Ultra Linear Output Stages.pdf

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Circulation: Since 3/23/9941365 Ultra-Linear Output Stages

This Issue

This month's issue tackles ultra-linear mode,but without an output transformer. The designprinciple is applied to line stages, headphoneamplifiers, and OTL power amplifiers.

In November, 1951 issue of AudioEngineering, by David Hafler and Herbert

Keroes outlined a new way of using tetrodesand pentodes, which they labeled "ultra-linear."This article is out of copyright and isreproduced in our new section ClassicMagazine Articles. Expect to see new (old)articles with up coming issues of the Tube CADJournal. Remember, if you have a request orsuggestion of your own for either an articletopic or circuit, please e-mail:

Editor

Ultra-linear output stage of an amplifier

Devised in 1951 by David Hafler and HerbertKeroes, the ultra-linear mode was created tocure the ills of both triode and pentodeamplifiers. Each amplifier type had itsadherents. The triode amplifier boasted a

smooth, low distortion sound and a low outputimpedance, but yielded only limited watts. Thepentode amplifier produced many more wattsand a more aggressive, exciting sound, but atthe cost of greater distortion and much higheroutput impedance. A middle ground wassought. If a new tube could be designed, itwould have to yield the average of the pentodeand the triode sound, argued Hafler andKeroes. But rather than create a new tube, theirsolution was to create a new transformer thatwould allow the pentode to be used in a newway.

In This Issue

13

44

69

1316

1717

19

611676

Ultra-Linear Output Stages Line Stage Amplifier The Ultra-Linear Cascode OTL Power Amplifiers Headphone Amplifier

Differential Input HP Amplifier Extracting Extra PS Voltages Ultra-Linear OTL Power Amplifiers

Heater Concerns Output Stage Design The Feedback LoopE-mail

Publishing InformationGlossary of Audio Terms

Glass-Ware.com ArticlesClassic Magazine articles

Pentode connected Triode connected
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In a normal pentode amplifier, grid 2 sees aconstant DC voltage. In a triode connectedamplifier, grid 2 sees the same DC and varying

voltage that the plate sees, as this grid isconnected to the plate (usually through a smallvalued resistor, 1k). The ultra-linear amplifierworks between these two extremes. Themethod is simple: a portion of the platevoltage's swing is fed back into grid 2 via alow impedance tap of the output transformer'sprimary winding. This voltage represents asignal that is negative or anti-phase to inputgrid's signal. Thus, the output impedance at thetube's plate will decrease and so too its gainand distortion. But as the DC center voltage ongrid 2 is at the same potential as the plate, thetube will share much of the pentode's higherpotential power output, as the greatly positivevoltage on this grid will make for much greaterpotential conduction at low plate voltages.

Ultra-linear operation of a tetrode or pentode,however, was said to offer much more than justa compromise between these two extremes. Itoffered an output impedance close to that ofthe triode connected pentode, a maximumpower output equal or even greater than the

strict pentode operation of the tube, a low leveldistortion figure almost as low as the triode,and a high level distortion figure lower thanthe pentode. The best of all possible outputstages, argued Hafler and Keroes. Well, maybe.

Although popular with tube amplifiermanufacturers because of the higher poweroutput it yields, Ultra-linear mode does notcommand a large avid following the way thattriodes do. But then how many of us have

heard a single-ended, directly heated pentode(e.g. 833), ultra-linear amplifier or a push-pullultra-linear amplifier without feedback?Furthermore, many of the ultra-linearamplifiers we have heard may have sufferedfrom the limitations imposed by thetransformer. I have tested a few transformerswhose ultra-linear taps seemed poorly tappedor whose output yielded a

Pentode connected 6550

Triode connected 6550

Ultra- linear connected 6550


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substantially different bandwidth andwaveform fidelity than its plate leads.Transformers are the loose cannons of an

amplifier's design.On the other hand, if the transformer isremoved from the circuit, but the ultra-linearmode is retained, we could more fairly judgethe ultra-linear topology. Because grid 2 is at amore positive voltage than the cathode, itreceives some of the electron flow from thecathode. Consequently this grid requires a lowimpedance source no matter how it isconfigured in a circuit. In other words, becauseit is conducting current, a simple two resistorvoltage divider will not work, as the currentflow into the resistor would shift the grid 2voltage too low. No, what is needed,unfortunately, is a more complex circuit.

Line Stage Amplifier As a design example of how the ultra-linearmode can be used in a circuit that is not apower amplifier, a line stage amplifier provesuseful. Of course, the technique used here canbe applied to phono stages, single-ended poweramplifiers, and virtually any other audiocircuit. While straight gain and a fairly lowoutput impedance are basic requirements ofany line stage amplifier, we will add ultra-linear mode. Remember, the goal here is tomimic the ultra-linear transformer functionalityby using resistors, capacitors, and activedevices, such as triodes or MOSFETs.

