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.
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.
**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
**Many surround sound receivers employ a second zone system, which has power amps dedicated for this purpose. That would be a good, economical choice.
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
E = sqrt(P*R)
then such an amplifier can produce about 28 volts RMS across the load. By Ohm's law:
E = I * R
I = E/R
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.
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.
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.
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.
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
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.
*OK, who stopped payment on my reality check?
Dave Plowman email@example.com London SW
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.
*Why is it that doctors call what they do "practice"?
Dave Plowman firstname.lastname@example.org London SW
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
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. ;-)
*To steal ideas from *one* person is plagiarism; from many, research*
Dave Plowman email@example.com London SW