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Can speakers be protected from amplifier damage? How do amps work?

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  • Can speakers be protected from amplifier damage? How do amps work?

    First and foremost I'm a music lover, I know next to nothing about the technicalities of producing a good speaker, but I would hope that I do know a good one when I hear it. I have aspirations to be a Harbeth owner, but am not one yet. One thing worries me about being an owner however.

    I have read a few times comments on these forums from members (and AS himself) that if an amplifier fails it can destroy the speakers. What is it exactly that happens that can cause a speaker to be destroyed? Given that Harbeth speakers are easily driven, and that I'm likely to be spending 5-10 times as much on my Harbeths as on my amp, it follows that my amp may not be in the same league, build-quality wise, as my Harbeths (which I hope would last me a lifetime).

    So, is there anything that can be done to protect them from amplifier failure, without diminishing the sound quality of course?

  • #2
    Thoughts about the workings of amplifiers ....

    Originally posted by broadsword View Post
    ...I have read a few times comments on these forums from members (and AS himself) that if an amplifier fails it can destroy the speakers. What is it exactly that happens that can cause a speaker to be destroyed?...
    First we have to consider what an amplifier actually is. It is nothing more than a high voltage reservoir - somewhere between 50 and 150 volts - which is sitting quietly minding its own business until some music appears at the input terminal and the volume control is advanced.

    You can think of the amplifier as a water reservoir high in the mountains, filled with water and contained by a dam. The dam has an opening, a gate, and under electrical control that gate can be opened a little, a lot or fully opened. Think of the fast-moving gate as being controlled by the music signal. When the music becomes loud, the gate rapidly opens and we (the listener, via the speakers), downstream, receive a gush of water. When the music falls quiet, the gate closes-off and we receive a mere trickle. Pause the music completely and there is no need for any water to flow - the gate is completely closed. Dam operation here.

    No, of course, there is far too much inertia in a heavy metal gate to be able to modulate the flow of water according to every note in the music, but the operational principal in the audio amp is exactly that. Rather than using moving parts, the 'gate' in the amplifier is literally the gate pin on the transistors and thermionic valves. See attached.

    What can go wrong then? The danger inherent in the storage of any potential energy source (water, high voltage, explosive, nuclear, heat etc.) is that the gate will fail and there will be a runaway leakage of the energy from where you want it (the reservoir) to where you don't (the loudspeakers). The implication of 'runaway' is that once the gate can no longer hold back the power, the power reserve is completely out of control and only when all the energy has dissipated (the reservoir has run dry) will the danger be over. But there will be consequent destruction to put right.

    What can you do to minimise the risk of the amplifier 'dumping' its entire power reserve on my speakers, and how long will they last if the amp's gate fails? From healthy to destroyed will take under ten seconds, too little time for you to realise what's happening and catapult yourself from your listening chair to pull-out the speaker plugs. Thankfully, electronics are very reliable and I've only seen this total destruction two or three times in the last 25 years or so. But there are some things you can consider which stack the odds for or against this happening to you.
    1. The older the amp the more likely it is to fail. Components do not last forever, and capacitors are notorious for ageing. And the PSU reservoir is nothing but a bank of big capacitors!
    2. Don't push your amp to the limits of its power ability. That will put strain on the components. So a 5W amp is just begging for trouble
    3. Electronic components that run hot are inevitably less reliable than ones that don't. Solid state obviously is going to be more reliable than tube gear, all other things being equal
    4. Budget equipment means that the specification of the electronic components has been made on cost grounds. That will inevitably impact on their durability
    5. Cost into your amplifier ownership the need to have it properly serviced every 10-15 years or so, as recommended. Think of owning an amp as bringing the same responsibilities as replacing the timing belt in your car engine
    6. Don't use an amplifier that is hugely more powerful than the speakers you are driving. You don't need the power reserve of the Hoover dam to fill one swimming pool: the more power the more destruction it can bring
    7. Super-audiophile amplifiers that are capable of hundreds of watts and can hardly be lifted are just pure machismo. Their reservoirs have the explosive potential of a small bomb. When they fail the result is devastating, possibly combustion
    8. For normal listening an amp in the 50-100W range is just fine and a good power match to most speakers
    9. Turn off the amplifier when not in use at the wall switch. That lengthens the operational life of the components (which is stated in powered hours)
    10. If you smell anything unusually hot in the amplifier or speakers, switch off immediately and seek professional advice

    Common sense about heat and durability.

