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How much power do I need? (How big or small an amp?)

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  • #31
    Input Sensitivity and Frequency

    I have read that dynamics thread and it is very well explained - completely acceptable. I listened to the set of four clips and appreciated the difference between #1 and #4.

    I have a question about input sensitivity. Mine is given as 600mV. I have taken that to mean that, given an input level of 600mV, the amp will output its maximum power possible.

    But at what frequency?

    Wouldn't a 5000 Hz, 600 mV signal require vastly more output power to reproduce than a 50 Hz, 600mV signal? Is there a nominal frequency at which amp manufacturers specify their input sensitivities?

    If this is, say, 1000 Hz, would it be reasonable to say that the amp is sensitive to >600 mV at lower frequencies, allowing the amp to deliver an output without clipping at closer to a CD player's maximum output level of ~ 2V?

    My reason for asking: I can't get my amp to sound like clip 4, even when cranking up Tchaikovsky's Violin Concerto to 4 o'clock on the volume control (max is 5). But perhaps clip 4 is too extreme an example.
    Ben from UK. Harbeth Super HL5 owner.

    Comment


    • #32
      Originally posted by BAS-H View Post
      I have a question about input sensitivity. Mine is given as 600mV. I have taken that to mean that, given an input level of 600mV, the amp will output its maximum power possible.

      Wouldn't a 5000 Hz, 600 mV signal require vastly more output power to reproduce than a 50 Hz, 600mV signal? Is there a nominal frequency at which amp manufacturers specify their input sensitivities?

      If this is, say, 1000 Hz, would it be reasonable to say that the amp is sensitive to >600 mV at lower frequencies, allowing the amp to deliver an output without clipping at closer to a CD player's maximum output level of ~ 2V?

      My reason for asking: I can't get my amp to sound like clip 4, even when cranking up Tchaikovsky's Violin Concerto to 4 o'clock on the volume control (max is 5). But perhaps clip 4 is too extreme an example.
      Clip 4, highly compress but at a moderate average loudness, is already crushed and it is unlikely that it's loud enough to drive your amp into itself clipping, and even if it did, you would have a clipped signal magnified and clipped again: you wouldn't expect to hear a difference. More relevant would be using the wide dynamic source clip (Clip 1) and let that push your amp hard.

      It's best to think of the power amplifier as a voltage reservoir. The amount of power sucked from the amp depends on several factors, especially the impedance curve v. frequency, but that (wildly) fluctuating impedance curve shouldn't impact on the speakers sonic output (frequency response) if the amp has sufficient reservoir to cope with the load. We're back to the issue of the small amp with a small reservoir trying to haul a speedboat up a mountain - the boat being the speaker load, the mountain being energetic music.

      The amp's voltage output from, say, 20Hz to 20kHz should be constant except perhaps at the extremes of the range. More power is drawn from the amp when replaying music in direct relation to the spectrum of notes in the music. Western music has a spectral tilt from low to high frequencies as we have seen here before, so in fact, much more current (hence power) is drawn from the amp in the lower registers where orchestral instruments (drum, piano, cello, double bass, organ etc.) are 'moving air' and harldly any in the middle and high frequencies - the opposite of what you suspect.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #33
        Many thanks, but forgive me - I was trying to drive my amp to clipping using an ordinary CD, turned up high. I'm still keen to understand what manufacturers technically mean when they quote their input sensitivity value.
        Ben from UK. Harbeth Super HL5 owner.

        Comment


        • #34
          Lawless specifications

          Originally posted by BAS-H View Post
          Many thanks, but forgive me - I was trying to drive my amp to clipping using an ordinary CD, turned up high. I'm still keen to understand what manufacturers technically mean when they quote their input sensitivity value.
          They can mean whatever they want to mean! It's a lawless jungle! What would you like that sensitivity figure to mean? Your view is as good as the next man's!

          What I would like it to mean requires some more words of clarification. What would be a watertight specification would be something like:

          'Input sensitivity of Aux sockets, 500mV rms @1kHz, load 47k ohms each channel; when volume control fully advanced and tone controls set to central bypass position, balance control central, 500mV/1kHz signal applied to both channels will deliver full output across 8 ohm speaker loads connected simultaneously to both output channels.'

          The key point here is that my expanded explanation unambiguously defines that the load (the speakers) is connected to both channels simultaneously. Ignoring this condition is an old marketing trick. It's one matter to connect one speaker to an amp, turn up the volume and let the PSU reservoir drain into that speaker load. It's a very different matter to couple two speakers and repeat the test: like having two calves at the udder - the reservoir drains really fast and the optimistic headline power rating of, say, 20W, in practice with real loud music and two channels driven, may be much less. This is unlikely to be an issue is the amp is rated at 100W with a big PSU because even if the reservoir is draining, there is still residual power to draw on, but a flea-amp which already is using PSU reservoir components pared down to the absolute minimum (that's why it is a tiny amp - it doesn't have worthwhile power reserves!) is going to have a performance greatly influenced by speaker loads (a big hungry calf or a newborn?) and whether one or two channels are being driven simultaneously and how much harmonic distortion would be generated at those end-stop power ratings, especially in stereo with two speakers. For the flea-amp, expect, say, 10-20% distortion and 3W true power, in stereo.

