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<title>The Class-A Amplifier Site - Hiraga 'The Monster'</title>
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<div class=Section1>
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<p align=right style='text-align:right'><b>The Class-A Amplifier Site</b></p>
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<p align=right style='text-align:right'><span style='font-size:10.0pt'>This
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page was last updated on 16 July 2001</span></p>
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<p align=left style='text-align:left'><b><span style='color:blue'><a
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href="hiraga.htm" title=hiraga.htm>[ Back to Index ]</a></span></b></p>
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<p> </p>
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<p align=center style='text-align:center'><b><span style='font-size:20.0pt'>“The
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Monster” Revisited</span></b></p>
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<p align=center style='text-align:center'><b><i><span style='font-size:16.0pt'>Jean
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Hiraga</span></i></b></p>
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<p align=center style='text-align:center'><i>(l’Audiophile No. 31)</i></p>
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<p> </p>
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<p> </p>
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<p>Described in Issues 27 and 29 of l’Audiophile, this amplifier is acquiring a
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very solid reputation in the small world of the perfectionist audiophile.
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Several hundred have already been built since May 1983, and the feedback that
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we have had from our readers is unanimous, this amplifier made the difference!
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Surprisingly, this amplifier, which was originally designed to feed the
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mid-range and treble in multi-amplified systems, gives extraordinary results in
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the bass. We have carried out various tests and it is true that, on signals
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that don’t require a very high energy level in the bass region, the 8W has a
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quality of reproduction in this register that is without equal. The sound is
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remarkably graded, revealing an unsuspected variety of sound colours that it is
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rare to hear from a reproduction system. It will be necessary to wait for the
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50W Kanéda, which (as you have seen in this Issue) is in preparation, to
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obtain these qualities at a higher power level. </p>
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<p> </p>
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<p>The various tests in the bass register that we have been able to make, very
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clearly highlight the differences that exist between configurations 1, 2 and 3
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mentioned in Issue 29. There is no doubt that the Monster proves to be superior
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to the two other configurations, especially version 1 without the battery. The
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advantage of batteries used on their own, or as a buffer for the mains supply,
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appears very clearly in broad band listening. The reason can be seen very
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simply and very clearly with a dual trace oscilloscope. One trace displays the
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output signal and the other trace the behaviour of the alternative power
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supplies. The amplifier is fed with a sinusoidal signal that is varied in
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frequency. It can be very clearly seen, starting at 50Hz, that when one
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decreases the frequency, the charge frequency, which is obviously that of the
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mains, is not sufficiently fast to feed the power supply filter capacitors. One
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would think that a capacitance of half a Farad, or even 1 Farad, would be
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sufficient to alleviate this slowness, but this is not so, and a modulation signal
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is found on the power supply which, similar to a piece of gelatine, fluctuates
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according to the signal. Of course, one could think that, below 50 Hz, a little
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distortion introduced by the power supply is not very critical. This would be
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to forget that the amplifier is required to reproduce higher frequency signals
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at the same time that, in addition, have much lower amplitudes in the mid-range
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and treble registers. Thus the noise introduced by the power supply will mask
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all the small amplitude signals. The overall performance will lose clarity and
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definition. </p>
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<p> </p>
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<p>The battery makes it possible to cure this deficiency by providing energy
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between the alternations of the mains. </p>
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<p> </p>
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<p>Following the article in Issue 29, many readers have asked if, in the
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"Monster" version, the mains supply was used only to charge the two
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large 40 A/h batteries. In fact, two modes of use are possible. The batteries
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can be used as the only power supply source, but take heed, the endurance
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hardly exceeds a few hours, lead-acid batteries are sensitive to deep
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discharges and their lifespan depends on this, so 12.2V is a lower limit that,
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above all, should not be exceeded. The second mode, which is the one that which
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we most usually employ, consists of using the batteries only as a buffer, with
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the mains charging the power supply permanently. Of course, in this solution,
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the power supply noise rises considerably, by 30 to 40 dB. However, in spite of
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this, the Monster remains an amplifier without rival, even if it loses in
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"luminosity" compared to its operation on batteries and without a
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mains supply. </p>
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<p> </p>
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<p>Whichever configuration is chosen, the construction of the amplifier does
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not pose any problems. For installation in the chassis, the reader can refer to
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Issue 15 of l‘Audiophile (unfortunately no longer available) in which the
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construction of the 20W Hiraga is described. It is advisable to follow the
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broad outline in the article that relates to the earth wiring in order to make
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the positive and the negative of the power supply perfectly symmetrical so that
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the filter ripple (as well as the rectifier switching peaks and transformer
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saturation non-linearities), which arrives in opposite phase on each positive
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and negative rail, is cancelled. It will be necessary to provide, even for
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configuration 1, a case with dimensions sufficient to accept configuration 2.
