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Thread: Electrical safety (risk of electric shock)

  1. #1
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    Default AC polarity etc. (dangerous)

    This thread is concerned with general issues, not amplifiers, sources or cables.

  2. #2
    Mank Guest

    Default Some Help From Alan Or Friends, Please.

    Hi Friends,
    As i don't possess technical knowledge to a high degree, I was told that there is a difference when the equipment is connected "in correct phase" to the AC current, than not in correct phase. That means that -theoretically- the "live" of the incoming plug should be connected to the "live" pin of in the socket of the device.
    I don't know how one could find which is the "live" pin on this socket.
    Does phase-correct to-phase from AC current really influence something, or is it an exaggeration? Does it really exist in AC current which becomes DC into the device?
    Then they told me that I should hear a difference in sound when I invert the plug to the AC feeding socket and play with it several times. When phase is correct, the sound -they said- should become clearer, tighter in bass, more open in treble...
    I couldn't find this difference when I experimented. Is this all serious and true, or am I losing time? I guess speaker red and black is critical, it's polarity. But with AC current, what?
    Any answers or comments very welcomed to solve my theoretical problem.
    Many Regards from Greece (AC current at 220v, schucko sockets standard).
    Cheers,
    Thanos

  3. #3
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    Default Re: Some Help From Alan Or Friends, Please.

    Quote Originally Posted by Mank
    Then they told me that I should hear a difference in sound when I invert the plug to the AC feeding socket and play with it several times. When phase is correct, the sound -they said- should become clearer, tighter in bass, more open in treble...
    Very simple answer: whoever put that idea into your mind is clearly barking mad and they should be ashamed of theselves.

    The only thing you have to worry about is that the speakers are wired in phase i.e. red terminal to the (+) tag on the amp.

    It is extremely likely (in my opinion) that in some or even many multi-mic recordings (or those with complex EQ gear through which the signal is routed) that some microphones or mixing channels are out of phase to others. Fortunately, on a large scale performance with many mics even if one or two are out of phase, their effect will be swamped by the other ones. I know, because I made a recording that way - I arrived rather later than intended and just cut corners. I'd borrowed mics and cables from various friends and as I faded up certain channels it just didn't sound quite right: I found out why later.

    Furthermore, and rather worryingly, I have never met a recording engineer who has bothered to actually check the phase continuity of his kit from the mics right through to the recorder with a 'scope and pulse generator. Everyone assumes it is wired 'correctly'. So if the front end of the chain is not controlled do you really think any serious effort should be wasted worrying about the absolute phase of the amp/CD at the back end? Not on your nelly.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  4. #4
    danrubin Guest

    Default AC and signal polarity

    AC polarity and signal polarity are two different things, are they not?

    Regarding correct signal polarity (which some digital sources and preamps allow you to switch on the fly), I generally can't hear it, but some people say they are very sensitive to it. And I'm told it varies from recording to recording, even from track to track, so you could drive yourself nuts.

    The guy who publishes Bound for Sound puts a lot of stock in getting AC polarity correct throughout a system. He has no credibility in general (my opinion), but there you go.

  5. #5
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    Default Electrical safety ...

    Quote Originally Posted by danrubin
    Regarding correct signal polarity (which some digital sources and preamps allow you to switch on the fly), I generally can't hear it, but some people say they are very sensitive to it.
    I can't comment upon individuals marketing claims, but just the general picture. I think the most important point is we strongly discourage any experimenting with mains voltages, plugs or wiring because it could kill you. As I understand it, in the UK the neutral pin is bonded at the power station to the ground, so it is nearly at the same potential as the earth pin. The switch inside mains operated equipment is correctly safe if it interrupts the live pin but if you reverse the mains plug then the entire equipment will be live, and the switch will be breaking the neutral, return side. That is dangerous especially is there is no earth pin connected to the case. You could be electrocuted.

    As I commented earlier, if the microphones can not be guaranteed to be signal in phase (that is, a positive pressure on their diaphgragms produces a positive going voltage at their balanced output pins) - and they can't unless someone personally checks every one, and every cable, and the entire mixing desk etc. - any conclusions anyone may draw about in phase/phase reversal at the far end of the reproduction chain are utterly meaningless.

