eightyeightkeys wrote on Sun, 03 October 2010 19:24 |
Continuing the "Unbalanced Noise" thread, I also became hyper aware of other noise in the studio. As I got rid of computer/printer high freq "whines" and Powercore computer hash, I started to become aware of other noise....damn it. I noticed that my external preamp was emitting a low-level 60 cycle hum...very low level, but, why the hell should there be any at all ? So, I pulled it out of the rack to make sure I isolated the preamp from the rack rails and, yes, I did....but, as I pulled it, I noticed that the hum went away ! So, wtf ?....I lifted the ground....nothing no help there. I decided to move it away from my Mackie mixer that I use for headphone cues and voila ! The 60 cycle hum is gone. |
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Is there a transformer in that preamp? EQ included in it as well? Dave, |
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Is that a metal rack with a ground strap? Bill |
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A Transformer is an inductor after all. Sixty Hertz straight from the Mackie PSU. Cheers |
Jim Williams wrote on Tue, 05 October 2010 11:10 |
"Does not play well with others" is the theme here. Throwing together audio gear, especially in racks haphazardly will lead to these problems. A quality install will test each piece with a noise vs frequency sweep using an Audio Precision or similar analyzer. The idea is to not make anything worse. Easy decisions include mounting gear with power transformers all on the same side. Avoid using dbx next to gear with right side power transformers. They are on the left side in dbx stuff. Avoid mounting dsp boxes with switching power supplies next to sensitive analog gear like mic preamps. An Eventide H-3000 mounted in the same rack as a dbx compressor will cause the dbx THD to rise well above 1/2% THD even if they are separated by several spaces. Remember: When gear makers test their stuff it's stand alone. They never test it with other gear nearby nor when mounted in racks with other pieces. You are the test engineer when you install equipment. It takes more thought than screwing in 4 bolts. |
Jim Williams wrote on Tue, 05 October 2010 11:10 |
"Does not play well with others" is the theme here. Throwing together audio gear, especially in racks haphazardly will lead to these problems..... Good stuff here. Dan, His racks are wooden boxes with no ground on the the rails, so it would be easy to create loops. A ground strap on a rail would be in order. Bill |
Bill Mueller wrote on Tue, 05 October 2010 12:12 | ||
I agree Bill, really good stuff here and thanks. This forum really is the best. How do you do a proper grounding strap on rack rails across multiple racks ? |
Jim Williams wrote on Tue, 05 October 2010 11:10 |
"Does not play well with others" is the theme here. Throwing together audio gear, especially in racks haphazardly will lead to these problems...... ....Avoid mounting dsp boxes with switching power supplies next to sensitive analog gear like mic preamps. An Eventide H-3000 mounted in the same rack as a dbx compressor will cause the dbx THD to rise well above 1/2% THD even if they are separated by several spaces.....It takes more thought than screwing in 4 bolts. |
Jim Williams wrote on Fri, 08 October 2010 10:32 |
Any DSP box with analog already inside is a "you are what you is" Frank Zappa scenerio. Do avoid pure analog processors next to DSP switching power gear. That includes line level EQ's, compressors, mic preamps, etc. I place all my DSP reverbs in their own rack, the analog gear is in it's own rack. I avoid placing anything with a digital clock near any analog gear. I do use metal racks as they have the best screening and mechanical mounts. I do use isolation washers on each rack screw. Electrical isolation is maintained this way. To check I use a meter to each chassis to ensure isolation. The reason for that is the "doesn't play well with others" problems you eventually come across. Some gear connects their audio grounds to chassis, some float it, some use a RC network. Without knowing how to read schematics I don't know how anyone can suss all that out and get each piece grounded correctly. That's probably why large well funded installs pay for professional help. The end result should be zero hum anywhere and very low hiss levels. |
Bill Mueller wrote on Fri, 08 October 2010 08:02 | ||
Jim, Great stuff. Over the years, I have seen it both ways. 1. Ground all chassis. 2. Lift all chassis and rely on the internal grounding scheme of each unit. My issue with lifting all chassis is unless all the units are transformer isolated, they are going to get chassis connected through their inputs and outputs anyway, defeating the lift at the rail. What do you think about this? Bill |