AudioNote-SP12Ankoru-pwr-sch维修电路图 手册.pdf

Issue 12 - SOUND PRACTICES 9 Hey, dont drop that thing on your foot! Musical information is a dynamic four dimensional continuum, like the one posed by relativity theory, consisting of three spa- tial dimensions and time, all inseparably interrelated. A system for music recording and reproduction must transfer this con- tinuum and faithfully reconstruct the orig- inal sonic performance in the listening room. Test instruments are supposed to ensure that the transformation of the con- tinuum is linear and accurate to certain parameters. Alas, these instruments and the mathemat- ical models that we use, such as relativity theory, lack the spontaneity and emotional content vital to music. Somehow our primitive method of recording scratches into a vinyl disc captures some of this emotion, and the lump of rock we call a stylus is able to extract the information and convert the vibes into a signal ready for the amplification chain. The amplifier, therefore, must not only perform well electrically. It must also convey emotion in order to fully satisfy both the analytical mind and the inspirational soul. These days, science is beginning to discov- er an essential, almost mystical, intercon- nectedness of everything. It is intuitively obvious that the character of the universe on a macroscopic scale should rely on the properties of the subatomic particles of which it consists. At the same time, the character and properties of those particles is defined by the universe at large, the whole system mysteriously holding itself up by its own bootstraps, each piece of the giant jigsaw fitting exactly into place without deficiencies or excesses. It is only by virtue of an intellectual ges- ture that we perceive a condensed, solid, and definable part of the web of reality, yet we have deceived ourselves into think- ing that our mental creation is the be-all and end-all of existence. Most of our old scientific “laws” including those currently used to judge sonic perfor- mance are only close ups of the whole picture. Im afraid we are not seeing the wood for the trees. Certainly these measured parameters do have some relevance in terms of overall performance, but to recreate a musical event, an amplifier must work on both a macroscopic scale as a part of a communi- cation system between the performance and listener and on a microscopic level as a Ankoru by ANDY GROVE Audio Note, U.K. RadioFans.CN 10SOUND PRACTICES - Issue 12 collection of valves and parts which must be tamed and optimized for the task at hand. A magazine article can only skim the sur- face of any design philosophy and, of course, there will be shouts of “what the *$! is this guy on?”, but I hope my dis- cussion of the Ankoru design will be inter- esting nonetheless. Starting with the basic precept that each part of the amplifier should fit exactly into place, and have a character defined by the overall requirements of the system, the validity of feedback and push pull opera- tion, two pillars of traditional amplifier design, are immediately called into ques- tion.These concepts are purely intellectual constructions, created in laboratorieswith no motivation from natural music, and I am convinced that they detract from sound quality as a result. In practice, the ultimate purpose of feed- back and push pull operation is to make amplifiers easier to make not better. In any event, reducing harmonic distortion to vanishingly small levels and increasing bandwidth from DC to cosmic rays does not make a more musically satisfying amplifier. Specs must give some satisfac- tion though, cos we all know a guy who slinks off to the bathroom with a copy of his tranny amp spec sheet! I agree that limitations such as distortion and bandwidth abberations unquestion- ably colour the sound and should be elimi- nated, but beyond that I maintain that there are more important areas to be con- sidered if musicality is the ultimate goal. According to my way of thinking, all of the above leads to the assertion that the over- all topology of an amplifier must be single- ended and there must be no feedback. Transistors and all things silicon sound unnaturalput sand in the signal path and you get gritty sound! So, lets proceed directly to valves and, in particular, the simplest and purest amplifying device. The materials used for the construction of the passive elements of the amplifier are just as important since the signal must pass through them. Every material has a tonal coloring effect, so only highly-specified, high-purity, listening-tested materials are suitable. For example, in the Ankoru we use only Black Gate and Cerafine electrolytic capacitors for the audio circuitry. These caps eliminate that electrolytic mushiness without going over to the brashness of cer- tain plastic caps. The valve selection was guided by the notion that the different sonic signatures of each type should be complimentary, leading to the goal of a sound that possesses both