          The absolute minimum statement an amp maker should make is whether his power rating is for one speaker or two speakers connected - expect a big difference in power potential unless the design is really well executed with expensive, appropriate, large components. It won't look good in the marketing material to print the truth if the amp really can't drive two channels simultaneously at full power.

          Incidentally, I attach a technical review of the great QUAD 405 amp. You can see from page 135 that the 100W + 100W manufacturer's rating is achieved in the test lab when both channels are simultaneously loaded and driven. Now that's what we call here a 'proper job'. Micro-amps would be exposed for their pitiful technical capabilities under brutal lab tests like these. Just as well for those on the bottom rungs of the power ladder that these expensive, impartial, time consuming but objective lab tests are no longer routinely undertaken.
          Attached Files
          Alan A. Shaw
          Designer, owner
          Harbeth Audio UK

          Comment


          • #35
            A responsible amp spec

            Originally posted by A.S. View Post
            What I would like it to mean requires some more words of clarification. What would be a watertight specification would be something like:

            'Input sensitivity of Aux sockets, 500mV rms @1kHz, load 47k ohms each channel; when volume control fully advanced and tone controls set to central bypass position, balance control central, 500mV/1kHz signal applied to both channels will deliver full output across 8 ohm speaker loads connected simultaneously to both output channels.'
            And that is essentially what a responsible manufacturer does. Take at look at the specifications for this Arcam amp - chosen entirely randomly - and you'll see that power rating is given for both channels into 8 ohms, and a single channel into 4. The specification for sensitivity (and input impedance) is also given, and although it's not 100% explicit, the clear implication is that the input sensitivity is for full output. I'm completely comfortable with those specifications.

            http://www.arcam.co.uk/_docs/a28e_manual.pdf

            Sadly, not all manufacturers are this diligent. And while engineers like Alan and I are able to recognise and cut through the meaningless specifications published by many manufacturers, the general consumer is not. And this really is the nub of the problem. Attempts at standardising specifications (e.g. DIN) come and go, but manufacturers aren't really interested in this, for sadly obvious reasons.

            All the best,

            Mark

            Comment


            • #36
              Dishonest specs

              The Quad 33 preamp also had a spec. for its gain with an added asterisc note, which I believe from memory stated, after the I/P voltage required for an O/P voltage of 500mV "* at 30% modulation", thus fixing the spec.

              The dishonesty allowed by incomplete specs is so useful to marketing men;

              My speakers have a wide range, 2Hz to 23kHz, and can be perceived as them having an amazing bass response, until I state that it is -80dB at 2Hz. (You can see the cone moving at 1Hz so there must be some output at that frequency).

              Comment


              • #37
                The output signal ramp and the small amplifier - demonstration

                Another example I've created to show what happens when you use an amplifier with inadequate power reserves - 'a micro-amp' - to drive a modern, low efficiency speaker, of that type that you'll see in your audio dealer as they represent 99.9% of all speakers made in the last 40 years or so.

                I've generated a 1kHz tone and synthesised what we'll hear when a tone generator is connected to an amplifier of generous power or alternatively to a micro-amp, typically 5-15w although could be rated higher or lower depending on the honesty of the specifications.

                If you refer to the attached picture, you'll see A is the signal at our ears when the amplifier has adequate reserves of power to push the speakers harder as commanded by the ramping-up of the test tone, which could, of course, be a musical signal. Example B shows the consequence of power limiting in the tiny amp. Once the level of the tone breaches a critical amplitude 'power threshold', the small power amplifier has no more power available to push the cones. It simply cannot deliver any more current, and regardless of how the incoming tone or music signal increases, the output from B has plateaued. In simple words, the amplifier has saturated and is now clipping - it has become an expensive audio compressor, the sort of tool the recording engineer would use to limit the dynamic range of a sound for artistic or commercial reasons.

                The only way to return the small amp B to its linear operating range is by turning down the incoming signal to below 'power threshold' point at which, obviously, the volume loudness diminishes to a loudness far below that of real life. It's really very simple: small power = small loudness with normal, modern speakers.

                Loading the player ...
                Clip 5: Example A:adequate power reserve in amplifier. Example B: a small amp which runs out of available power: output to speaker does not follow input signal from source

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

                Comment


                • #38
                  Amp personality

                  Thank you Alan.

                  This has opened up my understanding of clipping amps, dynamics compression and the perceived "low level" details and "full bodied" sounds of a small amp.

