Power Supply 7 Gnd.

Discussion in 'General Electronics Chat' started by kbyrne, Dec 25, 2011.

  1. kbyrne

    Thread Starter Member

    Dec 10, 2011
    I am working on a project and require assistance all thou it works. The project is a dual 12watt amplifier. It has two 24vac@2amp. transformers parallel wired in the primary side. The secondary side has 4 wires going into two different power supply boards. Ie vac-gnd. & vac-gnd into two bridge rectifiers then into a 100nf @ 250volt metal film capacitor then into a 4700uf @ 50v capacitor and then finally into another 100nf @ 250v metal film. Both power. supply boards are the same just duplicated. The output of the power supply boards is as it is needed. +36vdc & gnd. +36vdc & gnd. I get a small amount of hum out of both amps in a plastic box. Can I hog point this type of design when I put the entire set up into a metal box by running a wire backwards from circuit common
    on both boards to the green wire of a three prong plug without blowing up the box, capacitors ect. Does anyone know the correct way. The two amplifers power four 30 Watt speakers pretty nicely for my first attempt. I think I made a mistake in plastic enclosure thou. tanks
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    There are so many ways to pick up hum. A shielded box can help but is not necessary. Can you post the schematic? Do you have access to a scope ( if not scopes for audio work can be bought used on ebay or the like for short money )
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    As the last poster has pointed out, hum has many causes. A plastic case is not necessarily a bad thing for an amplifier, as there are many adequate commercial products using plastic enclosures. Some internal screening may be required however, to block out interference. The use of a non-conductive case may make this more important, but it may be needed against internal interference anyway. This will be particularly important when a sensitive amplifier is mounted close to a power supply.

    Interference may be by electric field coupling, which can often be blocked by electrostatic shielding, e.g. by using screened or coaxial signal cable.

    Magnetic field coupling can be more troublesome, e.g. hum loops, where a closed loop of common wiring picks up an AC magnetic field. Mains transformers in power supplies are a possible source of such fields.

    Another source of trouble can happen when a length of nonimally ground potential conductor is shared between an input circuit and for instance a DC supply line, where the current being supplied may be less than perfectly smooth. This is rather similar to a ground loop, but sometimes gets given a fancy name like "common impedance coupling crosstalk". It is a big nuisance in anybody's language.

    The power supply itself may not be smooth enough, or the amplifiers may react excessively to just a small amount of ripple on the supply. This is one of the situations where it is very helpful to have access to an oscilloscope - to check that the supply is clean.

    Often a mains ground is recommended or actually required for safety. Although it should be safe to connect a ground to your system care is required*. Problems, including possibly burned out parts, can happen if a mains ground is connected to more than one part of the circuit. Connecting a mains ground may also worsen hum loop problems. Where a number of grounded units have to be interconnected, it is sometimes necessary to use special methods to get around this.

    Finally, a question: what does "hog point" mean? I've never heard it before!

    *Edit: Connecting a ground is only possible if your system has a proper isolated power supply, but if this is not so you would have an inherently unsafe system. We are not even allowed to discuss that kind of thing here.
    Last edited: Dec 25, 2011