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The transfer from configuration 2 to configuration 3 is achieved by the
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addition of another box. Indeed, it would be a shame to deprive oneself of the
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almost unlimited upgrading capabilities of this amplifier. </p>
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<p> </p>
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<p>For the layout of the electronics, the construction is extremely simple, the
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two small printed circuits are fixed by means of the power transistors to the
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heatsink. It is very highly recommended a thermal compound be used. Two nylon
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spacers clipped to the heatsinks receive the card at the two corners opposite
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the power transistors. </p>
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<p> </p>
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<p>One should not lose sight of the fact that the 8W functions in class A. The
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quiescent current therefore has a prime importance. Its value should not be too
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low otherwise the amplifier will pass into class AB on strong signals, nor too
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high because it would impose too great a dissipation on the power transistors
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which, in addition to the fact of limiting their lifespan, can lead to thermal
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runaway. Indeed, the characteristics of the power transistors are related to
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the temperature of the junctions and beyond a certain threshold there is
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runaway, that is to say the more the temperature rises the more the current
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increases. The optimal value lies between 0.5 and 0.6A. </p>
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<p> </p>
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<p>To measure the quiescent current, it is sufficient to measure the voltage
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across the 1 ohm 5W resistors. The voltage must therefore be between 500mV and
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600mV. After sorting the batches of transistors, 2SD844/2SB754 on the one hand
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and 2SB716/2SD756 on the other, one can be placed in non-optimal operating
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conditions from the point of view of quiescent current. The remedy is extremely
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simple. If the quiescent current is too high, it is enough to decrease the bias
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resistors of 2SB716 and 2SD756 transistors, whose initial value is 1 kohm.
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These resistors coming from the bases of the transistors determine their point
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of operation and consequently the collector current, on which depends the
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quiescent current. In general, it is enough to change from 1 kohm to 910 ohm
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for the quiescent current to take the correct value again. </p>
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<p><span style='font-family:"Times New Roman"'> </span></p>
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<p align=center style='text-align:center'><span style='font-family:"Times New Roman"'><img
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border=0 width=690 height=408 src="monster31fig1.gif"></span></p>
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<p><span style='font-family:"Times New Roman"'> </span></p>
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<p>The value of the supply voltages must be between 12V and 13.5V. If however
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the measured value exceeds this voltage, it is appropriate to very slightly
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increase the 4 ohm 20W filter resistor. A voltage value that is too high does
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not present a risk to the circuit if the value of the quiescent current does
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not exceed the limits for the current mentioned previously. However, in the
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ultimate version where 0.47 Farad Supercaps are used, the supply voltage should
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not exceed 13.5V under any circumstances. </p>
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<p> </p>
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<p>To finish, it is advisable to carry out the adjustment of the output offset voltage,
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that is to say the dc potential difference appearing between the positive and
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negative loudspeaker output terminals. It is advised that this adjustment is
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made twice. Before power is first applied, place the wiper of the 100 ohm
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trimmer at the mid point of its track, measure the voltage (without your
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preamplifier or your active filter connected) and adjust the wiper of the
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trimmer to cancel the dc voltage at the output (voltmeter range 100 or 200mV
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dc). Let the amplifier find its point of thermal operation, 20 to 30 mins, and
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perfect the adjustment. Constructed well, the 8W amplifier has an exemplary
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stability, the offset does not exceed a few tens of millivolts. In any event,
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there is no absolutely point in tearing one’s hair out trying to obtain an
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offset of 0mV! Realise that 100mV offset represents a power of 1.25mW! Finally,
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at the request of very many readers, we publish the detailed power supply
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diagram for the 8W "the Monster" configuration. </p>
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<p> </p>
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<p align=center style='text-align:center'><img border=0 width=690 height=402
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src="monster31fig2.gif"></p>
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<p><span lang=EN-US> </span></p>
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<p><b><span style='color:blue'> </span></b></p>
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<p><b><span style='color:blue'><a href="hiraga.htm" title=hiraga.htm>[ Back to
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Index ]</a></span></b></p>
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<p> </p>
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<p> </p>
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<p><b><span style='font-size:8.0pt'>HISTORY:</span></b><span style='font-size:
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8.0pt'> Page created 12/07/2001</span></p>
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<p><span style='font-size:8.0pt'>16/07/2001 Text added</span></p>
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<p><span style='font-size:8.0pt'> </span></p>
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<p><span style='font-size:8.0pt'> </span></p>
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<p><span style='font-size:8.0pt'> </span></p>
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</div>
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</body>
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</html>
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