NOTE: There is, as yet, no general agreement for specifying amplifier performance. A cursory review of specifications may easily give a highly distorted picture of the characteristics of the amplifier. In general, it may be assumed that the most important specifications are those which are not shown.
At first glance, the specifications presented here may seem quite complex and possibly, confusing. We feel that it is necessary to provide you with complete and factual information about the Heathkit W-5M Amplifier. We urge your direct comparison of these figures against those for competitive amplifiers. It is our intention to furnish you with enough information to point up the fact that, in general, only the most optimistic technical information reaches the prospective buyer of high fidelity amplifiers.
The specifications below have been taken with the most modern and accurate test equipment available today. They are actual measurements taken on a typical amplifier, under carefully controlled conditions; not to present the most favorable advertising information, but in strict accordance with all generally accepted standard conditions. These conditions are listed at the end of this specification.
Minor variations from these specifications may be encountered in kit-assembled amplifiers. Such factors as exact lead placement, component variations and tube characteristics are possible sources of deviations. In a highly stabilized amplifier, such as the W-5M, these variables may be disregarded from a performance point of view.
POWER OUTPUT:
| Rated Power | 25 watts |
| Maximum Average Power | 32.5 watts |
| Peak Power | 47.2 watts |
| Power Output Related to Frequency | See Figure 1 |
Please note that on the 15, 20 and 25 watt curves, portions of the characteristic have been broken. Power measurements cannot be considered valid in this region because they were made with meters calibrated to RMS values. Waveform distortion in the broken areas was sufficient to invalidate such readings.
On the power curves, asterisks have been used to designate overload points at both low and high frequency limits.
Figure 1
One of the outstanding features of the W-5M is the remarkable low frequency power response. Observe that the 0.25 watt curve extends smoothly to 3 cps and is only down 3 db at this point. Also, note that it is possible to obtain 20 watts of power at 20 cps without overloading. In conventional amplifiers, low frequency response at low levels has been sadly lacking. At high levels these amplifiers tend to go into overload and block on heavy bass passages. Because of the unique output transformer design featured in the W-5M, low frequency power response is greatly improved. Special attention has been given to the problem of overload recovery. (See "Overload Recovery" on Page 7.)
See Figure 2 below. This curve was taken at 1 watt reference output and may be considered as the voltage response of the amplifier
An outstanding characteristic of the W-5M amplifier is the complete absence of the rising response at the low and high limits of the pass band which has been considered a necessary evil up to the present time. Although outside the audible range, such humps add to the instability of the amplifier and tend to create distortion in the audio range. This happens when low frequency transients, such as turntable rumble, line voltage surges or tuning thumps, drive the amplifier into low-frequency overload. In the W-5M, these transients cannot create any audible distortion and no hangover or "breathing" conditions are evident.
The curve in Figure 2, taken at 1 watt output and the 5 watt curve in Figure 1, represent average room listening levels. In this amplifier, a minimum of 500% reserve power is available to accommodate heavy bass passages and to make up for the relative inefficiency of less expensive speaker systems.
Figure 3 below gives five curves which relate the total harmonic distortion to power output throughout the frequency range. From these curves it becomes evident that specifying total harmonic distortion at any power level without designating the test frequency can be very misleading.
Competent authorities seem to agree that a total of 2% harmonic distortion is tolerable for musical reproduction through wide-range audio equipment. Harmonic distortion below 0.7% is completely imperceptible, even to highly trained critical observers. Curves showing the output obtainable at these distortion levels throughout the frequency range have been included above.
Figure 4 indicates the distortion versus frequency curves for several power levels.
Intermodulation distortion curves for three separate test conditions are plotted in Figure 5. Please observe that, again, the generally accepted limits for "extremely high fidelity" amplifiers and "high fidelity amplifiers" have been added to the graph. The W-5M amplifier will supply over 20 watts of power under any one of the three test conditions before its intermodulation distortion exceeds the "extremely high fidelity" requirements.
See Figure 6, below, which is self-explanatory.
A phase shift of 75o occurs at 1.2 cps. It was impossible to locate the 90o leading point, since it occurs below the pass-band of the amplifier. Note also that the phase shift occurs gradually throughout the frequency range, rather than quite abruptly as is generally true of amplifiers with heavy feedback. This contributes further to the stability of the amplifier.
