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Technical guidance for combo design

Discussion in 'Amps, Mics & Pickups [DB]' started by Raka, Jan 8, 2012.


  1. Raka

    Raka

    Aug 28, 2008
    I've technical background (electronics) and some experience designing and building hifi speakers (my main stereo is based on a 3 way active dipole system, design and built by me). I'm considering to build for myself a small combo to support a bit my double bass with the revolution solo pickup. So far, I've been playing in a trio (no gigs), with guitar and drums, playing in a room which is not big.

    My target is:

    Highly portable (guitar, bass, and three guys in a vw bora)
    Not high spl required. So far I use a 100w hartke combo, volume at 3 most of the times.
    Pizz only

    In order to define which the design criteria is, there are some ideas I need to clarify, confirm or rectify, where any kind of support would ve very valuable to me (I'm not stating the ideas below, just thinking out loud for someone else to ban or confirm, or give some light):

    - BG amps seems to be non linear, but strongly voiced (equalized)
    - bass cabinets for BG are also non-linear
    - all this can probably be explaines because the BG doesn't produce that much bass to start with.
    - specs from the commercially available items are not useful for frequency response because they don't say the limits (-3dB).
    - there seems to be people who likes linear amplification (alla hi-fi), and also relative small drivers (maybe because they have a more usable radiation pattern) for uncoloured sound (again, alla hifi)
    - double bass has a relatively good projection, meaning that the bass frequencies reach further than the mids or highs, and double bass has a good amount of bass to start with.


    The main technical parameters that I need to define are volume, cutoff frequency and roll off slope. I'm assuming that 92dB at 1mt is a reasonable outpout from the combo (headroom not included).

    Am I far off?
     
  2. brimo

    brimo

    Jan 3, 2012
    i have a BT 100 combo crate amp, a yamaha trb 6 string, a boss gt 10 b effects processor and a yamaha ne 1 equalizer. i need the settings for nathan east slap sounds.
     
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I've built DIY amps and speakers, including a small combo once upon a time. From a DIY standpoint, I finally decided to go with the new "micro" heads, to avoid messing around with potentially dangerous electronics.

    As you say, most electric bass amps have non-flat response. I have two amps. My EA Micro is flat. My GK MB200 head is not flat, but I figured out the EQ settings to achieve flat response. And if you're interested in getting into DIY, then equipping yourself with the audio analysis software is probably going to happen sooner or later anyway. This will tell you exactly what you're dealing with. You can also get flat response out of most amps by going in through the effects return, thus bypassing the built-in preamp.

    On the speaker front, better watch what you ask for. Most speakers in use by DB'ists tend to be non-flat, but instead, have a cutoff frequency somewhere in the 60 to 80 Hz ballpark. My DIY 1x12 is an example. With either of the micro heads, my entire amp rig weighs less than 20 pounds. On the other hand, you can give flat response a try. I think that the Eminence BP102 will let you go there, depending on the box design of course. Too bad Eminence hasn't come out with a neo version of that driver. They apparently had one in the works but pulled back because of the rare earth metal shortage.

    But... this is coming from someone in the amp gear business: See if you can get the band to work on rehearsing at volume levels where you can play without an amp. And maybe even gigs.
     
  4. Raka

    Raka

    Aug 28, 2008
    Thanks for the reply.
    The amp is not a problem, I built some and even have some smps to try. For the time being I intend to use my hartke250 as linear amp, using the return of course, until I figure out how much power I need.

    About the speakers, it's very useful the info about the cut off. Do you mean -3dB as cutoff freq., or the frequency they use for the ads? What would be the reasons that 60-80hz is enough, if the E is 42? Could be that the double bass has enough deep bass and that the support is more required in the frequencies that are not so good travelling without too much dampening? If so, my intuition tells me that a roll off slope of a closed box would integrate better in the mix bass-speaker response-floor coupling-damped acoustic load of the room.
    What do you mean with giving flat response a try? To reach 42hz -3dB? That would be or big, or with strong equalization at the penalty of excursion. That literally would mean to bring one of my stereos as combo amp.

    Playing without an amp? Aaahhh, I wish... I managed to bring my drummer down to very reasonable values, but I still need some dB extra (not much, hence my low requisite of SPL. I keep educating my drummer, anyway.
     
  5. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I meant the -3 dB point. As for giving flat response a try, I merely meant that you could build a test cab and decide if flat response is something that really works for DB amplification. Or, use one of your hi-fi speakers, as you point out. It would indeed be a big cab.
     
  6. Raka

    Raka

    Aug 28, 2008
    Having a full range speaker with measured flat and decent low extension as testbench for trying out what's the freq response required is not a bad idea. That would give useful information. If I could register also the voltage excursion on the amps terminals, the power requirements would also be given. At home, my speakers are -6db 20hz 6db slope, but it's unrealistic to bring one to the band place.
    For the electronics part, what would be the controls required apart from phase switch?
     
  7. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    For electronics, I'd just make sure that you're plugging your piezo into a high-Z input.
     
  8. Matthijs

    Matthijs

    Jul 3, 2006
    Amsterdam
    I traveled a lot of the route you're embarking on. What I found out is a flat response is probably not something you'd want. I tried that and found out through experience what is tought by the pro's on this site: The fundamental plays a relative small role in our sound. It's good to hear it, but it gets in the way if it's to loud. I myself found out I favour a very slow cutoff starting at about 120 tot 160 hz for double bass to about -6 to -12 at just above 40 hz. For electric bass I like the cutoff to be at 80.

    Have a look at the industry standards for small amlification for DB. An old time favorite and archetipical sound for a small jazz combo are the small closed box GK combo's. In no way are they putting out a lot of volume in the lowest octave. I don't know exactly, but I suspect the cutoff starts for those immediately below a 200 hz bump.

    I think a closed box cutoff rate fits with DB, but I managed to get my best sound up to now with a not so small ported design. You should look up Fdecks website: I found his quest for a small box for db and the math that goes with that really usefull.

    By the way I've had some reasonable results with a eminence legend b102, wich I think is the bp102 with a wizzer cone, in a small closed box. Very much like that GK sound with some added top end. I want a bit more low end myself though.
     
  9. Raka

    Raka

    Aug 28, 2008
    Thanks for the shared experience.
    If there is -12db at 40hz in a closed box, it means box pole is at 80hz, that is not a big box. I have two 8 cones with high qts that ina a realtively small box can do that, and then reach some kHz as they have wizzer. It would need equalization of the hump, and the excursion is not a impressive figure, but the efficiency is not bad (listed at 96 dB). Using the linkwitz transform, a small box would do.
     
  10. Matthijs

    Matthijs

    Jul 3, 2006
    Amsterdam
    About -6 db is what I favour for DB, but I use my current cabs for EB also, they're ported cabs and there's a bit more bass in there below 120 hz. For DB I try to EQ it to something in that ballpark. The legend B102 experiment is in a small closed cab.
     
  11. Raka

    Raka

    Aug 28, 2008
    I will calculate the excursion at low frequency, to consider a steeper rolloff from let's say from 40Hz downwards. I'm concerned about the low frequency output that a pickup can provide as artifacts or unexpected output
     

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