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Thread: Pre Amp for Bass Guitar

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    LSemmens
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    Default Pre Amp for Bass Guitar

    I have a bass with two humbucker pickups that I wish to modify. It currently runs a belcat preamp which introduces noise into the circuit whenever I attempt to boost any frequency via any of the three (Low, Mid, High) tone controls. If all are set at zero the signal is silent. I'm thinking that a basic one or two tranny pre amp would probably fill the bill and I'd then wire in tone control post pre-amp. My experience in cct design is about 40 years old, and I've forgotten more than I ever knew! Can someone please share a basic circuit to perform such. A google search tends to show scant detail on what I specifically need. A basic block diagram of what I am hoping to do is as follows


    I'm out of my mind, but feel free to leave a message...



Look Here ->
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    currently runs a belcat preamp
    model?

    introduces noise into the circuit
    define "noise" ....ie; distortion, hiss, hum, other....?

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    Hiss, or, as a bandmate would comment sounds "airy". Belcat ROHS Active EQ Equalizer Preamp Circuit For Bass Guitarl. There might even be some distortion in there too (can't replicate that problem, but it has occurred in the past,) Purchased from evil bay . Given that I don't have much invested in this Preamp and the fact that it is a sealed unit, repair is not an option.
    I'm out of my mind, but feel free to leave a message...

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    No wonder I was having issues finding it -- NOT a Belcat product, cheap chinese knockoff Kmise brand ... aka. junk =)

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    Chinese knock off makes sense. Anyway, I'm looking to build a replacement.
    I'm out of my mind, but feel free to leave a message...

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    Search for 'bass guitar preamp circuit' .....else 'bass guitar preamp kit' ...and if you wanna see how much you could be paying for quality.... 'bass guitar preamp' ..... =)

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    In the over 50 years experience(I started building Guitar amps when I was 15 and synthesisers at 17) with electrical instruments, I have never found satisfactory results using pre-amps in guitars or basses.
    The instant you add an active component (transistor) and apply gain you WILL add hiss.

    If you can't get a good sound, then you have a problem with the instrument, amp or the speaker cabinet. It is very hard to compensate this with a bandaid solution like that Belcat EQ.

    Not getting enough gain from a bass sounds more like a fault somewhere that should be addressed first. I would have a look at your amp(try out your bass on an amp in a music store) but maybe there is something wrong with the wiring in your bass. Have you tried bypassing all the electrics in the instrument and wire a pickup directly to the amplifier?

    I assume you have already tried different cables.

    Also make sure that the pickups are adjusted close enough to the strings.

    If you really still think after doing all the tests and ruled out any faults that you still need some sort of gain in the instrument here is a very good instruction and explanation of the design of a preamp using a single JFET transistor and very few components:


    The less components the less hiss, resistors can also introduce noise. Use only metal film resistors.
    Jaycar has the 2N5484 JFET.

    I have made similar circuits decades ago but for guitars for better distortion and bite(due to lacking of pickup and amp quality). Note the 10nF source capacitor that adds high frequency gain.
    You can play around with that value but I would also add an adjustable resistor in series to regulate that treble boost.
    I would use 47nF instead of 22nF for the input capacitor.

    JFETS are typically used in transistor amps to emulate thermionic valves where a little bit (or a sometimes huge amount) of distortion is a good thing.
    For my bass and guitar amps I ONLY use valve amps and NEVER have issues with low gain, dull sound or hiss in non overdrive settings.

    Pickup -> Preamp -> volume control -> output to amp.
    If you are getting distortion with that little circuit then you DO NOT HAVE A GAIN PROBLEM and don't need a preamp, assuming you made no error building it.
    Last edited by Uncle Fester; 27-01-25 at 11:01 PM.
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    You are correct Fester in that a preamp does introduce something that may not be needed. I have a Passive Washburn Bass and this active bass is a cheap Chinese clone of a Music Man. The original electronics were not even wired correctly. I put the "belcat" preamp in to, at least, get a sound out of it. I'm really just experimenting with active electronics for the tonal variation that they can offer. I might eventually turn this Bass into a Passive beast but, for now I'm just thinking of a basic pre-amp circuit to boost signal a little. I possess two Bass amps and run through FOH at Church the symptoms are the same with all, add any tone adjustment to the signal I introduce low level white noise. All tone controls rolled right off, signal is clean. Volume from the Bass is adequate in all situations.
    I'm out of my mind, but feel free to leave a message...

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    Quote Originally Posted by lsemmens View Post
    You are correct Fester in that a preamp does introduce something that may not be needed. I have a Passive Washburn Bass and this active bass is a cheap Chinese clone of a Music Man. The original electronics were not even wired correctly. I put the "belcat" preamp in to, at least, get a sound out of it. I'm really just experimenting with active electronics for the tonal variation that they can offer. I might eventually turn this Bass into a Passive beast but, for now I'm just thinking of a basic pre-amp circuit to boost signal a little. I possess two Bass amps and run through FOH at Church the symptoms are the same with all, add any tone adjustment to the signal I introduce low level white noise. All tone controls rolled right off, signal is clean. Volume from the Bass is adequate in all situations.
    OK now you have given us more vital information.
    I need you to measure the DC resistance of the pick ups but you must isolate the wires from any circuitry first and report here.
    If there are more than 2 wire strands coming out I need the resistance between each of them.

