MODEL SPA
 

ultra high definition
basic preamplifier

with built-in optional
headamp for moving
coil cartridges
 
 
 
 
 
 
 
 

WARRANTY

DAYTON WRIGHT ASSOCIATES LIMITED warrants the SPA PREAMPLIFIER to be free from original defects in parts and manufacture for a period of three (3) years from date of purchase.  This WARRANTY is owner transferable.  No other warranty is expressed or implied on the SPA PREAMPLIFIER and/or other equipment used in conjunction with the SPA PREAMPLIFIER.

DAYTON WRIGHT ASSOCIATES LIMITED restricts its liability to the cash value of the SPA PREAMPLIFIER at time of purchase.  No liability is assumed for injury, loss of time, and/or other damages.

The WARRANTY will be void by any of the following conditions:

1 . Failure to comply with the INSTALLATION AND OPERATING INSTRUCTIONS in this manual.

2. Abuse of the equipment.

3. Unauthorized repair or attempted repair of the equipment.

Exclusion for damage due to a lightening strike or power surge. Our warranty does not cover anything that is damaged by any incident like this. As transistors or other components may fail if subjected to a voltage spike, even after some months have passed, if we are called on to effect a repair, any such work will be excluded from any subsequent warranty repair. It must be understood, in advance, that should we attempt a courtesy repair, tat it is NOT under any warranty whatsoever!

4. Shipping the equipment for repair without original or replacement DAYTON WRIGHT packaging.

Please contact your nearest authorized DAYTON WRIGHT dealer for service.  Units returned to the factory without authorization will not be repaired.
 
 
 

PURCHASER'S RECORD
 
 

Model:  SPA M802A (with Headamp)
                                         M802B (without Headamp)
 

 FOREWORD
 Introduction
 General Product Description
 

 INSTALLATION AND OPERATING INSTRUCTIONS

 1. Unpacking the Unit
 2. SPA Power Module
 3. SPA Main Unit
 4. Appendices

 LIST OF ILLUSTRATIONS

 Fig. 1 SPA Packaging
 Fig. 2 Power Supply
 Fig. 3 Main Unit (Top View)
 Fig. 4 Trimpot Setting for Headamps
 Fig. 5 Location of Trimpots on Headamp Board
 Fig. 6 SPA Truth Table
 Fig. 7 SPA Block Diagram
 Fig. 8 RF Filter
 Fig. 9 Flow Chart

 F O R E W O R D

INTRODUCTION

Dayton Wright Group Limited welcomes you to its fine line of audio components.  Your newly purchased product represents years of dedicated research, design, and refinement, encompassed by carefully thought out design goals and features.  Every part and assembly in a Dayton Wright product is thoroughly inspected; every transistor and integrated circuit is hand selected in Dayton Wright designed test apparatus to meet our high standards for matched gain, low noise, and high stability; vigorous testing is performed at every stage of assembly, integrated with ample burn-in periods.  Before being hand cleaned and packed, every product is actually played and listened to in a stereo system - an extra effort to insure that a product in perfect condition will reach you.  All this in harmony with our basic, no frills approach to audio reproduction equipment, to bring you the highest attainable level of sonic accuracy possible.

IMPORTANT: Before using your Dayton Wright product please take the time to read through
this manual and familiarize yourself with the equipment This will enable you to set up your entire system correctly the first time, and to help you take full advantage of its high quality and features.

GENERAL PRODUCT DESCRIPTION

The Dayton Wright SPA Mk II preamplifier is an extremely flexible control center for processing signals from a wide variety of sources which require pre amplification.  As many as four phono inputs, three high level inputs, and two tape recorders are accommodated and selected with ease via the controls provided, all of which are preamplified through Dayton Wright State-ofthe-Art circuitry.  The result: a noticeably more complete and accurately detailed retrieval of your sources' musical information.  When used with compatible components exceptional results should be immediately evident.