Ultra-linear operation of a pentode requirestwo sub-functions: a sampling of the alterationsin the cathode-to-plate voltage and voltage

division of this voltage, and a low impedancemeans of driving grid 2 with this signal.Deriving the ultra-linear ratio of the output

tube's cathode-to-plate voltage requires only atwo resistor voltage divider. Ratios between10% to 50% have been used by ultra-lineartransformers. Consequently, the resistor ratiocan be anywhere between 9:1 to 1:1. If westick to Hafler and Keroes's ratio of 20%(actually, they specified 18%), the resistor ratiowill be 4:1. Thus, a 400k and a 100k resistorwill divide the plate voltage swing down to20%. High value resistor were chosen toprevent excessive loading of the plate. Ofcourse, if the plate load used is substantiallylower than usual, then substantially lowervalues could be used for these resistors. The obvious circuit choice for driving thisgrid, the Cathode Follower, is the best choice.In fact, this circuit works extremely well in thisapplication because the current flowing out ofgrid 2 will flow into the Cathode Follower'scathode and then to its plate on its way to the

power supply B+ connection. This means theCathode Follower's cathode resistor need notbe as low in value as might be first imagined,as the Cathode Follower will source most of itscurrent flow from the current flowing out ofgrid 2.

An ultra-linear linestage amplifier that

uses a

potentiometer to

set the ultra-linear

ratio for grid 2 of

the input tube.


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The first tweak to the basic circuit so faroutlined is to add a capacitor at the end of thevoltage divider resistor string. This capacitor

will shunt away the AC signal while preservingthe DC voltage of the pentode's plate, which isneeded to establish the correct DC outputvoltage of the Cathode Follower.

The second tweak is to replace the voltagedivider resistors with a single potentiometer.The potentiometer allows for some amazingadjustment to the pentode's mode of operation:at one extreme we have pure pentode; at theother, pure triode. In between these twoextremes lies ultra-linear operation.

What is the best potentiometer position? Bestfor what? Best for rock and roll or best foropera? Best for what amplifier? What speaker?What day? What wine? What mood? Thiscontrol gives the user a choice over the soundquality that is not frequency dependant. For thelongest time, I have wanted to build a preampthat sported several knobs that were labeled"Dynamics," "Darkness," and "Granularity"and that controlled such circuit parameters asidle current, coupling capacitor composition(from paper to Teflon), feedback ratio (without

altering the output level by reducing the inputsignal level correspondingly), and mode ofoperation (triode to pentode). Imagine StereoReview doing a review of this preamp: "But itmeasures the same no matter what the positionof the knobs!"

Since small triodes usually come two to theenvelope, a Cathode Follower for buffering thepentode's plate from the load can be easily had.The pentode specified in the schematic is afavorite of mine, the 6AC7. It is a low noise,

linear pentode in a metal envelope. It requiresabout 10 mA's of idle current to sound best.Avoid the Ken Rad versions of the tube as halfof the samples I have tested have provenseverely microphonic. Alternate tubes are the6SJ7, EF86, 12BY7, EL84, etc. One sleeperpentode is the 6AU6, which many tube fansregard as just being an RF tube; it isn't.

Ultra-linear

Cascode gain

stage. The two

100k resistorsreturn 50% of

the output

signal to the top

triode's grid and

provide a DC

bias voltage for

the top triode.

The Ultra-Linear Cascode

This circuit approximates the ultra-linearpentode circuit by using two triodes. Like thepentode version it boasts both lower outputimpedance and distortion than otherwise, butdiffers in that the top triode's grid has a veryhigh input impedance that is easy to drive withjust a simple voltage divider.

A further advantage this arrangement holdsover the stock Cascode is an increasedmaximum output swing, as the top triode'splate swings down, so too its cathode moves

down. This increase in output headroom wouldnot be needed in a preamp, but in the first stageof an OTL power amplifier it would provesignificantly better than a stock Cascodecircuit.

OTL Power Amplifiers Ultra-linear mode can be added to an OTLamplifier in much the same way as it wasadded to the line stage amplifier by samplingthe output and feeding this signal into a

Cathode Follower to drive grid 2. Here somecomplexity intrudes into the design. How dowe derive the correct ratios of output signal fortop and bottom output tubes? The bottom tubeworks in much the same way as an output tubein a conventional push-pull ultra-linear poweramplifier. Consequently, providing the bottompentode's grid 2 with the right percentage ofthe plate's voltage swing is easy enough toconceptualize.


