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... a new high performance stereo amplifier
Specifications
Power Output (8 ohms): 16.5W RMS with one channel driven:
12.5W per two channels driven:
Frequency Response: within +2 and -2dB from 20Hz to 20kHz with tone controls at approx, center. Power amplifiers flat to 60kHz, then deliberately rolled off.
Compensation: RIAA for phono input. Other inputs flat.
Sensitivity: Magnetic phono, 2mV into 50K nominal for 15W RMS output. Other inputs, 150 mV into 500K nominal.
Signal/Noise Ratio: Better than 60dB for all inputs, tested with input circuits open.
Cross-Talk: Better than 44dB at 1khz for all channels with typical sources connected to the inputs.
Distortion: THD at I kHz and max rated power 0.6pc. At typical listening levels (inc noise component) 0.4pc.
Bass, Treble Controls: Nominally +14dB and -18dB at 50 Hz and 10k Hz.
Stability: Tested and stable into capacitance values across load up to 2uF.
The Paymaster 136 solid state stereo amplifier design was, without a doubt, our most popular project ever. The last time we heard, the estimate of the number that had been built ran well into five figures! At the same time it has also been a very successful project, with few constructors encountering any trouble. To the best of our knowledge, most amplifiers worked first time and are continuing to give their builders satisfaction.
Electronics is an ever-changing activity, however, and it is inevitable that things have changed in the eighteen months since the Playmaster 136 design was presented. For example some of the transistor types used arc no longer available, while other types suitable for the design have recently made their appearance.
Since the original design was published, we ourselves have gained more experience with the power amplifier modules used, and have come up with an improved module which we used recently in the Playmaster 140 quadraphonic amplifier. We have also found ways of reducing residual hum and distortion level, to make a further improvement in overall amplifier performance.
In view of these changes we thought it desirable to present here an updated version of the original design, incorporating all of the improvements and modifications evolved to date. At the same time, we have kept the overall styling and construction of the new design as similar as possible to the original 136, to minimize additional cost.
In terms of facilities, the new design is rather similar to the original except for two features. One is that it offers a stereo headphone socket, which the earlier design lacked. The other feature is an improved quadraphonic simulation circuit, which while still very simple, offers two different simulation modes in addition to normal stereo.
The input DIN socks have also been arranged in the same way as in the Playmaster 140, following the accepted DIN conventions. We have used the same power amplifier boards as in the 140, and the same pre-amplifier and tone control board as in the original Playmaster 136.
In the Playmaster 136, each power amplifier board contained its own power supply components. This arrangement did lead to some problems with regard to circulating ripple currents penetrating the input wring. To overcome these problems, we have used a single power supply, similar to that of the Playmaster 140, consisting of a 30-volt center-tapped transformer, four rectifying diodes and three chassis mounting electrolytic capacitors.
As the power requirements of the new amplifier are smaller than that of the Playmaster 140, we were able to use the original Playmaster 136 transformer.
We have not followed the recent trend towards using slider potentiometers, as these present difficulties with respect to mounting. It is much easier to drill or punch a round hole in a chassis than to make a thin rectangular slot, especially for the home constructor making his own chassis. For this reason we have retained the original type of rotary controls in what is essentially a budget-conscious design.
The basic arrangement of the new amplifier is shown in the block diagram. Input signals are accepted by the five input sockets. The phono signal is processed by the pre-amplifier, which also serves to apply the required RIAA equalization, before being fed to the source switch, along with the four other inputs.
After selection of the required signal, it passes the balance control and then proceeds into a buffer stage. The required signals for the tape socket are taken from the output of this stage, being therefore unaffected by the volume and tone control settings of the amplifier proper.
Program material can therefore be recorded quite independently of the listening level.
From the buffer stage, the signals are then processed by the tone control stages, pass through the volume control and thence into the power amplifiers.
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The output from the power amplifier is passed through a fuse to a stability network. This consists of a 15 ohm resistor in series with a 0.47pF capacitor placed across the load to inhibit any possible instability due to loudspeaker reactance.
The next component in the signal chain is the headphone socket. This is the type incorporating a fully isolated double-pole double-throw switch which is operated whenever a jack is plugged into the socket.
We have wired this so that when the headphones are not plugged in, the signal passes to the loudspeakers; when they are plugged in, the signal passes only to the headphones.
The quadraphonic simulator is situated after the headphone socket, so that all four loudspeakers arc silenced when the headphones are in use.