The SPA remote POWER MODULE feeds only D.C. voltages via the control cable to the MAIN UNIT, eliminating induced hum and noise encountered with A.C. control cables.  All voltages are bridge rectified and generously filtered, as well as fully regulated with integrated circuits and heat sunk zener diodes.  The control cable terminates into a "D-connector", located in a recessed area on top of the unit, as do all the phono sockets.  These are hidden by a smartly silk-screened plexiglass cover, which serves as a guide to connections.

Preamplification is achieved through three separate amplifiers excelling in low TIMD (transient intermodulation distortion) and accurate "current mode" loading:

(1) a fully discrete cascode configuration for "moving-coil" type phono cartridges;

(2) a unique discrete differential bootstrapped cascode stage and integrated circuit phono amplifier combined with RIAA playback equalization for the more common "magnetic" type cartridge; followed by

(3) a dual voltage amplifier and voltage follower integrated circuit for "line level" gain of the previous and all other inputs.  Ample switching facilities are provided to achieve easy selection of inputs, tape sources, cross-dubbing, and mode.

The SPA Mk II was built in four models.
            A)   Without a pre-preamplifier
            B)   With a simple pre-preamplifier built on a single circuit board
            C)   With a more complex pre-preamplifier built on stacked circuit boards
                            (this requires a different power supply module - with different output voltages
            D)   The SPA case is deep-bronze-anodized aluminum. It has LED indicators
                            for both peak and RMS levels.
                            Its top is flat and it can be used with a parametric equalizer that plugs-into
                            the motherboard once a small jumper PC board is unplugged.
                            The power supply is larger as it has to supply the power to a larger
                            headphone amplifier.
 

DETAILED PRODUCT DESCRIPTION
INSTALLATION AND OPERATING INSTRUCTIONS
 
 

1. UNPACKING THE UNIT

The SPA packaging has been specially designed to protect the unit from shipping abuse.  It is suggested that you keep the packaging, should it ever become necessary to ship the unit.  UNDER NO CIRCUMSTANCES SHOULD THE UNIT BE SHIPPED WITHOUT ITS PACKAGING.  Please contact your dealer if you require a replacement set.

1.1 Carefully slit the tops of the outer and inner cartons, and lift all the flaps to expose an inner package.  This contains the MAIN UNIT.

1.2 Remove the main unit package from the carton.

1.3 DO NOT ATTEMPT TO PULL THE UNIT THROUGH THIS SLEEVED PACKAGE.  Open the sleeve and lift the MAIN UNIT out.

1.4 Carefully remove the plastic wrap.

1.5 From the carton remove the package containing the POWER MODULE. (The adjacent package serves only as a spacer.)

1.6 Open the sleeved package and lift out the POWER MODULE with attached A.C. cord and control cable.

1.7 Remove the plastic wrap.

1.8 Inspect the units for any external damage.  If any, please report it to your dealer at once.

1.9 The cases are protected with a unique velvet finish which is extremely durable and resistant to scratches.  DO NOT APPLY OIL OR FURNITURE POLISH TO THESE CASES.  They may be easily cleaned with a damp, lifter cloth.
 

1.9.1 DAYTON WRIGHT GROUP LTD., SPA Mk II'S

1.9.2 CASE Finish - the velvet finish was dropped in favor of a Bronze Pebble Finish.
                                    The the fourth model the of the series can be identified by its
                                    smoked plastic cover that is curved to fit the preamp's case.
 
 

2. SPA POWER MODULE

This unit houses the transformers and circuitry for the D.C. supply voltages.  A heavy duty relay is D.C. activated by a switch on the SPA MAIN UNIT, which applies power to the SPA and switched A.C. outlets.  There is a user accessible 115/230 V.A.C. mains slide switch, and an external fuse.  The power transformer is thermally protected and will shut down temporarily if excessive current is drawn.

