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How are onboard pre's made?

Discussion in 'Pickups & Electronics [BG]' started by rojo412, Mar 8, 2003.


  1. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    There are so many things that you can achieve with passive EQ options (capacitor value, pot value, phase, tone, etc). But how are active EQ's created? Take the OBP-3... it's a box. You send wires to pots and you control the box. How do they create it? Is it computer-style wizardry or a complex setup of micro-capacitors and resistors and such?

    Is there any info in books or on the web that shows you how to create an active preamp from scratch? Obviously, Mike Pope, The guys from EMG, Mr. Aguilar, Roger Sadowsky, and Bill Bartolini (to name a few) learned somewhere how to do this.
     
  2. geshel

    geshel

    Oct 2, 2001
    Seattle
    The basic idea is just a set of active filters - low, band, high-pass. Using op-amps, a few capacitors and resistors.

    Active filters are based on the same principles, but the fact that you have something that can boost voltage (the op-amp) in the circuit gives you flexibility to design more types of filters (including, of course, bass/etc boost, where passive is cut only).

    Basic electronics should get you there. They're not the simplest of filters as they generally are boost OR cut, but I'm sure there are plenty of texts out there that describe how to build such a circuit, and what to adjust for different frequencies and slopes. The topic is a very full one however, as there are vast numbers of different filter designs that have different qualities as far as phase response, dynamic range, etc, go.

    Dunno if there are any guitar-oriented design books out there.
     
  3. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    A good bit of skill goes into designing a musical sounding preamp that has low noise and low currrent drain. The other part of the design that is probably overlooked by those that haven't built circuits themselves is packaging... It would be really difficult to build your own preamp as small as the ones that you can buy.

    Ever see a photo of the J retro...HOLY COW...John East has a lot of surface mounted components packed in that one.
     
  4. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Well what about starting off a little less advanced. What about building a preamp like a Fodera or Sadowsky, a non-onboard pre? Not as advanced, of course. What does one have to do to learn how to go about doing that?
     
  5. Velkov

    Velkov

    Jan 17, 2001
    Lansdowne, Ontario
    I'm in the process of making my own preamp for my MIM jazz. I'm using JFET transistors and standard capacitors and resistors. I still haven't settled on tone controls that I like. I'm trying to achieve maximum variety of useful tones with a minimum amount of knobs.

    But anyway, you can use transistors or op-amps. I suspect that most commercial preamp builders use opamps since they are cheap, low-noise, and sound decent enough.

    Here's some places to start:
    www.albertkreuzer.com/preamp.htm
    He has built a rackmount jfet preamp, and an onboard version of the same thing. They both use relatively simple circuits.

    http://www.cafewalter.com/cafewalter/projects.htm
    This guy has an opamp rackmount preamp that can easily be turned into an onboard.

    Also, there is plenty of sites for guitar effect schematics. The most popular effects are overdrive/distortion boxes. A distortion box is a preamp that clips.

    Hope this helps.
     
  6. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    That's some crazy stuff... but it definitely helps.
     
  7. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Long post ahead. Folks who aren't interested in electronics should skip it!

    I built my first onboard preamp not quite 30 years ago. I've dabbled in it occasionally ever since.

    It's certainly easy to put together an op amp based preamp, especially with the chips available these days. For those just getting started, look at the ON Semiconductor MC33178 - this is the dual op amp chip Carvin uses, and it's got a near ideal combination of low noise and low power consumption at a very reasonable price. Other dual op amps worth looking at are Analog Devices' OP-262 and OP-284.

    For the basics, see Walter Jung's classic book "Audio IC Op Amp Applications". Ignore most of the data sheets in the book - modern chips are much more attractive for onboard applications.

    Designing a preamp from discrete transistors is definitely harder. It's difficult to balance low power consumption, low noise, and good drive into a highly capacitive load. Low noise and high drive generally mean lots of current, which is a bad idea for a battery powered device.

    Whether you use op amps or discretes, these principles apply:

    - You don't need a lot of voltage gain. It's often sufficient to use a unity-gain buffer or simple follower circuit.

    - The load is primarily capacitative: figure 1000 - 2000 pF is typical, long cables and large pedal boards can be even worse. The resistive load is at worst 5 Kohms if you go straight into a mixer, usually more like 500 K with a proper bass amp. But you need to be able to source and sink a lot of current to drive the cable capacitance.

    - The inductive nature of the pickups makes noise current the most crucial spec for the input device. Pickups have their highest impedance of around 100-250 Kohms at resonance, typically 2 to 5 KHz - which, unfortunately, is in the frequency range where the ear is most sensitive. This is the critical region for reducing noise. Voltage noise is most significant in the bass frequencies, where no one will ever hear it! Beware of using bipolar devices (either op amps or discretes) with a low voltage noise spec but a high current noise. This is where the MC33178 wins: it has modest voltage noise but lower current noise than most bipolar op amps. FETs, of course, have negligible current noise.

    I hope someone finds this information useful.
     
  8. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
  9. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    It appears I need to learn what half of what you just said means:confused:

    I know that Torres Engineering is a company that makes books that are up to date and deal with building amps. Is there similar text that is up to date or do the people who make pres know the old stuff and apply their own theories?

