Tone circuits

Search on Google for bass preamp schematic

 

Some more details about your background and what you want to do would be useful: Are you just looking for circuits to copy, or do you want to do your own designs? Do you have a basic knowledge of electronics and, in particular, of operational amplifiers? Do you plan to make your own PCBs?

 

Doing your own truly original design, without any experience or training in electronics, isn't really feasible IMHO.

Furthermore, there's not a real need to do an "original" design. There's plenty of really simple, effective onboard preamp designs that can be made on a little breadboard. (PCB fabrication not required) These range from a singe JFET, to a single op-amp, to two-and three JFET designs, with or without variations of tone controls... They're all out there on the 'net.

Would an "original" design improve on these? Probably not, transistor circuits are extremely basic and the existing designs have pretty much optimized the few parameters that can be changed.

I'd recommend finding a simple op-amp based preamp design, and make it first, this will help you learn what you're doing. Then progress to a version with some tone controls, learn how they work. The JFET designs are easy too. Here's a site with a nifty 3 JFET design and tone controls:

http://www.albertkreuzer.com/preamp_onboard.htm

 

One of the advantages of using opamps is that the behaviour of the circuit is almost entirely determined by the external components, you can design your circuit assuming that the opamp is "ideal" and get very good results, especially in a non-demanding application like a bass preamp (no need for high gains, narrow bandwidth, etc). This also means that you can use a circuit designed for, say, a hi-fi amplifier, simply replacing the opamp with one suitable for a bass preamp.

The main factors to consider when selecting an opamp for this application is low power consumption and high input impedance (especially if using passive pickups). Most of the preamps out there use the good old TL062 by Texas Instruments, though I have replaced it with the TL072 in a couple of basses and got slightly better quality at the expense of shorter (but still reasonable) battery life.

Another consideration is single supply vs. dual supply. Many circuits are designed with a dual power supply, with a V+, a V- and a 0 potential ground, while most bass preamps use a single battery. It's not difficult to convert a dual supply circuit to single supply, or you could use two batteries and use the connection point between them as your ground reference point (this is what I would do since it improves the dynamic range and simplifies the circuit).

If this is going to be just a one-off, as nashvillebill said there's plenty of circuits out there, search for "opamp tone circuit" and you will find them. If you plan to develop this into a hobby and want to have some more theoretical background, get a book about operational amplifiers. These are good:

http://www.amazon.com/exec/obidos/t...4/102-6806004-0132904?_encoding=UTF8&v=glance
http://www.amazon.com/exec/obidos/t...bs_b_2_1/102-6806004-0132904?v=glance&s=books

Another excellent source of information is the application notes of the opamp manufacturers. Sometimes even the datasheets of specific models include useful circuit examples.

This application note by Texas Instruments includes a section on single supply operation and a simple two-band tone control circuit: http://focus.ti.com/lit/an/slyt155/slyt155.pdf As you can see the circuit uses a single opamp. The TL062 is a dual opamp, which in this case is handy because you need the other "half" as an input buffer to ensure high input impedance and adjust the overall circuit gain, if required. There's an example in Fig. 7 of the OPA2604 datasheet: http://focus.ti.com/lit/ds/symlink/opa2604.pdf This one includes a 3-band tone circuit: http://focus.ti.com/lit/ds/symlink/opa2227.pdf

Finally, it is always a good idea to simulate your design before building it to make sure it works as expected. Linear Technology offers a free, fully working version of Spice, the most widely used circuit simulation software out there. You can download it here: http://www.linear.com/company/software.jsp

 

Another useful bit of information that seems to be very highly recommended is Duncan's Tone Stack Calculator.

Found here.

I'm not completely sure what sorta setups most onboards use for eq'ing but in the very least its fun to twiddle around with.

Possibly some of the nay-saying comments are related to semantics. You see, without prior experience in electronics, its difficult to come up with a circuit thats truly original, a completely novel idea or application of an idea. And for a large part its not really necessary, you can accomplish just about anything you'll want with what's currently around, its just a matter of taking what you need and applying how you choose.

From memory Kreuzer's onboard basically consists of independent buffers for each pup, a tone circuit and a final gain circuit. From that you can decide how you want to control it, you could fix the final gain stage and setup your first buffer circuits to control volume for each pup there, you could pan after the buffer and control volume at the final gain stage. Go further in depth and you decide how you design your circuits: JFet or Opamp for each stage, how you voice your tone circuit. You could even do your research and design one of the variable frequency, low pass filters like the Alembics use, that's pretty avant-garde as things go.

Josh D

 

Look for Craig Anderton's Electronic Projects for Musicians or Electronics Projects for Guitarists, as well. More than just ciricuits, but not so much information that someone with little experience feels buried in math or theory. I still use the first project I ever built, a preamp/eq design of his, in my stage rig. He lists some great starting points for modding all his stuff, too.

 

Respectfully, if you have the ability to design an 'original' preamp circuit you won't need to work in a PCB factory.