2.1 Set the "A.C. LINE VOLTAGE SELECTOR SWITCH" to either "l 15" or "230", depending on your voltage requirements.  The "l 15" setting will work over a range of 95-135V; the "230" setting will work over a range of 190-27OV; 50 or 60 Hz operation is acceptable with either setting.

2.1.2 The line cord may now be plugged into a wall outlet.

2.2 A.C. OUTLETS

2.2.1 There are four SWITCHED A.C. OUTLETS on the POWER MODULE rated "800 WATTS MAX.". The total power drawn by equipment plugged into these switched outlets should not exceed 800 watts, or approximately 6.6 amperes at 120 V.A.C.

2.2.2 Power will be supplied to the switched outlets when the power switch on the SPA MAIN UNIT is depressed.

2.2.3 There are two UNSWITCH ED A.C. OUTLETS, rated "400 WATTS MAX".  The total power drawn by equipment plugged into these unswitched outlets should not exceed 400 watts or approximately 3.3 amperes at 120 V.A.C.

2.2.4 Power is supplied to the unswitched outlets as soon as the POWER MODULE A.C. line cord is plugged in.

2.2.5 You may plug in associated equipment to these outlets to suit your requirements, so long as you do not exceed the wattage ratings.  Theoretically, some power amplifiers could be accommodated via a switched outlet, as the A.C. mains relay is rated at 1 0 amperes.  In practice, however, it is suggested that you plug your power amplifier directly into a wall outlet.  This is stressed for three reasons: (1) Heavy surge demands from the power amplifier are more easily afforded via direct connection to a wall outlet; (2) Swinging A.C. line variations between preamplifier and power amplifier will be minimized; (3) The possibility of ground loops (leading to numerous undesirable conditions) will be minimized.

2.3 PLACEMENT OF THE POWER MODULE

2.3.1 One should bear in mind that the SPA MAIN UNIT contains highly sensitive phono circuitry, and that all source equipment, along with interconnecting cables to these stages, is susceptible to hum and noise pick-up.

2.3.2 Keep the -POWER MODULE at least three feet from the SPA MAIN UNIT, source equipment, and signal carrying cables; otherwise hum, noise, and RF interference may be encountered.  See APPENDIX, Section 4.2, 4.3.
 
 

        This shows the top of the SPA that was built using the Leigh cases. After The Dayton Wright Group Ltd., purchased the company from Leigh, it took several months to develop a replacement pebble finish which was durable enough to withstand professional use. Leigh's case used a black flat-latex finish that had a tendency to suffer reversion when exposed to heat or sunlight.

3. SPA MAIN UNIT

3.1 POWER CONNECTION

3.1.1 Slide out and remove the plexiglass cover.

3.1.2 Insert the POWER MODULE control cable securely into the D-connector on the MAIN UNIT, located on the left rear side of the recessed area.

3.2 INPUT SELECTOR

NOTE: In order to protect your power amplifier and loudspeakers, it is good practice to
turn the VOLUME control completely down (counter clockwise) when depressing any of the INPUT SELECTOR buttons.  This will eliminate the possibility of spurious pulses injected into the high power associated components.

3.2.1 Moving Coil Phono Inputs

There are two inputs for moving coil phono cartridges.  These are designated "MCl" and "MC2" on the front panel, corresponding to the gold-plated input sockets at the right rear of the recessed area, designated "Low Level Phono Input" "1" and "2", respectively. (These inputs were specifically placed to be near the pre-preamplifier circuitry.  Hermetically sealed mercury thermo-relays pass the signal from the input sockets to the circuit board.  The INPUT SELECTOR buttons merely actuate these mercury relays with a D.C. voltage.  So, contrary to what one might assume, the moving coil inputs are as close as possible to the actual circuitry, while unwanted lengths of signal paths to the selector buttons are eliminated.) Insert the signal carrying cables from tone arms fitted with moving coil cartridges into these sockets.
 

