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    Hum Issue with VTA TCLA SP9 preamp and VTA ST70

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    MidMiHigh

    Posts : 3
    Join date : 2012-02-12

    Hum Issue with VTA TCLA SP9 preamp and VTA ST70

    Post by MidMiHigh on Wed Dec 26, 2012 2:34 pm

    I am being driven crazy by a hum problem & wondering if anybody else has had a similar problem.

    I have a VTA ST70 and a VTA SP9 TCLA preamp With a SP6 phono.

    The ST70 is dead quiet with inputs shorted and when using an iPod or a PC as input. The hum starts when the preamp is connected to the preamp. I sent the preamp to Roy who could find nothing wrong with it and said it was quiet for him.

    I have used cheaters to lift the ground from the preamp, switch the hot and neutral of both preamp & amp, no change. I even grounded the ST70 & lifted the ground from the SP9 which quieted the hum a little bit, but not much.

    The hum is there with the ST70 plugged into the out jacks of the SP9 even when the SP9 is shut off, but the hum stops when the SP9 power cord is unplugged.

    There is no hum when the ST70 is plugged into the tape out jacks of the SP9 with an iPod connected as an input.

    Any ideas?

    mrconclusion

    Posts : 15
    Join date : 2012-10-18

    Re: Hum Issue with VTA TCLA SP9 preamp and VTA ST70

    Post by mrconclusion on Wed Dec 26, 2012 5:59 pm

    I also have some crazy power issues. In my house some circuits are shared in odd patterns (like outlets in one room shared with exterior lights, basement outlets, etc.)

    Odd tip #1: Try running an extension cord to a different outlet on another circuit. If you can find a circuit with nothing else plugged into it so much the better! If the hum goes away, you know the noise source is on your circuit. Dimmers are notorious, ans some tv's, cable boxes, etc.

    Odd tip #2: Temporarily remove any compact fluorescent bulbs from that circuit. I've found some with very noisy ballasts.

    Odd tip #3: I have a lot of illuminated light switches, the kind that glow softly when they're in the off position. They're also on 3-way circuits and they bleed a little current in the "off" position while illuminated and a couple were noisy. I found Lithonia brand are quiet though.

    Stupid idea: If worse comes to worse, maybe try taking the preamp & amp to a friend's house for a test, or use/borrow an inverter & car battery to test them on completely independent AC source. Even though inverters tend to be noisy you'd at least know whether the hum is different when you plug/unplug the preamp.

    pieto1

    Posts : 1
    Join date : 2012-10-08

    Re: Hum Issue with VTA TCLA SP9 preamp and VTA ST70

    Post by pieto1 on Thu Dec 27, 2012 1:01 am

    I have exactly the same combination which had a hum on the left channel with identical symptoms - hum starts as soon as the power chord is plugged into the wall, even if the preamp is off. In my case I could verify the hum was coming form the preamp since it hummed when plugged in another SS amp I have. I checked and re-checked everything and also reflowed all the joints but could not find anything wrong. When I changed the wiring to take the output from TA, the hum was gone, so it came from the second half of the preamp which I could verify by inputting the signal at TB and then using the normal outputs. The scary part was after all my troubleshooting, the hum also showed up on the right channel and was a lot louder. Turns out the TB wires coming from the tone control was picking up noise form either the grey or yellow wire coming from the toroid power transformer - I did not cut these wires short when connecting to the PCB and tucked the excess under the PCB which caused the hum - by making sure the TB wires and the grey / yellow ones from the toroid are not in close proximity, the issue was resolved and the preamp is now dead quiet when connected to the ST70.

    MidMiHigh

    Posts : 3
    Join date : 2012-02-12

    Re: Hum Issue with VTA TCLA SP9 preamp and VTA ST70

    Post by MidMiHigh on Fri Dec 28, 2012 11:37 pm

    Thanks for the ideas, I will try these suggestions this weekend

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Hum Issue with VTA TCLA SP9 preamp and VTA ST70

    Post by tubes4hifi on Wed Jan 09, 2013 2:54 pm

    always remember that any AC power lines act as both transmitters and receivers of hum, so keep those wires internal to the preamp as short as possible and always AC wires should be tightly twisted to counteract these characteristics.

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