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Buzz from preamp stops when touching chasis. HELP!!

Discussion in 'Preamps / Channel Strips' started by mdb, Mar 2, 2011.

  1. mdb

    mdb Active Member

    I got my USED Trident S40 mic preamp and have been playing around. I really like it, but have a question about some buzz. Here's the scenario:


    1. Gating the signal at -63dB from within my DAW gets rid of the buzz.
    2. With NO input cables plugged in, if I turn the gain up it's relatively quiet (small amount of speaker static type noise). With the input gain over +45dB and the output gain at +5dB or more, the preamp begins to produce electrical noise, but I'm thinking that's fairly high gain.
    3. Plug the XLR cable in (no mic attached) and I get buzz (probably RF noise) at lower gain. Insert 48V phantom power and it gets a little worse.
    4. Attach a microphone to the XLR cable and the buzz significantly reduces, but is still noticeable with high gain settings.
    5. Using the instrument input (built-in DI) on the front panel produces the same as above.
    6. Hovering my hand centered and over the top of the amp increases the buzz significantly, but touching the amp chasis almost eliminates it completely.
    7. Adjusting the knobs increases the buzz and creates a frequency-sweep type hum when adjusting. I'm assuming this is due to the fact that the knobs are metal and transferring the voltage from my body (just a guess)??
    8. If I use my DI box and connect to the XLR input I can literally crank both input and output gain and the preamp is dead quiet.
    9. The buttons for EQ in, comp in, etc. all make very insignificant crackle when engaging/ disengaging, but I'm thinking this is fairly normal.

    Do I have a grounding problem, does the preamp just need a good cleaning or is there some other problem? I shut off all my other equipment except my A/D converter and Mac, but it made no difference. Should I record the sound it's making?
     
  2. Boswell

    Boswell Moderator Distinguished Member

    What are the "A/D converter and Mac" you plugging the S40 into to hear this mains-related noise? Is there a continuous ground connection from the case of the S40 through the other equipment to the ground pin on the wall outlet? Is the wall outlet properly grounded?

    This sounds to me like standard grounding troubles. I don't think what you describe indicates a problem in the S40.
     
  3. mdb

    mdb Active Member

    Mac Pro and Motu 2408 mk2. All equipment is plugged into the same wall jack. My Digimax D8 doesn't make any noise at all (that I've noticed).

    All equipment has a ground pin on their respective power cables. They're all connected to the same metal rack bars so that should tie all their chassis' together. Anything else that should be done?

    The wall jack does have ground going to the panel, but it is not a separate electrical panel. Should I be looking into a power conditioner or is that unnecessary?

    I'm taking the S40 up to my church tonight to test it in the system there. I know that the sound system is properly isolated to avoid grounding issues.
     
  4. Boswell

    Boswell Moderator Distinguished Member

    From what you say, it sounds as though it's all connected in a way that should not cause this sort of trouble. The Digimax D8 will not have similar grounding issues as its connection to the MOTU is presumably by fibre-optic lightpipe (no electrical conductivity between the two).

    I will be interested to hear what happens when you try the S40 connected to the church system, although I'm a little suspicious of your phrase "properly isolated to avoid grounding issues" (!)
     
  5. mdb

    mdb Active Member

    The Digimax is connected through the analog outputs so I don't have to worry about clocking (not ADAT or lightpipe). The church system has a completely different electrical panel and circuit so it is isolated from the rest of the building's electrical system.

    I have the output of the Trident connected to channel 3 of the Motu input. I used output #3 of the Motu to route previously recorded tracks into the Trident, but disconnected that 1/4" cable from the Trident before doing other tests. This routing cable was still connected to the Motu and left open-ended, but I had nothing from the DAW sending to that channel so I don't think it would have created any RF noise.
     
  6. mdb

    mdb Active Member

    Could using a balanced XLR for the input to the preamp (mic cable) and using an unbalanced 1/4" on the output cause a problem? I can't see this as it would then be an issue connecting any external preamp to the insert of a mixing board.

    I'm reading up on "pin one problems" to try and figure this thing out. When I made my XLR cables I soldered the ground shield to pin 1 and to the plug housing for chassis grounding. Was that a good idea? The connector for soldering to it was there. The housings of the cable jacks on the Trident are plastic so should I internally connect the Trident pin 1 to the chassis somehow? Bad idea?
     
  7. mdb

    mdb Active Member

    Talking to myself here... anyway,

    I tried the Trident on the church system and it has an awful buzz there too. With the DI connected it's quiet. A microphone hooked to an XLR creates crazy RF noise (using the XLR & mic as an antenna). If I hover my hand over the end of the XLR cable with the mic connected it gets louder and if I pick the mic up it quiets a little, but is still loud. Enabling the compressor increases the buzz too.

    When first plugged in and turned on there is almost no noise and it continually gets louder as if the unit is charging up over time and then it holds. I did a continuity test between the power cord ground, the preamp chassis and the input and output signal ground pins (#1) and the sleeve contacts of the 1/4" jacks. All have continuity so all are connected to the chassis ground.

    Is there a capacitor that is bad or something else? I might try disconnecting the ground from one of my XLR cables to see how that does, but that's not really a permanent fix I don't think. Just a test.

    Any smart ideas?
     
  8. Boswell

    Boswell Moderator Distinguished Member

    I think you have to try a commercial XLR lead. Connecting the plug shell to pin 1 is not generally a good idea as it creates an additional path to ground at the input stage. Having said that, I would not expect it to generate the increasing interference effect you describe. That must be due to another problem such as ageing capacitors in the pre-amp or its power supply.
     
