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Onboard DI (not a newb question)

Discussion in 'Pickups & Electronics [BG]' started by JimWasHere, Jan 18, 2012.


  1. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    I've been searching the interwebs for a week and haven't found my answer yet, so here goes.

    I am going to be gigging with a rackmounted preamp/power amp combo (avalon vt-737sp). The preamp has 3 inputs, unbalanced 1/4 t/s, balanced mic level, and balanced line level. I would like to have the option of coming directly out of my bass with a balanced mic level signal, to minimize the poop from having a 50' unbalanced ling out on the floor. This means having an onboard DI, essentially. I'm experienced enough with instrumend wiring and audio to make this happen, if I have a schematic to at least start with.

    Specifically, Here's what I'm looking for;
    • Both balanced and unbalanced lines from the instrument
    • DI must be active, capable of taking +48v phantom power
    • Small enough to fit in the instrument cavity of a bass with 2 pots and a 9v battery
    • No coloration. None.
     
  2. darkstorm

    darkstorm

    Oct 13, 2009
    Curious why you think that would be better then balanced and unbalanced line level? From instrument.
     
  3. lowfreq33

    lowfreq33

    Jan 27, 2010
    Nashville
    Endorsing Artist: Genz Benz Amplification
    The old Fender/Heartfield basses had an XLR balanced out. Maybe you can find a schematic for one of those.
     
  4. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    Unbalanced anything... The only place unbalanced will ever go is to an amp, and that's going to be extremely infrequent for these basses.

    Line level would require a preamp onboard, and I don't entirely trust that I can come up with a preamp that will show up with zero coloration.

    Also, making it clear, it's specifically active di work that I'm asking about, because Jensen rocks, and has a bunch of schematics for passive stuff.
     
  5. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    You want an active DI, but you don't want a preamp? What's the difference? And why does it mater if Jensen has "a bunch of schematics for passive stuff" when you want an active DI?

    Here's the thing, you don't need a balanced out. First off, your bass does not have a balanced pickup system. Everything goes to ground. It's unbalanced right in the bass.

    Second, you don't need it. Why? Because with a microphone, you have a feeble little signal to start with from a very low impedance coil. This is stepped up with either a preamp, in the case of a condenser mic, or a transformer for dynamic mics. But you still have a fairly weak signal. This then gets boosted at the board with a mic preamp.

    Now look at the signal chain for a typical mic. It starts on stage, then plugs into the snake, and might go through 100 feet of cable. Then that signal has to be boosted up to a decent level at the mixer. What you don't want is to boost all the noise you would get in all that cable.

    So that's where the balanced line comes in. It's only to cancel cable noise.

    Now with your bass, how long is you cable? Then on top of that, you have a lot more signal coming from the bass. If you have an active bass, you now have a low impedance output capable of driving a very long run of cable. But even with passive basses, you have to get up to about 50 feet before you hear a lot of high end loss. You wont pick up noise, but you will lose high end.

    So you don't need a balanced line out. To make your bass balanced with an active DI, it's going to take your signal and run it into two op amps. One is normal, and one inverts the signal. So it's not a balanced signal like with a mic capsule.

    If you use a passive DI, you are going through a transformer. That will color your sound somewhat.

    Now you mention you have a 9v battery in the bass. Is it active? Then you really don't need a DI or a balanced line, you are all set because of the low impedance output. Just get a good quality cable. And any good quality preamp or buffer in your bass will do more for you than a built in DI. If you want no coloration, get a Creative Audio Labs Redeemer. They even have a belt pack version.
     
  6. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    Because I already know how to do a passive DI. To make a better decision I would like to know how to do a good, simple, active DI.

    Exactly. I can control external noise at the bass by not having my cell phone in my pocket (oversimplification). What I can't do is control the sometimes upwards of 100 feet of cable between my bass and my amplifier. And no, I'm not exaggerating that, it's actually happened. :p

    This is where I'm starting to get fuzzy, both on how to do it, and exactly what's happening when it's done. Hence the questions.

    Also, I do understand the slight coloration from the transformer in a passive DI. The bottom line is that somewhere in my signal chain there is going to be a DI, and the closer to the pickups it is, the more it will be able to use the external noise canceling features of a balanced cable.