    I've prepared an image of how an audio amplifier works. Any audio amplifier, from $50 to $50,000 - they all work this way. The circuitry could be a little less complex (but not much) and could be 100 times more complex (why?), but the result would be the same functionality: the reservoir high voltage is permitted in a controlled way to pass through to the loudspeaker. What you find under the lid is three basic blocks: the gate stuff, the reservoir capacitor (bank) and the mains transformer - that's the lot.

    How it works: a second or two after you turn on the mains, the transformer charges up the capacitors. If they are in good condition and not old and leaky they will hold that charge. Every 25th of a second or so, another little pulse of power will be drawn from the mains supply and presented to the capacitor reservoir. If they have discharged a little since the last mains pulse (because they have delivered some music to the loudspeakers and that power had to come from somewhere) they'll greedily take-in that top-up. If they are already fully charged, that's it; the reservoir is full. So the capacitors are topped-up twenty five times a second or so if they need it.

    The music signal is applied to the input of the circuit, the gate control arrangement. This particular mono circuit has four small signal transistors (Q1-4) and two really meaty output devices (Q5-6). Q1-6 are all three legged devices and I have marked the gate pin on each. Because the big old output devices are not very sensitive (but can really handle a lot of power, a trade-off) the small signal transistors incrementally boost the incoming audio from left to right through the circuit until it is strong enough to usefully drive the gate pins of the output bruisers, Q5, Q6. I've marked their gates in red. OK so far?

    So the music signal instantaneously works its way from left to right through the amplifier until it reaches the gates on Q5 and Q6. Exactly as with the water dam, those gates let trough the voltage stored in the reservoir capacitors to the speaker. If the music at any one instant becomes louder, the gates progressively 'turn-on' more and really let the juice flow from the PSU to the speaker .... if the music drops in level, Q5-6 progressively shut-off and the voltage flow from the reservoir diminishes to a trickle as commanded by the much smaller music signal.

    What fails? Any of these components could fail at any time, but experience tells us that anything that's running hot, either because it is working hard (such as Q5 and Q6 when playing rock music very loud) and/or poorly ventilated is susceptible to fail. And we know that the reservoir bank of capacitors, upon which the whole beast depends, is very vulnerable to ageing. So you could argue, with good justification, that the circuit design is not much important for the sonics but that the design of the seemingly humble PSU is in fact, much more important to the sound we hear. And it's the one part of the design which is vulnerable to age which must imply, I suppose, that an amplifier with a linear PUS as I've shown, could indeed measure, perform and sound different on day one or day one thousand of its life.

    That's really all you need to know about audio amplifiers. They are simple, dumb closed-systems with wholly predictable performance that can be software modelled to very fine degree. There really isn't any room for black arts in amplifier design if minimising long-term warranty claims is your commercial goal. Hope that helps. (3 hours)

    >
    Attached Files
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

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    • #3
      Thorough ....

      Any more questions...? :) Thorough... As always.

      Comment


      • #4
        Bass pulse

        Huge thanks Alan for the time you've taken on that comprehensive answer. It will certainly help me decide which amp to go for and how to minimise my worries concerning speaker damage - I'm not into loud rock music but Stravinsky's Rite of Spring can be equally loud and demanding!

