I must have saved 10+ schematics for onboard bass preamps. The one I want to try first is a variation on the design of the Music Man Stingray pre. Since I prefer 1 volume and a blend control for 2 pickups, I am trying to work out an input buffer for each preamp, followed by a blend control, then to the preamp. Here's the schematic I have roughed out for buffering the pickups. See any issues? It's home made, and I don't mind flames if it looks hopeless.
Kiddies, don't try this at home (unless it works of course).
John
An externally hosted image should be here but it was not working when we last tested it.
Kiddies, don't try this at home (unless it works of course).
John
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Is 240K a decent load for the P-bass pickup? Typically that's roughly what it would see with a 250K volume pot and no tone control (I think).
Is 120K a decent load for the MM pickup? It's reputed to be a low-ish impedance pickup so I'm guessing it's okay. Especially if wired in parallel. I read on the music-electronics.com forum (pickup winders) that a pickup's output varies with the number of turns (double the turns = double the output), but the impedance varies with the square of the number of turns (double the turns = 4x the output).
The MM buffer is set for 2x gain to make it slightly louder, experimental since the bridge pickup usually is quieter than the neck pickup, at least if they're the same type and specs.
It looks odd for the Vref voltage of the MM buffer to be 2.25V instead of 4.5V, but I read somewhere that the boost of 2:1 affects this part of the circuit. If I'm wrong, please let me know.
I added load resistors to the output of the op amps because they want a decent load to play into under all circumstances. 1-2K are typical of the schematics I have seen. Won't see a dead short to ground and won't see a capacitive load.
The 1uF cap seems small. In my LTSpice sim, however, it provides a bass rolloff starting around 30hZ, which seems appealing to me. What's below 30hZ? Thumps from tapping on the strings while they're damped I suppose.
Is 120K a decent load for the MM pickup? It's reputed to be a low-ish impedance pickup so I'm guessing it's okay. Especially if wired in parallel. I read on the music-electronics.com forum (pickup winders) that a pickup's output varies with the number of turns (double the turns = double the output), but the impedance varies with the square of the number of turns (double the turns = 4x the output).
The MM buffer is set for 2x gain to make it slightly louder, experimental since the bridge pickup usually is quieter than the neck pickup, at least if they're the same type and specs.
It looks odd for the Vref voltage of the MM buffer to be 2.25V instead of 4.5V, but I read somewhere that the boost of 2:1 affects this part of the circuit. If I'm wrong, please let me know.
I added load resistors to the output of the op amps because they want a decent load to play into under all circumstances. 1-2K are typical of the schematics I have seen. Won't see a dead short to ground and won't see a capacitive load.
The 1uF cap seems small. In my LTSpice sim, however, it provides a bass rolloff starting around 30hZ, which seems appealing to me. What's below 30hZ? Thumps from tapping on the strings while they're damped I suppose.
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