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Acoustic box DB preamp

Discussion in 'Amps, Mics & Pickups [DB]' started by massi, Feb 5, 2013.

  1. massi


    Jan 17, 2013
    Hi guys,
    I have designed this high end preamp specific for DB to simultaneously enhance the definition of a piezo/passive transducer and the body frequency capture of an active mic.
    It’s the ideal choice for double bass musicians looking for character and definition, particularly when performing live with drummers.

    You can find some extra information in this forum

    I am here just in case you have some technical questions that I might be able to answer,

  2. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Due to the site's Commercial User Policy, commercial users are not allowed to start threads about their own products. Normally, all such threads are summarily deleted as spam. However, in this case for a brand new user of the forum, I'm going to skip this process, and only enforce the policy from here forward (so for practical purposes, pretend someone else started the thread). Please read the policy and understand that any deviation from it will result in post reports from other commercial users.

    Back on topic, it looks like an interesting box. If we can keep the comments to a discussion of the box rather than a promotion of it, the thread will be allowed.
  3. massi


    Jan 17, 2013
    Thanks Chris. I do understand the spam policy.
    That is why I avoid any commercial information including link to my web site.

    My whole presence here is to answer technical questions.

    I spent quite some time with different professional artists to tackle the DB problems in live performances.
    Hopefully I can provide some added information regarding mic piezo and correct equalization to the knowledge of the forum members.

    Thanks for allowing it.
  4. Is there a high-pass filter ?
    If so, what's its slope ? Frequency ? Adjustable ?

  5. massi


    Jan 17, 2013
    Hi François.
    I guess you are referring to the use of the high pass filter for removing proximity effect of the mic.

    We do add on request a knob to adjust the frequency cut off from 20Hz (no cut) to 200Hz. So when the knob is not installed the filter allows the whole range
    The filter is one pole high pass filter 20 dB/decade.

    We also experience that with the parametric eq. tuned around 100-120 Hz with reduction of 6-8 dB we can obtain a very good result and still allow the body sound (41Hz) to go through.
  6. Thanks.
    My pickup is extremely sensitive and provide infrasonics so a sharp-edged low cut is most welcome.
    Sending you a PM.
  7. massi


    Jan 17, 2013
    Hi François,
    most of the professionals I work with use the DPA4099 and the piezo Full Circle.
    Some of them combines DPA with the DYN-B.
    Others replace the DPA with the ATM-350.

    Which pickup are you using with infrasonic response ?
  8. Wow, if only they made preamps like this for EB.
  9. Look at my profile.
  10. massi


    Jan 17, 2013
    @ eukatheude
    Hi the CH1 works for any passive pickups. It does work well with the attack of an EB.
  11. massi


    Jan 17, 2013
    Hi guys few technical information regarding the Abox.

    Ch1 can be used either for piezo or magnetic coil. The wide dynamic and the high impedance allows both.
    The parametric eq on Ch1 is mainly used to remove the metallic sound of piezo or coil. For piezo the nasty frequency is around 2.4KHz. By reducing this freq. of 6-8 dB you obtain a more natural sound

    Ch2 can power up to 48V mics.
    Most used mics: DPA, AT350, C411 AKG.

    Tone controls are designed to be effective for DB.
  12. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004

    Actually, the benefit of a high-pass filter is to remove infrasonics that can be passed by many piezo pickups. Those can rob precious amplifier power and result in unwanted cone excursion. The typical design is a two or three pole (12 or 18 dB/oct, respectively) attenuation skirt. The most effective cutoff for this purpose is around 30 Hz or so. Higher corner frequencies can be desirable for reducing "boominess" which is why many are adjustable. By the way, a single pole filter will not yield 20 dB/decade unless you are using some very strange filter design. Even elliptic or Chebyshev will not produce that.

    What are the filter Qs of the parametric sections? What are the center frequencies and widths of the tone controls? Sliding turnover frequencies?

    In any case, for anyone short of a tone-control circuit, yours seems like it could be a handy device to have.
  13. massi


    Jan 17, 2013
    regarding the infrasonic response it is very important the loading input impedance of the preamp. Since the whole idea is to capture the best sound information of the piezo and mic, the input impedance of the CH1 (passive) is set to 1 Meg although an internal jumper can change it up to 8 Meg. When a Piezo is loaded with a low impedance (1Meg), the bottom end is attenuated while the high range is enhanced. A good bridge installation with 1 Meg input impedance should be sufficient to get out of the problematic area of the piezo.
    Furthermore the piezo's frequency contribution around 2.5Khz should be reduced several dB to remove the nasty metallic sound.