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Ultra-linear operation requiressomething like 33% of the plate'smovement to be fed back into grid 2.

This can be accomplished by the samemeans as was used in the ultra-linearline stage amplifier. The top tube, onthe other hand, works with a fixedplate voltage and a cathode the swingsup and down with the output of theamplifier. How is ultra-linear mode tobe applied to the top tube? Beforetackling the schematic, a review ofhow ultra-linear mode works isneeded. Ultra-linear mode uses aportion of the plate-to-cathode voltagemovement to define an input signal forgrid 2 that is in anti-phase to thesignal being applied to grid 1.

Totem-Poled, ultra-linear, OTL amplifier output stage

Because the ultra-linear signal is phaseinverted to the input signal, the gain is reducedalong with the distortion and the outputimpedance. In other words, it is a form ofnegative feedback. If the ultra-linear signalwere in phase with the input signal, the gainwould increase but the distortion and output

impedance would increase as well; in fact, theamplifier could easily become an oscillator.

Both grid 1 and 2 are referenced to thecathode: present either with a greater positivevoltage and the conduction from cathode toplate increases. So what is needed is an ultra-linear signal for the top tube that will work inanti-phase to grid 1 input signal. The key hereis to reference all voltage relationships to thecathode. If a two resistor voltage dividerbridges output to ground and its division point

is set 66% relative to the output of theamplifier, a +10 volt swing at the output wouldequal +6.6 volts at the voltage divider's output.Since a +10 volt swing at the output means thatthe top tube's cathode has also swung +10volts, the +6.6 volt increase in the grid 2voltage is actually equal to -3.3 volts relative tothe cathode.

Supplying the actual voltage may help makethis clearer. In the absence of a drive signal thecathode is at 0 volts; the plate, +100 volts; grid1, -15 volts; and grid 2, +100 volts. Thus, thecathode-to-plate voltage equals +100 volts; thecathode-to-grid 1 voltage, -15 volts; and thecathode-to-grid 2 voltage, +100 volts. In the

presence of a +14 volt pulse at grid 1, theresulting voltage relationships would then bethat the cathode is at 10 volts; the plate, +100volts; grid 1, -1 volts; and grid 2, +106.6 volts.Now, the cathode-to-plate voltage equals +90volts; the cathode-to-grid 1 voltage, -1 volts;and the cathode-to-grid 2 voltage, +96.6 volts.The voltage division ratio of 66% effectivelybecomes 33% relative to the cathode.

A string of three equally valued resistorsgives us all the sampling of the output we need

to make a totem-poled, ultra-linear, OTLamplifier. Two Cathode Followers and a fewcapacitors and resistors complete the outputstage. Each Cathode Follower must be biasedso that its cathode voltage is roughly equal toplate voltage of the output tube it will service.Thus, the Cathode Follower's grids needcoupling capacitors and bias resistors.


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In the schematic shown prior, two B+voltages are available not only to allow betterheadroom for the input and driver stages but to

distribute cathode-to-plate voltages fairly forthe Cathode Followers. The top output tube'sCathode Follower's cathode resistor terminatesinto the output; the alternative layout wouldhave this resistor connected straight to ground.The advantage to the first scheme lies in theunloading of some of the required currentswing through this Cathode Follower so that itbetter matches the operating conditions of thebottom Cathode Follower. The bottom CathodeFollower sees only 33% of the output voltageswing, whereas the top Cathode Follower sees66% of the output signal. By terminating theresistor into output, the top Cathode Followereffective sees only 33% of the signal.

Headphone AmplifiersA tube amplifier for dynamic headphone

serves two purposes. The first, obviously, is fordriving high quality headphones, such as thosefrom Grado and Sennheiser. The second is thata small headphone amplifier can be viewed asa test run for a large power amplifier forloudspeakers.

If a headphone amplifier does not performwell at driving headphones, then a scaled upversion probably will not work any better at

driving your speakers. On the other hand, if theheadphone amplifier proves stable, trouble free,quiet, low distorting, then the design conceptwarrants an expanded version. Basically, thelower the impedance of the load, the moreoutput tubes will be needed. Even if you do notown a pair of quality headphones building ascaled-down version of an OTL amplifier is agood idea. For example, if the final OTL amplifier willcomprise eight 6AS7s in the output stage, thenbuild the same amplifier but with only two6AS7s and test it with a 32 ohm load. Whatresults will be indicative of what will resultwith eight output tubes and an 8 ohm load. The amplifier shown below inverts the phaseof the input signal. The feedback is taken fromthe output and brought back directly to the gridvia the 100k resistor. The advantage of thisarrangement lies in the gain decreasing to unitywhen the amplifier is disconnected from itssignal source. The disadvantage to thisarrangement lies in the relatively low input

impedance presented by the 48k resistor.

