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DIY preamp

Discussion in 'Amps and Cabs [BG]' started by Bigwan, Apr 14, 2004.

  1. Bigwan


    Feb 22, 2002
    Ballymena (hey)
    Does anybody have a schematic for a DIY rackmount preamp? Valve or solid state - have the cafe walter one already. Or does anyone have an Ampeg SVP or Alembic f1x preamp schematic they could pass to me.

    This is just for personal use - not going to be ripping anybody off here. Just looking for an interesting project!


  2. Check out this DIY tube bass preamp kit from London Power. I've considered getting one to build as a fun project. I don't really know how this thing will sound, but the few reviews of LP's other stuff on Harmony Central have been good.

    JimEOT likes this.
  3. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    I would think that the SVP would be pretty messy to build.

    You can do a google search for bass preamp schematics... I did this once and turned up an FET bass preamp, the Alembic F2B schematic and probably some others...

    The Alembic preamps are basically Fender designs. The F1-X uses an opamp or two at the output stage to adjust the gain for mating with a modern power amp.

    You could search out a Bassman 100 schematic, lift the circuit for the Bass Preamp and do the same thing.

    I trust that you're wanting to do this for the fun of it, not necessarily in an attempt to spend less... If the later is your reason, beware of false economy... you could spend a good buck on parts, tubes, a case and tons of your time only to end up with a noisy, poorly operating mess...
  4. [SD]


    Feb 24, 2004
    hmm i guess on the ampeg web page there are some pdf files with the circuitry diagrams of the SVP's and all thier amps
  5. rwagne2


    Aug 2, 2001
    Here is a link to the alembic preamp (we have copyright permission only to link)

    <a href="http://www.geofex.com/FX_images/alembpre.gif">http://www.geofex.com/FX_images/alembpre.gif</a>

    Some caveats with building a tube preamp:

    For 'real' tube drive, you have to run the tubes with a relatively high plate voltage. This means building a high voltage transformered power supply to power the tube(s) in the preamp.

    This is the most difficult and dangerous part about building your own amplifier, and why most people don't like to build only tube preamps - its only a little more expensive to build a power suppy that can handle a preamp and a poweramp, and just as dangerous.

    The most important part of the circuit, bar none, are the filter caps in the power supply and the power transformer. If you don't use a good power transformer, you will wonder why it sounds like cardboard.

    Building the actual preamp part is relatively easy, since alot of the components can be soldered point to point mounted onto the control pots and the tube sockets.

    It would be tons easier and safer and probably sound just as good or better to just build a cleanly designed mosfet pre-amp. But if you just gotta have the tube sound, please be very careful building the power supply.

    Here is a link to a guy in cananda who sells kits (or prebuilt) acoustic 360 clones.

    <a href="http://www.lynx.bc.ca/~jc/vivaAnalog360.html">http://www.lynx.bc.ca/~jc/vivaAnalog360.html</a>
  6. Bigwan


    Feb 22, 2002
    Ballymena (hey)
    This project would be for my own interest - I'm not out to save money. It's a learning exercise.

    Cheers for all the help so far guys...

  7. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
  8. Bigwan, check out a book called "Tube amps" (or something like that) by a guy named Aspen Pittman. It has a zillion schematics of classic amps in it, mostly tube. I built a preamp from an Ampeg B25 schematic. It was very basic and gives that 60's thump, but it was a fun project. Also check out Audio Xpress for a bunch of books and general DIY stuff. They are mostly a HIFI place but their stuff includes tube and SS design info.

    Cheers, BT
  9. Bigwan


    Feb 22, 2002
    Ballymena (hey)
    BUMP again! :D
  10. GooseYArd

    GooseYArd Guest

    May 15, 2003
    This is a little off-topic but while some of you DIY guys are around, have any of you looked into building lightweight power amplifiers? I would really like to have a small, light amplifier around 200 watts or so. I've had my eye on one of the little stewart half rack amps that uses a switching PS, but I've also wanted to take a stab at building a little amp for a while. Building such a power supply is probably out of the question but if there are any designs that use existing modules, I'd like to take a look.

    Most of the kits I see in this power range come sans power supply, and once you add the transformer the weight goes way up.
  11. rwagne2


    Aug 2, 2001
    Here is the best looking 'kit' type power amp i could find with a bit of googling:


    his pcb looks really nicely laid out. layout is mucho important fo r good heat dissipation.

    remember though, that the heatsink and power transformer make up the bulk of the weight, like you've noticed. The thunderfunk amp gets by the heatsink weight issue by using an aluminium chassis as the heatsink (like those apple macintosh cubes) but a good transformer, like a toroid, is still weighty as you've remarked.

    I think GK had tried a switching power supply in one of their old amps - but it would always burn up and fail. they started retrofitting a normal transformer power supply into their returns for repair, IIRC. A switching power supply isn't a good idea in my opinion.

    good luck, whatever you do decide.
  12. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Do a google search for either ILP power amp modules or Plitron power amp modules. You still need a power supply though.

    Nothing wrong with switchers...you just need to have a good design. Plenty of pro quality power amps (like my QSC PLX-2402) use them with excellent, reliable results.
  13. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    Plitron is the only company I know that can make a toroidal output transformer, and for that reason I highly suspect that you'll find them in the 400w Aguilar tube amp. I doubt it's an off the shelf transformer though. Toroids are half the weight of "EI" transformers. But I looked at using one in my amp, and it didn't work out that way. The same weight and size at twice the price. Never prototyped it. Still in the future.

    For a preamp only, if you do opamps, you only need <1 amp of current @ +/-15 volts DC. A small transformer. If you want to do tubes, I'd look at Toroid of Maryland for their small, off the shelf toroid. They also have a ~500 volt toroid (with 5v rectifier and 6.3 filament) that will do almost 100 watts.


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