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Old 30th April 2009, 02:43 AM   #1
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Default help: preamp hum

I have recently aquired a couple of turntables, one including a Radio Labs ST-PH1 preamplifier. I'm new to vinyl, but here's the problem:
When I connect my preamp to my stereo reciever there is a loud alternating hum and no music. If I connect a turntable directly to my receiver I get little sound (no pnono input), but also no hum.

I will attach a sound recording of the hum. I can hear the hum through the speakers, and faintly through the preamp itself. Yes the circuit is making noise. I have tried grounding the amp and turntable to the receiver chasis, an unplugging each individually, and unplugging all; still there.

Please have a listen and let me know if you think this is ground loop hum, or a dead power supply/ voltage regulator.
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Old 30th April 2009, 07:39 AM   #2
mjf is offline mjf  Austria
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hello.
it listens to me like the preamp has failed - you can check the powersupply with a multimeter (voltage regulator and rectifier(diodes),supply elcaps..............).also have a look at broken connections/bad soldering points.
greetings...........
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Old 1st May 2009, 08:41 PM   #3
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Thanks, I'll try and pry the circuits out of the enclosure and test them. It looks like they were potted in there, but hopefully not.
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Old 1st May 2009, 10:07 PM   #4
eyoung is offline eyoung  United States
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Hola Y'all...


The question is what kind of cartridge moving magnet or moving coil and if moving coil , hi or low output??? The next kind of question concerns preamp what is it and does it have more than one phono input(with or with out capacitance and impedence loading). Finally what kind of amp/reciever is it??

Respectfully, Elwood
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Old 1st May 2009, 10:41 PM   #5
cbdb is offline cbdb  Canada
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Agree with bad preamp, its not a ground hum, dosnt sound right. Looks like the turntable/cartridge is working (it should put out low level whith no low freqs) But its a great buz, is it alright if I add it to my sound effects library?
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Old 2nd May 2009, 04:47 PM   #6
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eyoung: Both turntables use a MM cartridge. One has a Yamaha something or other, the other I tried a Stanton 500 and Shure m97xE (new).

here is a .pdf for the phono preamp:
http://www.rdlnet.com/pdf/Data_Sheets/st-ph1.pdf

cbdb: ha ha, feel free to add that sound to your effects library.
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Old 2nd May 2009, 10:51 PM   #7
eyoung is offline eyoung  United States
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OK...

So now we need to know what kind of amp/reciever you are driving and to which inputs???

Regards, Elwood
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Old 3rd May 2009, 12:35 AM   #8
kaos is offline kaos  United States
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Where are you grounding the turntable, and what kind of power source are you supplying the pre-amp with?
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Old 3rd May 2009, 05:46 PM   #9
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Turntables: Technics SP-25, Yamaha P-33

The Technics has a ground screw on the motor, the Yamaha I didn't bother grounding but it does have a ground connection near the phono output.

The power source for the preamp is made by the same manufacturer as the preamp: Radio Design Labs. It has a 24V 400mA wall wart.

As for connection I have tried one of the video inputs, and the AUX input on my receiver since there is no phono input. It is a Sherwood RD-7500 ( 6.1 DD, DTS, NEO, etc) that I purchased at Circuit City a couple of years ago. It has no earth ground on the power prong, but I tried running a ground wire from one of the under chasis screw to the preamp & Technics turntable just for kicks, but no improvement.

I'm just going to assume the preamp is dead since the unit itself makes hum ticking noises at the same frewency interval as the audio hum. I went ahead and ordered a TCC TC-750 phono preamp, so I'll try that out when it comes this week and let y'all know if that fixes the problem.
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Old 15th May 2009, 03:28 PM   #10
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Good news: I connected the TCC TC-750 phono preamp between my Yamaha turntable and receiver and there was only a slight ticking, which went away when I ran a ground wire from the turntable to the preamp.

I was worried that there were some apartment power wiring problems since we have an outlet that only works intermittently. I have yet to try the Technics deck, but assume it should also function properly with the new preamp.
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