 In order to extract the desirable qualities of moving coil cartridges, there are two variables on the SPA which deserve attention.  The first is the termination resistance.  MC1 and MC2 differ only in this aspect: MC1 is terminated by 12 ohms; MC2 by 47
ohms, as marked on the plexiglass cover.  However, these termination's, in conjunction
with your particular moving coil cartridge, will produce different sonic effects.  You
therefore owe it to yourself to try both inputs and determine which is sonically superior.
All undamped moving coil cartridges ring.  Bearing in mind that MC1 provides more damping than MC2, a cartridge exhibiting harshness in the high frequencies (dismissing mistracking or other anomalies) would benefit from MC1.

        The other variable is the setting of the output trimpots on the pre-preamplifier.  This adjustment matches the sensitivity of the pre-preamplifier to that of your particular cartridge, allowing for optimum operating ranges of the preamplifier stages.  The trimpots are located down the left side of the unit on the pre-preamplifier circuit board.  They are accessible through the holes provided. (Some earlier units will require removal of the bottom plate and inner shield with a 9/64" Allen wrench).  The trimpots are square, gray, and afford about 22 turns (i.e. full revolutions).  Units with Serial No.'s 240 or less were set at maximum (fully clockwise).  Units after No. 240 have been preset at 11 turns (midway).  By referring to the chart below and your moving coil cartridge's specification sheet, determine the optimum level setting for your cartridge.  Note that two stylus velocities are supplied for five different output ranges.
 

                Fig. 5A Leigh Circuit Board for PrePreamplifier
 
 
 
 
 
 
 

MAIN SPECIFICATIONS

 Generating system: Moving coil
 Output voltage @ 0.3 mV (1 kHz, 50 mr-ti/sec.,  horizontal direction)
 Channel sensitivity difference: 1 dB or less (1 kHz)
 Channel separation Over 25 dB (I kHz)
  Over 15 dB (30 kHz    PUF-  1 PU filter used)
  Electrical impedance: 40 SI ±20% (1 kHz)
  Compliance: 8 x 10-6 cm/dyne (on clisc) 25 x 1 0-6 cm/dyne (static)

 Stylus tip: Special elliptical stylus
 Stylus pressure: 1.8 ±0.3 gr.
 Playback frequency response@ 20 to 60,000 Hz
 Weight: 7-8 gr.
 Recommended  load resistance: 10012 or more (except   when transformer is provided)
 
 

EXAMPLE: Setting             the pre-preamplifier trimpots -

Shown above is the specification sheet of a typical moving coil cartridge.  The output voltage is rated at 0.3 mV at 50 mm/sec.  Since 50 mm = 5 cm, use the 5 cm/sec. column of the CARTRIDGE OUTPUT RANGE on the chart.

The output voltage, 0.3 mV, falls into the "0.25 - 0.45 mV" range.  Therefore, if your SPA has a serial #240 or less, you must turn back (counterclockwise) the trimpots 1 1 full turns.  Or, if the serial number is greater than #240, no adjustment is required.

3.2.2 Magnetic Phono Cartridges

There are two inputs for magnetic phono cartridges.  These are designated "Ml" and "M2" on the front panel, corresponding to the input sockets marked "Mag.  Phono 1" and "Mag.  Phono 2", respectively, on the plexiglass cover.  These two inputs are identical.  Insert the signal carrying cables from tonearms fitted with magnetic cartridges into these sockets.

3.2.3 Grounding

The turntables and tonearms may have ground wires.  Initially, secure them to the ground post.  Since there are no rules for grounding which apply to all set-ups, determine by trial and error the grounding set-up which yields the least hum and noise in your system. (For example, in certain instances disconnection of the ground wire(s) will decrease hum in the system.)
 

 3.2.4 Phono Cables

Long leads from tonearms should be avoided, but if they must be used, "off-the-shelf" extension cables are definitely not suggested.  The suitable solution is to cut off the cables' phono jacks, extend the cable (or better yet, replace it completely) with a high quality, low capacitance, low resistance cable, and terminate with new, high quality phono jacks.