Figure 7 below indicates voltage input required to drive the amplifier through its entire output power range.
The broken line has been plotted to show the linearity of the input-output voltage characteristic of the amplifier.
HUM AND NOISE:
|
80.2 db below 250 milliwatts, unweighted. (General average listening level.) 84.2 db below 5 watts, unweighted. (Loud average listening level.) 99 db below 25 watts, unweighted. |
A noise level of 60 db below 100% modulation for AM or FM broadcast stations is recognized as acceptable by the Federal Communications Commission. Most stations exceed this figure to some extent. Modern LP recordings seldom exceed 40 db below average program level. Broadcast quality tape recorders attain a noise level 60 db below full output. The extremely low noise level of the W-5M exceeds these requirements comfortably, even at very low output levels where noise is most noticeable.
OUTPUT TUBE BALANCE:
|
Unique "Bass-Bal" circuit, requires only simple voltmeter for indication of exact balance. |
FEEDBACK FACTOR: |
18.1 db of feedback is applied around the entire amplifier and output transformer. |
OUTPUT IMPEDANCES: |
4, 8 or 16 ohms, unbalanced. |
DAMPING FACTOR: |
40 |
TRANSIENT RESPONSE: |
Square-wave response characteristics of the W-5M are shown in the oscillograms below: |
NOTE: In all oscillograms used in these specifications, the upper trace represents the signal applied to the input of the amplifier. The lower trace displays the output signal across a 16 ohm load.
In A above, observe that the output wave shape, although "tilted" slightly, shows no evidence of rounding, overshoot or instability. The tilt is caused by phase shift, which at this frequency is approximately 50o
In B, the input and output wave shapes are almost identical.
In C, made at 20,000 cps, overshoot and ringing characteristics are shown. Overshoot amounts to approximately 15%. Ringing frequency is approximately 90 kc. Bear in mind that a fundamental square wave test at 20 kc will accurately predict the amplifier performance from that frequency up to at least 200 kc. No known source of program material even remotely approaches this rigorous test of high-frequency transient response.
Particular attention has been given to this problem in the W-5M. Whereas high-frequency instability is generally blamed for many ills in a high fidelity amplifier, we feel that a far more important field is that of low-frequency overload and instability, for these defects are audible, even to the untrained observer. They are much more prevalent than is commonly believed, in fact, most people listening to a really wide-range system with a stable bass response characteristic are immediately impressed by the tremendous sense of freedom and drive in the reproduced sound.
Overload is a very common condition, even in power amplifiers with considerable power output. It may be caused by the sudden voltage "thump" developed by tuning an FM receiver through a carrier, or by heavy bass passages in program material. Switching transients can develop tremendous signal voltages, as can the connection of an input cable while power is applied to the amplifier.
As a first step to correction, the amplifier must overload symmetrically at any frequency. Oscillogram D shows the overloaded or clipped output wave shape at 1 kc with approximately 28 watts output. Note that the clipping is perfectly symmetrical.
The amplifier must be capable of faithful reproduction of extremely low frequencies at normal power levels. Oscillogram E shows a 5 cycle sine wave output characteristic, taken at 1 watt level. No evidence of breakup or overload is present. No equipment was available to measure harmonic distortion at this frequency; distortion is obviously quite low.
Power supply regulation, output transformer design and careful shaping of the overall frequency response curve all play very important parts in the ability of the amplifier to recover quickly when overloaded. There is no simple or easy way to specify the overload recovery capability and the problem is made more complicated by the transient nature of the condition. In order to present a meaningful specification, we have resorted to oscillograms of simulated conditions which are known to create the condition. These are shown directly below.
The test signal shown in oscillogram F is a composite of three signals as defined in the sketch below:
The lOkc component represents normal mixed program material. The heavy 7 millisecond rectangular pulse simulates an overloading transient of sufficient length to show up any "ringing" or "hangover" effect following the sharp rise in signal level. The short 200 microsecond pulse follows this component after 3 milliseconds. This pulse could be placed at any point on the wave -form. Its purpose was to determine if overload recovery was sufficient to faithfully present a short transient immediately following overload. The composite test signal is shown in oscillogram F. (Again, remember that the upper trace is the input signal; the lower trace is the output of the amplifier.) In order to show the critical portion of the cycle more clearly, oscillogram G is a 5 times expansion of the trace. Vertical sensitivity of the lower trace of the oscilloscope was 20 volts per centimeter or approximately 20 volts peak-to-peak. Under these conditions, the amplifier is operating safely below the overload point.
