Aikido Headphone Amplifier

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John=20 Broskie's Guide to Tube Circuit Analysis & Design =

21 April 2006

Aikido PCBs and tube = sockets=20 available at the GlassWare Yahoo Store

Headphones=20 and headphone amplifiers
Ah, headphones: either = you=20 love them or you hate them. I love them. I like not hearing noise = from=20 refrigerators, air conditioners, barking dogs, the subwoofers of = rude=20 neighbors, cars, trucks, and especially loud minibikes and = motorcycles.=20 Moreover, I like hearing normally unfamiliar and uncharted = musical=20 events: the floorboards creaking beneath the 300-pound jazz bass = player,=20 the hall=92s reverberation, the guitarist=92s fingerprints across = the strings,=20 the vocalist=92s gasps for air, the recording engineer=92s clumsy = edits, the=20 sheet music=92s repeated turnings during every classical = performance, George=20 Harrison=92s thick-fingered flubs, the mechanical articulation on = a=20 saxophone, the pianist=92s humming=97well, strike that last one = item=97in other=20 words, the hundred diminutive sonic artifacts harbored by = recordings but =20 seldom heard. Some are annoying, others are embarrassing, others = still are=20 transporting, but all reveal a hidden world unmapped by even the = finest=20 loudspeakers.

What=92s wrong with loudspeakers? A = loudspeaker=92s=20 crossovers and massive diaphragms smear the sonic minutia, rolling = over=20 the barely audible information, washing away the recording=92s = sonic=20 fingerprints, like a blurry photograph that truthfully reveals a = dress=92s=20 color, but not its texture. Here is a simple experiment: visit a = music=20 store and listen to the sonic signature of each type of cymbal = displayed;=20 then go home and listen to your loudspeakers. What happened to = that=20 obvious and marked difference in timbre? Now listen to the same = recordings=20 with high-quality headphones. Can you hear more of the = distinctions in=20 size, brand, and composition of cymbals? (Yes, I know that CDs = make this=20 test much harder than LPs or tape did.)

To subtract further, the little that the = loudspeaker=20 can faithfully reproduce pours into a sonic coffin known as your = listening=20 room. Parallel walls, floor, and ceiling reverberate unevenly, = stressing=20 some frequencies, but attenuating others. I imagine that even the = air=20 between a loudspeaker and your ears subtracts from what the ear = could hear=20 directly against the loudspeaker; being no more than a mixture of = gasses,=20 how could we expect it to hold a precise imprint for very long = before its=20 natural sponginess smears the finest details? Here is an = additional=20 experiment: sit in the front row of a concert hall and listen to = the=20 amazing brightness, if not downright harshness of many of the = orchestra=92s=20 instruments. Then move to the rear of the hall and sit in an = untaken seat=20 and listen to the soft, murky sound, where many discrete violins = meld into=20 one fat, large violin.

Verily, headphones circumnavigate so many = sonic=20 snares that it is amazing that more are not ardent headphone = listeners.=20

The problem with=20 headphones
Of course, most headphones sound = dreadful,=20 but then so do most loudspeakers. And of course, many do not enjoy = listening that closely, like those who actually prefer the TV=92s = small=20 screen to the grandeur of the movies. If nothing else, TV erases = the=20 wrinkles and thick makeup from faces both aged and worn; for = example, Joan=20 Collins lived on as a sex symbol on TV well into her = sixties=97I=92ll grant=20 that she was once truly lovely, but the big screen would have not=20 permitted such a deception for so long. Back on topic, others may = claim=20 that headphones sound unnatural, but I have found that these same = critics=20 often believe that =93live=94 music is what comes out of = loudspeakers at=20 concerts; if that is what is "natural," I=92ll gladly take the = unnatural.=20

I know some are thinking: What about = electrostatic=20 loudspeakers? Surely they can match the headphone=92s = magnifying-glass sharp=20 detail? And I have to admit that the best electrostatic = loudspeakers can=20 come close to matching the best dynamic headphones, but I hold = that they=20 cannot equal the delicacy of the best electrostatic headphones. So = when we=20 compare apples to apples and oranges to oranges, the same = relationships=20 obtain.

Many shy away from headphones because they = miss the=20 visceral impact that big loudspeakers deliver, the shaking of the = floor=20 and body, which no headphone can reproduce. No debate here. In = fact, that=20 is why I have been using subwoofers with my headphone listening = for almost=20 thirty years. Just limit the subwoofer=92s high-frequency = bandwidth to 70Hz=20 to 100Hz and adjust its level to the point where it is only felt, = but not=20 heard with the headphones playing.

Finally, consider this: 99.9% of the = headphone=20 listening you have done in your life has been through solid-state=20 amplifiers. How many Walkmans have you owned that sported a = vacuum-tube=20 output stage? So, if you dislike headphone listening, are you sure = it is=20 not the miserable little solid-state OpAmps that you actually = dislike?=20

Now, when high-quality headphones meet = vacuum tubes,=20 real magic results. Gone are the obnoxious solid-state artifacts = that can=20 sometimes make dull loudspeakers sound sparkling. Gone are the = ridiculous=20 class-B OpAmps found in most receivers and in all MP3 players. (In = an MP3=20 player, battery life is critical and a class-A output stage would = be=20 lethal to playing time.)