    Also, how could the wiring be done NOT on a circuit board? Are there point-to point-wiring guides? If not, is something available from Radio Shack or some other, higher quality parts source that will work as a circuit board? Most of the stuff I saw looked like it was custom printed circuitboards. Who would be a company to contact about getting all of these parts listed above (the op-amps and such)?
     
  10. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Start with the Jung book I suggested.

    I don't know of any books specifically on the topic of onboard preamps.

    Try using some perforated, unplated circuit board material (known as "perfboard" in the trade). Since most old-style ICs have pins on .1" centers, you want the kind that has the holes spaced on a .1" grid. I highly recommend putting the chips in sockets if you do this!

    I've also used Veroboard, which is perfboard with copper stripes in one direction. I don't know if this stuff is still made; try a web search.

    Digi-Key, Jameco, and Mouser are three sources off the top of my head. These companies are happy to deal with small orders.

    The Analog Devices parts are going to be harder to find. Try Newark Electronics. They're more of a wholesaler to the industry.
     
  11. Here's another book you might want to look at. It's written with experimenters in mind, not so much engineering professionals. It's called the "Active Filter Cookbook" by Don Lancaster.

    Chucko58 - great post, I saved it on my computer for future reference. One op amp I designed with is the LMV358 from National Semiconductor. Very cheap. I don't know how the specs compare with On Semi.

    Stay Low,

    Basstrader
     
  12. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Great suggestion! Lancaster's books are excellent practical guides.

    I wouldn't use the LMV358, and here's why:

    - Maximum supply voltage is 5.5V - a 9 V battery will fry it!

    - Its input common range spec doesn't reach the power supply voltages. This, combined with the low max supply voltage, makes for limited headroom.

    - It has a much higher voltage noise spec than the chips I suggested.

    - It's kind of slow. Not prohibitively slow, just kind of slow.

    On the plus side, the LMV358 uses very little power, the current noise spec is reasonable, and it's cheap. But those advantages don't offset the major disadvantages.
     
  13. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Where might I procure this Active Filter Cookbook? Does it also have "recipes" for various other things in it, like effects, DIs and tube preamps?

    Let's say I were to plan on making a relatively simple box... volume control, bass, and treble. What does it cost to get the needed parts? How rapidly does cost on something rise, let's say if I wanted to add mid control, a gain function, a boost, or many other options? Does the book start you off with a simple device as a starting block or does it school you in what you need to know to start or both?
     
  14. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    You can try to get a copy through Amazon.com, but they didn't have any stock when I checked.

    The book describes circuits that can be useful for tone controls, equalizers, wah pedals, and other related signal processors.

    You're looking at US$20-30 in parts to start with, add another $7-10 every time you add a knob.

    Lancaster's book only covers filter circuits. Try Jung's book for the basics of using op amps for audio.
     
  15. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Will the second edition of the Jung book work?

    Also, as you can tell, I am a hobbyist in this field (electronic tinkering). If I got these books, what I would want to do is build a simple preamp and see if it works. The parts would most likely be mounted to something and it would remain an open unit (parts mounted to a piece of sheet metal or plastic) as I experiment with it... so basically, I would assemble the "active circuit", plug in a bass, then send out to an amp, see if it works. If I was successful with a simple device, I would then move on to advanced features. Do these books cater to that style of experimentation? Is what I said possible to do?
     
  16. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Yeah, Jung's second edition is fine. That's what I have.

    Your approach seems like a fine idea. That's how a lot of circuits are developed in real engineering labs.

    These books aren't really set up in a "first build this, then build that" fashion. Instead, they present common building blocks that can be combined, and occasionally complete solutions for certain small problems.

    One really nice feature of op amp based circuits is that they can be combined simply, like building blocks. They rarely have significant interactions between the output of one stage and the input of the next.
     
  17. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Okay... so they tell you basically "This is how to make a control for mid frequencies" for example? Or is it something more difficult? Does it lead up to steps like that?

    Also, there are books I was recommended to buy for amps, Torres Engineering's "Inside Tube Amps" and "How to service your own tube amp" by someone else (I forgot). Would these be in the same vein as they have sections regarding preamps, would these just confuse things, or would getting all 4 books give me enough info to start doing dangerous tone work?
     
  18. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    I got Lancaster's Active Filter cookbook... I may be getting the Jung IC-Op amp cookbook as well. I need to read further I'm sure, but is there a book that lays it down simply? Or even something that says "order these parts, then assemble them like this to do _____"? I'm trying hard to get into the Lancaster book, but it is very technical as opposed to practical (so far).

    I'm a guy who learns best by doing.
     
  19. Rojo,

    For practice's sake, you might want to check out an old Craig Anderton paperback book, "Electronic Projects for Musicians," which originally came out some time in the 80s. It has a variety of old style analogue effects you can build, including a simple preamp, which I put together without too much trouble. It has no tone controls, but it works great as a buffered signal booster (helps passive high Z pickups drive a long cable). It's very similar to something like an MXR MicroAmp. If you get hip to this, there's probably a more modern chip you could use to add various filtering to it for tone control, and the book has a separate project for that, but it doesn't integrate the two.

    Just thought you'd like to know.

    Whacker
     
  20. rojo412

    rojo412 MARK IT ZERO! Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Thanks. That's exactly what I want. I just sold my Active Filter cookbook because I couldn't understand it. What I wanted was like... a cookbook:
    1 of these, 10 of these, 6 of these...
    Take this and solder it to that...