        I was leaning towards an older second-hand amp in the 400-500 bracket to partner my Harbeths (when I buy them next year, once I've moved house and can target the model to my listening room) - the idea being that I could get more for my money. However since regular servicing is hard to prove on a second hand unit, and the risks of failure on older units would seem to be be higher, I think now that this kind of money would buy me a perfectly good new amp (with the emphasis on reliability and ease of service) - I see no real need to spend more than this on a dumb device.

        I recently bought an old Creek CAS4040 amp with some Monitor Audio (Monitor One) bookshelf speakers for a combined cost of under 100 as a 2nd system in my office - quite sweet sounding it is, though turning the amp on always puts an audible pulse through the speakers (and visible, the bass drive units almost pop out) - it hasn't done any damage but I certainly wouldn't want to drive some expensive high end speakers with such an old amp!

        Comment


        • #5
          It's a service issue

          Originally posted by broadsword View Post
          ...I recently bought an old amp with some Monitor Audio (Monitor One) bookshelf speakers for under 100 as a 2nd system in my office - quite sweet sounding it is, though turning the amp on always puts an audible pulse through the speakers (and visible, the bass drive units almost pop out) - it hasn't done any damage but I certainly wouldn't want to drive some high end speakers with such an old amp!
          Warning! That dramatic pulse-on-switch-on is indicative of a creeping fault in the amp which will, sooner or later, open the gates on one or other of the output transistors and dump the entire power reservoir onto your speakers. Basically, what's happened is that there is an imbalance in the circuit which mean that the positive PSU volts (perhaps around +30V) or the negative PSU volts (perhaps around -30V) are being momentarily applied to the speaker which gives the cone a "DC offset" whereas music has no DC offset component - the cone always return to rest.

          Eventually this amp will fail. Is it serviceable? Almost certainly - the fault is most likely to be circuit drift due to just one or two ageing components. But I strongly recommend that you do not use it with precious speakers.

          Just like vintage cars or aircraft, it's not the age which is the real issue, it's how well serviced it is. As I have been banging on for years, the primary selection criteria for an audio amp is, in my view, service backup not some ephemeral folklore about its sonic performance. You should make enquiries about who can you turn to in 5, 10, 15 years to reassure yourself that it is performing to original specification at a sensible cost.
          Alan A. Shaw
          Designer, owner
          Harbeth Audio UK

          Comment


          • #6
            Local service?

            Originally posted by A.S. View Post
            Warning! That dramatic pulse-on-switch-on is indicative of a creeping fault in the amp which will, sooner or later, open the gates on one or other of the output transistors and dump the entire power reservoir onto your speakers. Basically, what's happened is that there is an imbalance in the circuit which mean that the positive PSU volts (perhaps around +30V) or the negative PSU volts (perhaps around -30V) are being momentarily applied to the speaker which gives the cone a "DC offset" whereas music has no DC offset component - the cone always return to rest.

            Eventually this amp will fail. Is it serviceable? Almost certainly - the fault is most likely to be circuit drift due to just one or two ageing components. But I strongly recommend that you do not use it with precious speakers.
            These speakers are not precious no, but thanks - I'll look into who might be able to service the amp locally, but not if it costs more than the amp did itself (which is quite likely). If it does this Creek may just have to go to the old amp graveyard (my attic, I wouldn't have the heart to take it to the local dump).

            A good lesson to be learned here though, and not one to be repeated with Harbeth speakers!

            Comment


            • #7
              A little thump or a huge PULSE - different issues

              The Creek 4040 and many other British amps exhibit a 'switch on thump'. It's at a low level and causes no damage to the speakers.

              The 4040 is a great little amp which works well with Harbeth speakers. I actually used one during a demo a couple of weeks back.

              The Creek can be cheaply serviced by the manufacturer if required.

              {Moderator's comment: There is a big difference between an innocuous little switch-on thump and a huge offset pulse which moves the cones far from the rest position. The first is normal, the later indicates a drift in the circuit and inevitable failure... one day}

              Comment


              • #8
                Alternative amps

                As an owner of an Naim Nait 3, I should be looking to get my amp' serviced. I'll be speaking to my dealer about this on my next visit.