Aikido headphone amplifiers

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So is the Aikido amplifier, = with its=20 low distortion, low output impedance, feedback-free design and = excellent=20 PSRR figure, the best choice for driving headphones? No, I don=92t = think so.=20 In fact, I would argue against its use as a headphone amplifier, = as its=20 single-ended output stage greatly limits the potential current = swings from=20 the triodes used, and thus a push-pull circuit would=20 be=97regrettably=97essential.

But then there are the e-mails from readers = who tell=20 me that they are using an Aikido amplifier as a headphone = amplifier and=20 that they love the sound. Normally, I instinctively discount such=20 testimony, as just about any circuit or modification can provoke = such=20 enthusiasm. But I am reminded of my once dim view of using the Broskie=20 cathode follower as a headphone amplifier. One reader=20 told me that his Sennheiser HD-600s never sounded better than = they hooked=20 up to a 6922-based Broskie cathode follower. I couldn=92t see the = circuit=20 being happy driving such a low load impedance, but after modeling = it in=20 SPICE and after being suitably impressed, I tried it myself with = the=20 HD-580s I used at the time. Darned if he wasn't right. Recently, I = have=20 hooked up my HD-650s to a 6SN7-based Aikido amplifier (bypassing = my=20 excellent custom-made Tribute=20 Audio headphone transformers) and it also sounds great.

Still I worry. Although my wife finds it = hard to=20 believe, of all the audiophiles I know, I listen at the lowest = volume=20 levels. So I fear the that the headphone user who likes to share = his music=20 with those across the street from him will be disappointed by the = small=20 amount of ringing in his ears that an Aikido linestage amplifier = can=20 muster. In other words, I believe that if an Aikido amplifier is = going to=20 drive headphones, it must be designed carefully and robustly=20 overbuilt.

Getting = serious
=20 If only all headphones were high impedance devices, with = impedances=20 closer to 1,000 to 10,000 ohms, rather than 16 to 600 ohms, tubes = would=20 have a much easier load to drive. Unfortunately, portable audio = equipment=20 and low-voltage batteries dictate a low headphone impedance. (If = you know=20 Joe Grado, whom I understand is a marvelous fellow, plead with him = to come=20 out a 2k version of his top-of-the-line headphones, hell, even a = 300-ohm=20 version would be great; I=92ll buy one, as will many other tube = fans.)=20 Remember that 16 ohms is only twice the impedance of your = loudspeakers, so=20 can anyone really expect a 6SN7 to drive it without an impedance = matching=20 transformer?

So, our first design objective is to limit = expected=20 load impedance to no less than 300 ohms and to limit our choice of = output=20 tubes to those robust enough to drive 300 ohms effortlessly. In = other=20 words, no 12AU7s or 6SN7s or, even, 6N1Ps need apply. These tubes = have=20 their uses, but none can deliver enough current into a 300-ohm = load in a=20 single ended output stage. (Yes, I know that there is a company = that sells=20 highly respected headphone amplifiers that sport 6SN7s throughout, = but I=20 also know that the output stage is a push-pull one.) Single ended=20 operation means that the idle current is also the current limit = into the=20 load. On the other hand, a push-pull operation allows peak current = to=20 equal twice the idle current in a class-A amplifier and they sky = is the=20 limit in a class-AB or class-B amplifier.

How much current is needed to drive 300-ohm = headphones? It depends on the headphones. I have played my = Sennheiser=20 headphones at the loudest level I would ever want to withstand; = then I=20 replayed the track into an oscilloscope. The highest peak voltage = I ever=20 saw was about 3 volts, which translates into a peak of 10mA, which = in turn=20 equals 15mW. To buy some extra headroom, let=92s double this peak = current to=20 20mA. So, will 20mA always be enough? No, I doubt it, as some = 400-ohm=20 headphones are much less efficient; but at least the sizable = Sennheiser=20 crowd will be satisfied.

Octal Aikido = headphone=20 amplifiers
The best choices for an octal-based = Aikido=20 headphone amplifier are the 6BL7 and the 6BX7. (The 6AS7 or 6080 = or 6082=20 could be used, with a truly low B+ of only 50V, but these tubes = are not=20 nearly as linear and would definitely require a global feedback = loop.)=20 These triodes were designed to be used in vertical-deflection = oscillator=20 and amplifier circuits in televisions. Impressively, they can = withstand=20 high plate voltages and high cathode currents that make lesser = tubes=20 swoon=97but at the cost of a 1.5A heater current, the same as an = EL34 and=20 almost three times that of a 6SN7. However, in this application, = their=20 best feature is their ability to conduct 20mA with only 75 volts = on their=20 plates, something a 6SN7 simply cannot do. For example, the 6SN7 = can=20 conduct 20mA, but only with a lot of plate voltage, so much, in = fact, that=20 the plate runs into its dissipation limit. Actually, a 6SN7 could = also=20 conduct 20mA with 75 volts on its plate, but only with a positive = grid=20 voltage. So, there are effectively four operational boundaries: = the=20 0-volts grid line, the plate dissipation curve, the maximum = cathode=20 current limit, and the maximum plate voltage limit (we should add = the=20 bottom of curves because of its gross nonlinearity). Below is the = 6SN7=92s=20 curves:

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Now, the 6BX7=92s curves:

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Note how the 6BX7=92s 10W plate dissipation = limit does=20 not even intrude into the graph=92s confines. At 75V and 20mA, its = mu is=20 9.5, which gives us the ratio of 60.5% of power supply noise to be = injected into the bottom triode=92s grid. This ratio can be = realized through=20 a 64.9k resistor standing on top of a 100k resistor, or a 100k = resistor=20 standing on top of a 152k resistor.

Now, what remains is to determine the = cathode=20 resistor values. First of all, we want the lowest output impedance = possible, so we cannot tolerate the added resistance from the = cathode=20 resistor; thus, we forgo using a cathode resistor for the top tube = and we=20 bypass the bottom triode=92s cathode resistor.

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Well, there it is. The 6SN7/12SN7/12SX7 = makes a fine=20 input tube. Both channels draw a total of 48mA. The (external to = the PCB)=20 417-ohm resistor drops the raw 170V down to a fairly filtered = 150V. The=20 170V power supply is easily built from full-wave-bridge rectifying = a=20 120Vac isolation transformer.

So, how good is this headphone amplifier? I = haven=92t=20 actually built it, so I am only guessing here, but I am unhappy = about the=20 added bypass capacitor across the bottom triode=92s cathode = resistor. One of=20 the great features of the Aikido topology is that it uses cathode=20 resistance to its advantage, linearizing the triodes without = incurring too=20 large a penalty in increased output impedance. Besides, always try = to=20 avoid placing an electrolytic capacitor in the cathode circuit, as = it can=20 only harm the signal.

In addition, I worry about the 6BL7 or 6BX7 = not being=20 as linear as the venerated 6SN7, particularly without the top = cathode=20 resistor. So I would first build this circuit with both 180-ohm = cathode=20 resistors in place, unbypassed; if the bass is acceptable, then = let it be;=20 if not, try removing the top cathode resistor and tack solder a = bypass=20 capacitor across the bottom cathode resistor.

iPod = trick
=20 If the Aikido headphone amplifier is meant to be used with an MP3 = player,=20 such as the famous Apple iPod, the output volume is set digitally = within=20 the device, so no external volume control is needed. This means = that the=20 Aikido headphone amplifier can include a global feedback loop from = output=20 to input grid, as the Aikido inverts phase. In the schematic below = we see=20 the feedback loop in place. The amplifier=92s input impedance is = roughly=20 equal to the 20k feedback resistor at the input. Now 20k sound low = for a=20 tube circuit, but is wildly high for an MP3 player that was = designed to=20 drive a 32-ohm load. In other words, the MP3 player=92s internal = headphone=20 amplifier will be so unloaded by 20K, that its output stage might = even=20 remain in class-A operation.

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The schematic above shows an iPod-ready = Aikido=20 headphone amplifier that uses a global feedback loop to lower = distortion,=20 noise and output impedance, while keeping the output stage=92s = cathode=20 resistors in place, and unbypassed. The 6SL7/12SL7/6SU7 offers = much more=20 gain to power the feedback loop. (Note this circuit will require = some PCB=20 hacking.)

Output coupling=20 capacitors
A 300-ohm load only requires a = 27=B5F=20 coupling capacitor to ensure a -3dB frequency of 20Hz. The 47=B5F = coupling=20 capacitor just extends down to 12Hz. The 0.47=B5F coupling = capacitor=20 bypasses the large and cheap polypropylene 47=B5F coupling = capacitor (super=20 high-quality electrolytic capacitors might work, but I doubt it); = any=20 quality capacitor is useful as a bypass, even the same capacitor = type and=20 brand but in the smaller value (less inductance).

Although the Aikido octal PCB will hold = some=20 physically very large capacitors, such capacitors can work their = way free=20 from the board if the amplifier is shaken enough. Thus, I always = recommend=20 using some hot glue or tie strips to hold large capacitors in = place. (An=20 added advantage to holding large capacitors down is that all = capacitors=20 have a microphonic quality, which the glue or strips helps dampen. = I=20 always place a small section of Sorbothane below each coupling capacitor, = for this=20 reason.)

What if you plan on driving both power = amplifier and=20 headphones, but not at once. A rotary switch can be configured to = allow=20 giving the line output only the high-quality 0.47=B5F coupling = capacitor,=20 but paralleling both coupling capacitors when driving headphones = (with a=20 center position of mute). Below is how to wire such a switch.

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Next = time
=20 We will cover some 9-pin Aikido headphone amplifiers.

//JRB

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