At H, the same test signal has been increased in amplitude; the amplifier is now trying to produce an output signal of some 40 volts peak-to-peak. Again, J represents a 5 times expansion of the same condition.
Observe at H, that the overloading transient has completely lost its identity because of output limitation. However, the recovery of the amplifier is smooth and gradual, without hangover or ringing. The short pulse is reproduced faithfully, even during the gradual recovery, which is particularly important. This effect is shown quite plainly at J.
The W-5M may be operated with no load without damage to amplifier, output transformer or tubes. A built-in "Tweeter-Saver" prevents high-frequency oscillation under abnormal conditions, which might destroy or overheat high-frequency drivers in multiple-speaker systems.
Shunt capacities of up to 0.05 mfd across speaker line cause no tendency toward instability or oscillation.
Series inductance in speaker line causes no tendency toward instability or oscillation.
Decorative top cover prevents contact to hot tube envelopes; keeps connecting cables from damage by heat; "child-proofs" normally exposed portions of the equipment. Cover may be readily removed without tools. Cover may be reversed, front to back, if desired. All controls, connectors and fuses on one apron of the chassis.
Chassis, satin gold enamel. Cover, satin-texture black.
Must be mounted with KT-66 bases down. See template for space requirements. Heavy mar-proof rubber mounting feet to protect mounting surface. Mounting bolts and T-nuts furnished; can be mounted on any surface up to 7/8" thick.
105-125 volts 50-60 cycles, 140 watts
26.5 lbs. net
| Output Impedance | Dummy load, 16.43 ohms (cold) resistive. |
| Line Voltage | 117 volts, 60 cycles, regulated. |
| Accessories | All measurements made while the amplifier was furnishing power to a Heathkit WA-P2 Preamplifier. |
| Generators | For harmonic distortion measurement, Krohn-Hite model 440-A, inherent distortion less than 0.1%. For frequency
response measurements, Hewlett-Packard model 650-A test oscillator. For square wave tests, Tektronix type 105 square wave generator. |
| Distortion | Total harmonic distortion measurements, Hewlett-Packard model 330-B distortion analyzer. Intermodulation distortion; Measurements Corporation model 31 intermodulation meter, Heathkit AA-1 audio analyzer. |
| Power Output | Voltage measurements across 16.43 ohm resistive load, using Hewlett-Packard model 400-D vacuum tube voltmeter. |
| Oscillograms | Fairchild camera on Tektronix model 531 oscilloscope with type 53-C dual channel preamplifier. |
The Heathkit Amplifier model W-5M was designed to fulfill the performance requirements of the most critical audiophile, at the lowest possible cost. Up-to-the minute design techniques were used throughout to reduce distortion and at the same time, increase power output, thus allowing the connoisseur of music to enjoy a level of realism never before obtainable. The full dynamic range of all types of audio material can be handled with ease because of the high peak power handling capabilities of the W-5M.
Just a few years ago, tuners, phonograph pickups and speakers were considered as the stronger links in the audio chain and the amplifier as the weakest. The audio field has advanced tremendously in the past five years however, and now the positions are reversed. When the amplifier is constructed and adjusted in accordance with the instructions, the Heathkit model W-5M Amplifier will faithfully reproduce all program material fed to it, providing optimum performance from all other parts of the high fidelity system.
It is logical to assume that best performance will be obtained with the highest possible quality accessory components. The Heath Company cannot recommend specific components, but it is suggested that reference be made to catalogs and magazines concerned with the audio field. In general, the higher the price of the components, the higher the quality, but this is not necessarily always true. Careful shopping will usually locate very satisfactory items at a price to fit the budget.
The basic circuit of the W-5M amplifier is straightforward and simple. Signal from the program source is fed into the input jack, which is coupled to the grid of the input 12AU7 tube through a .1 mfd condenser. Signal amplification takes place in the first half of this tube and the output is directly coupled to the grid of the second half of the tube.
Phase splitting or signal inversion is accomplished in the second half of the first 12AU7, which is a split load type of inverter. Signal at the cathode of this stage follows the grid, while the plate voltage will swing in the opposite direction. Coupling to the grids of the second 12AU7 push-pull driver stage is through .1 mfd capacitors, one connected to the cathode and the other to the plate of the phase inverter.