3.2.5 Tuner and Auxiliary Inputs

These three inputs are identical.  One of them is designated "TUNER" for the sake of user convenience only.  Connect to these inputs your tuner and other auxiliary ("line level") sources (except tape decks .... these are accommodated by the tape facilities, to be discussed below.  A third tape deck, however, could be connected for playback only to an AUXILIARY INPUT).  If the tuner is equipped with output level controls, it is appropriate to adjust them for approximately the same output level as that obtained with the phono inputs.

3.3 TAPE

Facilities for two tape decks are provided, featuring extensive monitoring and cross-dubbing combinations.  Observe carefully the instructions for input and output connections.  Studying the "TRUTH TABLE" will supply the user with the necessary information to yield the greatest flexibility from this section.

3.3.1 Connect the output of the tape deck to "TAPE INPUT 1 " (refer to plexiglass cover) of the preamplifier.

3.3.2 Connect "TAPE OUTPUT 1" of the preamplifier to the INPUT (sometimes designated LINE-IN) of the same tape deck.

3.3.3 If a second tape deck is to be used, repeat the above two steps, this time with "TAPE INPUT 2" and "TAPE OUTPUT 2" of the preamplifier.

3.3.4 Depressing "MONITOR 1" on the front panel will override the INPUT SELECTOR and preamplify the signal being fed into the "TAPE INPUT 1" sockets.  Similarly, "MONITOR 2" will preamplify "TAPE INPUT 2".

3.3.5 The TAPE OUTPUT levels of the preamplifier are at "line level" and are not affected by VOLUME, BALANCE, and MODE settings.  The TAPE INPUTS of the preamplifier do pass through these controls just as -a selected source would.  Therefore, an uninterrupted recording can be made while effecting changes to the monitored signal.

3.3.6 The SPA "TRUTH TABLE" explains in detail the signal routing possibilities with respect to the TAPE pushbuttons.  Note that some combinations are not recommended ... their results are included for reference only.

3.4 MUTE

Depressing the "MUTE" pushbutton will reduce the MAIN OUTPUT level by 15 dB, regardless of the VOLUME control position.  It is convenient to use when the VOLUME control has been pre-set to a particular level.

3.5 MODE

3.5.1 Depressing the "LEFT ONLY" pushbutton will send information of the left channel input to both left and right outputs.

3.5.2 Similarly, depressing the "RIGHT ONLY" pushbutton will send right channel input information to both left and right outputs.

3.5.3 Depressing the "INPUT REV" (reverse) pushbutton crosses the channel's input information: left channel input information is fed to the right channel output, and right channel input information is sent to the left channel output.

3.5.4 Depressing the "MONO (MIX)" pushbutton will parallel left and right channel inputs, resulting in an identical main output for both channels.

3.6 PHONES

The "PHONES" socket on the front panel is to be used with stereo headphone sets of good efficiency and medium to high impedance only.  Headsets rated below 600 ohms will clip the SPA into premature protection and severe distortion (this will not damage the SPA).  The PHONES output is in parallel with the MAIN OUTPUT; therefore, headphones should be disconnected when using the MAIN OUTPUT (i.e. when operating a connected power amplifier).

3.7 BALANCE AND VOLUME CONTROLS

The potentiometers used for the BALANCE and VOLUME controls of the SPA are hand selected for excellent tracking and smooth, continuous signal transfer.  Heavy knurled knobs are custom built at our factory for ease of rotation and positive control.

3.7.1 Rotating the "BALANCE" control to the left or right diminishes the output level of the opposite channel.  From the 12 o'clock position, -1 60 degrees of rotation provides only +- 2 dB in level changes.  This affords the most scrutinizing balance adjustments.  Of course, channel extinction is achieved at the ends of rotation.

