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Volume control at the speaker?

Discussion in 'Electronic Design' started by DaveC, Aug 2, 2005.

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  1. DaveC

    DaveC Guest

    In a distributed audio system in a residence, how can volume control in each
    room be accomplished.

    I realize that it's more complex than just putting a potentiometer in the
    speaker leads.

    Is this accomplished via 70v distribution system (ie, high-impedance
    amplifier output)?

    Or is some kind of acceptable variable attenuation possible in each room?

    Google turns up L-pads. If I understand, an L-pad keeps 8-ohm impedance on
    the line from the amplifier, while providing an attenuated signal to the
    speaker.

    As long as it is properly chosen, are there any cautions I should know about
    installing an L-pad for each pair of speakers in a room?

    Are L-pads reliable (no noise, etc.)? Brands to recommend? Or avoid?

    Other ideas? Speak your peace.

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  2. Beachcomber

    Beachcomber Guest

    Strictly speaking, L- Pads are fixed attentuators that match different
    impedances.

    What you want is a variable attenuator pad that will keep the
    impedance (as seen by your amp) constant (commonly 8 ohms but it can
    vary) as you vary the volume. Additionally, the controller has to be
    able to handle the power being applied to your speaker and may be
    ganged together for a stereo application.

    No affiliation, but here is a link to one such product.

    http://www.hometech.com/audio/volume.html

    70 V distribution systems are used for PA systems (music and paging)
    where there are particularly long runs with multiple speakers.

    With a home audio 8 ohm speaker system, you can't just keep putting
    parallel speakers on the line as each unit will lower the load
    impedance and you will very quickly get to the point where you are
    overloading your amplifier.

    Beachcomber
     
  3. **There are a bunch of methods. The best is to distribute the music, via
    line level (balanced) cables and amplifiy in each room. There are many more
    methods, which are generally inferior.
    **Not necessarily. It depends on how much quality you are prepared to
    sacrifice.
    **It can be. If you want to increase cost and throw away any chance at good
    sound quality.
    **That is one way to do it. Quality will be sacrificed, however.
    **Not really. Just be aware that the L-pad needs to be able to cope with the
    maximum output of your amplifier. They can get very hot.
    **If the power is kept to reasonable levels (say: around 10-15 Watts
    AVERAGE), they can last a few years.
    **They all come out of Asia and porbably from the same factory. Just look
    for the biggest, heaviest one you can find. That would probably be around
    70-80mm diameter.
    **Many surround sound receivers employ a second zone system, which has power
    amps dedicated for this purpose. That would be a good, economical choice.
     
  4. Arfa Daily

    Arfa Daily Guest

    It can also depend on whether the driving amp is a tube or semiconductor
    type. The general rule of thumb is that tube amps don't like open circuits
    ( or anything in between ) across their outputs whilst semiconductor types
    don't like shorts ( or anything in between ). By this, I mean that if a tube
    amp is designed to deliver its rated power into say 16 ohms, then it won't
    like having 50 ohms across it when you wind the wick up. Likewise, a
    semiconductor amp ( transistors, ICs or STK hybrids ) won't like 2 ohms
    across it, if it's designed to run into an 8 ohm load.

    So, to run several sets of speakers, each adjustable, from one amp, it is
    not a problem to put a wirewound pot in series with each speaker, assuming
    that you are running a semiconductor amp. You can calculate the total load
    easily by series addition, and ohms law. It's not quite right, as the '
    ohmage ' quoted for a speaker, is its impedance at a particular frequency,
    not its DC resistance, but near enough.

    I would suggest using series resistors as well, to balance up the levels in
    each room, and still leave a good adjustment range, as well as making sure
    that the whole network cannot drop below the minimum load impedance before
    the amp is being overloaded.

    So let's say you are going to feed out to 5 rooms and use some 8 ohm
    speakers that you've already got. The quoted minimum load impedance for your
    semiconductor amp, is 8 ohms. If you just hook them all up in parallel, the
    impedance presented to the amp will be 8/5 or 1.6 ohms - clearly an
    overload.