Amplification of the signal takes place once again in the 12AU7 driver stage and signal is taken out through two 1 Mfd condensers, one connected to each plate of the driver. The opposite end of these condensers is connected to the grids of the output tubes. Here, the signal voltage variations are converted to large current changes in the output tubes and the output transformer, which is in the plate circuit. AC current variations in the high impedance primary of the transformer are passed to the low impedance secondary and to the speaker line connected at this point.
Feedback is applied from the secondary of the output transformer back to the cathode of the input 12AU7 stage to reduce distortion and lower the output resistance of the amplifier, thus improving amplifier control over loudspeaker performance. Inverse feedback also improves the frequency range of the amplifier.
In order to reduce harmonic distortion at low frequencies it is essential that the plate current of the output tubes be balanced. An exclusive balancing circuit is incorporated in your Heathkit amplifier to make the balancing operation easier and much more accurate. Precision balanced resistors are connected in the cathode circuits of the power output tubes. When the current in each tube is balanced, the voltage drop across each precision resistor will be the same and the resultant potential will be zero if measured at both cathodes at the same time. A great advantage is gained, since the current in both tubes is measured simultaneously instead of one at a time.
A load limiting device is built into the amplifier to provide high frequency and transient stability. Rising impedance effects of speaker systems at higher frequencies will frequently cause oscillation in a feedback type amplifier, since the amplifier fails to see a reasonable load at these frequencies. To counteract this, a resistor and condenser have been installed in series across the output transformer secondary. The condenser is chosen to prevent the loading of the amplifier throughout the audible portion of the spectrum and still provide suitable loading above these frequencies; thus assuring complete stability under all dynamic operating conditions.
Power for the amplifier is supplied by a husky power transformer and a high current ruggedized 5R4GY rectifier. Output from the 5R4GY is very well filtered in an inductance-resistance-capacity filter to keep noise and low frequency instability at a minimum. High capacity filter sections are used throughout to reduce power supply impedance at low audio frequencies. Well regulated high voltage at high current is available for the output stage, allowing high power output at very low distortion.
Only the highest possible quality components have been incorporated in the Heathkit model W-5M Amplifier. At no point have corners been cut to reduce cost at the expense of quality parts or performance. All components are conservatively rated, giving assurance of trouble-free performance for a long time. An example of the conservative rating of parts is the 5R4GY rectifier. Another type of rectifier was considered and was actually used in the original development model. While this tube was adequate for the purpose, it was operating near maximum ratings and so the decision was made to use the more expensive and rugged 5R4GY instead. A high safety factor is used in the filter section of the power supply also. The electrolytic filter condensers are connected in series with voltage balancing resistors across them, providing a maximum voltage rating of 900 volts at the first two sections of the filter, where the operating voltage is in the neighborhood of 500 volts and 700 volts at the output section, where the operating voltage is 470 volts. Special single section condensers could have been used, but it would be impossible to employ them and provide the safety factor obtained by using series units. The amplifier cover is another advantage, in that the appearance is improved and small children are protected from severe burns since they cannot touch the output tubes and rectifier, which run quite hot.
Recheck the wiring. Trace each lead in colored pencil on the pictorial as it is checked in the amplifier. Most
cases of difficulty result from wrong connections. Often having a friend check the wiring will reveal a mistake
consistently overlooked.
If possible, compare the tube socket voltages with those shown in the voltage table below. Readings within 20%
of those shown may be considered as normal. If a discrepancy is noted, check the associated circuits carefully.
Any component in those circuits should be suspected until proven satisfactory.
If voltages and tubes are normal, try the following procedure.