3.7.2 The MAIN OUTPUT level is adjusted by the "VOLUME" control.  Clockwise rotation increases the level.  One may notice this control to feel slightly more positive than the BALANCE control.  This is normal, as the VOLUME potentiometer contains four sections, while the BALANCE has two.

3.8 MAIN OUTPUT

The "MAIN OUTPUT" sockets are located in the recessed area toward the rear.  Connect them to the power amplifier via high quality phono jacks and cable.  See APPENDIX, Section 4.1.

3.9      POWER

3.9.1 The illuminated pushbutton "POWER" switch is located at the right end of the front panel.  When depressed, a timing circuit (approx. 15 seconds) flashes the lamp in the switch, indicating that POWER is on and the circuitry is stabilizing to its optimum operating range.  During this sequence, the MAIN OUTPUT remains off (i.e. shorted to ground).  On completion of the sequence, the lamp will stay on, denoting that the MAIN OUTPUT is on and the SPA preamplifier is ready to function.

3.9.2 With the SPA's timing circuit, you need not worry about turn-on pulses to your power amplifier should it be on as you depress the POWER switch on the SPA. (At most, a very small pop will be heard on completion of the timing sequence).  Nonetheless, it is good practice to turn the preamplifier on first; then, about 20 seconds later, the power amplifier.  Conversely, shut off the power amplifier first; then, about 20 seconds later, the preamplifier.  This is a fail-safe method for turning component systems on and off.
 

                                         SPA  Circuit Diagram
 

                                    SPA Power Supply Circuit

                           Pre Preamplifier as fitted to Leigh SPA
 
 

NOTE: Dayton Wright Group Ltd., offered a 'stacked board' #777 PrePreamplifier with
            Noise &  Distortion Figures comparable with the #999

        There was a major flaw when the SPA was designed by Leigh's engineers. There was a double capacitor + resistor network on both the low-level preamplifier outputs. When it was omitted, the output above 18 Hz, rises at 6 dB/octave and then the rise above 35 kHz is even steeper. The predecessor to the SPA reduced TIM by using a combination of feedback and feed though to lower phase shift thereby rendering the preamplifier unconditionally stable. When this network is not used, not only can the output stages overload at high levels, but the power amplifier has to have an excessively high slewing rate to avoid adding TIM itself.

        I had been about to add a muting switch to the switching system when Leigh bought the company. On the Leigh SPA, the holes are still there. A separate small switch similar to a leaf actuated unit. would be depressed every time any input was selected or changed. By using a quick acting reed relay and a circuit that would impose a small time delay after any input was selected, there wouldn't be any sound or pop.

        When Behnke bought out his predecessor, we started to encouter sheet metal problems. Benoma wasn't used to handling coordinate dimensioning and the rejects skyrocketed. We had been using THA sheet metal before, and they had used punched-tape-controlled presses like the Weidamen's I had become accustomed to using in Boston.

        We had to hand fit the parts together and, although the pc boards were fine, trying to make them fit the sheet mrtal was a nightmare! We has become accustomed to the use of 'go no-go' test fixtures in preperation for larger production runs,

4. APPENDIX A

4.1    INTERCONNECTING CABLES

The importance of cables cannot be stressed enough.  Most "off-the-shelf" audio cables are probably the weakest link in a high fidelity sound system.  They invite intermittent problems, hum, noise, RF interference, and can actually decrease overall sound quality even if functioning properly.  A few "professional" audio cables are surfacing on the market, offering superior phono jacks and cable, multi-shields, etc.  Some are very good, others completely unacceptable - all are costly.  Enthusiastic audiophiles can make their own, employing metal phono jacks (preferably with nylon insulation) and a high quality audio cable*.  As low as 12 picofarad/foot capacitance cable is now available commercially.  If one wants to raise the capacitance slightly to optimize the frequency response of a particular magnetic cartridge, small capacitors can be accommodated inside the phono jack shell.  In any case, always check the contact of the phono jack ground shell against the mating socket.  Crimp the flanges of the jack slightly to ensure a good contact.