OTL ultra-linear headphone amplifier


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The 6BM8 will also work as the input tube, asits pentode will serve as the voltage amplifierand the triode will work well in the Split Load

phase splitter. (I want to build this versionsometime soon, as a 2x3 array of these tubeson a printed circuit board would certainly beclean looking. Additionally, using all the sametube has the advantage that they can all betested for noise and the winners can be placedin the input tube position.) If a discrete pentode, such as the EL86, isused for the output stage, then a 6DJ8 or12AU7 type tube can be used in the CathodeFollower position. The 6DJ8 or a 6AQ8 willalso work perfectly as an input tube and phasesplitter.

If you wish to keep the amplifier lookingmore like a Futterman, then an EF86 or 5879would be a solid choice for the input tube. Onetrick would be to feed the output directly intogrid 2 of the input tube. With the output stageconfigured as a unity gain, non-phase invertingbuffer, the output signal should equal that onthe plate of the input tube. In other words, theoutput signal will force the input pentode tofunction in triode mode, a mode that

intrinsically entails a form of feedback. This isa marvelously short and direct feedback loop.

OTL ultra-ultra-linear headphone amplifier
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Differential Input

Headphone Amplifier The problems of how avoid phase

inversion and how to easily applyfeedback can be overcome by switchingto a differential input stage and a bipolarpower supply. At first glance, the inputstage looks as if it functions just aDifferential amplifier, but closerinspection shows that it work more like aCommon Cathode amplifier in that theplate of the first triode does not swing asgreat a voltage as the second triode. Thisis the result of feeding the output of the

amplifier back into the first triode's plateresistor. It is this arrangement that bothequalizes the voltage swings of theoutput tubes and allows both top andbottom output tubes function as CathodeFollowers, i.e. unity gain and low Zo.

Differential input stage for headphone amplifier

To get the logic behind this circuit, workingbackwards may help. If a positive going pulseis applied to the output of the amplifier, thispulse will be transmitted to the top of the firsttriode's plate resistor. Since the very littlevoltage

division occurs at the plate, the pulse is thenrelayed via the coupling capacitor to thebottom pentode's grid 1, which will forcegreater conduction from this tube, which inturn will pull down the output voltage of theamplifier.
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As the top pentode is clearly acting as aCathode Follower, it will experience a decreasein current conduction because the positive

pulse will force its cathode to be even morepositive than its grid, all of which in turn willserve to decrease the voltage at the amplifier'soutput.

So far the output stage has an outputimpedance equal to 1/2 Gm, where Gm is thetransconductance of the output tubes. Theoutput impedance can be further decreased bywrapping a feedback loop around the amplifier.Here is where the second triode's grid comesinto play. This grid is at ground level and itrepresents a high input impedance. Thesefeatures serve to make adding a feedback loopeasy: no additional capacitors, no criticallyvalued, low resistance feedback resistor.Instead, all that is needed is a two resistorvoltage divider. While we must be mindful ofthe Miller effect capacitance from the secondtriode's grid to its plate, we enjoy greaterlatitude in the choice of resistor values thanpreviously. Notice that when the positive goingpulse is forced on to the amplifier's output, thefeedback will send a correcting signal to both

output tubes. Let's follow the path.

The pulse cause the second triode to increaseits current conduction, which develops agreater voltage across its plate resistor, which

then forces the top output tube's currentconduction to decrease, thereby pushing theoutput negative. Now the pulse which causedthe second triode to increase its conductionalso causes an increase in the common cathoderesistor, which will cause the first triode to seeits cathode moving positive while its gridremain fixed at 0 volts. The first triodeconducts less and thus its plate resistor seesless current flowing through it, which results ina positive pulse being fed into the bottomoutput tube's grid. The bottom output tube thenconducts more current, which will work to pullthe output voltage down.

Of course, the gain of the triodes will greatlymagnify the errant pulse at the output and thisgain is what feeds the feedback. If the open-loop gain of the amplifier is 10 and we feed allof this signal back into the amplifier, theoutput impedance will decrease by ten fold, asshould the distortion and noise. Furthermorethe feedback will extend the bandwidth of theamplifier. Magic?