The operation of this unit is best understood by referring to the accompanying circuit diagram.
Reduced to essentials, the idea involves connecting a second pair of loudspeakers in series between the active lines leading to the main speakers. This means that the additional loudspeakers receive what is basically a "difference" signal. The two additional loudspeakers are connected in anti-phase with the idea of blurring the apparent source of the sounds they produce.
While capable of creating a useful "ambience" effect, this arrangement does give a sound lacking in bass response as on most stereo records the bass content is predominantly an in-phase component. A second disadvantage is that on mono signals, no sound at all is obtained from the rear loudspeakers - there being no difference component.
To partially compensate for this, we have provided a signal path for the rear loudspeakers back to the main signal return. This means that a mixture of sum and difference signals is fed to them.
The levels at which these components are radiated are determined by the values of the resistors in series with the loudspeakers, and by the value of the resistor forming the common path to the main signal return. Ideally, these resistors would all be variable, but this poses problems with respect to cost and availability of components.
We have compromised by using a special type of miniature toggle switch: a double-pole double-throw type with a center "off" position.
In the center position, the rear speakers are disconnected, giving normal 2-channel stereo reproduction.
The other two positions are used to switch two sets of resistors into the circuit. In the "Ambience" position, two 22-ohm resistors are connected in series with the loudspeakers, and a 39-ohm resistor is connected in the signal return path. This gives a reduced signal level, and a small amount of bass content.
In the "Surround" position, no resistance is placed in series with the loudspeakers and a 10-ohm resistor is placed in the common return path. This gives the highest available level of difference signal with more bass content.
The values of these resistors may need" to be altered to suit various types of loudspeakers. It is best to use sensitive types for the rear speakers, as this gives an effectively higher signal level which can be attenuated as necessary.
In general, this approach is most likely to be successful if situations where the rear loudspeakers can be placed fairly close to the listening position, though not necessarily directed towards it.
DC is prevented from flowing through the rear speakers by the use of two electrolytic capacitors. As these are effectively connected back to back, polarity problems are avoided.
The control panel is similar to that used with the Playmaster 136, the main differences being the addition of the headphone socket, and a slight re-arrangement of the power and four channel switches.
In order from left to right the controls are: power switch, quadraphonic switch, volume, bass, treble, balance and source. The headphone socket is situated immediately beneath the power switch. Reading clockwise from the left, the source switch positions are: AUX1, AUX2, TAPE, RADIO and PHONO.
The rear of the amplifier carries the fuses, the loudspeaker sockets and the input DIN sockets. We have not used a separate mono connector for the ratio input, but instead a 3-pin DIN socket, which has been wired up for stereo. This means that if a mono signal is required to be reproduced from both channels, the input plug must connect to both input pins of the DIN socket.
For the phono socket, we have used a five pin DIN socket, wired up so that it is compatible with either a three pin or a five pin DIN connection cord.
Internally, the construction and layout is broadly similar to that of the Playmaster 136, although we have rearranged some of the component positions.
As will be evident from the diagram the power transformer is situated in the rear left-hand corner, just in front of the fuses and speaker sockets. Note the orientation of the core, and the tact that the secondary output lugs face inwards. To minimize hum induction into the steel chassis, we mounted the transformer on brass spacers, using brass machine screws - an idea that .might be worth applying to existing 136 amplifiers.
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----------- May be constructed on Vero board
The method of mounting the plastic BD235 end 8D236 power transistors is apparent from the line drawing directly above
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In front of the transformer is the tag strip containing the rectifier diodes and the dropping resistor for the pilot light. Next to this, to the right, are the three chassis-mounting electrolytic filter capacitors.
The main pre-amplifier and the tone control board is mounted in the front right-hand sector, while the power amplifier boards are behind them. The quadraphonic components, stability components and headphone dropping resistors are mounted on a piece of tag strip fastened I to the center of the rear of the chassis, between the input and output sockets.
Construction of the new amplifier is quite straightforward, as most of the circuitry is contained on the three printed wiring boards. Two tag strips are used to hold the components of the power supply and the stability and quadraphonic networks. The remaining components are fixed direct to the chassis.
Commence construction by fitting the following components to the chassis: power transformer, filter capacitors, input and output sockets, fuse holders, potentiometers, and the source switch. For those components requiring mounting bolts, we used j inch machine screws and nuts.
