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I have the following setup:  From the top down, the whole list is Marantz TT 1200 turntable (ancient, stolen from my parents) Headamp 6 Pro headphone splitter DBX 215 15-band equalizer DBX 266XL 2-chan compressor Presonus Firestudio Project firewire recording interface Furman power unit and Behringer PP40 MicroPhono preamp Amplifier and speakers are not pictured. When the setup is just [turntable -> preamp -> amplifier] (all RCA) everything works as expected. I figured since I have this EQ sitting here, I might as well put that in the chain, so I can shape the sound a bit, depending on if I'm playing a recent-times Amy Winehouse record versus some old 70s Bootsy record. I'm assuming the preamp should com before the EQ (instead of the other way around), so the EQ can deal with a line-level signal. Confirm/deny? So I set up [turntable -> preamp -> EQ -> amplifier], and although everything "works" (changing the EQ bands does change the sound), I also have this noise coming out of my speakers: http://epswing.com/shared/sound.m4a. The noise in the recording changes based on moving my new-age recording device (iPhone) to the top and bottom cones of my bookshelf speaker. Maybe it's the EQ. So just for fun, I do [turntable -> preamp -> compressor -> amplifier]. Same noise. The headphone amp has a pass-thru so I try that too, [turntable -> preamp -> headphone amp -> amplifier]. Same noise. To get in/out of the EQ (and comp, and headphone amp), I'm using RCA-to-1/4" adapters:  Everything is plugged into the Furman power unit, so it's not a ground loop. Am I doing something unbelievably stupid? Help!
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Nov 6, 2015 01:10
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| # ? Apr 19, 2024 00:43 |
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I see that the preamp has a dedicated ground screw - out of curiosity, does the turntable also have one? If so, try connecting them together and see if your noise goes away. Years ago when I used to DJ at a club, the SL1200 turntables they had would start causing almost that exact same noise if one of the ground leads somehow came disconnected. They were connected to a ground screw on the mixer, but it served the same purpose. (edit) Even if the turntable doesn't have a dedicated ground point, you can use pretty much any screw in the chassis of the unit, as long as it's actually in contact with the metal case. Acid Reflux fucked around with this message at 00:33 on Nov 7, 2015 |
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Nov 7, 2015 00:30
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Yep, the preamp has a ground screw, and the turntable's RCA cable run includes RCA-L, RCA-R, and a ground wire (which is hooked into the preamp ground screw). The turntable doesn't have a ground screw. (Aside: just curious, would it make sense for the turntable to have both a ground wire, and a ground screw? Would hooking a device's ground wire back into the device's own chassis accomplish anything?) Just clarify, when you say "try connecting them together", do you mean run some more wire from preamp ground screw to turntable chassis?
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Nov 7, 2015 01:15
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epalm posted:Yep, the preamp has a ground screw, and the turntable's RCA cable run includes RCA-L, RCA-R, and a ground wire (which is hooked into the preamp ground screw). quote:The turntable doesn't have a ground screw. (Aside: just curious, would it make sense for the turntable to have both a ground wire, and a ground screw? Would hooking a device's ground wire back into the device's own chassis accomplish anything?) quote:Just clarify, when you say "try connecting them together", do you mean run some more wire from preamp ground screw to turntable chassis? And with that, I'm already out of ideas, unless maybe disconnecting the ground wire from the preamp changes something? There shouldn't be any ground loop happening, as you said, but maybe there's something funkadelic in your particular setup. Sorry I'm not any more help, it's been a looooooong time since I've really had my hands on any audio stuff. 
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Nov 7, 2015 01:50
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Acid Reflux posted:And with that, I'm already out of ideas Yeah me too, I'm kind of at a loss here. As a software engineer, I'm getting that familiar feeling where "everything looks good, but the actual result is still not the expected result" which 9 times out of 10 is simply due to a lack of understanding. Are we at least pretty sure we're dealing with a loop here? Acid Reflux posted:unless maybe disconnecting the ground wire from the preamp changes something? I'll try this. Plus I'll try connecting the ground wire to the chassis of the rack itself. Will report my findings if I haven't electrocuted myself.
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Nov 7, 2015 03:56
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| # ? Apr 19, 2024 00:43 |
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I have that weird feeling that I'm missing something obvious, but I can't quite suss it out. I'm going to attribute that to the 20+ year gap in my audio equipment troubleshooting knowledge. It *sounds* like an improperly grounded table, but I couldn't say for sure if it's actually a loop or some other oddity. Hopefully someone both smarter and more up to date can chime in and make a fool of me soon.  If there's any real danger of electrical shock, just get a friend (or someone you actually don't like that much) to do the work while you stand by with a long stick at the ready. You need to pry 'em off the equipment pretty fast, or the smell will linger for hours.
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Nov 7, 2015 05:33
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