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Homemade Onboard Pre?

Discussion in 'Pickups & Electronics [BG]' started by NicJimBass, Nov 23, 2004.


  1. NicJimBass

    NicJimBass Is this thing on!? Supporting Member

    Nov 22, 2004
    Lancaster, OH
    Endorsing Artist: DR Strings, Source Audio, Hipshot
    Anyone ever made their own onboard pre-amp? I like the sound of mine, but I'd like to learn how to do this... maybe be the next Mike Pope! Schematics and easy to understand directions appreciated!
     
  2. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    I'd love to make an all descrete component preamp - I mean two or three individual solid state components, as opposed to an integrated circuit with twenty or thirty transistors etched on it. I assume all the preamp boards available out there use ICs for everything, but I don't know -- are there descrete-component onboards out there? I think one of the problems with high-end descrete sound as compared with IC is that the highest-performance circuits use a lot of power; I think if I designed one, it might be a battery-sucker, and you'd have to load the thing up with a handfull of AA batteries or somthing to get through a show. Well - it probably wouldn't be that bad.

    Maybe FETs would be more conservative than transistors. FETs actually act alot like tubes in their transfer curves - maybe I could try a class-A FET front end - maybe with a very low amount of feedback for a nice even harmonic series, and get a tube-ish sound.

    I think that active cut/boost tone controls with descrete components could be a real bear to design too. I suppose one could cheat, and design a high-gain - I guess multi-stage - descreet 'main' signal path, and then put op amps in the degenerative feedback circuitry for EQ controls. That way it would be mainly descrete, but the circuit feedback would be processed with op amps... Hm.

    I guess if we went with the FET preamp we could use old tone controls designed for tube circuits. I should maybe look in my old 1952 Radiotron Designer's Handbook. Yeah - or we could copy the tone controls out of a Marshal or Fender head or something (no 'presence' control, I don't think; that comes from the far-side of the output transformer). That might work good if you want even harmonic distortion from the input stage, because these tone circuits really attenuate the signal, so you have to start with pretty much voltage into them. The more gain you try to pull from a class-A FET, the more even harmonics you'll get.

    I used to have a pretty nice home electronics lab, but I put all the stuff away as I started swinging away from engineering interests, and toward art about five years ago. Now all I do is 35mm rangefinder photography and bass.

    Hmm.

    Any engineers out there? I'm a highschool-dropout, ex-electronic hobbyist schnoid.

    Joe
     
  3. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    I have a collection of links to web sites with schematics: http://www.bassviews.org/phpBB2/viewtopic.php?t=232

    Please feel free to post any additional sites that you find. BTW, I am an engineer, but you don't need to be an engineer to build a useable preamp. I have found it fun to build homemade preamps.
     
  4. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    I'm not an engineer... but I play one at home.

    I've been playing with IC based onboard preamps since the early '70s. The chips have come a long way since then. There are a number of op amps available today that can yield low noise, full use of the available supply voltage, and low power drain.

    You'll find it's difficult to design a discrete onboard preamp that has both low noise and low power drain - ESPECIALLY if you use FETs. For FETs, low noise requires high drain current.
     
  5. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    This is very true. Noise is related to resistance and that is something FETs are known for. I would not recommend doing a discrete design as a first project. Op amps actually do a great job if you spend the time to find the ones that offer low noise at the lowest available current draw.
     
  6. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Bob, there was a thread about this very subject a while back...low noise, low current draw and packaging were the main concerns...

    Back in the late '70s, early '80s a friend of mine designed the dB Flamethrowing preamp (dB are his initials) using one FET from Radio Shack.

    I'm including the schematic. It was originally designed for guitar but do with it as you please (the patented part is a private joke).
     
  7. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    That will work, but you don't need the 1uF cap or the 470k resistor at the input.

    I agree with xillion that FETs generally love current to sound their best, and that's where the problem lies; you need the current for sonics and low noise, but increased current means decreased battery life. Discrete preamps can sound excellent, but can be a lot of work to get sounding right. For a first project, I'd suggest opamps.

    Or, you could do like I do; design for best sonics, irrespective of current requirements, and use an external supply to power it. No battery issues then either, 'cos if my external supply has failed, the entire rig will be down.
     
  8. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    I've often thought that to be a good solution. Look at Alembic. That is the approach that they take with the Series I and Series II electronics. The down side is that you then have a custom bass that you can't plug in to another person's rig.
     
  9. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Good sound at any cost is cool, but I prefer something I can plug in at a jam session without having to wheel in my own cart full of gear.

    I've also found I don't use onboard tone controls much. I prefer to do most of the tonal adjustments at the amp.
     
  10. Passinwind

    Passinwind I Know Nothing Supporting Member

    I have one bass set up this way. I could easily wire in a bypass, but it sounds so much better with the onboard preamp that I always just bring the matching preamp/power supply if I want to use someones else's rig. The power supply need be no larger than a stompbox, hardly a huge burden to carry. My old one fit in my bass case no problem.
     
  11. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Here is the other down side. I personally use a wireless system at gigs. I'm sure that there are many others that do as well. If you are running wireless then you don't have external power availalble.
     
  12. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    Not so. I can either take my pre, which is a 1RU design with the correct PSU and connectors built in (+ the ones for my Alembic) or take the small portable PSU which has high and low (std passive) 1/4" RTS jacks so it can be plugged into any standard amp. The 'little' PSU is about the size of 4 decks of cards and weighs maybe 2lbs. It's a bit smaller then the DS5 that Alembic ships with their basses, but has quiet regulators.

    The only disadvantage I see to this setup is if you want to use a UHF to go cordless on stage. However, my pre's have a bypass so you could run from the 1/4' on the front of the bass to your belt mounted UHF. For simplicity, I don't build tone controls into my onboard pre's, so bypassing and going passive doesn't lose any flexibility.
     
  13. viccpa

    viccpa

    Mar 15, 2005
    Hi all! I'm a DIYer and I would to know if there anybody have schematics of Mike Pope onboard preamp?
    Thanks in advance.