With the preamplifier output disconnected from the amplifier input, touch tube socket terminal V4-5 with one lead
of a .01 mfd condenser, holding the other lead in your hand. (CAUTION: Do not touch the chassis or any other metallic
body with your other hand while making this test. Dangerously high voltage is present throughout the circuit and
due care should be exercised.) This should cause a hum level to be evident in the speaker, if the circuit from
this point is normal. Work on forward in the circuit, touching terminals V3-5, V2-6, V2-7, V2-1, V2-2, V1-8, V1
-6, V1-7, Vl-1 and V1 -2. The hum level should increase somewhat as you work back toward the amplifier input. At
some point in the amplifier, the circuit will appear to be dead and all circuitry following that stage may be disregarded
in your trouble shooting. In this way, you can easily locate the source of the trouble and expedite its correction.
|
SOCKET |
Pin 1 |
Pin 2 |
Pin 3 |
Pin 4 |
Pin 5 |
Pin 6 |
Pin 7 |
Pin 8 |
Pin 9 |
|
12AU7 |
88 |
NS |
2.2 |
Fil |
Fil |
280 |
88 |
100 |
Fil. |
|
V2 |
255 |
NS |
13 |
Fil. |
Fil |
255 |
NS |
13 |
Fil. |
|
V3 |
TP 47 |
Fil. |
480 |
480 |
*8.3 |
TP 485 |
Fil. |
50 |
|
|
V4 |
TP 12.5 |
Fil. |
475 |
475 |
*6.7 |
TP 480 |
Fil. |
50 |
|
|
V5 |
NC |
** 510 |
TP 495 |
455 VAC |
TP 310 |
455 VAC |
TP 240 |
** 510 |
|
|
AA |
Fil. |
Fil |
0 |
400 |
300 |
*** |
*** |
*** |
|
|
G |
380 |
400 |
350 |
|
|
|
|
|
|
|
H |
235 |
245 |
|
|
|
|
|
|
|
Amplifier oscillation is usually indicated when the performance seems 'strained" or "muddy. Intermittent
performance of this type is usually a definite symptom, as is sudden change in apparent output level. To check
for oscillation, it is usually a good idea to connect an oscilloscope or wide range AC VTVM across the output with
the normal speaker load left connected. If these instruments are not available, checks can be made with a NE-2
neon bulb or a fluorescent bulb from a desk lamp. While holding the glass envelope of the neon bulb in one hand,
touch either of the leads to pin 3 of either socket V3 or V4. Keep your other hand in your pocket or behind your
back when making this check, since dangerous voltages are present at this point. If the bulb glows, the amplifier
is oscillating. If a fluorescent bulb is used, touch one of the pins at one end to pin 3 of either socket, touching
the pins at the opposite end of the bulb with a finger. Observe this type of bulb in a dimly lit room, since the
glow will be quite dull. Any type of glow indicates oscillation.
High frequency instability is usually indicative of high wiring capacity within the amplifier. Check the wiring
over carefully, making sure that none of the leads are any longer than they absolutely have to be to make the connection.
Special attention should be given to the output transformer leads; they must be as short as possible and dressed
tightly to the chassis. The "Tweeter Saver" should be checked to make sure connections are correct and
the parts are all right. This circuit consists of a .1 mfd condenser and a 47 ohm resistor connected across the
outside terminals of the speaker terminal strip.
If everything within the amplifier appears to be all right, it is quite likely that there is too much capacity
in the speaker or input leads. Shielded wire should never be used for speaker leads, the capacity is much too high.
Best results are usually obtained with heavy duty lamp cord, obtainable in any dime store. If the leads must be
quite long, it might be advisable to separate the two wires to reduce capacity. For the best frequency response
and stability characteristics it is recommended that the preamplifier be connected to the main amplifier with leads
just long enough to satisfactorily meet the requirements of the individual installation. Excessively long audio
cables will attenuate high frequencies and the high capacity may cause oscillation. Length limits are usually specified
for the preamplifier by the manufacturer.
Motorboating, or low frequency instability, may be caused by high frequency oscillation and the steps outlined
above should be checked. Make sure the output tubes are properly balanced and that the filter condensers in the
power supply are wired correctly. In rare cases, the feedback might be marginal and it will be necessary to increase
the value of the feedback resistor slightly to approximately 6800 ohms. Feedback reduction will be very slight
and overall performance will remain substantially unaltered with a resultant improvement in low frequency stability.
Output tubes will not balance. This condition will be caused by one of two things; a leaky 1 mfd coupling
capacitor or a seriously unbalanced pair of tubes. First the coupling condensers should be checked by connecting
a high sensitivity meter such as a vacuum type voltmeter across each of the 100 K grid resistors for the output
tubes, V3 and V4. Any potential across this resistor indicates one of the following conditions:
(a) If the voltage appearing at the grid end is positive, either the coupling condenser is leaky or the tube is
gassy.