    Now change things so that at each speaker, you have a 22 ohm wirewound
    resistor, in series with a 22 ohm w/w pot, hooked as a variable resistor, in
    series with the 8 ohms of the speaker. With the pot at minimum resistance
    ( maximum audio ), each speaker will represent 30 ohms. So with all 5 rooms
    set like this, the total paralleled load presented to the amp, would be 30/5
    or 6 ohms. Unless you're going to play drum and bass at full volume, then by
    the time you've added in a bit of cable resistance, your amp is not going to
    mind this slight reduction in the minimum presented impedance.

    Now turn each speaker to minimum vol ( maximum pot resistance ) Each speaker
    will now be 22 ohms + 22 ohms + 8 ohms = 52 ohms. Parallel all these up, and
    the impedance presented to the amp will be 52/5 or about 10 ohms.

    You would set all this up by setting each pot to about half way, then
    setting the amp volume control to get the desired level in the ' loudest '
    room. The other rooms can then be adjusted down to the desired levels, but
    could also still be adjusted above the loudest room if that became a
    requirement. If the quieter rooms are still too loud with the pots at max
    resistance, then just go for a higher value resistor until you reach the
    level you want, bearing in mind that the higher you go with the resistor,
    the less range the pot will have if you keep with the same value.

    The best thing is to try it in the garage or wherever first. A few different
    values of resistor and pot to play with, bought from your local Radio Shack
    store, will be a lot cheaper and easier than getting special pads. The
    values I've used are just to make the math easy to understand. If you follow
    the principle, you should be able to adapt it. 22 ohms is probably a good
    starting point though. It's hard to calculate a definitive power rating for
    the pots and R's because it depends on many factors, but 3 watt wirewound or
    cermet pots would probably be ok, with 3 or 5 watt resistors. Just try it
    out and see if they get more than ' 3 watts hot '. Assuming that they don't,
    then they will last for ever, not just a few years.

    Finally, by presenting something other than the design impedance to the
    driving amp's output, audio enthusiasts will tell you that you are
    compromising the audio quality. Whilst this is strictly true if you start
    looking at damping factors and other such esoteric quantities, I defy Joe
    Average-Listner to hear anything untoward.

    Geoff
     
  5. In the days when amplifiers were expensive, there would have been one
    central amplifier, for economy of scale, with its output at 70v (US) or
    100v (UK) fed around the building by a line.

    Each loudspeaker unit would have a transformer built-in to reduce the
    line voltage to whatever was required for the speaker. A
    multiple-tapped primary (or sometimes secondary)was used with a rotary
    switch as a volume control.

    Unlike a potentiometer, this method wasted no power at intermediate
    settings and reduced the loading on the amplifier when loudspeakers were
    set to low levels.

    Nowadays, you may find it better/cheaper in a small building to
    distribute the signal at a lower line level or just from the loudspeaker
    terminals of your source and use individual amplifier-speaker units in
    each room.
     
  6. Guest

    That's an interesting set of assertions, considering the fact that
    it's not at all unusual for a speaker's impedance to rise to 50 ohms
    and above within its operating frequency range.
    While this might be true of some units, as a generalization,
    it's not.
    Series resistors and series pots may not be the worst way to
    accomplish this, but until someone suggests something worse,
    this will stand as the likely candidate.

    Given that the impednace of the vast majority of speakers is
    a frequency dependent function and can easily vary by as much
    as a factor of 1 to 10, any substantial series resistance WILL
    result in fairly gross frequency response errors. Let's take
    your suggestion below and see what happens.
    In such a scenario, each "nominally 8 ohm" speaker will be looking at
    44 ohms in series with it. Let's reasonably assume that the speaker's
    impedance varies from a low of about 7 ohms in the midband to say 40
    ohms at resonance. With 44 ohms in series, the attenuation at the
    high point of the impedance will be:

    G = 40/(40+44)
    = 40/84
    = 0.48

    which is equal to -6.4 dB. At the minimum impedance of 7 ohms,
    the result will be:

    G = 7/(7+44)
    = 7/51
    = 0.14

    or about 17.3 dB.