                Here's a list of some amps you may wisah to check out broadsword. The higher priced ones are well worth saving for. I can vouch for the Naim as I'm a long time owner / user of the intergrated. Does not have a huge power pc but it does deal with music very well and is a match for cheaper units that (may) pack more of a power punch.

                Naim Nait 850
                Exposure 1010 450
                Nad C316BEE 250
                Primare R32 850
                Rega Brio R 500

                Comment


                • #9
                  A bigger than usual pulse

                  Originally posted by hifi_dave View Post
                  The Creek 4040 and many other British amps exhibit a 'switch on thump'. It's at a low level and causes no damage to the speakers.

                  The 4040 is a great little amp which works well with Harbeth speakers. I actually used one during a demo a couple of weeks back.

                  The Creek can be cheaply serviced by the manufacturer if required.

                  {Moderator's comment: There is a big difference between an innocuous little switch-on thump and a huge offset pulse which moves the cones far from the rest position. The first is normal, the later indicates a drift in the circuit and inevitable failure... one day}
                  I think this is more on the lines of the offset pulse - the movement in the cones is quite significant. But I have read elsewhere that it's normal to hear a thump on powering on with old British amps like the Creek CAS4040, so I'm not 100% sure that isn't what's happening here. The thump does seem to be more severe the longer its been off though. Another symptom is that when the source stays on and I switch the amp off, it continues to feed the speakers for about 10 seconds - not sure how normal this is, but it doesn't happen on the newer amps I've owned.

                  hifi_dave - do you have any contacts for old Creek servicing? It might be worth doing if it's reasonable - it would be a shame to discard it.

                  Comment


                  • #10
                    A really big amp .... how to make

                    Next question then has to be

                    ... how do you make a bigger, more powerful amplifier? How do you take the design from, say, a modest 30+30W up to 300+300W or even more?

                    In my post #2 I showed a circuit of a hi-fi amplifier. The power output of the amp wasn't specified, so how would we know what it is capable of without running a lot of tests and/or relying on the manufacturer's published data?

                    First, some common sense. As I explained, the power source in an amplifier can be thought of as the water in the dam, regularly topped up by the mains supply. In fact, the height of the dam above sea level and the volume of water therein is really what defines the power reserve. That's why hydroelectric dams are as high off the ground as possible - often up in mountains, and why they hold a lot of water so that when they start producing electric power it's not all over in a flash and the water all gone. The equivalent of the height of the water mass above sea level is the voltage or potential energy of the PSU capacitor bank, and the amount of 'oomph' that the capacitors can supply - the force behind the water when it starts to flow, the destructive force if uncontrolled, is current.

                    What defines the current drawn from an amp?

                    The load resistance. And we know that's fixed for a given speaker. Most Harbeth speakers present a load of about 6 ohms or so. The only way the speaker can gobble more current is if it is playing louder, which is under your control via the music and the rotation of the volume control on the amp. It's really important to appreciate that current is pulled from the amp to the speakers. It's not pushed into the speakers whether they like it or not. The loudness of the speakers determines how much current they pull from the amp - nothing else. So, regardless of how humongous, sexy and expensive your amplifier, if you are playing at, say, 90dB there is exactly the same amount of power (current x voltage) going into the speakers using a 10W amp as a 1000W amp. You cannot force the speakers to consumer more power in any other way than turning up the volume. Amplifier power ratings only tell you what they could, under extreme conditions make available to the load. If the load isn't calling for that power, it is merely a power potential, but an unused one.

                    What defines the voltage potential in the reservoir capacitors?

                    The design of the power supply circuit. Nothing to do with the amplifying circuit at all.

                    So how do we scale-up the 30+30W amp to something much more powerful?

                    Easy. Since the current drawn for any given speaker load will remain the same for any given loudness all we need to do is increase the 'height' of the reservoir capacitors voltage. In other words, we need to increase the voltage a lot. Maybe to + and - 80 or 100V or even more.