strength and finesse. Schematic diagram of authors design - Audio Note Ankoru SE parallel 845 70 Watter RadioFans.CN Before I go on to describe the Ankoru cir- cuit in detail, I would like to say a few words about transformers and transformer coupling, since transformers play an important role in the design. In any valve, waveform distortion is caused by the characteristic parameters of the valve changing in sympathy with the applied signal. In a standard RC coupled triode circuit, the valve is set up with a quiescent current (Iq) flowing through it and the load resistor, yielding a particular quiescent voltage on its anode (Vq). With a negative-going input signal, the current is reduced and the anode swings positive due to the reduced voltage drop across the load resistor (Rl X Iq). The reverse is true with a positive-going input signal, the valves anode current is increased so the voltage on it reduces due to increased drop across Rl. There is a problem with this, however, because as the anode swings positive and the current decreases, the transconduc- tance of a valve goes down due to the cur- vature of its characterisic. Of course, the reverse is true with a positive-going input signal, the transconductance goes up with with the current. This means that the positive part of the anode swing is compressed and the nega- tive part is expanded waveform distor- tion. Usually, this distortion only becomes serious with very non-linear valves and/or large voltage swings. When we want to drive a fairly meaty output valve we need to swing a lot of volts because the mu of these types is necessarily low to keep the anode impedance down to keep loud- speaker damping up. In this circumstance, waveform distortion can easily rear its rather ugly head. We need a system for keeping the current through the valve as constant as possible over the anode swing, i.e., a high load impedance. Increasing the load resistor on an RC coupled stage can only go so far, however. One soon runs into problems of resistor dissipation and PSU voltage if the anode current is kept at the optimum level. The SRPP stage and his other active loaded cousins, such as the mu follower, have never really delivered the goods for me. Close listening reveals a lack of focus and immediacy compared to even the humble RC coupled stage. Anyway SRPP is a feedback device and quite often that scheme doesnt work very well electrically either, especially with the low impedance valves we would like to use as drivers. Simply pretending that youve got a low output impedance just doesnt cut any ice in the world of real audio. For large power valves, a low AC drive impedance is necessary because large valves have large and therefore highly capacitive grids. Thankfully, the low gain keeps down the Miller Effect, but its still there, so for good HF response, there is no getting around using a good low imped- ance driver. From the standpoint of sound quality, for a strong sound we need a beefy, low imped- ance driver. Wimpy driver equals wimpy sound. Drive two 845s with an ECC83 and itll be like putting a model aircraft engine in a Chevy Impala. Not exactly awe inspiring. The DC resistance of the grid circuit must also be kept low to control the effect of another rather annoying bugbear, grid cur- rent. Unfortunately, the vacuum in many modern valves is far from perfect, so there are quite a few gas ions floating around inside the bottle. Some of these ions will collide with the grid and draw electrons from the grid circuit. If the grid resistance is high, the grid bias will be modulated in tune with the signal, a real no-no in my book. Also the grid may occasionally be driven positive on signal peaks, causing the grid- cathode diode to conduct, rectifying the input voltage in the manner of a shunt diode supply with the coupling cap as the reservoir. This action makes the bias volt- age more negative, reducing the quiescent current through the tube, sometimes to the point where it will only conduct on peaks (Class C). In fact, a severe peak can cause the amp to cut off altogether, result- ing in a total loss of output. Worse still, the grid resistor/coupling cap combination acts as an RC time constant, so the effect lasts for some time after the overload has passed in sort of a time-delay distortion mechanism. Reducing the grid resistor to combat these effects is no solution. We want a DC grid resistance similar in magnitude to the impedance of the driver valve, i.e. a few hundred ohms not a few hundred kilohms. Making your grid resistor 600 ohms will likely kill the driver stage and, anyway, would require a coupling capacitor so big that the RC time constant would put us right back where we started. To cure the voltage swing problem requires a circuit element which has low DC drop but a high AC impedance. Plus, we need a low DC resistance in the grid of the output valve. And the device should efficiently couple the driver valve to the output tubes grid. The driver transformer is exactly