Alternatively, "pop rivets" could be used, although these are of a more permanent nature, and do not permit easy disassembly if this is required.
The power transformer must be mounted on brass spacers using brass nuts and bolts. The spacers should be about 10mm long, in order to provide approximately the same clearance at the top and bottom of the transformer.
The 240V AC wiring can now be completed. The mains cord enters the chassis through a grommeted hole at the rear, and is securely anchored used a suitable clamp. The active and neutral wires are terminated at a small insulated terminal block. The earth wire is threaded through, and soldered to a solder lug attached to the cord clamp.
Regular lamp cord or suitably insulated twisted hookup wire runs from the terminal block to the miniature toggle switch, and from the terminal block to the primary of the transformer. The exact connections at the terminal block are shown in the wiring diagram.
Once the connections have been made and checked, a wise safety pre-caution is to tape the primary connections to the transformer and the power switch, as these points are possible shock hazards. Two or three layers of insulating tape should be sufficient to prevent accidental contact with the mains. ' Assemble the power supply components on the tag strip, taking particular care to mount the diodes correctly. The tag strip is held in place by a metal spacer, which, in conjunction with a solder lug, forms the chassis earth for the two RF bypass capacitors. Do not forget to scrape the paint from underneath the spacer and screwhead where they contact the chassis.
Loosen the clamps holding the filter capacitors so that they may be rotated in their bases, and align them as shown in the wiring diagram.
The two nearest the rear of the chassis have their positive leads facing the right, while the third has its negative lead facing right. Connect the three left-hand terminals together with heavy gauge tinned copper wire to form the main power supply earth.
A second piece of wire is used to connect the two positive terminals of the two capacitors nearest the rear of the chassis to form the positive supply rail. The negative supply point is the unattached terminal of the third capacitor.
The wiring from the transformer to the tag strip and from the tag strip to the filter capacitor^ can now be completed, along with the wiring to the pilot light. The pilot light, in series with a 470ohm 1 watt resistor is wired across the negative supply rail. This also serves to discharge the capacitor. A 2.2k resistor serves the same purpose on the positive capacitors, being wired directly across the capacitor terminals.
The next stage in construction is to make the connections between the input sockets and the source switch. This wiring is concerned only with the two auxiliary inputs, the tape input and the radio input. We will discuss the wiring of the phono input at a later stage.
First of all, connect pin 2 and the shell connection of each socket together. (A cut off component lead makes an economical and easy to obtain wire.) Then join each of these together with short lengths of hookup wire, and then to a solder lug attached to the chassis by the mounting screw of the AUX1 socket. This will ensure a good earth connection for the inputs.
The signal is carried from the input sockets to the source switch using figure-8 shielded wire with outer PVC covering, to prevent random contact between the shield and the chassis.
It is absolutely essential to follow the earthing procedure suggested in the diagram. Failure to do so will almost certainly result in hum problems and possible instability as well.
The braids of the cables are connected to pin 2 of the respective sockets, while the inner conductors are soldered to their respective pins. Refer to the wiring diagram for details of which pins are used.
If the cable comes with inner conductors which are color coded, i.c. with red and white insulation, follow a suitable convention with respect to the connections, such as "red equals right','.
If your cable is not color coded, use colored insulating tape or a scrap of PVC sleeving to mark the inner conductors. This is best done by using say red tape for the right channel, and marking both ends of the conductor before soldering the cable into position.
At the source switch end of the cables, no connection is made to the shield. Only the inner conductors are soldered to the switch, the braids being cut off short and insulated with the colored tape or sleeve already mentioned. Refer to the wiring diagram for details of the connections to the switch. Once all connections have been made, the four stereo cables may be taped or bound into a single cable, using either nylon tubing or insulating tape, and tucket down into the angle of the chassis.
For the phono socket, pins 1 and 5 are bridged, allowing either a 3-pin or a 5-pin DIN plug to be used. Do not make a connection between pin 2 and the shell connection of the socket. Two small RF chockes made from ferrite beads and a small length of thin enameled copper wire are placed in series with the input leads immediately adjacent to the socket.
These chokes, intended to combat radar and other RF interference, are made from 3.5mm diameter and 5mm long ferrite beads by winding about 5 turns of 28 B & S gauge wire, or similar, through them. One end of each choke is anchored to pins 3 and 5 respectively, while the other ends are terminated on a small piece of tag strip secured to the socket mounting screw. This also serves as the termination point for the shielded cable leading to the magnetic pre-amplifier board.