    Just one example comes to mind: it is a well used trick - and tbe BBC engineers were probably the first to develop the skills - to use the Lexicon reverb unit to add a little live to dry acoustics. I'm guessing but I would think that it was probably used on many (if not most?) BBC classical recordings. What does it do to signal phase? Has anyone actually checked?

    Did you know that the output from QUAD preamps is phase inverting? The QUAD power amps are also phase inverting which means that the speaker feed is inverted-inverted i.e. back in phase with the output arriving at the preamp's input socet. But if you used the QUAD preamp with another power amp ... who knows?
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  6. #6
    danrubin Guest

    Default Tube preamps and phase

    Many tube preamps are phase-inverting and we are generally instructed (by the manufacturers, by audiophile conventional wisdom) to compensate for this by reversing phase at the amplifer output terminals or at the speakers. But as you say, it's such a pig in a poke, what's the point? Even if you can hear a difference.

  7. #7
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    Default Absolute phase ... and musical instruments

    Quote Originally Posted by danrubin
    Many tube preamps are phase-inverting ...
    I didn't know that because I've never investigated it (or even thought to). But if the manufacturer admits that is his situation, and proposes a neat and intuitive solution, I think it would be wise to follow it. But maybe the phase inversion you mention is not applicable to all tube amps. After all, the circuit cost penalty of implementing non-inverting would be small, and could even be a by-product of one particular design approach.

    In which case, maybe a routine recommendation to those without the test equipment to check for themselves would be to ask the manufacturer "From the CD input to the loudspeaker output is your amplifier inverting or phase inverting? Does that change if I select any filters or tone controls?"

    Maybe we could keep a record here of the answers. But I seriously doubt that in music, as opposed to the acoustics lab, this phase issue is audible.

    {A thought: imagine a violin string. When the bow pushes against the string the string starts oscillating with the sound wave's leading edge radiating in a positive direction away from the instrument. But when the player's bow pulls against the string, the positive motion is arrested and becomes a negative going, with the sound wave's leading edge radiating towards the player's ear. Hence, the phase of this note has reversed but it's the same note. Can you hear its phase? No. Imaging that situation multiplied up across the entire orchestra where some players are pushing the string and some pulling ..... do we now think that 'absolute phase' is a real issue in music?}
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  8. #8
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    Default Re: Electrical safety ...

    Here is a link to an article which describes the process.

    http://www.audaud.com/audaud/JUL01/E...uip3JUL01.html

    Don

  9. #9
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    Default Re: Electrical safety ...

    I urge our vistors here NOT TO MAKE ANY INVESTIGATION, MEASUREMENT OR ADJUSTMENT OF THE MAINS ELECTRICAL CONNECTION TO THEIR SYSTEM WITHOUT QUALIFIED ADVICE. Leave well alone.

    Follow the manufacturers instructions to the letter. In my opinion there is a serious risk of electric shock even attempting to measure mains voltages. Whether or not there is any sonic improvement, nothing can be worth putting your life at risk.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  10. #10
    Miss j9 Guest

    Default Re: Getting the best from your system

    Hi all,

    At long last managed to log in! Problems with my PC or something. WRTO the thread on phase inverting signals etc, all I would say on the matter is that if you have a "figure of 8" socket into your amp/cd/whatever (which means that it does not matter which way round they are connected) then from my own theories and experiences, you CAN hear a difference in sound. Not directly, but as a process of the way the gear inside is wound to the transformers. Think of at one specific moment Live = lots of current and neutral = near earth therefore not much happening, so when you wind a transformer generally the live end goes nearer the core of the transformer and the neutral end is the top of the winding nearest to where the secondary is wound. If you had them connected the other way round then nothing much would be happening near the ferrite ring and most of the current nearest the secondary, so performance of the transformer is not so good.

    More noticeable in UK as mains is not balanced, but in USA it is.