  9. Link555

    Link555 Well-Known Member

    Here is a good reference for grounding....

    Grounding and Shielding Audio Devices

    I think you may have a cabling issue somewhere or the S40 is misbehaving....
    Phone John Oram and Ask him what he did for the Pin one (or sleeve) connections in the S40....
    http://www.john-oram.com/index.php?/Company/

    I have always found him very helpful and easy to talk too.
     
  10. mdb

    mdb Active Member

    Thanks for the suggestions. My XLR cables are all Mogami cables with Neutrik ends. They're good. I was thinking about having the unit looked at, but am worried about how much it will cost to troubleshoot and repair.

    Link555, I was reading that article from Rane yesterday. Very good resource. I think the S40 is misbehaving and maybe I should contact Mr. Oram. I need a manual too...
     
  11. Link555

    Link555 Well-Known Member

    Did you buy it from Rob Dewar?
     
  12. mdb

    mdb Active Member

    No I didn't, but Penticton is about 2 hours from where I live (checked out his website).
     
  13. mdb

    mdb Active Member

    Here's some sound samples:

    Trident S40, DI box - ground lift on, gain @ +75dB
    is with a DI box attached and both the input and output gain fully on (+75dB)

    Trident S40, no input connections - gain @ +75dB
    is with nothing attached to the inputs and the gain fully on (+75dB)

    Trident S40, XLR and/or mic connection - gain @ normal settings
    is with an XLR cable attached with a microphone and the gain set for typical mic use. I used multiple variations of cable mods, grounding configurations and output cable types. They all produce the same noise indicated in this audio file.
     
  14. mdb

    mdb Active Member

    The Trident S40 is apparently fixed now so I can go get it and try it out tonight to be sure. The report is that the pots needed to be cleaned (which I knew) and there were some internal grounding points that required re-soldering. That's actually a relief because it could have been far worse. The real test will be when I get it home.

    I'm probably the only one here that has one of these things. It usually goes that way with me and wouldn't surprise me at all. Always have to be different.

    I'll maybe post some sound samples when I get it hooked up. I've never really heard what it sounds like FIXED...
     
  15. mdb

    mdb Active Member

    I'm going to talk myself through this. Maybe it will help someone else one day.

    The shop did not fix the preamp although they claimed that they did so... forget them. I'll try it myself (even though I have no idea what I'm doing). I took it apart at home and there were two pieces of bare aluminum wire connected (soldered) across the top of the pots - one on the top row and one on the bottom row. There was also two pieces of green wire soldered to the negative terminal of the VU meter and attached to the nearest pot (connecting all the potentiometers in series to ground via the VU meter negative). These components did not look factory so I removed them (they were present before I took it to the service shop too). Am I correct and okay to do this? The pots are grounded to the chassis via their mounting nuts anyway and by removing the additional grounding some of the noise the amp was making has been eliminated (although it still gets noisy with the gain turned up high). The buzz is less when the EQ stage is engaged.

    Should I replace all the caps and if I do, is the rated operation temperature of the caps critical? What about the type of cap (aluminum electrolytic, film, copper, poly, etc.)? I hear Panasonic are good, but they have so many different "series" of caps. Which ones should I pick? I really don't know anything about them, but I can operate a soldering iron. It can't hurt to change them although I've read changing them may change the sound (color) of the pre.

    These are the radial caps that I found in the preamp:

    Main circuit board
    2200 microF @ 16V x1
    100 microF @ 50V(RE) x2
    4.7 microF @ 63V x2
    10 microF @ 50V(LL) x2
    22 microF @ 35V x1
    47 microF @ 35V x2

    Power supply board
    220 microF @ 35V x2
    2200 micro-M @ 25V x2 **What is micro-M?
    100 micro-M @ 63V x4
    220 microF @ 100V(RE) x1
     
  16. Link555

    Link555 Well-Known Member

    If your in North Van in the near future bring it by.
     
  17. mdb

    mdb Active Member

    Thanks Link555. Maybe I will. I'll PM you for info.

    I drilled a hole for a grounding screw and attached the power supply ground directly to the chassis, but it's also grounded direct via a circuit board mounting screw so it made things worse. I removed my mount. I tried a direct ground from the shield (XLR housing) to the chassis and that helped, but the ground signal was still going to pin 1 too. I used a modified cable where a sent the shield separately to the chassis and not to pin 1 and that helped a bit. I pulled the Trident out of the rack and moved it away from the other equipment and that also helped. Sitting on the XLR cable also helps... LOL. For a couple minutes the buzz went away completely (even with a standard XLR attached)... dead quiet at really high gain. It was wonderful... and then it came back. There's a secret code of touching the right combination of cables, the chassis, wrapping the cable around your head and rolling on the ground or something and then... the noise goes away... and then comes back.
     
  18. natural

    natural Active Member

    wrap in aluminum foil and face due north.

    Intermittent problems are the worst. Sounds like something becoming ungrounded as the unit heats up perhaps.
    I have had on occasion some luck in connecting the ground of one unit to another unit to provide more solid grounding. (either through an unused 1/4" connector or sometimes temporarily with a wire attached to a screw on both chassis as a means of testing - or ground lifting at the plug- again as a test )

    There is also the possibility of the dreaded dual problem, where there are 2 or more factors that must be identified and repaired in order to fix the anomaly.
     
  19. Link555

    Link555 Well-Known Member

    Its sounds like you need to check the units XLR connections first. cold Solder?
     

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