The next step in construction of the amplifier is to assemble the simulated quadraphonic and stability components onto the 15-pair tag strip. First place in position all the straps, as shown on the wiring diagram. Do not forget to insulate these to prevent short circuits.
Once this has been done, the resistors and capacitors may be added.
The next job is to make up and fit the wiring harness for the headphone socket and the ambience/stereo/surround switch. This runs from the tag strip at the rear of the amplifier behind the transformer and up the side of the chassis to the relevant components at the front. A branch runs from behind the transformer up to the speaker sockets and the fuse sockets.
Using the chassis as a guideline, and using multicolored wires, make up this harness before installing it. Take care that no mistakes are made, as this could lead to expensive damage, particularly to the output transistors.
The next stage is to assemble the power amplifier modules.
While the power amplifier modules have been derived from the original Playmaster 136 design, there are important differences.
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The most obvious has already been mentioned, in that the new board does not carry any power supply components. Power input is by three wires; power supply earth or common, plus 21V and minus 21V. A fourth lead from the same end of the board feeds the active side of the loudspeaker circuit. The remaining connection is the shielded signal input lead which also provides the earth link back to the preamp board.
Adjacent to the plus 21V lead is a link into which a milliammeter can be inserted to measure the quiescent current of the output stage. For simplicity, we used a loop of hookup wire with a soldered joint in the ' middle.
The passive components, resistors and capacitors on the wiring diagram require no special comment.
Note, however, that we have added one resistor to the amplifier, a 6.8k bridging one side of the quiescent current adjustment potentiometer.
This is to protect the output transistors in the event that potentiometer wiper or element becomes open-circuit. What happens in this circumstance is that the quiescent current control transistor, TR5, is turned off and the output transistors arc turned hard on, drawing heavy current which can cause them to burn out.
While the possibility of an open circuit potentiometer is fairly remote, the 6.8k resistor provides cheap insurance. Now, in the event of an open circuit pot, the control transistor is turned on and the output transistors draw zero quiescent current. In this condition, cross-over distortion occurs but no damage eventuates and the situation can be rectified.
The transistors themselves call for special comment. Considering first the small signal transistors, there are several important points to be observed. . t The original 136 board and the new board were designed for Fairchild transistors of the "glob top" variety, having their connections in a triangular configuration. The relevant type numbers have been retained to assist those who may want to use existing transistors or rebuild modules from a 136 unit.
These types of transistors are no longer available, due to changes in fabrication techniques. Instead, similar types are supplied in the "TO-92" configuration, in which the pins are in line.
In the meantime, manufacturers of semiconductors have produced transistors which are a direct electrical replacement for the earlier types. Some care is required in fitting these transistors, as they do not all have the same pin configurations. Details of the device numbers and configurations are shown in the accompanying table and diagrams. With TR1 and TR2, the transistors are fitted simply by bending the leads sufficiently to fit the triangular pattern of holes, so that they sit about one centimeter or so above the board.
The real difficulty has to do with the other three which need to make physical contact with the output transistor heat sink to provide thermal feedback.
With the glob top transistors, the requirement was met by providing three suitably positioned dimples in the underside of the heatsink, each partially filled with silicone paste. The small-signal transistors were dropped into position on the board but not soldered. The heatsink carrying the power transistors was then locked in position with the power transistor leads just emerging through the copper pattern.
This done, the glob tops were pushed up into the silicone-filled dimples and the leads soldered.
The TO-9 style transistors don't lend themselves to this approach. They don't sit down snugly on the present board and the small, Hat top does not mate naturally with a dimple. We are therefore suggesting that holes be drilled in the heatsink, which will be a clearance fit for the TO-92 bodies, Allowing the transistor to sit part way through the heatsink. A blob of silicone compound can blend the two thermally.
The mounting method has a possible bonus in that the transistors are no longer trapped under the heatsink. They could, if necessary, be extracted and replaced through the holes.
The original circuit board and heatsink was designed for use with the TO-66 style transistors, specifically the Fairchild types AY8171 and AY9171. These are no longer available, but Fairchild have suggested the use of the 2N4232 and 2N3740. These pose no problems with regard to fitting, as they are also TO-66 types.