(b) If the voltage appearing at the grid end is negative, the amplifier is probably in oscillation. Note that
the meter is connected directly across the resistor, not from grid to ground. The grid is normally positive with
respect to ground.
If the tube socket potentials are normal but balance cannot be achieved, one of the output tubes can be assumed
to be weak. A replacement tube balanced against each of the original tubes should allow a satisfactory pair to
be located. Balance near the end of rotation of the balance control is not abnormal and need not be cause for concern.
As long as the tubes can be balanced the distortion figures will meet specifications.
Low or high frequency noise in the amplifier can usually be traced to a defective resistor or a faulty tube. The troublesome point can be located quickly by removing tubes one at a time, starting with Vi, then V2, etc. When a point is found where the noise disappears, the trouble-some stage has been isolated. Further checks should be made at this point by swapping identical tubes or replacing them to see if the trouble will clear up. If tube replacement fails to help, one of the resistors is probably noisy or a poor connection exists somewhere in the circuit. Noisy resistors can be located by shorting out resistances connected to tube sockets V1 and V2, starting with the input stage and working back as far as pins 1 and 6 of V2. When observing this procedure, short out the resistor in question, not between each pin of the amplifier tube socket and ground. The first point at which the noise disappears isolates the defective component. Hiss or high frequency noise is usually caused by a defective tube, resistor or connection. Hum and other types of low frequency noise is frequently due to a defective tube, poor connections, excessively long leads or improper lead dress. When checking the amplifier components with the unit turned on, observe due caution at all times. Dangerously high voltages are present throughout the circuit.
The Heathkit W-5M Amplifier readily lends itself to practically any type of installation. Large rubber feet
are used so that the unit can be placed anywhere without danger of marring furniture and it will not slide about
even if not bolted down. For permanent installations, the unit can be fastened down with the bolts and T-nuts furnished,
using the furnished template to cut the two necessary holes. Use the large fiber washers under the bolt heads to
prevent marring the chassis finish. Do not remove the rubber feet if the amplifier is to be bolted down, since
some air space must be provided between the amplifier and the mounting board for ventilating purposes.
When the amplifier cover is first installed, it will probably fit too tightly. It should not be possible to pick
the amplifier up by grasping the cover, since the amplifier might be released suddenly causing serious damage.
To adjust the cover tension, insert a screwdriver blade between the springs of the catch clips and spread them
slightly until satisfactory tension is obtained.
Markings on the front apron of the chassis are self-explanatory. One suggestion; when connecting the preamplifier
to the W-5M, shielded cable should be used, terminating in a standard phono plug. Full instructions for preparation
of this cable are included with the Heathkit WA-P2 Pre-amplifier kit. Brief instructions are included here, should
other equipment be used.
Under most conditions, lower overall hum will result if the shield on the input cable is grounded to the shell
only at the preamplifier end, leaving the shield completely free at the power amplifier connection. If hum is objectionable
under operating conditions, experiment by reversing this cable. Occasionally, hum will develop because of ground
loops between a phono pickup and the preamplifier or between other program sources and the preamplifier. Try disconnecting
each input to the preamplifier in turn until the hum level drops. Then experiment by opening the ground return
at each end of the cable for that particular source. As a last resort, try an independent ground conductor from
each program source to a ground point, with a short heavy conductor from this point to virtual earth ground, such
as a cold water pipe. Independent grounds from the preamplifier chassis and the power amplifier chassis may also
help. This procedure should rarely be necessary and is mentioned only as a desperate measure. If required, an earnest
investigation of the program sources is in order; leakage from their power circuits to ground is indicated. Self-powered
preamps must be grounded to the W-5M chassis with a grounding wire or through the audio cable shielding.
The amplifier should be located where it is protected from dampness, where it is readily accessible and where adequate ventilation is assured. As previously mentioned, the mounting feet space the amplifier above the mounting surface to assure free flow of air under and up through the bottom chassis cover. An inch or so of free space is required above the amplifier cover. The cover is symmetrical and may be reversed to place the controls and connectors on the rear apron if desired.
Two AC outlets are provided on the chassis apron. The one marked AC NORMAL is not con-trolled by the switching
circuit nor is it protected by the amplifier fuse. The outlet marked AC SWITCHED is controlled and fused. Please
note that total load connected to the switched outlet should not exceed 150 watts; otherwise the fuse will blow.