    If these are indeed low impedance coils you will need an entirely different circuit to make them active, a totally different animal than the circuit above.

    This is what you might need:


    Note that the coils must be separated and fed to the positive and negative inputs of the operational amplifier for noise cancelling.

    For a bass C1 and C2 should be 47nF.
    There is some discussion on that thread about R3. In the data sheet that resistor is also 30kΩ but EMG apparently chose to omit it. Try it out.

    The LM4250 has been discontinued. It is suggested to use the OPA1637 which you can get from Mouser Australia but only in SMD.

    You can try to experiment around with a TL071 (Jaycar) but battery life will be poor and noise higher.
    Last edited by Uncle Fester; 28-01-25 at 10:48 AM.
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    I shall pull the bass apart again and measure them. I might actually remove all of the electronics and attempt to wire it passive anyway.
    I'm out of my mind, but feel free to leave a message...

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    A good article with practical designs.

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    Quote Originally Posted by Reschs View Post


    A good article with practical designs.
    Paywalled, only introduction visible with no useful info.
    Update: A deletion of features that work well and ain't broke but are deemed outdated in order to add things that are up to date and broken.
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    Quote Originally Posted by Uncle Fester View Post
    Paywalled, only introduction visible with no useful info.
    Did not know if you are a reader or not.
    I can PDF from my copy and send if you want.
    Basically it gives you the classic designs of famous guitars and basses.

    Sometimes available through local library.
    Last edited by Reschs; 29-01-25 at 09:06 PM.

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    Quote Originally Posted by Reschs View Post
    Did not know if you are a reader or not.
    I can PDF from my copy and send if you want.
    Basically it gives you the classic designs of famous guitars and basses.

    Sometimes available through local library.
    PM sent, thanks.
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    This is a fantastic reference about the EMG active circuit I posted above and the reason for the omission of R3 to create different resonant peaks:

    I have installed EMG81s in both of my Washburn guitars.

    You can of course do similar tone modelling in a passive setup if you access the centre of the coil wires on your humbucker and solder a capacitor parallel to at least one single coil to achieve two different resonant peaks.
    I often use a 12-way rotary switch instead of a tone pot where I can select 10 different capacitors with increasing value parallel to the pickup.
    The two other end positions are no cap and a 100kΩ resistor that flattens the resonant peak.
    When I rotate the 12way switch swiftly it sounds like wha wha effect.
    This works best using a single coil, which I would also enable on a bass pickup with a switch.
    There is also the option of parallel hum bucker using both coils that sounds almost as bright as a single coil but it still cancels noise.
    You can find on Ebay suitable 500kΩ pots that have a 2way 2pole push pull switch to do all kinds of passive magic with your hum buckers with out having to add annoying ugly extra switches on your pick guard and of course none of this will add any hiss as it all remains passive.
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    Thanks Fester, I am considering going all passive. I just like the concept of having one active bass in my arsenal, however, if it's more trouble than it's worth, I might just go down that route.
    I'm out of my mind, but feel free to leave a message...

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    Thanks to Reschs I had a look at the Silicon chip passive mods.
    Lsemmens, I strongly suggest to ask him to send them to you too, if you have not already.

    Figure 6 shows the above mentioned series/parallel humbucker switching with S1 that you can do with the switches on these pots:


    I recommend to do this at least with the bridge pickup to get the more treble as you mentioned above you have trouble 'adding tone' but have enough bass,
    assuming you can separate the wires carefully on your pickups.

    Simply swapping/reversing the wires on one pickup (no split/wire separation needed) and playing with both pickups gives you the classic out of phase sound: a lot of treble but a lot less bass but if each pickup has it's own volume control you can mix this sound in and maybe find the right tone. Definitely the simplest mod to get more clarity.

    Also you should not directly plug into the FOH mixer pad. Even a 'line in' can flatten your highs and any attempt to compensate with the mixer tone setting will add hiss.
    Always use a line out from your amp if you have one or a mike in front of the amp's speaker.

    Back to Silicon Chip. The fuzzbox circuit V2 D1 D2 is a bit of an oddball. To be honest I have not tried this 'passive' overdrive but I assume you might not like this. At least not for church.
    If you like John Entwistle from the Who then that might be a different story. This might even emulate a fretless, although I would think poorly.

    However the choice of BAT43W is not a good one for this with it's 0.22V forward voltage at 1mA, is too high. You would only get a bit of buzz on the initial attack of the string and the pickups must be of good quality (high output).
    Better to use high power Schottky Barrier diodes. I have the SBR12A45 and they test at 0.088V @1mA and 0.43V @12A !
    That is as close as I have seen to the "Ideal Diode" and should provide a more sustained distortion.
    I might try this out on my Ibanez SDGR some time soon and if I like it I will install one of those switching pots for the ton control so I can switch between the diodes and the tone cap.

    ...and yes I do like John Entwistle's playing style and sound, although I hardly believe it will come close but who knows until I try my 12A Schottkys
    Last edited by Uncle Fester; 04-02-25 at 11:28 PM.
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    Have to put this project on hold for a while as I am currently broke (yes I can afford to live, but that's it) and I have too many other more pressing projects. My other bass is currently my main one for the above reasons. As for DI into FOH we use and have no amps on stage.
    I'm out of my mind, but feel free to leave a message...

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