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Author Topic: Buzz from preamp stops when touching chasis. HELP!!  (Read 6911 times)

mdb

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Buzz from preamp stops when touching chasis. HELP!!
« on: March 04, 2011, 11:45:36 am »

I'm not a newbie (well, here I am), but not pro either. I purchased a USED Trident S40 mic preamp and have been playing around with it. I really like it, but have a question about some terrible buzz.

Quick background:

I'm hooked up to a Mac Pro using a Motu 2408 mk2 A/D converter/ interface. All equipment has a ground pin on their respective power cables and are all connected to the same metal rack bars so that should tie all their chassis' together. The wall jack does have ground going to the panel, but it is not a separate electrical panel. I have a Digimax D8 preamp (connected via analog outs and not the ADAT out) as well and it does not produce the same buzz.

Here's the scenario:

    * Gating the signal at -63dB from within my DAW gets rid of the buzz.
    * 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.
    * 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.
    * Attach a microphone to the XLR cable and the buzz significantly reduces, but is still noticeable with high gain settings.
    * Using the instrument input (built-in DI) on the front panel produces the same as above.
      Hovering my hand centered and over the top of the amp increases the buzz significantly, but touching the amp chasis almost eliminates it completely.
    * 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)??
    * 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.
    * 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.

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.

Any suggestions?

**UPDATE**

I tried the unit at my church on their live system and got the same results. 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 unit's 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.

HELP!!
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John Roberts {JR}

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #1 on: March 04, 2011, 12:02:08 pm »

If the noise is literally changing and building up over time, that sounds like something is broken.

While it may just be poor RF rejection in the preamp or poor input grounding strategy internally.

Hopefully somebody here has first hand experience with that preamp.

You say it is quiet with a DI plugged in... What Di? active, passive, ??

I am not a fan of lifting grounds.

JR
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Bruno Putzeys

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #2 on: March 05, 2011, 03:15:29 pm »

When merely touching stops the buzzing I would go and look for RF problems.
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mdb

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #3 on: March 11, 2011, 01:21:14 pm »

You say it is quiet with a DI plugged in... What Di? active, passive, ??

JR
The DI is passive. It's a Radial AV2 multi-media box.
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John Roberts {JR}

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #4 on: March 11, 2011, 01:43:33 pm »

I would look for the DI perhaps adding some capacitance across the input, which could change the RF behavior.

JR
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mdb

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #5 on: March 11, 2011, 04:38:07 pm »

I got a call from the service shop and it is supposedly fixed now. So I will confirm that tonight when I pick it up and test it at home.
It needed the pots cleaned (knew that) and there were some internal ground connections that needed re-soldering. Here's hoping...
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JGreenslade

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #6 on: March 11, 2011, 04:42:55 pm »

Have you got access to an oscilloscope? It will show you if the preamp is oscillating ultrasonically, and you're just hearing that which is demodulating into the audio band. I had virtually the same problem with a preamp a few years back. I changed a cap to limit the opamp's bandwidth and the oscillation went and has never returned.

I don't know what kind of feedback arrangement the Trident uses, or whether it has external compensation for the opamp it uses. It could be worth experimenting, putting say, a 100pF or 220pF cap in the feedback loop. Likewise, if it has external compensation, trying a larger cap could do something. It's also worth investigating bypass caps on the opamps.

On a commercial product, it'd be very unusual for an OEM to release a product with a 'hair trigger', that could be triggered into oscillation at the drop of a hat. However, stranger things have happened...

BTW - is it easy to change op-amps? If they're 'normal' types, i.e. 072 / 5532 etc, for what it costs, you'd be mad not to try new ones.

NB - I found this by putting 'Trident S40 oscillation' into Google...there may well be more hits: http://www.gearslutz.com/board/music-computers/22802-percussion-grooves.html


Justin

edit - you posted while I was writing. I'll keep the post as-is in case you get further grief. Let's hope the shop fixed it.

edit - reading back, you say it acts up with the mic plugged in, but not the DI... Each source will have a different output impedance. In an oscillating scenario, it's not unusual for different source impedances to affect whether oscillation is triggered.
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mdb

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #7 on: March 14, 2011, 02:29:46 pm »

The shop did not fix it so forget the shop. 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. 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). 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. I really don't know anything about them, but I can operate a soldering iron.

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
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John Roberts {JR}

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #8 on: March 14, 2011, 03:15:06 pm »

I'm not sure why replacing the caps would make a difference.

Repeating myself a little, it the buzz stops when you plug in a DI, perhaps there is something about the termination, either R or C to ground that stabilizes it.

You might want to look at the very input  (connections between pins 2 and 3 to ground or 1).  perhaps also look at the output of the DI.

JR
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mdb

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #9 on: March 14, 2011, 03:46:56 pm »

I thought I posted audio samples here, but I guess I didn't. Here's what it's doing...

Trident S40 DI w/ Lift Noise

Trident S40 - No Input Connection Noise

Trident S40 - XLR Connection

I wouldn't say the external DI completely eliminates the noise, but it sure attenuates it. If you turn your speaker volume way up you can still hear the buzz. With no input connection it sounds like white noise (gain full @ +75dB). With a cable connected to the input with or without a mic attached it buzzes loudly (the change in sound is from me adjusting gain and EQ pots).

Maybe the samples will help to understand what I'm experiencing.

Can I connect a ground lead from the power cord input direct to the chassis? There is an unused grounding tab on the inside of the power cord plug-in socket. Should I also connect pin 1 of the XLR input directly to the chassis using a 0.01 micro-farad capacitor (I've heard this can help)? I really want to eliminate the buzz/RF noise from this amp.
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Bodoc

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #10 on: March 14, 2011, 04:30:10 pm »

While you're attempting to chase the grounding ghost in the machine, try the Ebtech Hum-X (http://www.youtube.com/watch?v=YyYb266R2-M)

It is, apparently "safe" (http://www.ebtechaudio.com/humxdes.html).  So, while you try other solutions, you just might be able to use your gear in the meantime.

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0dbfs

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #11 on: March 17, 2011, 02:15:15 pm »

I recently saw these exact symptoms in two different facilities with different pre-amp's. In both cases it was that either the + or - were swapped with shield and as a result, chassis.

Hope that helps!

Best,
-jb

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Jonathan Burtner
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mdb

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #12 on: March 24, 2011, 02:19:54 pm »

Thanks for all the help here. I sent the unit to Vancouver to a most excellent technician. I do not have the Trident back yet, but am confident it will be great when returned. There was some cold solder joints, oxidized input XLR posts and no ground lift circuit. Jumping a ground from earth to the XLR pin#1 stopped the noise.

While it's opened up I am also changing the original preamp with a THAT Corp 1512 pre IC and some of the opamps with Burr-Brown OPA2032's. I might be replacing the rest of the opamps with Burr-Brown OPA627 or with the OPA2227 ops, but it depends on whether the pins match and they can be fit onto the PCB. It may not come back sounding like a "Trident", but it will be the best sounding Trident S40 around!!

If you need something fixed, Steller Studios in North Vancouver is excellent!!
http://www.steller-studios.com
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mdb

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Re: Buzz from preamp stops when touching chasis. HELP!!
« Reply #13 on: March 29, 2011, 01:48:06 pm »


The Trident S40 mod is complete and there are sound samples between it and a Neve Portico 5012 and a Steller Lucid pre here:

http://recording.org/diy-pro-audio-forum/48946-trident-s40-by-john-oram-mo ds.html
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