    The bottom end is beautifully captured by a condenser microphone placed on channel 2. All professionals I have met so far are using either a DPA 4099 or the ATM350 for this purpose

    Tone control and parametric eq. of the both channel have been tuned manually according to the taste of two very experience professional Dutch players using piezo and DPA mics: Frans van der Hoeven and Tony Overwaters

    Since our tone control and the parametric eq. are very effective for DB and considered a real added value on the preamp, I would like to keep these information confidential.
  14. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    I'm surprised that, given you've attempted to design the pre-amp for DB amplification using piezos, you seem to be unfamiliar with what occurs in practice. Even when piezos are optimally loaded, they can pass infrasonics. As I've posted here before, you can easily demonstrate this for yourself. With a piezo installed and optimally loaded (e.g., 1 Mohm - 10 Mohm), adjust the gain on the rig to the level used on a gig. Now, grab the E string and wiggle it back and forth. There should be no discernible movement of the speaker cone. If there is, then the system is passing infrasonics. This will occur unless there is some type of high-pass filtering that is operative. It's a rather common occurrence. This is not merely theory, a fact to which many players here can attest.

    When a piezo is optimally loaded, it is true that a high-pass filter is formed. The idea, though, is to place the high-pass cutoff as low as possible. It's not really that the high end is enhanced. This has been described in detail here by me, by fdeck, and by others. The loading is chosen to yield a desirable low-frequency response. Take a look here.

    While listening is the best way to judge the suitability of an amplifying device as a last step, specifications can be very valuable and informative in terms of an initial evaluation. They are also important because, of course, not everyone is able to "try before you buy." I can tell whether the design of a tone control section would be useful for me and, in fact, whether it would be generally useful for others, by having the information I requested. It's actually of little value to know that the tone controls were designed according to what two players found useful when coupled with unknown amps and speakers in unknown rooms.

    You started this thread by touting your DB-suitable pre-amp that works particularly well with piezos. From what you've written, the pre-amp lacks an extremely important function for using such pickups. That is, a properly designed high-pass filter. A single-pole optional high-pass filter just won't do it. That's not simply speculation.

    For some reason, you wish to keep the characteristics of your tone controls a secret. Those are typically specs that are published, and for good reason. In my opinion, you aren't doing yourself any favors by adopting that approach.
  15. massi


    Jan 17, 2013
    Hi Drurb,
    As I mention before, with a proper installation on the bridge and low impedance your infrasonic effect is dramatically reduced.

    I do understand your comments. Nevertheless, it appears that for you combining mic and piezo is really out of the picture. Is that true ?

    In my view the piezo is not the ideal transducer to capture the low end. If the DPA 4099 is largely diffused by prof. DB players it must be a reason for that.

    On the other hand, musician are afraid of mics on stage due to feedback.
    So we design a preamp to enhance the performances of the two (piezo and mic) and reducing the feedback.
  16. Clarkybass

    Clarkybass Supporting Member

    Apr 30, 2010
    FYI, Francis Deck and Drurb are two different people ;)
  17. massi


    Jan 17, 2013
    Thanks for let me know,I correct the post.
    I think a demo video using the preamp with mic and piezo should proof the concept better that any technical spec.
    I will try to arrange that if you guys are interested.

  18. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    I do not combine mic and piezo. When I used a piezo, it was not in combination with a mic. I now use the Ehrlund transducer. In any case, this is not about me.

    You keep asserting that a proper installation of a piezo on the bridge of a DB, coupled with optimized impedance loading will vanquish the nasty infrasonics. That is simply not true.

    My experience squares with that of many players who used and use properly installed piezos loaded by impedances in the Mohm range. I started this discussion right here in these pages back in 2005. Amp designers and players are now well aware of the issue (I was the one who brought it to the attention of Gary Gibilisco at AE) and it's one of the primary reasons that fdeck built his first pre/HPF.

    So, what you believe should happen carries far less weight than what does happen. I'll try again:
    If your amp does not incorporate a high-pass filter with the parameters proven to be effective for use with a piezo, then it is simply not suited to that purpose. Sure, if you combine a piezo and a mic and attenuate the piezo channel sufficiently, the problem can be reduced but the necessity of such an approach would seem to limit the usefulness of the pre-amp to a limited set of circumstances.

    Please keep in mind that you started this thread. As you said:
    So far, flexible as your device seems for some limited purposes:
    1. It does not incorporate a high-pass filter and is, by itself, not generally suited to use with piezos. The single-pole high-pass filter you offer is not sufficient.
    2. The filter Qs of the parametric tone-shaping section are a secret.
    3. The cutoff/turnover frequencies of the tone controls and the specific type of controls employed are a secret.
    So, you won't provide the answers to some legitimate technical questions even when those answers would not reveal proprietary aspects of actual circuit design.

    It may be the case that the parametric and tone-control sections work very well. There's no way for one to evaluate that until one tries it. Common practice is to publish such specs and I can't recall ever having a manufacturer refuse to do so. As for the missing high-pass filter, well, that issue would not be solved by trying it out.
  19. massi


    Jan 17, 2013
    Hi Drurb,
    if I would implement a two pole low pass filter without having it programmable which frequency cut off would you considered as most common used for cutting of the infrasonic effect ?
  20. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    30-35 Hz. Of course, infrasonics are lower but placing the cutoff there provides two benefits. First, given a two-pole filter, the attenuation in the infrasonic region would be substantial. Second, it places the cutoff near the typical resonance frequency of many cabs where the driver is least well controlled. Some players greatly prefer a three-pole high-pass filter but a two-pole is really the minimum in order to achieve desirable results.

    Now, about those tone-control specs... :)