Because of the inherently low impedance of moving coil cartridges, the capacitance of the cable is not as significant as proper shielding and good low resistance electrical connection.  The optimum cable for moving coil cartridges is therefore a cable with good shielding and low resistance such as RG58 or RG59.

Speaker wire requirements will vary depending on the amplifier and speakers used.  With a difficult speaker load (inductive, capacitive, or very low impedance), the amplifier would probably prefer to see (i.e. it will sound better) some added resistance by using, say, at least 20 feet of 18 gauge lamp cord to each speaker.  On the other hand, easily driven loads might yield better results with 10 feet of 14 gauge wire.  Varying these parameters affect the VI limiting of the amplifier and, inevitably, the sound quality. *See also SPA MAIN UNIT, Section 3.2.4.

4.2 SYSTEM INSTABILITY

Instability is analogous to microphone feedback encountered in PA systems.  Unfortunately, it is usually electronic in nature (as opposed to acoustical) and therefore not quite as easy to detect.  High frequency instability will exhibit certain characteristics.  These include overall increased distortion, apparent decrease of power or efficiency, overheating and/or shut-down of the power amplifier, and high frequency overload sensitive to increases in volume.  All of these conditions may be apparent on one or both channels.

· The problem can often be alleviated by adhering to a few rules for cable orientation and grounding.  Cables carrying different voltage levels should be kept separate.  For example, phono input leads can be grouped together; line level input leads in another group, preamplifier output leads in another, and speaker leads in another.  Also, where groups of wire cross each other, cross them at right angles.  In this manner the possibility of
· feedback induced into the more sensitive groups is minimized.  Ensure that there are no ground connections (be it wires or case to case contact) between the turntable, preamplifier, and power amplifier, save that of the signal grounds carried in the interconnecting cables and beneficial turntable/arm ground leads.  Of course, A.C. lines should be kept as far as possible from all other leads.  Any other grounds will only provide a feedback loop for oscillation to occur.  Even the ground pin on the A.C. line cord of a
 power amplifier may provide this path, as well as A.C. coupled loudspeaker systems, lest we forget the infamous "rack mount".

Severe instability problems of an oscillatory nature should be confronted by monitoring the power amplifier output with an oscilloscope.  High frequency envelopes will demonstrate the presence of instability and the effectiveness of the above procedures used to eliminate it.
 

                                                    Fig. 8 RF Filter
 
 

4.3 HUM, NOISE, AND RF INTERFERENCE

Stray magnetic fields and RF waves, the carriers of unwanted noise sources, are present almost everywhere.  Although high-gain, wide bandwidth components are more susceptible to picking up these nuisances, Dayton Wright has designed its circuit configurations to eliminate most of these problems.  Furthermore, if set-up procedures recommended in the previous sections of the appendix are followed, the chances of noise pick-up will most likely be extinct.

However, for the seemingly incurable noise problems, there are more steps one may take to alleviate them.  An extra layer or two of shielding over interconnecting cables will often solve the problem.  The shielding is available as "braided tinned copper" in various diameters.  Each cable pair should be fished through the braiding.  Do not electrically connect the braid to the signal grounds of the cables.  Instead, ground the braid to the case of the unit to which the cables terminate.  By determining which input picks up the noise, you need shield only that particular cable pair.  Cable and unit orientation is another possible solution.  You have already grouped the various levels of cables'.  Now, by merely twisting and turning the cables coming from, say, a turntable, hum and RF can disappear completely.  Similarly, moving a unit a few inches up or down, or to the sides, can improve the situation 2. When all else fails, an effective RF filter can be constructed in a simple metal case, employing RF chokes and feed-through capacitors in series. (See diagram for an appropriate configuration).  This type of filter can be inserted between any source unit and the preamplifier.

See APPENDIX, A - System Instability.

