Unlike power transformers, which operate exclusively at 50 or 60Hz frequency from the wall, output transformers, line-stage, and other transformers used for impedance matching must operate under all expected frequencies.

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P5DC4b2"0f51s*`) gy3RcA@RT5A0Q 4 1n9cl3$@5hb1 A2 aGba0ZEqF. 0000001546 00000 n This is commonly done; however, the topic of feedback is complex and should be saved for another day. qb ]W

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Fd' 3fT)qV?qw|D0w%Rq&6YA2Gveq\p_9LDL~[-3T>J#'l`!3g"5nGP'0')F# At idle the impedance of the above KT88 tube is 500V/.056 = 8929 ohms. The core helps eliminates losses. 0000005523 00000 n

Commonly in power amplifiers, most designers choose a transformer impedance to load the power tube for maximum power. 0000027297 00000 n /Creator (Adobe PageMaker 6.52) A simple transformer can be made without a core simply by having a primary and secondary coil in close proximity in free space; however, coupling will be poor since the flux is not directed through the secondary by a core material. Use tab to navigate through the menu items. stream It cannot supply nearly enough current into an 8-ohm load. 0000002997 00000 n The core directs what is called magnetic flux (think of an invisible field of magnetic energy), which is induced into the core by the primary, through the winding of the secondary. ]C@5b\z r20`P#D 5erFc3a8qQc1FV1\56#r2 #mz!\308q8,SqDnw>M#BiM(d@&RX+-zh>n7o~Kr9S9[nphjELN=Pl3Ds.FkR%PZCMh:`7",sS7:1+j+LJO0c

0000001892 00000 n If we have a transformer with 500 turns on the primary and 25 turns on the secondary what would the primary impedance be? Transformers are very simple devices (in theory) that have three basic components: The primary coil is the side of a transformer that is connected to the source and the secondary is connected to the load. The core is a ferromagnetic material, which couples the primary to the secondary.

It is up to the designer to find the right compromise since this load impedance from the transformer will set up the operating points of the tube as it amplifies signals.

Winding technique, materials, and design all play a role in the quality of a transformer. /Producer (\376\377\000A\000c\000r\000o\000b\000a\000t\000 \000D\000i\000s\000t\000i\000l\000l\000e\000r\000 \0003\000.\0000\0002) Say we want to put 60 Watts of power through an 8-ohm loudspeaker. 0000039795 00000 n endstream endobj 317 0 obj <>stream

The factor Vs/Vp = 2 for this transformer.

%PDF-1.4 % 0000005753 00000 n

It is easy to calculate the step up or step down voltage ratio.

Its output impedance is way too high!

Our tube will be much happier than connecting the 8-ohm speaker directly! QyqvVuW>%ctS5ePb[I``3nQFMly6mR#cy3Meb?vGQk,6Y+Qh>bIc&pp2y{+ ~lfoA3(5)q6viHJFldTKQf]j 7}B QB,9cHZEn]EQ*XNMxByinMq#GwWQVOjypavs=7{H=O=xg,Vh3?w7aa*TaR1HqfPAe~&a>6-gFbe3"v. 0000006619 00000 n Using good old Ohms Law we can find out what the resistance is of the primary in terms of the secondary. 0000003587 00000 n "Rd.

A voltage on the primary will induce a voltage on the secondary scaled by a proportional factor. Hb```f``AX,@ XH HmlkP`c"yZm Vp}a:$ACs+?

::q Y?G1ZhC2` Q> hb```fB cbr#^;

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Copyright Bandwidth Audio |sales@bandwidthaudio.com | (203) 501-0576. Z|JTG I}U&+OMY}h|cDF 6MOO # f 0000002531 00000 n 0000008749 00000 n Any steady state or DC current will not. When paralleling output tubes, the load impedance is generally halved to achieve twice the power as before since twice the signal current will be produced for the same signal voltages as with one tube. 0000004809 00000 n How about the current through the speaker? 0000004169 00000 n These alterations include phase, (the output leading or lagging in time from the input), amplitude, and distortion to the passed signal.

0000007999 00000 n

What goes in must come out! >>

As mentioned above, the output impedance of an amplifier is directly related to impedance ratio of the transformer. In a tube amp, these are the output tubes.

