R/E/P > Bruno Putzeys (Designer) - Dave Hecht (Master Tech)
Buzz from preamp stops when touching chasis. HELP!!
mdb:
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!!
John Roberts {JR}:
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
Bruno Putzeys:
When merely touching stops the buzzing I would go and look for RF problems.
mdb:
--- Quote from: John Roberts {JR} on March 04, 2011, 12:02:08 PM ---You say it is quiet with a DI plugged in... What Di? active, passive, ??
JR
--- End quote ---
The DI is passive. It's a Radial AV2 multi-media box.
John Roberts {JR}:
I would look for the DI perhaps adding some capacitance across the input, which could change the RF behavior.
JR
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