                  Comment


                  • #39
                    The reality of international manufacturing

                    Originally posted by rantwb View Post
                    Thank you Alan.

                    This has opened up my understanding of clipping amps, dynamics compression and the perceived "low level" details and "full bodied" sounds of a small amp.
                    Pleasure. I'll trade a secret with you!

                    At the Munich show a few weeks ago, Pluto and I put out feelers in various directions concerning the Harbeth amplifier project. We know that there will be a market, so we don't need to even talk about that - that's a certainty. The issue was, who should we trust to partner with us here to design and make this fabled amp? So you can now hear the facts directly, first hand from me; Pluto is welcome to give his independent opinion as to how he sees things.

                    We have five prospective partners in the wings, each offering a different balance of skills and competencies, a different way of interfacing with us, and a different set of contacts. The days when you could turn to one single designer and ask for a complete, worked-up, production-ready consumer electronic product of the relative complexity of a modern audio amplifier are long gone. These days, the circuit design is as likely to be in, say, Glasgow, the PCB layout in Mumbai, the parts list co-ordination and Bill of Materials management in Shanghai, electrical safety work in Toronto and casework in Croatia. Final production ..... anywhere. We learned rather a lot more than we perhaps should have (because we're good listeners and can hold our beer) about the realities of subcontracted electronics. The reality is that unless you are a Sony or Panasonic, you are using a string of subcontractors (as defined above) to bring your amp/CD player/DAC etc. to market. And you are, as the brand and a product, only as strong as the weakest link in that very long and in places, highly technical and vulnerable human chain.

                    The reality is that, even the ex-British electronics brands that are now in overseas hands, are having tremendous difficulties with QC. That's because in their former UK cottage industry guise, there was a figurehead, legally the Managing Director, who had a vision over the entire little business, stuck his nose into every corner and knew what was going on. Add to that the western culture where individuals are not (usually) reprimanded for highlighting weaknesses in design or management, and the product was well designed, well executed and lasted. Those little businesses thrived. Management and employees cared, but of course, capital and imagination limitations meant that they remained little businesses.

                    What crystallised for me was the reality that transfer to the far east, solves immediate capital, personnel and even imagination limitations but brings serious new challenges. And the most serious of all is the irresistible temptation for the parent company, the one we see as the brand owner, to virtualise production. From R&D through to production, testing and packaging, the product and its vital parts may be farmed out to an array of subcontractors, some little more than sweat shops, all of them beaten down on price and on 2% gross margins. That's a huge management and coordination operation as the piece parts are sent hither and thither around the countryside, bounching around on rickety vehicles on unmade roads. And the result can be, as we heard in graphic detail, that quality has to take care of itself. So, a situation reported by a reliable old friend was that a famous ex-UK brand has a >50% dead-on-arrival reject rate as it arrives in sealed cartons in Europe. I one case, a rather attractive rework of an old design didn't work despite various QC stickers because the tube heaters were not wired-up.

                    Now, subcontracting is fine, but what about after care? The reality is that HQ places a PO on subcontractor A for 500 main circuit boards. A makes 500 main circuit boards, cuts an invoice, sweeps the factory and readies for another day's work, perhaps making printer parts or whatever. HQ forgets that there are global spare part requirements for 10 circuit boards. HQ should have raised the PO for 510 boards, or if really on the ball, 550 to put some on the shelf. But because everything suffers from short-termism, and that would have screwed-up the profit calculation for the eventual 500 complete amplifiers, the 10 spares are just ignored. And ignored. And ignored. The poor customer, sitting in Antwerp or Stockholm bought into this good old fashioned brand, living the dream, and is now waiting for months or forever to hear music. My point is that the BOM complexity, the international supply chain, the cultural and legal factors, obsolescence of electronic parts, relatively short production runs in a start-stop manner all combine to one conclusion: if you pick the wrong partners, electronics manufacturing will be a nightmare.

                    However, it can work. Clearly, it most often does, but that's mainly a testament to good cross-border management. And that eats resources, not the least of which is time, the most precious of all.

                    So, by the end of July, I will make a decision to proceed with the Harbeth amp at least to the point of a prototype. Then we can pause and reflect. But note please: I have not once, in conversations with prospective partners said that this product must be of 'audiophile grade'. First, that is wholly ambiguous and can mean what anyone wants it to mean (it usually means heavy and expensive, weighed down with bling); second: we put solid functionality and reliability as paramount.

                    Hope this is of some interest.
                    Alan A. Shaw
                    Designer, owner
                    Harbeth Audio UK

                    Comment


                    • #40
                      Very interesting!
                      I have just finished reading the original Harbeth amplifier thread. Exactly as you, Alan, wrote in July 2012, you took half a year before taking the next step. I'm really enthusiastic that you will get to the final, long awaited by many, Harbeth amp. Looking forward.

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