low. Remember, from the factory this pickup was designed for acoustic flat top guitars with no tailpiece, and has no string ground. Totally quiet (stacked humbucker design) in original configuration. So should I try ultra low (25K ohm or lower) resistance pot? Thanks. Thinking of adding Peegoo's copper foil bridge plate grounding trick next. No manufacturer online specs or component values available on line, but physically appears to be a good quality clone of the similar L. R. Baggs design.
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pdf64
Wholenote
Posts: 553
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Post by pdf64 on Jan 14, 2020 4:15:27 GMT -5
Lrbaggs make a bunch of pickups www.lrbaggs.com/acoustic-guitar-pickup-systems to avoid incorrect assumption / wasted time, please could you point to the one that’s relevant? In the absence of any other detail, if you’ve got a meter, please report your pickup’s resistance.
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Post by Peegoo 🏁 on Jan 14, 2020 10:20:37 GMT -5
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pdf64
Wholenote
Posts: 553
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Post by pdf64 on Jan 16, 2020 14:49:07 GMT -5
Thanks Peegoo! And great tip on string grounding for flattop acoustics.
As the pickup isn’t active it probably won’t cope with a low value vol pot, eg 25k. I guess the 2 coils might be wired in parallel in which case it may manage a 100k pot.
Any news on the pickup’s resistance? Have Dearmond been contacted for advice on the matter?
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Post by reddeluxe on Jan 17, 2020 22:50:19 GMT -5
Quarternote, there is no specification info available that I can find regarding this DeArmond pickup, either through the Guild or DeArmond web sources. As stated before, pickup works properly without the added volume pot. FYI, I'm familiar with most standard guitar industry wiring schematics for both electric and acoustic guitar pickups/circuits and I have never experienced this anomaly....first time for everything. Foil shielding tape on the bridge plate is next.
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