Electrically but not mechanically similar transistors are also available from other manufacturers, and these require a different mounting arrangement. This will be described immediately following the description of the mounting arrangements for the TO-66 style transistors.
The first step is to inspect the heatsinks, which should be of aluminum, not less than 16 gauge. Make sure that the two power transistors sit flat against the surface, with mounting holes aligned and with adequate clearance around the base and emitter pins. If there is any inaccuracy, lead the holes as necessary with a small round file.
In fact, we rubbed the inside surface of our own heatsinks witli a large flat fine-gauge file to remove any high spots and then buffed the surface all over with steel wool.
Now check the heatsinks against the wiring board. Make sure that the bolts securing the power transistors can pass straight through, and the pins likewise. If there is any fit problem, the mounting holes in the board can be elongated as necessary.
If the small-signal transistors happen to be the older glob tops, the heatsink will need dimples or countersunk holes in the underside.
If, as likely, the transistors are of the TO-92 configuration, the heatsink will need snug clearance holes instead. In fact, dimple type heatsinks can be adapted by drilling appropriate holes. They may not be concentric with the dimples, however, since the TO-92 transistors sit most naturally between the collector and emitter pins, with the base lead kinked outwards to fit the triangular pattern in the board.
With all this sorted out for each of the boards, the heatsink assemblies can be completed. Smear the underside of the power transistors with silicone compound and secure the transistors firmly to the heatsinks with 1/8-inch long bolts and nuts, cither p Whitworth or 5BA. The transistors do not need to be insulated from the heatsink. In fact, the heatsink and mounting bolts form part of the collector circuitry.
You will need four spacers per board, either 4mm or 5/32, such that when the heatsinks arc mounted, the base and emitter pins of the power transistors just come through the copper pattern. We found some brass nuts of a larger size which we turned into spacers by running a p-inch clearance drill through them. This done, the power transistors assemblies can be put aside for later installation.
On the wiring board itself, it is wise to smear the copper around the heatsink mounting holes with a thin layer of solder. If this is done, the nuts will bite into the solder when they are finally tightened and make good contact between the transistor collectors, heatsink and mounting bolts and the copper pattern on the board.
At this stage, the remaining components can be installed as per the diagram. Use an iron with a clean, slender tip and flow the solder around each component lead as quickly as possible, to avoid overheating cither the pattern or the component. Make sure that you install the electrolytics with the correct polarity.
The alternative types of transistors come in an SOT-32 style case. This is a plastic package, having three pins in line, the center one being the collector. The collector is also connected to a metal plate which forms the coupling to the heatsink. Unfortunately, different manufacturers have different configurations for the base and emitter leads.
The Philips types BD235 and BD236 must be mounted underneath the heatsink as explained below.
The reader is referred to the accompanying sketch for full details of the method used to mount the plastic encapsulated transistors. Care should be taken that the following points are observed while fitting them.
The power transistors are fitted underneath the heatsink, and bolted to it through the holes provided to mount the TO-66 type transistors. They must be mounted with the metal part next to the heatsink, as this forms the collector connection. As a result of this, the collector lead provided can be carefully snipped off close to the body of the transistor. (The collector lead is the center one.) To ensure that the base and emitted leads are not transposed* it is vital that the mounting hole furthest from the original center clearance holes be used in each case to mount the plastic transistors. On the heatsink, these happen to be the holes nearest to the dimples for the driver transistors. The transistors are secured to the heatsink using suitable nuts and bolts. A small washer should be used under the head of the bolt, as shown in the diagram, and silicon grease applied to ensure good thermal contact between the transistors and heatsink.
Do not use excessive force when tightening the nuts, as this may damage the transistors. Take particular care that the actual transistors arc not interchanged. The heads of the bolts must be in the underside of the heatsink to ensure that the transistor leads are long enough to reach through to the copper pattern on the board.
Once the transistors have been fixed to the heatsink, their base and emitter leads may be bent as shown in the diagram. Use the holes in the heatsink as a guide, and hold the leads next to the body with a pair of small long nosed pliers to prevent them from breaking off.
The heatsink is held in position, and the collector connection made by bolts through the remaining holes. We used two nuts as spacers to position the heatsink at a suitable distance from the board. Do not forget to tin the copper pattern underneath the nuts to provide good electrical contact. - 1 The Texas Instruments TIP31B and TIP32B arc packaged in a plastic TO-126 style case, similar in appearance to the SOT-32 types. However, they differ in having the base and emitter leads transposed with respect to the Philips types.