    If you can enlighten me more on the function of an active DI, I'd certainly appreciate it though. :smug:


    EDIT: After I reread it, this post seemed a tad defensive. I didn't mean it that way at all. I'm heading into a very non-traditional rig, and the next logical step seems to be a somewhat non-traditional signal path between the instrument and the rig. Hence, exploring some options.
     
  7. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    We can also go at this problem from a different angle.

    The signal chain (breaking it down) will be strings-->pup-->pup pan-->volume-->[something balanced]-->rackmounted preamp (Avalon channel strip)-->Mesa power amp-->cabinet.

    The pickups don't necessarily have to be the active ones I'm currently using, and in fact, I'm kinda looking for an upgrade. I want to color the tone as little as possible before hitting the preamp (onboard preamp is out). I definitely want balanced cabling between the bass and the rack. I need 2 balanced outputs (for the racked preamp and for foh), but the location of the second could be anywhere, such as a splitter at the rack.

    Goals are to maximize cleanliness and warmth and minimize noise. I'm very happy with the power amp/preamp combo, and with the basic tone of the bass. It's a good sounding chunk of wood. The preamp can supply +48v phantom power. What do I insert in the make stuff balanced place?
     
  8. Any active DI box has a preamplifier built in, so "No coloration. None." is an unrealistic Goal
     
  9. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    ... wow, tough crowd tonight. I asked for some ideas, not your approval. Perhaps tomorrow there will be some helpful posts here. I mean, not things I'm already aware of.
     
  10. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    [​IMG]

    Balanced Transmitter and Receiver II ...The one piece missing there is his design runs on +/- 15V, and you need it to run on + 48V, so you'd need to make a little voltage divider and figure out best practices for wiring the XLR for power. There are some suggestions on that ESP site, but nothing concrete that I saw. Some potentially useful info here: Powering microphones
     
  11. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    You don't need a balanced output. It will do nothing to prevent your tone from degrading. What you currently have is all you need. The active pickups you have will drive a long cable fine. You say you don't want a preamp, but you already have a preamp in the pickups. That's all you need.

    If you want other non active pickups, and you don't use a buffer in the bass, then the cable will color tour tone just like it does on every other passive bass. So the bottom line is you don't need a DI, you need a buffer. The DI would be active like a buffer, but you don't need the balanced outs. It serves no purpose in this instance.

    But don't take my word for it. Let's look at a very expensive and hi-fi bass: an Alembic. They don't have balanced outs. They do have an onboard preamp however.

    If you want to drive the cable with passive pickups, get a clean buffer. It wont color the tone, and what you end up with at the preamp will be what's coming out of the pickups. The Redeemer is a good choice. I tried one out and it sounds exactly like the bass. It did nothing to the tone at all.
     
  12. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    A better way to implement that is with pin 3 being driven by an inverting op amp. That's not balanced really.

    Like this one:

    [​IMG]

    In both cases however the input impedance is too low for passive pickups, but would work with active pickups.

    Here's a simpler one made for guitar/bass:

    opamp3.

    Code:
    Parts List
    
    R1, R2 - 1k
    R3,R5,R6,R7,R8,R11,R12 - 10k
    R4 - 1M
    R9 - Jumper
    R10 - 100 ohms
    R13 - 10k reverse log potentiometer
    C2 - 1uF
    C4 - 100pF
    C1,C5,C6,C9 - 10uF
    C7,C8 - 47uF
    IC1 - TL072, RC4558, or similar dual opamp
    
    All resistors are 1/4w and capacitors are in uF.
    AMZ Multi-Purpose Opamp Booster and Buffer

    The input impedance is 1M as to not load down the pickups.

    But these are still not needed at all.
     
  13. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Just for the sake of discussion, ESP had this to say about that particular circuit:
     
  14. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    OK, but it's still not suitable for passive bass pickups.
     
  15. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    Thanks! This is helpful to me. :D

    Now, I understand that the preamp in the pickups I currently have prevents coloration from the cable over reasonable runs, and I do get it that any preamp at all, as well as a passive DI, will cause a change to some tonal characteristic. The reason I'm exploring the possibility of doing it this way is that I know I will almost always have to provide a balanced xlr output for Foh before the preamp. My understanding and experience with balanced lines is that they help combat outside interference in cabling. Like the drummer that kept his cell on so he could text when we weren't playing, stuff like that.

    Also, I'm not at all set on continuing with the active pickups, and definitely want to move away from the brand I'm currently using. So I suppose my morning's reading will be the buffer.

    Thank you both!
     