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Well, sort of, the problem with feedback liesin that it can do all of these very desirablethings, but not all at once. Without a load the

open-loop gain was 10, but with a 32 ohm loadthe gain may fall to 2. Now if all of the outputsignal is fed back into the amplifier, the outputimpedance will only be halved. But thedistortion may even be greater than in theunloaded, open-loop state. Why? The outputstage might have left Class A operation, all ofthe tubes might have left their linear region ofuse, and the power supply might have beguncollapsing because of the increased currentdemand and its noise component greatlyincreased. In other words, the feedback couldbe used to lower the output noise and extendthe bandwidth of the amplifier with no load, orit could be used to maintain a constant unitygain at the output in the presence of a load.

Realizing the limits of feedback to convertsow's ear unto silk, we must make the amplifieras close to silk as we can before we add thefeedback. This makes noise reduction of primeimportance to designing this amplifier. Byusing as many noise canceling techniques aswe can, the feedback will have much less work

to do. In order to balance the output stage, thetop pentode must see no noise on its grid andthe bottom pentode must see all of the negativepower supply rail noise on its grid. Why thedissimilar demands? In fact, there are not any. Think in terms relative to each pentode'scathode. The top tube's cathode connectsdirectly with the output of the amplifier;therefore, its cathode should have zero noise onit as should its grid. The bottom tube's cathodeconnects to negative power supply rail;

therefore, it will share all the noise on it and,thus, so should its grid. If both tubes have theirgrids tied to their cathodes and if this two tubeseries is placed across a bipolar power supply,the noise at the junction of the two tubes willnull and equal no noise. (Imagine two rubberbands tied together and pulled equally inopposing directions.)

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We know what is required to make thisamplifier naturally quiet: along with the musicsignal, no noise on the top tube's grid and all

the negative power supply rail noise on thebottom tube's grid. The advantage a bipolarpower supply holds over a mono-polar powersupply, is that much of the noise on each rail isboth equal amplitude and anti-phase with theother. Of course, this assumes an equal load foreach rail voltage. This means that the midpointbetween these rails is noise free, just as theknot in between the two rubber bands remainsstill while both ends are vigorously pull to andaway from each other. (Actually, wherever wedefine ground to be will also define thereference point that against which both signaland noise will be measured. This point,obviously enough, has no signal and no noiserelative to itself.) As the inputs to the differential input stageare referenced to ground, the common cathoderesistor remains fixed at one end and at itsother end bounces up and down with thenegative power supply rail noise. This varyingvoltage defines a varying current through theresistor, which, in turn, is relayed through the

two tubes into the two plate resistors. The firsttriode's plate resistor is effectively grounded atthe output of the amplifier. Thus, the noiseinduced current change develops an in-phasenoise signal at the connection between plateresistor and plate, which is then transmitted tothe bottom output tube's grid.

On the other hand, the second triode's plateresistor is effectively connected to the positive100 volt rail of the power supply. Thus, thenoise induced current change develops an in-

phase noise signal that expands and contractsthe voltage across the plate resistor to the samedegree as the positive rail voltage expands andcontracts, which effectively nulls at the plateand the top output tube's grid.

In order for this noise nulling technique towork requires that the plate resistors valueequal twice that of the common cathoderesistor.
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On the first half of the cycle, the capacitorattached to the transformer's secondary ischarged and the output capacitor receives no

charging. On the second half, the bothcapacitors are charged simultaneously. So thetransformer's secondary experiences a currentconduction on both halves of the cycle and isthus notsubject to the magnetizing of its core.

The full-wave voltage doubler circuit, whileit does experience conduction through bothhalves of the cycle, works differently from thehalf-wave version. Basically, it charges onecapacitor during one half of the cycle and then

in the second half of the cycle, it charges thesecond capacitor. Because these capacitors arein series with each other, the voltages andfrequency of the noise double.

The full-wave voltage doubler circuit reducesto two half-wave rectifier circuits placed inseries. Now, if two half-wave voltage doublersare placed in series, the result is a full-wavevoltage doubler. This will require three morecapacitors and four more diodes. Allcapacitors must be rated for at least the fullvoltage doubled rail voltage and 150% of this

value would be safer. So to the diodes mustrated for at least the full doubled voltage. (Donot forget that this circuit only works with acenter-tapped transformer.)

Half-waverectifiercircuit

But in the simple half-wave voltage rectifiedpower supply shown above, the current willflow through the secondary winding in onedirection on one half of the AC cycle and thenit not flow on the other half of the cycle: up,off, up, off. These repeated single directionconduction cycles are similar in effect tohooking up a DC voltage source across thetransformer's secondary, which woulddefinitely work to magnetize the transformer'score.

The principle of operation for a half-wavevoltage doubler, however, is different fromthat of the simple half-wave rectified powersupply, being much closer to the full-waverectifier circuit. They share the "half-wave"name because of noise frequency and halfcycle charging of the capacitor to which theload attaches, but not because of the

conduction cycle through the secondary. Thehalf-wave voltage doubler's transformerconducts through both halves of the AC cyclejust like the full-wave bridge rectifier circuit.

