Friday, July 29, 2016

The Mysterious Case of the Turntable Hum

So there's this project down in SoCal; the client is a huge vinyl rock 'n roll and blues fan with what you'd call a multi-use media room. After the initial commissioning of his system, everything was groovy (pun intended) save for some serious hum and noise issues with the high-end turntable rig. Unfortunately, scheduling required us to knock off before we could track down all the issues. 
Over the next while, the installer's guys and some outside consultants gave it their best shot - to no avail. Eventually, I said, "Enough is enough; we've got to get this guy squared away."  

So, I loaded up my test gear, tossed in my trusty Technics SL1200 turntable with a basic preamp of my own, and headed south.
After a good bit of head banging (the bad kind) and hair-pulling, I replaced his turntable and phono pre with mine. Presto! Hum/noise gone. Taking systematic approach of slowly inserting the client’s gear into the scheme, I found a “perfect storm” of issues that caused the unacceptable levels of hum and buzz.
First off, a (much) closer look at his turntable revealed that the connector box that took the cartridge lead-in wires to the RCA connectors was powder-coated. Uh-oh. The ground lug, barrel connector for the lead-in, and the RCA connectors were all touching a powder coat finish instead of actual metal, and there was no ground contact anywhere. So there I was for an hour scraping off powder coat from the contact points with a tiny screwdriver. Good times! A reasonable person would expect that to fully fix all the issues, but no such luck. 
I then observed that the lead-in wire that looped up and out of the tone arm wasn't shielded and picked up RF interference. Also, we still had some oscillation and hum with his phono pre, but not mine. It was a vacuum tube device; lo and behold, one of the tubes was probably bad! We replaced the preamp with a fresh one from stock, thanks to some forward thinking on the part of the installer. Better, but still not perfect.
That's when I noticed the single-ended link between the phono preamp and the main pre-processor - a distance of about 25 feet - ran near lights and other EMI/RFI-spewing electronics. I don't want to point fingers, but that's kind of a no-no. Such a run needs to be balanced in order to reject the noise. Luckily, the wire used for the connection was actually a twisted pair with shield. So we upgraded the connection by inserting a Jensen transformer-based balanced converter after his phono pre and running the other end of the wires into a set of XLR balanced stereo inputs of the main system pre-processor.
There was still a tiny bit of noise audible when listening at a few inches from the tweeters, attributable to the non-shielded lead-in wire and noise floor of the tube-based phono preamp. The shield issue is simple enough to fix with a very flexible braided shield connected to the main ground lug. The quiescent phono stage noise floor is then probably low enough not ever be heard at the seating position.
The end result was a great improvement in the hum/noise problem - also some renewed lessons learned about verifying that gear actually comes from the factory working right!

No comments:

Post a Comment