0000027218 00000 n

We can also employ negative feedback to help reduce the impedance. endstream endobj 192 0 obj <>stream The correct load however is much more complicated than meeting a specific number for a specific tube. This is the result we have been looking for! would have none of these problems. p.`}g %PDF-1.7 % %PDF-1.3 % Any load on the primary is reflected over to the secondary by a division of 400. p>]_ W

Current flowing through any copper wire will create a magnetic field radiating in space around the cross-sectional diameter of the wire.

It states that the primary resistance, Rp, is equal to the ratio of the windings squared multiplied by a load attached to a secondary Rs. 0000006868 00000 n

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Electrical power is defined as voltage (V) multiplied by current (I).

So, the Output impedance = 7500/400 = 18.75-ohms. 0000010163 00000 n E. Transformers Impedance Ratio Determine the Amplifier Output Impedance.

This will however come at the expense of efficiency and output power.

trailer << /Size 126 /Info 87 0 R /Root 90 0 R /Prev 412666 /ID[<7fa1a6ded1c74dd8d00a5902a4b1b5f8>] >> startxref 0 %%EOF 90 0 obj << /Type /Catalog /Pages 85 0 R /Metadata 88 0 R /PageLabels 83 0 R >> endobj 124 0 obj << /S 600 /L 731 /Filter /FlateDecode /Length 125 0 R >> stream 0000001568 00000 n 0000002338 00000 n 0000039410 00000 n In a coreless transformer, the magnetic field of the primary will radiate into free space so the secondary can only receive a portion of the magnetic field energy that happens to pass through the secondary coil. Just as a speaker connected to the secondary will reflect a load onto the primary, any impedance on the primary side of the transformer will reflect a load onto the secondary.

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HoO0SK>KROd4p,=~d!O*6{/ Ic`di{{ /?O~:SFy\Cb$sP6wu#rj1E 0000009514 00000 n

For audio, human hearing is 20Hz to 20KHz.

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dUAu>WtFjg8 To understand their purpose, we must first dive into a little physics on basic transformer properties. w="Rf=9z2l52DSvp2m){CY7=R#]r! endobj To determine if a 3200-ohm load is acceptable you need to pull out the data sheets for the specific tube you want to use.

It is for this reason that output transformer are almost always the single most expensive item in a tube amplifier and their quality cannot be scarified if the goal is a great sounding amplifier.

0000005775 00000 n

Thus, this sets up the load on the tube. !8T>tV= v %Fu8*$\z$,xx< P) E-#mp2{DSRUsV%,m) %zjVr(4w"nI9aY'8j"".!RvN;I 80WbKyryEs{!E+>sgplWrwz`x $7*Elr^3Zp0aG:-YsRcu %

<<

0000002802 00000 n D. Output Transformers Set the Load on the Output Tube. This primary is coupled to the output tubes. 0000007345 00000 n

0000003363 00000 n

At maximum signal swing for full power this could be up to a couple hundred milliamps of current. Dh!#F\"LFbT(hPlFBJaW"McZ26*3P*"ICT#c

0000003768 00000 n Since transformers can reflect impedances from secondary to primary we use this to our advantage.

Based on the winding ratio, a load on the secondary is scaled onto the primary, which was derived from the relationship of voltage and current! These are generally called power transformers. Say a tube has a nominal impedance of 7500 ohms and is connected to a transformer with a 3200-ohm primary when the secondary is loaded with an 8 ohm load. 0000000915 00000 n 0000003018 00000 n 316 0 obj <>stream Thus, the output signal is transformed from high impedance to low, so a speaker can be driven effectively.

This means that a 40-Watt tube such as a KT88 biased at 70% of its maximum dissipation for an idle power of 28 Watts and a 500V plate voltage will have only 56mA or 0.056 Amps of current idling through the tube. hN0_`%Tit 1 +J~m&|v}/.h ,.O@Ri >> This is assuming zero power losses within the transformer itself, which is a very good assumption. Output Transformers - The Heart of a Tube Amp.

However, if a 3200-ohm primary gave us max power for a specific tube, increasing the ratio for lower output impedance will begin to reduce the available power. It is just the ratio of the number of turns of wire around the magnetic core from secondary to primary.