· See also Placement of the Power Module, Section 2.3.
 

4.4 THE DAYTON WRIGHT TROUBLESHOOTING FLOW CHART

Locating the source of a problem is often more difficult than correcting the fault itself, considering the fact that so many problems in a system can be traced to poor connections or cables.  Therefore, the purpose of this section is not a course on learning how to repair (?) your precious components, but a step by step procedure for tracking down a faulty component or cable.  At that point, you will know whether you can correct the problem yourself or not.  If you have a malfunctioning or absent channel, this procedure will prove invaluable to you.

The Dayton Wright Troubleshooting Flow Chart tells all, but please read the instructions below before proceeding to the chart itself.

The SPA Flow Chart

HOW TO USE THE FLOW CHART

First make note of the channel (i.e. the loudspeaker) on which the fault occurs.  Each trilogy of boxes in the flow chart requires you to cross the channels at a particular stage in the set-up.  By observing whether the fault remains on the same loudspeaker, or if it crosses over to the other loudspeaker, you will have either located the fault, or continue on to the next box, respectively.  Be sure to cross back the wires to their original position before performing the next trilogy.  This will maintain your reference faulty channel.

If the fault occurs only within a particular stage or input of the preamplifier you may proceed directly to boxes 13, 13B, or 13C, according to the stage in question (line-level, magnetic phono, or moving coil phono, respectively)
 

4. APPENDIX B

The SPA MK II Premplifier with LED level indicators.

5. APPENDIX C

Preamp modifications:

      Leigh's SPA had some problems. I, in 1975, had laid out a prototype of a new preamp using part of the old SPL board. In March 1975, I changed the very high frequency part of our RIAA compensation by adding another RC circuit in parallel with the existing circuit. I had been usinga feed-forward phase compensation circuit to insure greater stability. Without a passive RC circuit following the RIAA compensation, the grequency response rose above 18 Khz at 6 Db/octive. This could exceed the permissible slewing rate of some Power Amplifiers - leading to supersonic overload and the conquent clipping of transients.

        I found that an RC network was needed to insure  that the RIAA curve was followed over 17 Khz. This network was loaded with a parrallel RC network.

        The values in the series network are a 4n7 capacitor (mylar) and a hundred ohm resistor.

        The values for the shunt RC network are a 7n3 capacitor in parallel with a 47,000 ohm resistor. These can be seen more easily on the enlarged photo on the right hand side. (Right click on the photo to see the enlargement).

        The resistors can be 5% deposited resistors as 2% metal film don't make any difference. Avoid wirewound resistors for the low value resistors as their inductance can cause erratic peaking!

        About the same time, I had designed a switching transient suppressor to insure that a sudden change in minor DC levels would not be propagated to the Power Amplifier. The preamp already had a switch that was actuated any time another input was selected; however it needed an additional time delay to efficiently surpress any "CLICK". Therefore, I added a storage capacitor to the reed-relay that already was there to insure that there was about a minute delay  after the preamp power was switched on. (this is why the power light blinks when the preamp is turned on). The extra pair of leads 'mutes' the output whenever any change is made to the input selector.

        Unfortulately, when LEIGH took over control they ignored my prototypes, and, supposededly because the guy that made the pen and ink layout for the PC boards (I switched to red and blue layout tape in 1968 - but LEIGH didn't change until the early '80's) had taken a lunch break when he was on the RIAA section of the PC board.

        When I first looked over the SPA in 1977 (at a dealer in Toronto) I was suprised. I called a friend who had worked for me in 1975 and told him. He was the chap who let me know about the draftsman. He was well aware of complaints about the SPA sounding 'glassy' on many amplifiers and the sometimes loudspeaker-blowing 'clicks' whenever an input was changed. But he said that he was out of the loop now as was Dave Reich (who founded "ClassA" when he left Leigh.
 

© 1980, 1999 Wright Electroacoustics

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