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In Reply to: RE: Hum on FPIII - A Definitive Troubleshooting Checklist posted by breakfastchef on September 21, 2009 at 14:21:28
BTW, the difference between hum and buzz is high frequency content. Since you are not looking on an oscilloscope just listen. If it is all low frequency, you will hear something that really says “mmmmmmmm.” If there is a high frequency component it sounds nasal. You will probably hear something from your tweeter.
Ok, let’s see, these are mainly for hum. But can apply to buzz.
My first question, are your outlets grounded, did you check with a three prong tester?
Remove the interconnects and leave your components plugged in and turned on. Then take your meter and measure the AC voltage from each chassis to the screw on the outlet (the screw is grounded). Then turn them off and measure the resistance from the screw to the chassis of each. This last one tells you if the chassis is grounded.
If the voltage on the chassis (I was recently informed that this is correct for the plural) are different by as much as 20V AC then there will be noise introduced. Each chassis should be grounded to the ground lug of the power cord. The readings should be 0V AC.
That done, moving on. The FP III has many improvements that eliminate hum. And the fact that it didn’t hum before would indicate that they worked. The move must have changed the grounding scheme or interconnect integrity or broken a ground point in the preamp.
You say that it hums with all inputs, more with two of them. First, you should short each pair of inputs, at the RCA input jack from center to grounding tab. Try each individually. If this eliminates the hum it probably isn’t in the FP III. Even if this works, eliminates the hum, check if you have continuity between the hot and shield of each interconnect. Moves can be hard on them. Winding them up puts strain on the RCA connectors.
Since you say that the hum goes with volume I would guess that it is injected before the volume control. Good! There are few things to look at. You should touch up, resolder, each of the RCA jack hots and grounds. Do the same thing where the TSP enters the selector switch. And again where the signal leaves the selector switch and enters and leaves the volume controls.
That brings you to the voltage amplifier stage and it shouldn’t be involved since the noise follows the volume setting.
Hopefully this will find the problem. I’m not certain that an inline resistor will do more than pad the input. It is used in some really sensitive systems that are loud at low volume settings on the FP III volume controls.
Remember, YOU are the only one who needs to be happy with the sound of your system
Grainger Morrison
There Is Only One (Grainger Morrison, it seems)
Follow Ups:
Grainger:
Again, thank you for taking time to post your comprehensive short guide to hum reduction. After some time taking measurements, plugging and unplugging ICs and playing with ground schemes, I have not yet solved my problem. That said, these are my impressions.
First, perhaps I am being overly critical of my BH system (Paramounts, Foreplay III and Seduction). It may be that these components cannot produce inky-black noiselessness.
Second, without extra tubes, I cannot test for sure that I may not have a tube problem. This is on my to do list.
Third, the Paramounts, no input, hooked to a pair of speakers are nearly noiseless. There is a very, very low level hum (mmmmm), but not anything close to awful. Interface the FPIII to the Paramounts with no inputs connected to the linestage and the hum increases. Increase the volume of the FPIII and the hum increases.
Fourth, grounds are solid on all BH components and there is no AC voltage from one chassis to another.
Fifth, I did find that my DAC and Squeezebox (both running off power supplies with 2-prong plugs) do inject more hum into the system when the power supplies are plugged into an outlet. Even if I plug these two devices into other wall outlets, they seem to immediately increase the hum heard through the speakers.
Sixth, with the Squeezebox and DAC connected to the FPIII and powered on, the hum is reduced. Do not know why that would be, but the Seduction input to the FPIII is way noisier.
Lastly, my IC's are fashioned after the NAWA cables on the BH website where the shield is lifted on the non-source end. I thought this would work out well by providing a shiled to the signal wire.
I will try to find some other tubes to see if noisy tubes may be my problem.
Let's see...I think the system can be silent. One of the tests is to short the hot to the ground (common) of each input at the RCA jacks and measure the hum (volts AC) at the output of each device. This gives you some measurement of what the component can do. In the case of the Paramounts, short the input and put your meter on the speaker leads. Then tweak the hum pot for the lowest reading. (I think that is in the manual, but if not here it is)
Second, if there is more hum in one channel swap the tubes from right to left. In the case of the Paramounts one tube at a time and adjust the hum pot when swapping the power output tube.
Third, the Paramounts should have a shorted input for this test. I have a cheap pair of RS RCA plugs with the hot and common shorted I use for this test. You say here that hum follows volume on the FP III. On the FP III for this test the input you are using should be shorted (what again?). If the FP III volume control still increases hum (and it could be the interconnects picking up some AC as they go between the components) you have very little to check. That means the hum is mostly before the volume control. Check the connections at the RCA jacks, the connections to the selector switch and the wiring between the selector switch and the volume controls. Again, maybe one channel is different than the other.
In the fourth example, there should be no interconnects between the two components. I probably forgot to say that. But the common wire of the interconnects grounds the two components together.
Fifth, good clue, there are boxes to isolate these two digital devices from the other animals. (Animal Farm reference here) And both devices can cause all kinds of noise in the system when running. Check with a search. Too many Bottleheads have banged their heads against a wall searching for noise that was caused by digital devices (not hum as I remember). Different wall outlets can be on a different incoming phase of the power. It is best to have all components on the same circuit breaker.
Sixth: Check the ground between the Seduction and FP III (is the chassis of the FP III firmly attached to the Seduction?). The Seduction will have more hum than other components as it has much higher gain for the tiny Phono signal. Test with the ground wire and turntable interconnects off and inputs shorted.
And finally, it is a standard practice in industry to use a + and a - signal plus a shield that is grounded at one end. You will have to have a connection to both hot and common coming through the interconnect. Plus a shield that is connected on one end.
Hope this helps.
Remember, YOU are the only one who needs to be happy with the sound of your system
Grainger Morrison
There Is Only One (Grainger Morrison, it seems)
Edits: 10/07/09 10/07/09
Thanks for sharing your knowledge so generously. I have some work to do tracking down the hum based on your input. I shall report back with my findings in a few days. Thanks, Grainger!
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