what we need for the job. Its primary inductance presents an extremely high AC impedance to the driver valve and reflects the anode impedance of the driver into the grid cir- cuit of the output valve. A good driver trans will have a primary and secondary DC resistance on the order of 300 ohms, so the problems associated with grid cur- rent are more or less eliminated. This is a resistance 1000 times lower than Ive seen in some designs. Ideally, the transformer secondary is left unloaded, i.e. there is no “damping resis- tor” put across it to cut ringing. An unloaded transformer sounds better and it gives the driver valve a higher impedance load. There are two large-scale problems with driver transformers,HF frequency response and LF frequency response.These two requirements are mutually exclusive to a certain degree and many commercially available transformers sacrifice one for the other. The Tango transformers, for exam- ple, seem to go for impressive-looking HF specs but they have diminutive primary inductances Issue 12 - SOUND PRACTICES 11 RadioFans.CN which limit the LF performance. The problem is compounded by the unbal- anced DC current imposed by SE opera- tion, which requires that the number of primary turns must be increased to coun- terbalance the loss of permeability caused by the air gap in the core. Leakage induc- tance is proportional to the square of the primary turns so its a real pain in the butt. The driver transformer in the Ankoru has to handle 45 mA and still have superb bass, so it took some heavy calculator work and a few trees worth of paper to get it all working! The Ankoru interstage trans will be available as a DIY parted. I love the sound of large triodes like the 211 and 845. The 845 was used in this amp because it offers greater power in Class A1. The 211 is a more voltage sensi- tive valve than the 845, its mu is higher but then so is its internal impedance. It cant swing a lot of current at the low volt- age end of the anode swing without having the grid driven positive into Class A2. When pushing the grid above zero volts, it no longer reacts as a high-impedance ter- minal. It starts to draw appreciable cur- rent, corrupting the input signal in a most unattractive way unless the driver imped- ance is extremely low. The grid-cathode diode impedance of a 211 is about 2k, so we would need some- thing around 100 times lower or hideous distortion would result. The waveform dis- tortion could be corrected using feedback but why build an amplifier that is intrinsi- cally nonlinear? The 845 can sink a lot more juice where the 211 starts wheezing, but since the mu is so low, it requires a driver stage capable of considerable voltage swing. The 845s in the Ankoru are biased at -100 to -200 Volts for an anode current of 75 mA at 1200V B+, they look into a load imped- ance of around 6k, and put out a formida- ble 70 Watts. The output transformer has to cope with 150 mA DC and hold its 6k impedance down at LF, requiring a high primary inductance. This takes a serious hunk of iron, but the Ankoru output is just such a beast and the bass is awesome, if I do say so myself. To keep the drive signal to the output valves clean requires a driver valve of excellent linearity. One could use an indi- rectly heated valve such as the 6BX7, very linear, or the slightly less linear 6BL7, but low impedance, low mu directly heated valves are definitely the best choice. Since this amp has to be built using valves which will be available for some time into the future, so that replacements can be made throughout its life, it was necessary to use modern versions of either the 2A3 or 300B. I originally experimented with the 2A3 as I wanted a measure of its clari- ty and immediacy, but these valves have a very nasty habit of making toilet related noises even in the output stages of amps, and using one as a driver was impossible. I even tried some NOS samples but many were only marginally better, only the best and therefore rare and expensive examples were quiet. So the 300B was chosen, and it brought its characteristic warmth and musicality to the amp as well a greater impact to the bass. The 300B is operated with 300V across it and an anode current of 45 mA so it will last for ages, no more current or voltage was necessary for driving the 845s to full output. The 300Bs output is in fact so large that the 845s will be freaking (and so will your wife and the neighbours) before it runs into trouble which makes its jobs and the job of the input stage easier. Various input configurations were tried, all using the E182CC/7044 valve for its pow- erful sound. The original and best sound- ing configuration gave the amp so much gain as to be impractical. Long speaker leads acted like antennae and transmitted the amps output into the input leads causing instability. Super high quality cables and careful system setup would eliminate the problem but as this is a com- mercial amp it has to be dealer proof so a simple, single stage R