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Replacing my amp's balanced out

Discussion in 'Amps and Cabs [BG]' started by Pokeroo, Aug 23, 2007.

  1. I posted a while back about the noisy balanced line out on my SVT-II. Well I've tried it at a couple of gigs since then and sure enough it was deemed unusable by the soundguys.

    What I'd like to know is: would it be worthwhile to somehow replace/modify/convert that output to make it usable? I'm thinking that maybe I could get an electrician/tech to install a passive DI transformer, like a Jensen or one of the others I've read about on this forum.

    I know I could just buy a DI box but I'd love to make that output useable. It would also be nice not to have the extra little cable run between a DI box and my amp.

  2. anonymous02282011

    anonymous02282011 Guest

    Jun 27, 2007
    get a repair guy to fix it.

    Jensen's nice. There's one in my EvilTwin (DI/preamp).
  3. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Yes, of course it will depend on the specific amp, but it seems like a tech should be able to retrofit a "good" DI in there. They might have to mount the xformer to the side of the housing rather than to the circuitboard, if there is no xformer space already on the board; I've seen that done sometimes.
  4. deaf pea

    deaf pea

    Mar 24, 2005
    Cuernavaca 1 hr S Mexico City
    Seymour Duncan/Basslines SMB-5A Endorsing Artist
    Is this an SVT-II or an SVT-2PRO?

    If it's the SVT-2PRO, the "Bal Out" should be similar to the "Bal Out" on my SVT-3PRO's.

    The "pre/post" switch should be out ("pre") and the "Bal Vol" should be at maximum, with the knob pushed in.

    If there is buzzing or hum at the "Bal Out" to the FOH OR from YOUR speaker, try pulling the "Bal Vol" knob out (to lift the ground).
    If it STILL doen't work, adjusted like that, then it's broken and needs repair . . .

    If it's an SVT-II, ask "I got a Mohawk" for help . . .

    good luck!
  5. Thanks, deaf pea. It's an SVT-II, not an SVT-2PRO and I run the Bal. Out PRE.

    Don't think it's actually broken -- more likely it's just one of the few shortcomings of this model. I've heard it a couple times on this board and the guy who sold it to me had another one with the same problem. I think most of us just shrug and plug in the DI box.

    Hopefully someone here is familiar enough with this amp that they can offer suggestions as to which way to go with this.

  6. I've had this experience with tube Ampeg heads, my SVT-VR it's noisy. I think it's better to go for a DI, the Ampeg one is awesome, not too expensive, and has a cool tube sound.
  7. deaf pea

    deaf pea

    Mar 24, 2005
    Cuernavaca 1 hr S Mexico City
    Seymour Duncan/Basslines SMB-5A Endorsing Artist
    If it's anything like the ones in the SVT-PRO series amps, the DI takes the "pre" feed from the input "gain" volume pot, before ANY active circuitry . . . and then there's a transformer just before the XLR connector.

    Have you tried using the DI "BAL Out" with another bass (or even with a guitar) to be sure that the problem isn't just with YOUR bass only?

    Another idea . . . PM "Jerrold Tiers" asking for help. He used to work in design at Ampeg in St. Louis IIRC.

    And, like I said, PM "I've got a mohawk" to ask HIM for help . . . he's the SVT-II specialist here at TB . . .

    Good Luck . . .
  8. I've got an SVT5-Pro with the same problem. It's a pain in the butt.

  9. figuredbass

    figuredbass Supporting Member

    Jul 11, 2007
    NYC vicinity
    There are several options available to you:

    1. As already posted, retro-fit a high quality transformer D.I. i.e. Jensen with its proper peripheral circuit. If you want to try it from scratch, I believe Jensen has actual schematics available on how to do this with their transformer products:
    Be very careful with a transformer installed inside the amp itself though - careful magnetic shielding should be done to the transformer, and it should be placed and oriented in a way that it does not pick up induced magnetic hum from the power (or any other) transformer already there, or else you may wind up with more noise than you have now!

    2. Retro-fit a different active D.I. circuit. Your tech would have to know how to provide the proper voltage and current to it from your amp. It can be from an existing direct box or from scratch. Such a circuit (balanced line driver) is available in the Analog Device's OP275 opamp datasheet:
    (I use this circuit and IMHO it works great. And you can use different opamps.)

    3. Modify the existing D.I. circuit itself. This is often possible as long as you have a detailed schematic of the amp for your tech to refer to. You can change the circuit of the DI itself, or the feed point (i.e. post, pre, etc.)

