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Started by RandomGruve, December 31, 2014, 04:49:59 PM

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RandomGruve

200
200 
My piano very recently had a Retrolinear Warneck 200 amp and Vari vibe installed. I have no idea if the hum is related to this.

I get a loud hum through my Fender Vibrolux Reverb (or any of my other amps) when connected to the aux out of the Wurlitzer 200. Even when the piano is turned off. If the piano is off but plugged into the wall anywhere in my house, I get this hum through my Vibrolux. I tried plugging the piano into a Furman PL-8C. No change. I also tried a different piano power cord. No change.
*NOTE - The hum through the Vibrolux amplifier is louder as I turn the amp volume up. If the Vibrolux volume is at zero, no hum. There is also no hum with my Rhodes or my guitars plugged into the Vibrolux.

Also, I do not hear this loud hum from the pianos onboard speakers. However, if I dime the piano volume, there is a light hiss. If I also dime the vibrato volume and I can hear the vibrato tapping. (while not playing).
I also tried a new digital piano with the Vibrolux (and other amps)  No hum from the digital.

Any ideas?

Tim W

#1
Hi RandomGruve,

If you get hum with the piano off, you have a ground loop.  The loop is through the piano's 3rd prong safety ground on the AC plug and your amp's 3rd prong ground (ground path 1) as well as the signal cable (ground path 2).  The Aux out jacks in 200s are usually metal and grounded to the piano amp rail, which is also directly wired to 3rd prong safety ground via the sheet metal chassis.  In later Wurlitzers (200As) they used a sleeved 1/4" jack to allow the audio out ground to be tied back at the amp board to help reduce the hum. 

Your other instruments will not have this problem if they are passive or not safety grounded to the AC line, since the only ground connection between your amp and the instrument is via the signal cable.  Ground loops are funny, but there is a clear way to troubleshoot and eliminate them if you are familiar with the problem.

You can verify this by floating the piano's ground temporarily by using a 3 prong to 2 prong cheater adapter.  If you float the piano's ground, and the hum goes away, the ground loop is verified.  You can also verify this if you don't have a cheater adapter by leaving the piano off and just pulling the piano's AC cord out of the wall while your amp is on.  The hum should go away. 

You may also still have the cap that goes from the AC line to chassis ground in your Vibrolux and it may be exacerbating the issue.  First try the polarity switch on the back of your amp.  Even better, have someone qualified remove the cap from your amp.  By today's standards, it is no longer considered safe unless it is a special kind of cap and you can guarantee that the amp will always be properly grounded in use.  Not possible if you play out in clubs and bars.

I don't recommend keeping anything floating from ground as it may present a safety risk.  This is why pros, in practice, in the stage and studio use direct boxes and isolation transformers to eliminate ground loops and hum, while still ensuring safety.

Please call us next week if you'd like to further discuss the issue.  We are closed for the New Year's holiday, but may have a few additional ideas for you. 

Happy New Year,
Tim
retrolinear.com

RandomGruve

Hey Tim,

Thanks for the help. I tried the two prong adapter. The noise is gone. I also pulled the piano power plug from the wall (while the piano was off). This also stopped the hum.  :)
Enjoy the holiday

Mike Borish

If you like old gear and have a lot of musical equipment set up in your house, I highly suggest that you get something like this: http://www.amazon.com/ELECTRICAL-RECEPTACLE-TESTER-OUTLET-PRONG/dp/B002Q3R7HI

Without getting into too much detail, I've run into a lot of issues with houses that are just plain wired wrong and neutral is switched with line.  The above tool will help you communicate with technicians and verify that your space is set up correctly.  Also, as Tim said, switching the ground switch on your amp might make a difference, assuming a tech didn't go in there and wire it out or switch neutral with line.  Has your power cord on the amp been replaced?  Sometimes the "Death" cap can cause issues too as Tim explained.   Another issue that I've run into is idiot electricians that ground to conduit when they wire up a house.  This one is a lot harder to catch and far more complicated to explain.  Fortunately, you can eliminate this variable by plugging in the amp and Wurlitzer into the same outlet.

So, to recap.. Buy an outlet tester and verify correctness.  Also plug the Wurlitzer and amp into the same outlet.  If you still have trouble, I can help.
www.borishelectronics.com

We're a Chicago company that repairs amps, pro audio, DJ gear & synths!