another pre-amp question

Discussion in 'Amps, Mics & Pickups [DB]' started by oliebrice, May 27, 2003.

  1. oliebrice


    Apr 7, 2003
    London, UK
    has anyone ever tried a normal microphone pre-amp instead of a specialised db one?
    I can buy a microphone preamp for about £20 from my local Maplin (general electronic stuff shop), whereas a K&K or similar looks like costing about £90.
    What would I be missing?
  2. lermgalieu

    lermgalieu Supporting Member

    Apr 27, 2000
    Palo Alto, CA
    I believe it comes down to input impedance rating. You can find more on this in some other thread we have already had. Basically, aside from EQ capabilities, a preamp just takes a very low level signal and makes it into something the amp is expecting, and a mic preamp does the same thing. So if the input sensitivity is comparable to a good instrument preamp, this might work just fine.
  3. mje


    Aug 1, 2002
    Southeast Michigan
    You had it right on impedance. A preamp does one more thing in addition to amplifying the signal: It matches the impedances. A microphone preamp has an input impedance of 50 ohms and expects a balanced input: +/- signal plus ground. For piezos you want something in the MEGOHM range- preferably 5-10 megohms- and unbalanced input: signal plus ground.

    Using a microphone preamp will probably result in a very thin sound.
  4. rdalpra


    Mar 2, 2003
    Reading UK
    Some of the normal rules for impedance matching don't seem to apply for piezo pickups. Normally you try and match the output impedance of the source with the input impedance of the amp to get the best signal transfer.
    But the strange thing about piezo is that the the output impedance is very frequency dependent. I believe that is the reason why feeding it into a lo impedance amp sounds so dull, but the suggested 10M input impedance is so much higher than the output impedance at any usable frequency you get a better sound.
    A trick I tried to see what difference the input impedance would make was to solder a 1M resistor in the centre lead. This looses a lot of the signal so you need to turn up the gain/volume to compensate and is not suitable to a performance environment as the signal to noise is poor.

    I liked the tone and then bought a preamp which gave me the tone and low noise. I also liked the phase reversal and the ability to have control even when I was remote from the amp.

    By the way if a 1M resistor does not make much difference to tone or volume, then you probably have a high input impedance amp anyway.
    But any soldering etc is at your own risk!

    Regards from a first time poster
  5. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    I've not used a mic preamp instead of a regular K&K-style preamp, but I have used on in addition to a K&K. It gave me a lot of tone colors to play with, along with the ability to dial in a very aggressive sound if I wanted it. As it turned out, I like the sound of it for BG a lot but not so much for DB. This being the case, I ended up dropping it from the gear I carry. But your ears will always be the best judge of these things - maybe you could borrow one to try out? Good luck.