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Post by atherisinnovations on Feb 8, 2024 18:39:03 GMT -5
Hi all,
Long-time listener, first-time caller here, I was wondering if there could be some discussion/explanation on the proper pot and cap values to use for a passive bass-cut pot within an active circuit (like 9v EMGs or SD Blackouts). I am pretty familiar with wiring and using the passive PTB circuit, and, from reading the pinned PTB thread, it was mentioned that it is not ideal to use a volume pot with a value less than A250k, as the downstream (volume) pot value affects the magnitude of bass-cut, so I am not quite sure how to work out what pot values to use with active circuits. I am sure most of you know this already, EMG and Seymour Duncan both recommend using A25k volume pots for their active pickup systems (the preamps are integrated in the pickups themselves), though SD used to ship out A100k pots with their earlier sets.
Stepping back to view the larger picture, I don't know enough of the 'why' to know if the interactions present in the 'normal' passive high impedance PTB circuit would still exhibit the same behavior/interactions/tendencies regarding rule-of-thumb values for pots and caps in a low impedance active circuit. I would very much appreciate some insight (or to be pointed to another thread or article that could explain it) on this facet of the topic.
I also wanted to mention, I am in awe of the expertise shown by the outstanding members of this board, thank you for putting your wisdom within the grasp of those who would seek it out! I would appreciate any guidance you may have..
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Post by reTrEaD on Feb 9, 2024 0:09:14 GMT -5
Hello atherisinnovations and welcome to GN2. Stepping back to view the larger picture, I don't know enough of the 'why' to know if the interactions present in the 'normal' passive high impedance PTB circuit would still exhibit the same behavior/interactions/tendencies regarding rule-of-thumb values for pots and caps in a low impedance active circuit. I would very much appreciate some insight (or to be pointed to another thread or article that could explain it) on this facet of the topic. Regarding the value of the (treble cut) tone pot and tone cap, use the values recommended by the manufacturer. That's dependent on the output impedance of the active pickups. For EMG, if I recall correctly, their recommendation is 25k and 0.1 µF. However, the value of the (bass cut) pot and cap should be scaled from the G&L values, according to the value of the volume pot you use. For instance, if the value of your volume pot is 1/10 the value of the volume pot in the G&L circuit, divide the G&L bass cut pot value by 10 to get the value of the bass pot for use with your EMG and multiply the G&L bass cut cap value by 10. This will give you the same amount of bass cut, and the cut will start at the same rolloff frequency.  If you're using a 25k volume pot, your bass cut cap should be 0.022 µF and the bass cut pot should be 100k, C taper. If you can't source a C taper pot, you can use an audio taper pot but instead of using the wiper and CW lug, use the wiper and CCW lug. Your maximum bass will be when the pot is fully CCW, but you'll have good control of the sweep from minimum to maximum. |
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Post by atherisinnovations on Feb 17, 2024 16:16:10 GMT -5
reTrEaD: Thank you very much for this schematic! I ended up using values close to those the formula would suggest. This is all in a 2008 Schecter Hellraiser C-1 FR with an EMG 81TW/89R active pickup set: - A25k master volume (push-pull) with treble bleed ('cap only TB', 221 (220pF)); - A25k treble-cut tone (push-pull), 393 (39000pF); - A250k bass-cut tone, reverse wired so '10' is max treble cut, 272 (2700pF); - 18v battery system. The signal runs from: - the 3-way selector switch to the treble-cut tone; - parallel sent (at that pot's input, 'lug 1') to the bass-cut tone 'lug 1' (lugs 1 and 2 are jumpered); - out from 'lug 3' to the master volume 'lug 3'; - and finally sent out to the output jack via 'lug 2' (with treble-bleed of course across lugs 2 and 3). I am listing this out because I use custom quick-connect components and the routing isn't readily visible, and also because the 'PTB' may have a distinct wiring path but it is not the only way to wire a bass-cut into a guitar. This way any ambiguity can be avoided for those that don't know yet (as I didn't know 10 years ago) what assumptions to make.. I am very happy with this setup, this guitar can cover a lot of ground now! I will post some pics if I can figure it out..
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Post by atherisinnovations on Feb 17, 2024 16:59:38 GMT -5
reTrEaD: Thank you very much for this schematic! I ended up using values close to those the formula would suggest. This is all in a 2008 Schecter Hellraiser C-1 FR with an EMG 81TW/89R active pickup set: - A25k master volume (push-pull) with treble bleed ('cap only TB', 221 (220pF)); - A25k treble-cut tone (push-pull), 393 (39000pF); - A250k bass-cut tone, reverse wired so '10' is max treble cut, 272 (2700pF); - 18v battery system. The signal runs from: - the 3-way selector switch to the treble-cut tone; - parallel sent (at that pot's input, 'lug 1') to the bass-cut tone 'lug 1' (lugs 1 and 2 are jumpered); - out from 'lug 3' to the master volume 'lug 3'; - and finally sent out to the output jack via 'lug 2' (with treble-bleed of course across lugs 2 and 3). I am listing this out because I use custom quick-connect components and the routing isn't readily visible, and also because the 'PTB' may have a distinct wiring path but it is not the only way to wire a bass-cut into a guitar. This way any ambiguity can be avoided for those that don't know yet (as I didn't know 10 years ago) what assumptions to make.. I am very happy with this setup, this guitar can cover a lot of ground now! I will post some pics if I can figure it out.. 2008 Schecter Hellraiser C-1FR Hellraiser PTB; EMG 81tw/89r; AtherisPickguards.com wiring; Notes, pots' and caps' values  Hellraiser PTB; EMG 81tw/89r; AtherisPickguards.com wiring; Angled pic  Hellraiser PTB; EMG 81tw/89r; AtherisPickguards.com wiring; Overhead pic  |
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Post by reTrEaD on Feb 18, 2024 3:07:01 GMT -5
- A25k master volume (push-pull) with treble bleed ('cap only TB', 221 (220pF)); A treble-bleed network isn't very important with a 25k volume pot. The cable capacitance is pretty low in comparison. 220pF is small enough that it won't present a problem with over-compensating, so I don't think that will cause any problems. |
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Post by atherisinnovations on Feb 19, 2024 15:21:28 GMT -5
- A25k master volume (push-pull) with treble bleed ('cap only TB', 221 (220pF)); A treble-bleed network isn't very important with a 25k volume pot. The cable capacitance is pretty low in comparison. 220pF is small enough that it won't present a problem with over-compensating, so I don't think that will cause any problems. I feel like I can notice it, but there is the distinct possibility that is just my confirmation bias speaking. FWIW, the guitar is running into an Avid Eleven rack, with the TrueZ set to 1Meg, via a 10-foot Fulltone Gold Standard instrument cable, so, not a typical amp stack, and without other pedals or cabling in front. As far as the PTB goes, the guitar can definitely get a bit crispy as it is now, with the values I am using, but I do like it that way. And the pots allow a great deal of tonal range to dial it back to wooly if needed.. |
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