This means that they cannot be mounted as shown in the photograph.
Instead, they must be mounted on top of the heatsink, and the leads must be bent downwards so as to pass through the holes in the heatsink.
As before, the center lead must be cut off, as the collector connection is made via the heatsink.
Do not forget to thermally bond the transistors to (the heatsink using silicon grease, and. do not omit the washer under the nut of the mounting bolt. This is to prevent damage to the case of the transistor.
Excessive force must not be used when tightening the mounting bolts.
The heatsink is mounted in the same way as before, using $-inch machine screws, in conjunction with spacers made from nuts.
The fitting of the completed power modules can now be checked against the holes in the chassis. The modules are mounted on spacers, which do not have to be insulated, as appropriate clearances have been provided on the boards.
After checking the fit of the modules, remove them from the chassis.
The next step it to check the power supply and only then install the modules. This procedure has less chance of damaging anything should any fault exist.
Check that the chassis and associated power supply wiring, as described last month, is correct, and that no trailing wires are resting against the chassis. If all is in order, plug the mains connector in to a suitable receptacle and switch on. If all is in order, the indicator light should come on and voltages, plus and minus 21.5V approximately, should appear across the respective supply rails at the filter capacitors.
The first module may now be wired up to the appropriate points, as shown in the wiring diagram, and installed in position. At this stage, do not switch on.
Insert a milliammeter in the link, with plus to the supply, and set to the 250mA range. Rotate the current set potentiometer, as viewed in the chassis fully anti-clockwise, and set the volume control pot at full off. Since the earthy side of the pot may not at this stage have an earth return, run a temporary link to a chassis earth.
Now watch the current meter and switch on. If the meter slams over, there is something radically wrong. Switch off instantly and check. You may have the power transistors interchanged, or one of the other transistors the wrong way round. Whatever you do, don't tempt fate by switching on again and repeating the overload condition.
In fact, the current flow with the preset pot retarded, should be zero.
If it is, reset the milliammeter to 50mA and carefully rotate the potentiometer clockwise. Bring the current up to 12mA and leave the module running for a few minutes. If all is well, switch off, remove the milliammeter and close the link.
Note that the current should be set without a loudspeaker or other load connected. When the loudspeaker is plugged in, current distribution in the output stage will change due to the small offset voltage (0.2V approx.) across the loudspeaker terminals.
By now connecting a loudspeaker and feeding a signal to the volume control from any source capable of producing a reasonable signal across 50 k-ohms, ii would be possible at this stage to check the module for sound, or yet again to run instrument tests.
Once satisfied that this module is working, the second one can be connected, and similarly tested. It is better to check the modules in this fashion than to wire them both in at once and switch on simultaneously. One with an inadvertent fault could be "cooking" for several minutes while the other one is being tested! When completing the wiring to the power modules, take pains to ensure that the wiring is in accordance with the wiring diagram. In particular, ensure that the earth leads are as shown. From each module, they are run towards each other, and then twisted together and run by the shortest direct path towards the earth rail on the filter capacitors. This takes them directly underneath the left-hand power module.
The remaining wiring is run as shown in the wiring diagram, to complete the connections to the filter capacitors and to the fuse holders.
This wiring should be kept as close to the chassis as possible.
Having installed and checked the power modules, the next obvious step is to build and install the pre-amplifier and tone control module; this is secured to the chassis floor on four pillars in the space between the power modules and the panel controls.
For your guidance, we show the schematic circuit of one channel of the preamp-tone control module.
The board is drilled for transistors having the traditional triangular CBE lead configuration. Transistors of this type can simply be dropped straight in, with little risk of confusion. However, if you are supplied with TO-92 style transistors, check the base connections carefully against the circuit diagram.
Electrically, the pre-amplifier provides enough gain (75 times) to ensure full drive to the main amplifier from an input of 2mV RMS, with good signal/noise ratio and adequate tolerance to peak level input signals. It provides a nominal loading of 50k ohms for a magnetic cartridge and compensation which conforms closely with the required RIAA' characteristic.
Output from the compensated pre-amplifier goes to the "SOURCE" switch, where it is made available, along with signals from other sources: radio tuner, tape player and auxiliary inputs. Since the signal levels ,at this point are normally 150mV or higher, shielding is not necessary.