                    And then what?

                    Well, the amp designer has to make some assumptions. He has to assume that if a buyer deliberately invests in a super-powerful amplifier that he will probably try and use that power. So he will have to beef-up the output stage Q5-Q6 (at the least) and ensure that they can deliver the higher current (and greater heat) - and the ventilation will have to be improved to keep the components cool too.

                    Does the circuit need to be more complex just because the power rating is increased?

                    Not necessarily. Although the amp designer may include some sort of temperature monitoring circuitry out of the audio path to compensate for the effect of temperature on distortion etc..

                    Is there any advantage having an amplifier with a power rating dramatically higher than the speaker rating to which it is connected?

                    Not that I can see, since the speakers resistance defines the load and loudness. Most home listening at about a healthy 85dB or so with normal speakers only needs a handful of watts. Of course, a power reserve to allow the speakers to pull more energy when the music really does go loud (1812 overture etc.) is like having a 4.8ltr engine when around town you only need 1.2ltr. It's nice to have but not really usable under normal conditions. The Class A amp is working flat out all the time regardless of how loud you listen.

                    If I listen to the same speakers at the same loudness is there any advantage in having a much more powerful amp?


                    Not on electrical grounds. If the volume is fixed at your ears, then the amount of power (measured in watts) is the same. So if that loudness is comfortably generated by a 10W amp, then if you change to a 100W amp, 90W of the available power is not being used. But regardless of how much power is being drawn by the speakers, if the amp is Class A (tube or solid state) the total amp rated power is continuously being drawn from the mains supply and turned into useless heat, as you'll see from your electricity meter spinning round regardless of how loud the music. The same as revving that 4.8ltr engine with your foot on the clutch - power literally going up in smoke.

                    Is there an increased danger of failure with a more powerful amplifier?

                    Hard to say. But the combination of running hot, the needlessly high voltages in the reservoir capacitors and poor or no servicing is a toxic combination and imply a very big and expensive bang
                    Alan A. Shaw
                    Designer, owner
                    Harbeth Audio UK

                    Comment


                    • #11
                      Next question ...

                      Thanks again - next logical question would be why separate pre/power amplification into two or more boxes - and why bi-amp - any advantages etc.

                      EDIT -- not that I want to take up too much of Alan's time on a subject that doesn't interest him!

                      Comment


                      • #12
                        30 years old ....

                        They always made a significant switch on "thump", so I doubt it's a problem. Music from the source for a few seconds after the amp is switched off is also normal. It's there until the power supply discharges. Best bet is to turn the volume right down before turning off.

                        The 4040 is getting on for 30 years old now and was a budget amp, so a service is definitely desirable. I haven't got contact details but if you Google Creek Audio, you should find them easily enough. I believe they are in Friern Barnet, NW London.

                        Comment


                        • #13
                          Moderator's comments

                          Originally posted by hifi_dave View Post
                          The Creek 4040 and many other British amps exhibit a 'switch on thump'. It's at a low level and causes no damage to the speakers.

                          The 4040 is a great little amp which works well with Harbeth speakers. I actually used one during a demo a couple of weeks back.

                          The Creek can be cheaply serviced by the manufacturer if required.

                          {Moderator's comment: There is a big difference between an innocuous little switch-on thump and a huge offset pulse which moves the cones far from the rest position. The first is normal, the later indicates a drift in the circuit and inevitable failure... one day}
                          May I ask that the moderator stops placing his responses in our correspondence? This issue has been brought up more than once. Even Alan does not intersperse his comments among us.

                          {Moderator's comment: Actually we asked Alan for a view, he dictated it and I typed it.}

                          Comment


                          • #14
                            Thanks.

                            Thank you Alan, an easy to follow explanation in layman's terms, brilliant.

                            Comment


                            • #15
                              'Microsize me'

                              More here about selecting an amplifier power ....
                              Alan A. Shaw
                              Designer, owner
                              Harbeth Audio UK

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