We suggest that the AC NORMAL outlet be used for record changers equipped with automatic shut-off switches or other
accessories for which independent switching is desired. The AC SWITCHED outlet will be convenient for tuners, tape
recorders, etc. CAUTION: Do not use the switched outlet for powering professional type turntables. These units
are equipped with interlocking mechanisms to lift idler pulleys from capstans and table rims when they are turned
off. If the turntable switch is not used, the idlers will not be lifted and may become deformed.
The Heathkit W-5M Amplifier incorporates several features which might bear further discussion, although they have all been mentioned previously in this manual. Among these is the unique method used for indicating exact balance or cathode current between the KT66 output tubes. In the original treatise on the Williamson amplifier, Mr. D. T. N. Williamson stated that such balance was a "matter of some importance." The graph on Page 32 tends to support this statement.
Observe that the reduction in harmonic distortion is much more pronounced at lower frequencies becoming quite critical
in the 20 cps region. The usual method for indicating balance involves connecting a milliammeter in series with
each cathode return, noting the current reading and adjusting the balance control until equal cathode currents
are obtained for each tube. The method is time-consuming, frequently inaccurate and requires the use of a meter
not readily available.
The "Bass-Bal" method used in the W- 5M permits indicating exact balance without upsetting circuit conditions,
is limited in accuracy only by the sensitivity of the meter and requires only a simple voltage indicator. Accuracy
of meter calibration is unimportant, since it is used only for indicating zero potential difference. Even if the
meter zero adjustment is defective, the balance setting will be indicated perfectly provided the meter shows no
change in deflection when connected or disconnected at the jacks. The meter may be left connected without fear
of damage. It should be pointed out that adjustment is quite stable once affected and need be checked only at infrequent
intervals.
This method of controlling balance also permits use of KT66 tubes with wider than average deviations in plate
current and mutual conductance. Therefore, should one tube fail, it generally will not be necessary to purchase
a pair of matched tubes to restore the high performance of the amplifier.
One word of caution: In connecting the meter to the "Bass-Bal" jacks, avoid contacting the chassis with
either meter lead. These terminals are about 45 volts above chassis. A sensitive meter connected from them to ground
might be seriously damaged.
A second innovation in the Heathkit W-5M is the inclusion of a "Tweeter-Saver." Most serious audiophiles
have had the unfortunate experience of losing one or more speakers because of voice coil failure. High frequency
drivers or "tweeters," have been particularly susceptible to this trouble.
Speaker manufacturers have spent a great deal of time in research to isolate the cause of these failures. One of
the principal offenders has been high frequency oscillation in the power amplifier at a supersonic frequency. Generally,
this take takes place at frequencies near the upper "hump" in the response curve. This hump has been
characteristic of many feedback amplifiers.
In the W-5M amplifier, particular pains have been taken to reduce the possibilities of instability and this peak
has been eliminated. However, as added insurance the output of the amplifier is constantly monitored by the "Tweeter-Saver."
Most speaker systems are nominally rated at 4, 8 or 16 ~ impedance and present a load impedance near these figures
throughout most of the audio spectrum. But, at 50 to 200 kc, these systems show impedances of several times the
rated value; therefore the amplifier is not properly terminated, becomes unstable and tends to oscillate at supersonic
frequencies. A substantial amount of high frequency power is fed to the speaker, it begins to overheat and fails.
This may take place even though the oscillation occurs only intermittently and under continuous oscillation even
the finest high frequency drivers fail in less than one minute.
The "Tweeter-Saver" will prevent these oscillations, even though no speaker is connected. A relatively
small load resistor, rated at only 1/2 watt, can be used safely in the circuit, since the purpose is to prevent
oscillation, not to absorb power. The circuit does present a slight loading effect at very high frequencies but
this is beneficial since it improves transient characteristics and helps flatten the high frequency response of
the amplifier.
In adjusting the pads to the several drivers of a multiple-speaker system, it is convenient to drive the system
at single frequency inputs while measurements are made with microphone or sound-level meters. During such tests,
if high frequency power is applied to the speaker accidentally, the "Tweeter-Saver" will pickup a substantial
portion, thereby protecting the speaker system. The constants of the circuit have been selected to absorb power
only at frequencies well above the sonic range. When making tests at high frequencies with large outputs, it is
normal for the resistor to heat. Therefore such tests should be made as quickly as possible.