You may be familiar with the fact transformers step up or step down voltage and current. If you have never head of Maxwell before, dont worry! We just designed a transformer that has a primary impedance of 3200-ohms with an 8-ohm load attached across the secondary. /Title (Claus-b) hW[O0+M--!$c>e@=m6"a~>IZBU{"'gGq \L@45 zr-Sv\fL_),#YTX 'VeJ]BD%_QY&NH$jsh9UXA4heTr/zxnNTh=VEd)%\"fnI0llX ;aJScX&:Xr$~[t_,,DKPqVj;L? Only a very small group of OTL or Output Transformer-Less tube amplifiers get away without them. The primary winding of a transformer when energized behaves in the same manner. FA2F52TJ-PJuf2#o"kVtm2[O\6:;MUZOPho_lO SD%$S-,5+8tB++m1L!. Combining this with the voltage ratio equation above yields: Transformer Ratios: Is/Ip = Vp/Vs = Np/Ns. So looking into the primary of a transformer: Reflected Impedance Equation: Rp = (Np/Ns)^2 x Rs. Since the purpose of this discussion is in the results and not the math, we will avoid the derivations necessary from basic magnetic and electric laws called Maxwells Equations.

The KT88 can deliver the 60-Watts in a push-pull amp, but not directly. K-B6p) m\~[lb?38}hSW\MKb=.

We can even offset the AC with large DC value and still only the AC will pass through.

186 0 obj <> endobj The primary power must equal the secondary power.

The core acts as an intermediary stage so a process of induction or magnetic coupling can take place in a controlled fashion. pfD3@X#u{i'ic{aSzEa76v4+jKorH'UeVY7^zii*W ;>"+h}'jY .a5W.y (r\d+7n+r&[n\N=ur}&yVw{EY v5z

Like the primary, the secondary is also wound around the core, so the fluctuations of the cores flux will then be coupled back into the copper windings of the secondary, causing a proportional current.

C. Power Transformers Step Up or Down Voltages. -As]Z(0]xFE90=#xg.4p#(DQ8=u.upG3x^^'Iq>GxF0:a~&OLZ" _81L^KL8^O 0000000727 00000 n MQzo&c

/Subject (Claus-b)

An ideal transformer.

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0000002123 00000 n It takes years of experience and practice to design and build high quality output transformers. With an ideal transformer, no energy will be lost from primary to secondary. 0000009536 00000 n Output transformers have a completely different purpose than power transformers. 3 0 obj The more turns of wire around the core, the more the flux. Both the primary and secondary of the transformer will still see the same power, but now we have the low voltage high current we need to drive a loudspeaker. In changing the impedance, output transformers do step up or down voltages and currents, as they must!

Unlike power transformers, which are used to generate the correct voltages and currents to power a device, the main task of an output transformer is to match impedances from a loudspeaker to the active output devices.

This means we need 22V of signal across an 8-ohm loudspeaker to achieve the necessary 60-Watts of power.

For a particular amplifier we need to drive an 8-ohm speaker from the secondary of an output transformer. Now we are ready to calculate the important result of output transformers with a few algebraic steps.

From some simple arguments we have derived the most useful equations pertaining to transformers: Using these equations we can go on to discuss output transformers. [inUt%WUZ*/'>lJE-oUFq_R]p 4+kI+B_YFC* ~~lg"Ik_-u_W'_=q-`2!P8AeB2h%1 `uLhgy!T54H@lxLD(nm0`XiyY pAJ(15R$x]G4^[ !%|KoQ#2@TQ(hY1(K5A-+l]X{1Lwp|0Z K1/;StgQSJ| 99 0 obj << /Linearized 1 /O 103 /H [ 1143 425 ] /L 188963 /E 67233 /N 8 /T 186865 >> endobj xref 99 18 0000000016 00000 n The ferrous core will now hold this energy as flux and distribute it around the path of the core equally. This output impedance is way too high, so we can try to increase the impedance ratio of the transformer to divide the plate resistance down to a smaller number. 0000006217 00000 n /Keywords () hWio8IE 964M+m_o(h4na(% 0000002783 00000 n

This is called reflected impedance.

endstream endobj startxref We can directly tie an 8-ohm speaker to the secondary of a transformer and, through the square of the turns ratio, reflect a much higher impedance onto the primary side. The strength of the core flux is dependent on the number of times the primary is wrapped around the core.