From the source switch, the signal passes to an emitter follower, which presents a fairly high impedance to the various signal sources, and a low source impedance for the tone control circuitry. At the same time, it provides a convenient point from which to derive signal for an external tape deck; this signal is independent of the amplifier's own volume and tone controls.
The tone control circuit is of the feedback type, favored because of its inherently low distortion and its tolerance to higher level input signals.
The point should be made, however, that signals derived from the source switch are applied directly to a transistor base and can overload the circuitry between this and the volume control, if the level is excessive. Signals from radio tuners, tape players, etc., must be limited in some way if there is any suggestion of overload, or if you find that the amplifier is being fully driven with the volume control only fractionally on.
In constructing the preamp-tone control module, a useful first step is to check the fit of the board and chassis mounting and make any adjustments necessary by filing holes, etc. We used 0.5-inch threaded pillars, . with 5-inch long, y-inch diameter countersunk screws securing them to the chassis and 0.25-inch long, 1/8-in. diameter roundhead screws securing the printed wiring board.
Next solder a solder lug onto the copper pattern so that it will earth the pattern next to the mounting hole nearest to the magnetic preamp inputs.
This forms the main earth of the amplifier. We used a double-ended solder lug, so that it formed 2 bridge across the gap in the pattern which , had previously been provided for when the earth was not at this point.
No earth connections should be made at any of the other mounting positions.
Dp not overlook the cut which is desirable in the board pattern to eliminate a possible troublesome earth loop. The exact position is shown on the component layout diagram.
For the rest, the components drop into place, as per the accompanying drawing. Polarity is important only in respect to the electrolytic capacitors, and this calls for some care. Note that we have shown an alternative position for the main decoupling capacitor, in case it happens to be larger than the one originally planned. An extra hole may be required and, for this, you will need a fine twist bit and, preferably, a small "egg-beater" hand-drill.
The most tedious job with the module has to do with the leads which must ultimately connect to the potentiometers and switches. The leads range from about 3 to 7 inches long and are unshielded, except where otherwise indicated. It is a good idea to use as many different colors as possible to facilitate lead tracing, and to use thin rather than thick hookup to retain maximum flexibility.
The leads should be anchored to the wiring board during initial assembly and left trailing. Please yourself whether you make them all generously long, or save wire by cutting each one discretely to suit the requirement.
But, whatever you do, plan for each wire to follow a gently curved path so that the board can be unbolted and lifted up for testing or service.
For the same reason, leave some slack in the phono input and tape output leads, which run back to the DIN sockets.
Numbers 1-19 on the board wiring diagram relate to numbers on the main diagram and indicate where each lead goes.
These leads are best soldered at their ends after the board has been fitted in position. Do not forget to scrape off any paint underneath the spacer forming the earth connection. A good idea is to "tin" the steel beforehand, thus ensuring a good connection.
The general layout of the leads is as shown in the main diagram. Try and keep all leads as close to the chassis as possible, as this will minimize hum pickup. We found that a neat appearance could be obtained by tucking as many leads as possible underneath adjacent boards: this can be seen in the photographs.
The final step in construction is to fit the front panel, which is held in place by the switches and potentiometers. Care must be taken during this process, as a scratch on the panel can be quite unsightly, and very difficult to remove.
The amplifier is best tested by actually using it. Connect up a suitable pair of eight-ohm speakers and turn the amplifier on. Switch to stereo, and set the tone and balance controls at the mid-range position. Switch to PHONO, and turn the volume control full off. The amplifier should sound dead quiet from the listening position, and only if your head is placed right up next to the speakers should any hiss or hum be audible.
When the volume control is advanced about halfway, the noise level should not increase audibly at a normal listening position.
Connect up a suitable input, and check the operation of all controls. If a suitable pair of speakers are available, the simulated quadraphonic feature may be tested. Note that true operation is not obtained unless the main speakers are connected. Using a suitable pair of headphones, check the operation of the headphone switch. All speakers must be silenced when the phones are plugged in.