What is wrong with magnetizing the core?Transformers are able to pull off a fairly amazing feat: they are able, in a small package, to

convert one AC voltage into another. This feat works by using the magnetic field that results fromhaving a current flow through an inductance (the primary) engulf another inductance (thesecondary) so that an AC voltage will develop in the second inductance (secondary). While this

process does not necessarily require a core, it does greatly benefit from using a core. The coreconcentrates the magnetic field so it more effectively couples to the secondary and it greatlyincreases the inductance of both the primary and the secondary. But when the core becomessaturated (magnetized), it cannot do either very well.

A toroidal transformer is particularly susceptible to core saturation, as its core is very tightlymade. On the other hand, the usual stack frame transformer's core cannot as easily be made tightand this sloppiness creates an untended air-gap in the transformer, which will serve to reduce thetendency towards saturation by the core. As the toroidal transformer lacks this small air-gap, itwill not withstand much unidirectional current flow before saturating.


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Combing two transformers not only serves tobost the rail voltage, but it also serves toflatten the aspect of the enclosure that willhouse the power supply. Two flat-pack style

PC mount transformers are not nearly as tall asingle horizontal mount power transformer thatholds the equivalent windings. Furthermore,having the two lower voltage windingseffectively center-tapped allows for easilyestablishing a +/- 12 volt rails about theground. These rails will come in handy, if DCservos, slow start circuitry, or startup/faultsmuting circuitry are used, as these circuits willundoubtedly use ICs in their implementation.Of course this means the low voltage windings

must be full-bridge rectified, which we shouldanyway to provide DC voltage for the heaters.

Full-wave voltage doubler circuit

Finding a 140 CT transformer is tough, asthis is not a common value. Three fixespresent themselves. The first is to have onecustom winded. This will cost more muchmore money than the off-the-shelf option,but designing power transformers is notsecret art like designing output transformersis. And a custom transformer quicklybecomes cheaper if you can get a few ofyour friends to order one as well. Thesecond fix is to obtain an over-specified 120CT transformer, as 20% regulation on thetransformer will easily bump the secondaryvoltage up to 140 under a light load. Thefinal fix is to use two transformers in series.This is my recommendation. Since at least one extra winding will beneeded for supplying the heaters, a secondtransformer would probably be neededanyway. Wiring two transformers in seriesis easy, as a common transformerconfiguration is to have four windings on

one core.

Ancillary support circuitry: DC servo to correct for DC

offsets at the output and an automatic muting circuit for

protecting the headphones from a fault condition in the

amplifier
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The Real Thing:

Ultra-Linear OTL Power Amplifiers Only the brave need apply. OTL amplifiers

have a well-deserved reputation for breakingdown and often damaging loudspeakers in theprocess. This makes sense, as the protectionfrom the lethal voltage within a tube amplifieris usually the isolation provided by the outputtransformer. For example, if a cathode-to-plateshort occurs in a transformer coupled push-pullamplifier, nothing much happens to the speakereven though the tube is blown (and probably, ahandful of resistors, diodes, and wire). AnOTL amplifier, on the other hand, is like

performing a trapeze act without a net. To getan idea how much potential energy is inside anOTL amplifier imagine how many watts yourloudspeakers would have to endure if one ofthe output shorted plate to cathode. The mathis simple voltage squared divided byresistance: Power = VR.

Assuming 170 volt rails in the amplifier, thismeans 3612 watts into an 8 ohm speaker!Protection circuitry helps, but there is always

the chance that it will not be fast enough.Adding an output coupling capacitor looks likea better idea than first imagined. Having scared away the weak-hearted, wecan move on to describe what an ultra-linearOTL power amplifier would look like.Actually, a power amplifier would not look toodifferent from the headphone amplifierexamples already given. More output tubes,beefier tubes throughout, much higher powersupply voltages, certainly, but notfundamentally different in design. The firststep is to forego any temptation to build aClass A ultra-linear OTL amplifier. Here iswhy: 100 watts would require 850 watts ofplate dissipation alone. We must add to thisfigure the heater and support circuitdissipation. A stereo amplifier would trip yourhouse's circuit breaker at turn-on. Five channelsurround sound would require the powercompany to set up one step-down transformerfor just your house.

