Tech guys - Attenuation of a Piezo

Discussion in 'Amps and Cabs [BG]' started by Petebass, Apr 20, 2004.

  1. Petebass


    Dec 22, 2002
    QLD Australia
    A friend of mine has a small, cheap vocal PA which sounds too bright. I heard it at a gig and to me the horn needs attenuation. I suggested an L-Pad but being a "technically challenged", she doesn't want to know about it being "adjustable". So I said I'd install a couple of resistors in a proper attenuation cell that wouldn't change the overall resistance of the speaker.

    When I opened the speakers up, things got interesting. The horn is a Piezo with no crossover in sight. The only electronics inside the cab is a single 10 Ohm resistor which I presume is to attenuate the horn. It's wired in series.


    * Wouldn't that change the overall impedance of the speaker? If so, why hasn't she fried an amp?
    * Calculating resistor values seems to be dependant on the horn's nominal impedance. How do I calculate resistor values for a piezo which has no nominal impedance? Actually while I think about it, how would you calculate crossover values?
  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Hey Pete,

    Piezos are capacitive in nature. As the frequency drops, its impedance goes up (which is why they don't require a crossover). They're also very efficient devices and may present a higher than normal impedance anyway...I'm not sure.

    I presume that the resistor is only in series with the tweeter and not the woofer. The woofer's impedance being much lower will probably dominate the effective impedance that the amp sees anyway.

    I would simply try increasing the value of the resistor. Perhaps during the research phase you could make a series chain of resistors so that you could try different taps until you found the one that seemed "just right".

    Keep in mind that the room that you heard it in will affect how bright it was. If she plays a room with lots of people, acoustically dead surfaces, you might find the cabinets too dull.

    Can she just turn down the treble on the PA amp?

    I looked through literature that I have. In his book about building musical instrument enclosures, Bill Fitzmaurice uses a 4 ohm resistor in series with pairs of piezos.

    The Parts Express catalog recommends placing a 20 ohm, 20 watt resistor in series with a piezo tweeter. They say that it increases stability and reduces the chance of burnout.

    Seems like an inexact science and the best way to find a solution is to play around with resistor values until you find the right one.
  3. Petebass


    Dec 22, 2002
    QLD Australia
    Yep, you assume correctly.

    Trust me Billy, they're bright :) They sounded bright at the gig, and bright again at my place. I even ran them side by side with whatever speakers I had at hand, bass, PA, and Hi-Fi and just for reference. Didn't matter wether I was playing recorded music, Bass , or mu acoustic guitar, the spitty zing sounds dominated.

    EQing proved fruitless. Besides I like to use EQ to fine tune sounds, not fix inherant problems.

    That's easy then. All I need is a second 10w, 10 ohm resistor like the one that's already there, hook it up in series and see how it sounds....... then experiment from there.

    Thanks Billy. You're a ledgend!
  4. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    What I would do is put an L pad in between the 10-ohm resistor and the piezo horn. It'll save on a lot of repeated cycles of disassembly, inserting a different resistor, reassembly, and listening.

    Once you have it set the way you think sounds good enough, then your friend then doesn't actually have to adjust it.

    Many manufacturers who use piezos are too cheap or lazy to put a crossover network in (or they figure the end users are too cheap to pay a little more for a system with a crossover). As Billy said, piezos don't need crossovers, but they can often benefit from them. Sometimes the main problem with a piezo is too much overlap with the low frequency driver, resulting in a lot of high midrange honk, even if the highs are reasonably balanced with the lows.
  5. Petebass


    Dec 22, 2002
    QLD Australia
    Ya know what, I think that's what's going to end up happening - the L-pad I mean. I had a play with it this afternoon and boy that's one wierd-ass piezo. Adding resistors didn't change the output of the horn. It stayed as loud as ever not matter what I tried. I even tried bypassing the existing 10 Ohm resistor and guess what? That's right, it didn't change anything. That resistor was only there for show. I grabed a piezo horn I had floating around, and this one responded to the changing resistor loads. Any ideas what might be going on here?

    And Bob you're right about the midrange honk where the 10" speaker and Horn overlap. I had to cut about 12dB of 4K to make recorded music sound decent. A crossover is also on the agenda, maybe 12dB/octave at 2k, and I have enough caps and coils lying around to whip that up myself.

    I'm gonna get these things sounding good is it kills me :)
  6. Petebass


    Dec 22, 2002
    QLD Australia

    The L-pad worked. I might set it and then anchor it inside the cab somewhere where she can't get at it.

    I also whipped up a 12dB crossover at roughly 2k (6.8uF caps and 8.2mH coils). It didn't do much to the sound of the horn but it's no longer recieving low frequencies which can't hurt. But it made a huge difference to the 10" woofer. These speakers are actually starting to sound musical. They don't produce much bottom end though, so just for kicks I ran them alongside my JBL 15 and together they sounded pretty darn good. I then ran the "hot-rodded" speaker in an A/B comparison with the unaltered speaker, and even my singer friend commented on the improvement. YAY!

    Next step, re-adjust the porting. I ran the woofers T/s parameters througn Winisd Pro. The cab size seems OK, but it's current cab tuning seems too low. I guess that's designed to protect from over-excursion. But with proper porting, and a High pass filter to relieve the woofer of those "too low" frequencies these things should sound better again.

    Thanks for your input guys!