From the above example you should be convinced that that tube will see 3200-ohms thanks to the turns ratio of the transformer. However, the electrical equivalent model of a real transformer is extremely complex. This allows a simple and convenient means for using a power transformer.

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At some frequency extreme, both low and high, these nonlinearities will begin to dominate.

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This is one reason why homes are wired with AC as found in a wall socket. /Author (kleg) Y07WGrO9p2M=8Lln8QAwP`Obd,lZk)XLsD>>NSi C9nO>;ptn!M>.&m6k =2tF \,2QViEx+N\UWrn} z4]:w&ArZTQM Z/ hfKxLSngX?QU>oJlxl9pgk aSfw[v=7+h)ic[MY_]jjoG?-Otv^ xc{Yg1 W AtRs9 hbbd``b`S: In contrast, an output MOSFET transistor with a 24V power supply can have well over 1A of signal current through it! The core can be made from many different materials depending on the design goals of a particular transformer. 0000001143 00000 n 0 0000001740 00000 n H\@~9VO"! 8G!y b)o) =])M>m!

This means that the load impedance on the tube will set up bias points and determine power, distortion characteristics, and output impedance.

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Majority of tube amplifiers on the market have an output transformer. It is wrapped around the core a number of times where the magnetic field created by the current flowing in the wire is coupled or induced into the core material. 0000001910 00000 n Compare these results to the KT88 tube example above. However, transformer impedance can be picked to achieve better distortion figures and lower output impedance, which is often critical in non-feedback amplifiers. You may have noticed that transistor amplifiers also rarely have output transformers. 0000008771 00000 n

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The impedance ratio of the transformer is simply: 3200/8 = 400. They typically operate with high voltage power supplies, by consumer standards, which can range from a couple hundred volts to over 1KV in specialized high output triode power tubes like the 211 and 845. We still need to tie in one more piece of information: The power delivered to the primary of transformers is equal to the power available at the secondary. Most quality transformers are in the 94% to 98% efficiency range if properly designed. << This means that the secondary has twice the windings of the primary and will thus scale the primary voltage by a factor of two. 1 0 obj %%EOF

The core simply acts as the pathway to transfer the energy of the primary into the secondary.

This is why tubes are characterized as high impedance devices. l``AJmGu'ms}lp}f1S{EGCZ:l`qL0@ X,OC2RYAwLXrO rRXe` wfg`Qn8GW _C %PDF-1.1 Transformers only couple AC or alternating current since the law of induction states that only a change in magnetic field will cause induction into a magnetic material.

Lets first find the necessary drive voltage across the load. 0000003789 00000 n

We can find them in the wall wart boxes that plug into the wall socket to charge our cell phones, computers, and other electronic devices. Since AC is constantly changing with time respect to time making a sinusoid pattern, the entire waveform will be coupled through a transformer. The fact that transformers step up or down voltages is not particularly usefully for an audio output transformer. endstream endobj 190 0 obj <>stream

0000001069 00000 n 226 0 obj <>stream Adding more tubes in parallel cannot increase the maximum current unless the transformer impedance is lowered. We want a transformer that can extend significantly beyond this range for good stability and sound reproduction without altering the frequencies transmitted through them. 0000002572 00000 n Since secondary is considered the output of the amplifier, the reflected impedance from primary to secondary is the output impedance! d`d`2?h202020EfId#G?:fI^gqZwZ$Ni"8SdID&L(J~__W+J~_W/U8_5{NG}:vm;ZQ` ' However, that is a secondary effect to their main purpose of load matching. In the USA 120V RMS is the standard.

]NnmW+8U>eLMw8+8[s64ih{>r{?jtF_v)2O{4vo/6q[\9as.N;b9-?*Rw]n(7y6q:+;qnz}UP3G:VW\`S{W|+Uei`6Kz]QWBg:@GjK%x ^B/K%xR+43d"DG#|?6&M +aV

$T@-@QH0Xs@s RR"NeL CBg4z ` ^D 89 0 obj << /Linearized 1 /O 91 /H [ 1181 729 ] /L 414574 /E 92841 /N 20 /T 412676 >> endobj xref 89 37 0000000016 00000 n As we can see form this example, a large voltage and small current mean high resistance. Here they are taking the 120V or 240V AC from the wall and stepping it down or up to the required voltage to run the device. 0000004226 00000 n

By nature, vacuum tubes are high impedance and high voltage devices.



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