    Thus, the scheme suggested will introduce a frequency response error
    or nearly 11 dB on such a speaker. Hardly subtle.

    Second issue, since all these resistors are in series and thus
    the same current must pass through both the speakers and the
    series resistors, and since the power dissipated goes as the
    resistance times the square of the current, it becomes quickly
    apparent that MOST of the amplifier's power will be used to
    heat up the series resistors. Let's assume that the amplifier used
    can produce 100 watts into a nominal 8 ohm load. Since:

    P = E^2/R

    thus

    E = sqrt(P*R)

    then such an amplifier can produce about 28 volts RMS across the
    load. By Ohm's law:

    E = I * R

    thus

    I = E/R

    so

    I = 28/52

    or about half an amp. That half amp into the speaker will produce:

    P = I^2 R
    = 0.5^2 8
    = 2 watts

    while the series reistor and pot will dissipate:

    P = 0.5^2 * 44
    = 11 watts

    Fully 85% of the amplifier's output will be devoted to heating
    up those resistors, only 15% will find its way to the speakers.
    now consider the very likely possibility that the speakers used
    GIven that the total series impedance can vary from 22 to 44 ohms,
    and assume, for the purpose of simplicity, that the speakers DO
    represent a resistive 8 ohm load, the minimum and maximum gain
    of the proposed arrangement will be:

    Gmin = 8/(22+22+8)
    = 8/52
    = 0.15
    = -16.3 dB

    Gmax = 8/(22+8)
    = 8/30
    = 0.27
    = -11.5 dB

    A range of 4.8 dB, or only +- 2.4 dB from the "midpoint" setting.
    That's completely insufficient for compensating for differences
    in speakers, differences in room acoustics and positioning,
    differences in the amount of level desired or, for that matter,
    to make a substantial audible difference.
    The difference being, of course, that "special pads" have at least
    a chance of working.
    Okay, using that math, I think it can be shown that the scheme is
    uworkable.
    Actually, no, as shown above the math is VERY straightforward. To
    generalize it even further, the amount of power dissipated in
    each serires resistance is directly proportional to the ratio of
    that resistance to the total serires resistance. Take that ratio,
    multiply it by the total power output of the amplifier, and that's
    your power requirement.
    I might suggest that such "audio enthusiast" are unaware of the
    fact that almost EVERY speaker on the market presents an impedance
    which varies rather substantially from the "design impedance" of
    amplifiers. From your assertion above, it follows that almost
    every speaker on the market "compromises the audio quality."

    In fact, all audio amplifier MUST cope with the fact that the
    load impedance is LIKELY to vary widely. And since most amplifiers,
    even most tube amplifiers (with some notable pathological
    exceptions) behave essentially as low output impedance voltage
    sources, this is not an issue.
    I might posit that an 11 dB error on the frequency response is
    likely to be detectable by Mr. Average Listener. That's equivalent
    to adjusting an equalizer to have an 11 dB boost in the bass, about
    a 3 dB boost in the midrange, and a 3-4 dB boost at high frequencies.
    Are you asserting that such is NOT audible?

    If the system is used for primarily background listeing and not
    the utmost in fidelity, then a 70V distribution system with
    levels taps on the speaker transformers is the most reasonable
    way to achieve moderate quality, adjustability, safety and
    reliability with an existing system at moderate cost. However,
    the best way, as suggested by another poster, is to distribute
    the audio signal at some low level and then use local amplification
    at the listening point. The distribution could be low-level analog
    signals over appropriately shielded, twisted pair or preferably,
    via either multi-drop digital or even via networking. This would
    orovide the maximum quality, efficiency and versatility, but
    at the highest cost.
     
