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DIY On-board Preamp

Discussion in 'Pickups & Electronics [BG]' started by John the Bassist, Jul 27, 2020.


  1. Greetings,

    Please excuse the very long post. I am a long-time lurker and first-time poster on TalkBass, and have found the forum immensely helpful (and quite time-consuming) over the past few years. I'm currently halfway through my third year of electronic engineering studies (our academic year begins in January/February).
    I have a Warwick Rockbass Corvette Basic five-string (with active MEC jazz-style pickups) that I have recently re-finished in TruOil (a preference over the original opaque, black finish). The bass has unfortunately not seen much use over the past few years due to faulty electronics, which I didn't much like from the start (it came stock with a Chinese pre, not an MEC one). I do eventually hope to replace the pickups with passive Delanos or Aguilars, but that is rather unrealistic at this stage as our South African currency is a bit weak, local range is limited and so is my budget. For the time being, I've decided to attempt to design and put together a circuit to compliment the MEC pickups myself using predominantly components that I already own. Apart from simply not being able to afford importing elecronics at this moment, I see this an an interesting project and look forward to learning what I may from it. I certainly have much to gain asking here, and would value any input from those with much more experience than myself (even if it doesn't address every question) :)

    I generally make little use of on-board preamps, and more often use the pickup pan an coil-tapping (where it is available). If an on-board pre allows it, I do often like boosting the mids a little (particularly around 800Hz). I therefore hope to include an active pick-up blend, a mid-band control, and passive tone control or an equivalent active low-pass filter (to tame the bright character of the pickups). This is by no means a conventional on-board preamp, but I feel that it suits my preferences fairly well without being too complex and hopefully minimizing current draw. When I am able to purchase a nice set of passive jazz pickups, I will likely also change the on-board circuit (or at the very least adjust it to accommodate the higher pickup impedance of passive pickups). For now, what do I have to lose but time? I have three main questions:
    1) What are the impedance and maximum output voltage of the MEC 'gold-lettered' active jazz pickups. I have tried to source this information, but the best that I could find (from 'mec-pickups.de') is that they are black in colour, are active and use ceramic magnets.
    2) I have several MCP6022 dual-opamp ICs from Micaochip. These are rail-to-rail, wide-bandwidth opamps with low noise. I have been impressed with the noise performance of these opamps relative to others in previous projects and would therefore be willing to give them a shot if possible, but their other specs may be a bit over-kill for audio applications (e.g. a 10MHz bandwidth).
    2.1) Their maximum power supply range is 5.5V and the absolute maximum value is 7.0V. I would therefore need to attenuate the 9V supply of the battery if I use them. Would this be a problem if the pickups are supplied with the full 9V? I would still ensure that the supply ranges for each fload around the same central point (i.e. the opamp supply would range from 1.75V to 7.25V, or as close as practically possible). Would it just be best to find a more suitable opamp?
    2.2) Would the limited maximum power output allowed by this supply voltage be sufficient for an instrument output? Again, should I simply find a better opamp?

    Under normal circumstances, I would have free access to an oscilloscope in one of our campus laboratories, but campus is now inaccessible due to the 'rona. So, the best that I can do is multimeter measurements and LTspice simulations. I have imported non-ideal spice models of the MCP6022 opamps, which I obtained from Microchip's website and could do so for any more suitable opamp model. I will send a schematic of the full envisioned circuit as soon as possible.

    Cheers,
    John
     
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    Feb 28, 2021

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