    HOWEVER, if there is already an unacceptable amount of noise in the amp itself, the problem may NOT actually be with the DI! Even with a better DI circuit it will only ultimately be quieter if you take the signal going into the DI right from the amp's input jack.
  10. deaf pea

    deaf pea

    Mar 24, 2005
    Cuernavaca 1 hr S Mexico City
    Seymour Duncan/Basslines SMB-5A Endorsing Artist
    And even then you COULD have more noise problems if the amp's got a "noisy" ground ("noisy earth" for the brits).

    Just a reminder . . . the Ampeg SVT-xPRO amps ALREADY HAVE a tranformer-isolated DI out! (But it's NOT a Jensen DB-E by any means . . .) They DON'T have an "active" DI circuit!!
  11. There's no gain pot on the SVT-II.

    I'm sure it's not the bass -- it's an Alembic ;) . I have tried it with other axes tho, and yes same problem.

  12. Wow, what an informative post. Thanks very much figuredbass!

    I'm currently asking around for a good tech (wasn't happy the last time I used my current guy) and I will definitely ask about these three options.

  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I would NOT start with the transformer.

    The ones in the units are not Jensens, but they work. And they won't INSERT NOISE, since they are passive devices.

    "Noise" is a term that has many meanings...... "HISS" noise, "HUM" noise, each has a different cause.

    There WAS a problem with hum noise particularly with some amps. The 4 pro, for instance had it, and has a simple fix, two PC board traces get connected, which takes care of most newer units.

    Other units have a "boost" circuit, which can insert "hiss" noise. And some units have other circuit oddities which have caused problems.

    Having a circuit that connects to widely separate places in the amp is often troublesome. I wanted them to put the DI on the FRONT, because that would be closer to all the circuitry, and not that much of a hassle to use.
  14. Thanks Jerrold.

    I've sent the balanced out to my mixer at home and it's definitely a HUM. I also tried it POST and the hum is so bad it's completely unusable (I assume because my EQ settings are simply boosting the existing hum).

    Any idea if the solution you mentioned would work for the SVT-II as well?

  15. anderbass


    Dec 20, 2005
    Phoenix. Az.
    I think this amps noisy DI problem is similar to the Mesa 400+, in that the DI signal is tied in from the amps power section.

    Have you tried simply connecting your SVT-II "preamp out" jack through a DI box ?
    (if you don't have a DI box, try connecting this jack directly to your mixer for a quick and easy test)
  16. This is what I do at home to get the signal to a mixer when I can't mic up (late at night or when the kids are napping).

    It's an option but I'd rather send a PRE-preamp signal and give the soundguy less to worry about. My guitarists both have TONS of different patches, depending on the song. They also tend to get louder as the night goes on :rolleyes: so depending on how things sound on stage I sometimes have to tweak.

    With four vocalists all pointing at our wedges and giving "up" signs ;) , a keyboardist plus sequenced tracks to worry about, plus the aforementioned dueling guitars, I'd hate to force him to deal with a changing bass signal. More than likely he'd just turn it down and we certainly don't want that!

    Plus it's killing me that I can't use that Bal. Out.

    Hopefully Jerrold can offer more details on the PCB trace fix or some other model-specific solution.

  17. deaf pea

    deaf pea

    Mar 24, 2005
    Cuernavaca 1 hr S Mexico City
    Seymour Duncan/Basslines SMB-5A Endorsing Artist
    OK . . . that DOES limit the possible solutions!

    A "hum" problem is USUALLY caused by having 2 paths to ground (through the amp AND through the mixer/recording console). If they (the amp AND the mixer/recording console) are already plugged into the same AC outlet, the BEST solution is isolating the amp's "ground" from the mixer/recording console "ground" by using a transformer-type DI with the "ground lifted".

    There COULD be a design flaw with that model (Jerrold?) that causes power line (60 Hz) induction into the DI circuit.

    Good Luck.
  18. I could see that being the case at home. But at a club with the console and stage at opposite ends of the building?

  19. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    This is the situation that a balanced line with ground lift is supposed to remedy.
  20. I just re-read deaf pea's post and realized that I completely misunderstood. I think the second part of that bit (the part that comes after "..." in my last post) confused me.

    So I guess I would almost certainly have "two paths to ground" in this situation.

    Anyway I think I've found a good tech -- hopefully we can make this work!


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