The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    SP14 Hum reduction and Chassis Grounding

    Share

    mijohn

    Posts : 33
    Join date : 2013-06-19

    SP14 Hum reduction and Chassis Grounding

    Post by mijohn on Thu Feb 06, 2014 4:28 am

    While looking at the build photos of the various VTA SP14 preamps on this site I noticed that the primary and secondary pairs to and from the power transformer are not twisted. I thought it was good practice to twist all AC carrying wires, but here the only AC wire twisted is that from the IEC socket to the power switch. Twisting will at least do no harm and may help reduce hum.

    In the photos and wiring diagram for the SP14 the chassis has no safety ground connection and could become live in certain fault conditions. The exception is Roy's latest build photos posted in November 2013 where the bus bar wire appears to be connected to the chassis earth screw on the back next to the phono input jacks which makes sense to me. If I'm correct, in all the other builds, that screw is connected to the chassis but nothing else and therefore ineffective and the chassis is not safely grounded.

    I started building my SP14 before I saw Roy's latest build photos and have earthed the chassis at the IEC socket and connected it to the bus bar wire via a "High Current Safety Loop Breaker ", also known as a "Hum-loop block network", which consists of a 10 ohm/5 watt resistor in parallel with a 100nF capacitor and a bridge diode to by-pass fault currents. When a ground loop is created it will now have a 10 ohm resistance in series with it, which should reduce power supply ground-loop currents to negligible levels, according to Valvewizard. See the links for details:

    http://sound.westhost.com/earthing.htm

    http://www.valvewizard.co.uk/Grounding.pdf

    The Valvewizard article also gives good reasons why the ground-chassis connection should be close to the inputs as is the case in Roy's latest build photos. When I have finished my build I will report the results.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: SP14 Hum reduction and Chassis Grounding

    Post by tubes4hifi on Fri Feb 07, 2014 7:16 pm

    if you are building a preamp such as the SP14 in an aluminum chassis with the parts that I provide (which includes an IEC AC input line filter)
    the line filter and earth ground are connected and so is the chassis, and then the PCB is also grounded to earth ground.
    As for transformer windings, it's a little difficult to twist 18g wires that are only 3-4" long.

    mijohn

    Posts : 33
    Join date : 2013-06-19

    Re: SP14 Hum reduction and Chassis Grounding

    Post by mijohn on Sat Feb 08, 2014 10:24 am

    OK thanks, I hadn't realized that those IEC line filter sockets that you use have the earth internally connected to the outside metal case.
    If constructor's have a painted metal chassis like I have, I think it would be a good idea to scrape off some paint around one of the chassis screw holes that the socket is fitted to, to give a really solid earth bond.

    I've managed to twist the AC wires without too much trouble, although the long cloth sleeving that Antek use on their transformers makes them look less elegant than I would like. But that's not a problem of course.

    mijohn

    Posts : 33
    Join date : 2013-06-19

    Re: SP14 Hum reduction and Chassis Grounding

    Post by mijohn on Tue Mar 04, 2014 4:20 am

    [url=

    Well, I've finally completed my SP-14 build and it sounds very good indeed. Not a trace of any hum, but had some hiss in one channel that was fixed by swapping input and output tubes around.
    I can't say for sure if the "hum-loop block network". and AC wire twisting helped but won't do any harm and may be worth trying if you have a hum loop problem in your amp.

    The network consists of a 10 ohm/5 watt resistor, a 100nF capacitor and a 35 amp diode bridge connected as indicated in one of those links attached in my earlier post.

    Sponsored content

    Re: SP14 Hum reduction and Chassis Grounding

    Post by Sponsored content Today at 10:04 am


      Current date/time is Tue Dec 06, 2016 10:04 am