Once we accept a lean Class AB as the onlypractical choice (along with using feedback tokeep down the distortions that result from

Class AB crossover problems [or Gm doublingeffects] and to lower the output impedance),we must determine how many output tubes wewill need. Even the most muscular sweep tubecan only conduct a peak current of about 2.2amps (the 8908) and this peak current is onlypossible with a grid 2 voltage of 250 volts.The 6LF6 limit is about 1.2 amps. Theproblem with these tubes is that they arealmost impossible to find at a reasonable cost.This leaves the EL509/6KG6 as the onlychoice.

The EL509 is the only sweep tube in currentproduction as far as I know and its cost isunder $30. Although its maximumrecommended continuous cathode currentdraw is only 500 mA, it can put out peaks ofover 1 amp. Now the question is how manywatts we want from our amplifier. Theformula for peak current demand based onRMS watts into a load is:

Ipeak = (2 x Watts x R)R.

Assuming 8 ohm speakers, if we want100 watts, we will need a peak of 5 amps;

80 watts, 4.5 amps;60 watts, 4.5 amps;50 watts, 3.5 amps;40 watts, 3.2 amps;30 watts, 2.7 amps;20 watts, 2.2 amps;16 watts, 2 amps.

Thus, a 100 watt amplifier would need 10

EL509s to play it safe, although 8 wouldprobably prove adequate. This number ofoutput tubes results from dividing the peakamperage by 1 amp and doubling that number,as the amplifier must be able to swing thepeak amperage negatively as well aspositively. Of course, we should always roundup to build in some safety margin. Besides, themore output tubes, the lower the outputimpedance.


17/20

Heater ConcernsNow, where you live might help you decide

on how many output tubes to use. In Japan, 8

would be a good choice; but in the UnitedStates, 10 would be the better choice. Why?These numbers lend themselves to a nicetrick. One of the problems with any poweramplifier is having to feed the heater string.The EL509 heater draws 2.5 amps. whichmeans given 10 tube output stage, we willneed a 25 amp at 6.3 volt tap on the powertransformer. They make such transformers, butthey are huge and the wire coming out of themis usually something like 10 solid-core

(definitely a headache to work with).Now, if all 10 tubes heaters are placed inseries, the total voltage requirement would be63 volts and the current draw per heater stringwould be only 2.5 amps. If the heater stringfrom one channel is placed in series with theother channel's string the current draw wouldstill be 2.5 amps, but the total voltagerequirement for the string would be 126 volts.126 volts is very close to the 120 volts wehave coming out of our wall sockets in theUnited States. But if 8 output tubes are usedper channel, then the total series requirementfor this heater string would be 100.8 volts,which is very close to the 100 volts wallvoltage in Japan. (Understand that running theheaters directly off the wall voltage is not thesame thing as running the amplifier's railvoltage directly of the wall voltage as in someof the Futterman amplifier and many cheaptabletop radios from the 1950s.)

I would never trust the wiring to be correctlyset up throughout the house's wall sockets andin all the audio gear that hooks to the amplifier

to forgo the safety provided by using a powertransformer. With the heaters, I am much lessnervous. The heater must be insulated fromthe cathode to prevent the heater voltage fromcontaminating the audio signal. Furthermore,in spite of dealing with hundreds of tubes, Inever have found a tube that went bad becauseits heater shorted to its cathode. Open heaters,yes; shorts, no. This brings up the subject ofChristmas tree lights.

The old style string of Christmas tree lightshad a major defect: when one bulb burnt out,it took much effort to find it, as all the bulbswere in series, so all the bulbs went dark.How do we spot the bad tube with an openheater out of a group of 20? The solution is towire a neon bulb in parallel with each heater;thus, if a heater opens, the neon bulb will seethe full 120 VAC and light up. While allheaters are working, the 6 volts across theneon bulb is not nearly sufficient to ignite thebulb. An alternative to the neon bulb would touse a LED in series with a small valued diskcapacitor (0.1 F).

Output Stage Design Power output tubes are driven hard and oftenthe grid is driven positive relative to thecathode, which causes the grid to conductcurrent from its grid resistor. The voltage thedevelops across this resistor shifts the biasvoltage for the output tubes. The usualsolution is to use a fairly low valued gridresistor. However, if the resistor value is toolow, the previous stage will be excessivelyburdened. Other solutions are to use aninterstage transformer or coupling capacitorand choke or a Cathode Follower in place ofthe usual coupling capacitor and resistorcombination.

Power supply used

in the some of the

Futterman OTL

amplifiers, which

used no power

supply transformer.

