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My passive preamp finished!

Discussion in 'DIY Projects' started by DIYer, Nov 19, 2010.

  1. DIYer

    DIYer Well-Known Member Famous

    As mentioned about a year ago (on the other forum) that it will be a long process and it was a long process. It's done at last.

    Here's chassis with holes cut out (what a pain in the a$$ to do!).

    Here's front view with the parts mounted.

    The rear (need to buy a set of RCA plugs to fill the remaining holes).

    With cover on.

    Mounted on the rack and power on.

    Prototype enclosure I built a year ago with scraps.
  2. Towen7

    Towen7 Well-Known Member Staff Member Moderator

    Nice job...

    How does it sound? Would you consider it an upgrade over what you were using before?
  3. Botch

    Botch I.Y.A.A.Y.A.S! Superstar

    Cool! I wish I understood electronics well enough to try something like this.

    You call it a "passive" preamp, yet it has a power supply. Educate me...?
  4. soundhound

    soundhound Well-Known Member

    Looks great. I am considering buying the same remote volume control. The volume control unit comes with a relay bank to do the input switching; did you use this?
  5. soundhound

    soundhound Well-Known Member

    Passive preamps shouldn't have any sound of their own since they are essentially just a passive volume control and some input switching. In practice though, since the impedances are pretty high, things like cable capacitance can alter frequency response if the external cabling is too long from the output of the preamp to the power amp. This particular volume control is 100,000 ohms which is moderately high. Passive preamps basically translate the output impedance of the source component which is usually a few ohms up to maybe 500 ohms, to the impedance of the volume control, with the worst case position being roughly mid-rotation where the output impedance would be about 50,000 ohms (since this is a log taper potentiometer, mid-rotation resistance is not at the physical mid-rotation position). At full volume, the output impedance of the preamp is the same as the source component.
  6. DIYer

    DIYer Well-Known Member Famous


    Yeah, what soundhound said. :handgestures-thumbup:
    The power supply is for the remote control (infrared system and motor) and the indicator light. I don't use its relay feature. I use manual selector which is the dial on the right. I made one input to go through volume pot for stereo listening and the others are just bypass.
    It sounds very neutral which is what I wanted. The whole reason I built this was to have as little circuit in the signal chain as possible, at least for stereo listening.
  7. malsackj

    malsackj Well-Known Member

    Ok where is your brand name, label and such, is the safety label hazard not to be opened by consumers?

    Looks good, great job!
  8. DIYer

    DIYer Well-Known Member Famous

    Oops, those things totally escaped me. :doh: I guess I can't sell it. :cry:

    Thanks though.

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