    Simply switching over the figure of 8 socket would do no harm and and you could use your ears to see if the change produces a beneficial result. On IEC plugs don't switch Live for Neutral!

    On an aside, can you hear a difference if the + is at different ends of a speaker coil? I.e. if you swapped round + for - on BOTH speakers could you hear the sound eminating from the speaker being sluggish/slower/muddled?

    Miss J9

  11. #11
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    Default Re: Getting the best from your system

    Dear Miss J9 (wish I knew your original name- did you publish it in the past?),
    I feel that -as a scientist/engineer- as to the transformers' behaviour- you are probably right. Personally, I cannot feel/hear/realize any difference. Do you have any device to suggest that -with one simple click- will help us find the right "plug in"? If not, lets remain reasonable, giving more gravity in understanding Alan's above comments, so much a wise engineer as he is.
    AC current has no polarity, practically speaking. It is the Answer from chief engineer of McIntosh Labs, N.Y.
    Thanks everybody,
    Thanos from Athens.

  12. #12
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    Default Electrical safety (risk of electric shock)

    I noticed when handling two metal phono plugs a slight tingling. After some investigation I found 118V AC difference between the two grounds. Both equipment continued to work correctly.

    UK mains supply is 240V 50Hz, and we use 3-pin mains plugs. The middle (safety, ground) pin is connected to the building's earth point. I discovered that the earth pin in my mains plug looked as if it was connected but in fact, metal fatigue had severed the connection from the internal cable. Hence, the equipment was not earth bonded but was floating with dangerous potential. I was lucky. Please regularly check your mains cabling and if you are in any doubt about electrical safety consult with an expert.

    If you ever feel a tingling when handling electrical equipment there is a fault, have it investigated imediately.

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    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  13. #13
    eelekim Guest

    Default Re: Electrical safety (risk of electric shock)

    Thanks for reminding us the safety issue. It's really important as we should not take any risk of our life for careless handling of power connection or "improved" sound by floating the ground. I've once tried the latter for my DAC and even my amp! Thanks god that I've been back to the straight way not so long afterwards.

    However, I'm curious how to treat those products with 2 power prongs only. For example, those from the old States are usually without the ground...

  14. #14
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    Default Re: Electrical safety (risk of electric shock)

    Quote Originally Posted by eelekim
    I'm curious how to treat those products with 2 power prongs only. For example, those from the old States are usually without the ground...
    As I understand it there are two categories of mains powered equipment here in the UK and the UK Govt. publishes this useful guide to wiring safety. I quote from it ....

    ==================================================

    "?How do I know if equipment is [or should be] earthed or not??


    Equipment which is not earthed is usually called ?double insulated? or ?Class II? and is marked with the ?double square? symbol. The cable has two wires. Equipment not marked with this symbol is usually earthed and is called ?Class I?. The cable has three wires".

    ==================================================

    Many Japanese consumer electronic systems have been double insulated Class II for years and the supplied mains cord does not have a metal earth pin. Even though the case may be metal, the internal live (mains) parts are two layers of insulation away from the user's body. The presumption is that it is extremely unlikely that the case could come into contact with the live parts. Class II has the big advantage of avoiding earth hum loops.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  15. #15
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    Default Ground loops with Class 1 (earthed) equipment: CAUTION

    Most stereo systems will have at least one Class 1 component. Ground loops arise when there are multiple paths for current to flow to ground. Since the Class 2 components are not bonded, the shield of interconnecting audio cables act as a single path for stray currents.

    Attempting to ground a 2 wire (double insulated) component will, in most cases, cause audible hum. A capacitance exists between a component's transformer's primary and secondary windings and between both of these windings and the chassis. As a result of this parasitic capacitance, you will measure a substantial A/C voltage between the component's chassis and circuit ground. Generally one connection will produce a higher voltage than the other. This can be tested with a VOM (multi meter) by reversing connections and measuring voltage from chassis to ground. The connection which results in the lower voltage on the chassis is preferable.

    {Moderator's comment: Use EXTREME CARE when investing anything related to mains power. It can and will kill without warning.}

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