List of component parts for the new amplifier
Main Section
1 Transformer, 240 V primary, 30 V C.T. secondary, 1 amp
4 EM401 or RS276-1139, 1NA005, BY 113 silicon diodes or similar
2 47uF 100 VW ceramic capacitors
1 470 ohm 1 watt resistor
3 2500uF, 35 VW electrolytic, chassis mounting capacitors
1 22k 0.5 watt resistor
1 Pilot light, 6 V 50mA 2 1.5A quick acting fuses and holders to suit (chassis mounting types) 4 2-pin polarized speaker sockets 1 3 lug tag Strip 1 6 pair tag strip 1 15 pair tag strip 1 10 ohm 5 watt resistor 2 22 ohm 5 watt resistors 1 39 ohm 5 watt resistors 2 220tiF 16 VW electrolytic capacitors 2 0.47tiFpolyester capacitors 2 330 ohm A watt resistors 2 15 ohm ' A watt resistors 3 5-pin DIN sockets 2 3-pin DIN sockets I rubber grommet II mains cord and plug 1 mains cord clamp 1 3-terminal block, 240 V 1 headphone socket, with double pole insolated switch ,1 miniature on/off toggle switch 1 miniature double pole double throw with center off 2 500K linear dual gang potentiometers
1 50K log. dual gang potentiometer
1 2 2M linear potentiometer
1 5 position 2 pole rotary switch 5 knobs to suit 1 front panel, 355 x 75mm 1 chassis and cover, 360 x 270 x 83mm I
Power Modules (2)
2 wiring boards 12.3 x 7.6cm 2 aluminum heatsinks, 16g; 76 *40 x 27mm overall 14 transistors - see text
RESISTORS (0.5W, 10 pc)
2 100 K, 2 82K.
2 27K.
6 150 ohms
2 220 ohm preset tab pots
2 1 OK, 4 6.8K, 2 4.7K.
4 560 ohm, 4 390 ohm, 2 IK, 6 ¦ 27 ohms
CAPACITORS
2 470 uF 16V vertical electrolytic
2 100 uF 25 V vertical electrolytic
2 0.47pF 160 V polyester
2 390pF polystyrene
2 270pF polystyrene
Preamp/Tone Control
1 wiring board
2 BC109, BC549 or similar transistors
2 BC108, BC548 or similar transistors
4 BC108. BC548 or similar transistors
2 Ferrite RF beads 3.5mm dia, 5mm tong
4 0.5 in or 1cm spacers with screws
RESISTORS (0.5 W or 0.25 W, preferably 5-pc)
2 3.3M, 2 470k,
. 4 27k.
3 680 ohm
2 560k.
2 47k.
7 1k, 2 1.5M, 2 82k.
2 10k.
2 2.2 M, 6 100k.
2 15k.
2 2.7M, 2 330k.
2 22k.
CAPACITORS
1 220 UF 25 VW vertical electrolytic
1 100nF 25 VW vertical electrolytic
4 4.7pF 12 VW vertical electrolytic
2 1.5pF20VW tantalum electrolytic
2 0.47pF 20 VW tantalum electrolytic
2 0.27pF 100 V polyester
4 0.1 pF 100 V polyester
4 0.022pF 100 V polyester
2 0.004 7pF 100 V polystyrene
2 0.0015 pF 100Vpolystyrene
2 680pF 100 V polystyrene
4 100pF disc ceramic
Miscellaneous
Brass spacers, brass machine screws and nuts, washers, lock washers, solder, solder lugs, colored hookup wire, silicon grease, insulating tape, shielded cable, rubber feet.
Fairchild Pro-Electron Texas Tandy Archer Other Types
TR1 2N5088 BC549 BC184L RS276-. BC109 BC548 BC183L RS276-
. BC108 2N5818 2013 SE4010 2N5818 2009
TR2 PN42S0 BC559 BC213L RS276- BC214-309-514 2N6003 BC213-308-513 2N6015 2021 2N4250 BC558 BC214L RS276 2021
TR3 PN3643 BC549 BC184L RS276- BC109 BC548
. BC108 2013 / 2N3643 BC183L RS276- 2009
TR4 PN3638A BC559 BC213L RS276- BCW63/BC257 2N5366 2021 2N3638A. BC558 BC214L RS276 2021 MPS3638A
TR5 PN3565 BC548 BC183L RS276-. BC108 BC549
.BC109 2009 2N3565 BC184L RS276- 2013
TR6 AY8171 BD235 TIP31B RS276- BD177-439-441- 241A 2N4923.
BDY79 2N6123 or 6101 2017 2N4232 BD201 RS276 2017
TR7 AY9171 BD236 T1P32B RS276- BD178-242A-440 2025 442 2N4920-3740 BDX14 2N6125 or 6134 2N3740 BD202 . RS276- 2025 .