  16. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    In terms of noise cancellation this is correct. In terms of the receiver being able to process the signal and noise properly this is correct. In terms of the signal to noise ratio (SNR) this is wrong. Noise cancellation with a balanced line is never perfect because the balance is never perfect. There will be residual noise. By driving both sides of the line with out of phase signals you get a 6 dB improvement in SNR. Maybe that is important to you, maybe it isn't and if it isn't today it might well be tomorrow. If I were going to the trouble to build a balanced system I would certainly drive both lines. Why go to all the effort only to leave 6 dB on the table?

    Ken
     
  17. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    You should give them the DI off the Avalon. I give the FoH the DI off my Trace Elliot. If they want it from the bass they give me a DI box. You still don't need a balanced output to give the FoH a DI.
     
  18. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    In theory, fewer parts, easier to fit in a cramped control cavity (bearing in mind the cavity already contains a preamp and battery). Whether the space savings in reality is enough to be worth that tradeoff, I don't know yet.
     
  19. edwinhurwitz

    edwinhurwitz Supporting Member

    May 13, 2003
    Boulder, CO
    Endorsing Artist: DR Strings, SMS
    Jim, I've done a lot of crazy onboard electronics experiments (including building and installing Craig Anderton's Super Tone Control into a bass, which is a state variable filter with mixable high pass, low pass and band pass filters. The sweeps were crazy wide and the resonance could get to the point where the preamp could oscillate and create tones of its own, all the way down to 20 Hz) and used a lot of different types of basses in some hellaciously noisy places with long cables. Perhaps the most insanely long cable on an instrument I've seen was Albert Collins and his 150' regular guitar cable on a reel that he'd use to walk through the club and out onto the street with his Telecaster. Teles are noted for being noisy SOBs and even with that really long cable, the Iceman was still able to get a clear tone with no serious noise added to the sound.

    I'm with Dave, there is no really compelling reason to go with a balanced out from the bass. A good buffer should take care of driving a long cable and proper shielding should take care of keeping the bass quiet. I like buffers. I like a setup where the first thing the pickup hits is a buffer, before the volume or blend controls. My favorite onboard electronics is the Alembic Series II (I just installed a full Series II setup in my '67 Guild Starfire). To my ear it's very clear sounding. The pickups have a resonant peak that's not in the middle of the frequency response of a bass (mine have a resistance of about 800 ohms!) like most pickups designed for a passive system, and the electronics have huge headroom and the ability to get a clear, clean sound and it can probably drive a cable a mile long. The Alembic stuff is massively over engineered, so it lasts forever (the pickups and electronics I installed were ca. 1973 and still work perfectly. The opamps that they used back then were apparently designed for satellites, so they are very precise and robust). It's also unbalanced. One of the really nice things about an unbalanced output is that you can use it with a variety of devices, whether effects, amplifiers, inputs to a computer, etc. A balanced output on your bass can make things complicated if you want to sit in with a band, if your special cables and/or other devices break down. There's a lot to be said for doing things the conventional way.

    That said, if I were going to do what you want to do (and I have to admit that it's crossed my mind way more than once), I would install a stereo buffer, to individually buffer the pickups before the blend control, and then attach a good transformer in parallel to the regular unbalanced output. Jensen would be fine. I have some old Reichenbachs that I would probably use that are similar and there are others out there. This way, the pickups are isolated by the buffers from the transformer and the rest of the world and the output transformer doesn't add noise like another layer of active circuitry would. A good transformer can be remarkably transparent. As you can tell from all the responses, you are on the outer fringes of bass practice here and there are a number of ways to accomplish what you want. I would just keep in mind what happens on the road when the gear that you need to operate the bass malfunctions, goes missing or is for some reason or another not available. If you've got a showcase set with back line as the only option, how will you interface your bass? I used to go on the road with a stereo setup that had a 5 pin cable that also provided MIDI control right off the bass and while it was a great setup, it had a couple spectacular failures during some important gigs. I'm living on the edge a bit with my Starfire, but I'm not touring these days and a backup bass provides enough security. Plus, my soldering chops are a lot better.
     

    Attached Files:

  20. JimWasHere

    JimWasHere

    Sep 8, 2010
    Portland, OR
    After SGD Lutherie put me on the buffer track, this was exactly what I came up with. It's good to hear I'm not the only one out there with an active imagination :D

    Thanks to all of you, you've been very helpful.
     

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