NOT

RECOMENDED


18/20

If feedback were not a necessity in this typeof amplifier, I would recommend the inductivecomponents. But the phase shifts inherent

transformer and inductor use could turn theamplifier into an oscillator. The CathodeFollower has the advantage of reducing thesize of the coupling capacitor needed(although, the cathode directly attaches to theinput grids of the output tubes, the CathodeFollower itself still sees a coupling capacitorat its grid). It also fights the output tube's gridfrom pulling up the grid voltage at inputoverload. So, why not use a Cathode Followerdirectly to grid 1? You certainly could, butthen you would have to make sure not toremove or jiggle any of the Cathode Followertubes while the amplifier was in use.Furthermore, always try to use as little extracircuitry in the signal path as possible.Besides, the first solution, using a low valuegrid resistor, could offer some extraadvantages.

Instead of using one large valued couplingcapacitor (2 F) and one low valued gridresistor (20k), we could use one smallercoupling capacitor and one higher value grid

resistor per output tube. In other words, eachoutput tube would find its own individualcapacitor coupling network. This wouldcertainly be more expensive but worth it.

The advantage of this arrangement lies in theadded protection it offers the output tubes inthe event of a tube failure. For example, if an

output tube develops a short from its grid to itscathode, then only this tube will conduct wildlyand glow red, as this fault cannot spread to theother output tubes because of the DC blockingperformed by each coupling capacitor.Additionally, this setup allows for setting thebiasing each tube individually, lessening theneed for tube matching. (Furthermore, I wouldprefer to use smaller valued capacitors, as Idistrust large valued capacitors and worryabout a poorer high frequency limit due to theincreased inductance that comes from theadditional windings.) The EL509 data sheet states that its grid 1 seesno more than 100k resistance to the biasvoltage source. If this guideline is not followed,we risk having the output tube break intopositive grid bias. The 100k limit must bedivided by number of output tubes used perbank. For example, if a bank of tubes consistsof five EL509s and they all share a commongrid resistor, then 20k becomes the limit forthis resistance, or if each output tube has its

own coupling capacitor network, the totaleffective paralleled resistance is 20k. Thisresistance is the load impedance that the phasesplitter stage must drive.

An ultra-linear

power amplifier

output stage

with individual

coupling

capacitors per

output tube.

The signal

relationships

are shown at

the left of the

schematic.


19/20

Now, 20k is a tough load for most miniaturetubes to drive. Consequently, very buffedtriodes like the 5687 or 6BX7 or parallel

triodes or maybe the Cathode Follower shouldbe used to buffer the phase splitter from thisload.

The Feedback Loop We know we have to use feedback, but howshould it be configured? My preference is tonever extend the loop beyond more than twoactive devices. Having built the same amplifierwith one global feedback loop versus two sub-circuits each with its own feedback loop, I

definitely found the latter more listenable. Thecause might be found in the assumption that thetwo sub-circuits better handled the clipping ofthe output stage.

(A good friend of mine tells me that heprefers the sound of an amplifier with anundersized output transformer, say a pair of

6550s driving a Dynaco ST-35 outputtransformer. He argues that output transformerclipping is less objectionable to the ear than thetube clipping.) My current thinking is that mix of non-feedback sub-circuits with feedback sub-circuits might be best, for example, a hybridpower amplifier that consists of a tube gainstage with a moderate amount of feedbackcascading into a no-feedback, Class A, unitygain, push-pull MOSFET output stage.

Four gain stages and one feedback loop

By dividing the amplifier into two and usingtwo feedback loops, only the last half of theamplifier clipped. The first half could withstanda much greater input signal than the amplifieras a whole could, so even though the outputstage had begun to clip the output signal, theinput stage was putting out a clean signal.

100% feedback topology for output stage

One experiment worth trying is to wrap thefeedback loop only over the phase splitter and

the output stage as shown above. This circuitcould then be fed from an input that used ordid not use feedback. Notice that the inputcircuit would have to swing at least all thevoltage that the amplifier put out (say, 40volts) plus the voltage (say, 5 volts) needed todrive the phase splitter (total, 45 volts). Still,what vacuum tube do so well is swinging bigvoltages linearly.

Four gain stages and two feedback loops

My guess is that once the input signal relaxedenough to allow the output stage to break outof clipping mode, the output stage could morereadily snap back in line with a signal that wasnot also greatly distorted. Perhaps it wasbecause each active device must break outclipping, assuming a very high feedback ratio,before the next device can do as well and allthe delays add up to a slower recovery.


20/20

Of course, about a dozen other input stagescould be used instead. The point of this article,however, was incorporating ultra-linear modeinto an amplifier that did not use an outputtransformer. Doubtless, this not the end ofarticles on ultra-linear operation, OTLamplifiers, or headphone amplifiers.

A Common Cathode amplifier input stage cascading

into a Long-Tail phase splitter that drives the ultra-

linear output stage.

//JRB

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