  7. You can often cludge together a system using 100 volt line transformers on
    a normal amp, and use the power output taps via a switch to give some
    control of level. Not ideal, but any form of resistive system will sound
    worse.

    But a far better way is to have an amp per pair of speakers, and feed that
    at line level.
     
  8. BoborAnn

    BoborAnn Guest

    I did this in my home using NXG Impedance matching volume controls ( part
    number NX-VCM80) which you can get on EBay for under $20 each this device
    allows up to 8 sets of speakers to be connected to a receiver.It has taps
    based on how many you are using which allow too to present a constant
    impedance to the receiver. They have 12 steps of attenuation which is plenty
    for me
    hope this helps
    Bob
     
  9. Arny Krueger

    Arny Krueger Guest

    (1) Separate amp at each location, not totally impractical
    in these days when you can get a pertty fair 100 wpc amp for
    under $80 if you look around.

    (2) Transformer-type stepped volume controls. Ironically
    they are around $30 each, so you're talking a bit less than
    half the price of the receiver.

    http://www.hometech.com/audio/volume.html
     
  10. DaveC

    DaveC Guest

    Thus spake Dave Plowman (News):
    You being in the UK, I presume you mean 240:100v line transformers, yes? In
    the USofA this would be half that: 120:50v transformers, yes?

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  11. DaveC

    DaveC Guest

    Thus spake Arny Krueger:
    Sources for small amplifiers? Place in wall? Attic?

    Google turns up so many amps, but are stereo system component types.

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  12. Guest

    No, he doesn't mean that. You're talking about AC power. He's talking
    about audio signal distribution and matching.

    There is a means of distributing high-level audio (speaker level)
    using impedance matching transformers. Two different level conventions
    exist: 70 volt and 100 volt. Matching transformers are provided at
    each speaker and are used to determine how much power that speaker
    can get. Some transformers have switchable taps that allow adjusting
    level.
     
  13. DaveC

    DaveC Guest

    Thus spake :
    Ah, yes. I'd known about 70v, hadn't heard of 100v.

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  14. No - 100 volt line as used in PA systems for long runs to speakers, etc.
    You use a transformer to match a normal low impedance speaker to the
    'line' and use the turns ratio to set the power going in and therefore the
    level. With this method you can hang as many speakers across one amp as
    you want - up to its rated output - unlike paralleling low impedance
    speakers across a normal amp. Most such transformers will have taps for
    several different output settings, so can also be used to control level.
     
  15. DaveC

    DaveC Guest

    Is there a wireless solution to distributing audio throughout a residence to
    8 rooms? Digital?

    Something similar to wireless computer networking...

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  16. Arny Krueger

    Arny Krueger Guest

  17. jclause

    jclause Guest


    Yamaha makes a system called MusicCAST that does this. Has a hard
    drive, stores 1,000 CD's in MP3 format or 100 in original format.
    My son sells these in his audio store. See Yamaha.com.

    JC the elder
     
  18. My solution was to make them, as if hidden don't have to look pretty.
    Plenty of kits on the market. I used a DC controlled pre-amp for volume
    etc, as then the controls take up a tiny space and can easily be fitted on
    a face plate of the type used for sockets etc, and there are no safety
    implications if used in a wet room.

    This was some time ago - a remote control might do as well.

    But I've got an easy to wire house. Victorian with a cellar and dry lined
    walls. A modern solid concrete one would be a different matter. ;-)
     
  19. Pooh Bear

    Pooh Bear Guest

    Seperate amplifiers in each room ( or alternatively so-called 'powered speakers'
    with inbuilt amplifiers ) is the only solution that preserves audio quality. I
    assume you don't want quality trade-offs which is all that any other approach
    can offer.

    Graham
     
  20. I used the Niles speaker selector and the Niles volume controls. The
    selector I chose allows 6 pairs of speakers to be attached. It's kinda
    pricy. I think the selector is about $100 and the volume controls are about
    $50 each.

    I'm using it for sound in 4 rooms and it works very well.
    Bill
     
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