YORKVILLE AP-1200 Service Manual

1200

HIGH EFFICIENCY STEREO POWER AMPLIFIER

WORLD HEADQUARTERS

CANADA

Yorkville Sound

550 Granite Court
Pickering, Ontario

L1W-3Y8 CANADA

Voice: [phone]

Fax: [phone]

Quality and Innovation Since 1963

U.S.A.

Yorkville Sound Inc.

4625 Witmer Industrial Estate

Niagara Falls, New York

14305 USA

Voice: [phone]

Fax: [phone]

SERVICE MANUAL

Printed in Canada

IMPORTANT SAFETY INSTRUCTIONS

AUDIOPRO 1200 SERVICE MANUAL

M1050 “THE INPUT BOARD”

The input board processes the audio signal from the input jacks to the voltage amplifier board, (M1049).
Each channel consists of a balanced gain stage, defeatable bass boost filter, and a pre-emphasis filter

network.

INSTRUCTIONS PERTAINING TO A

RISK OF FIRE, ELECTRIC SHOCK, OR

INJURY TO PERSONS.

CAUTION:

TO REDUCE THE RISK OF ELECTRIC SHOCK,

DO NOT REMOVE COVER (OR BACK).

NO USER SERVICEABLE PARTS INSIDE.

REFER SERVICING TO QUALIFIED

SERVICE PERSONNEL.

Read Instructions:

The

Owner's Manual

before operation of your unit.

instructions for future reference.

Packaging:

Keep the box and packaging materials, in case the unit
needs to be returned for service.

Warning:

When using electric products, basic precautions should
always be followed, including the following:

Power Sources:

Your unit should be connected to a power source only of the
voltage specified in the owners manual or as marked on the unit.
This unit has a polarized plug. Do not use with an extension cord or
receptacle unless all three blades can be fully inserted to prevent
blade exposure. Precautions should be taken so that the grounding
scheme on the unit is not defeated.

Power Cord:

The AC supply cord should be routed so that it is unlikely that it will
be damaged. If the AC supply cord is damaged

OPERATE THE UNIT

Service:

The unit should be serviced only by qualified service personnel.

should be read and understood

Please, save these

DO NOT

.

INSTRUCTIONS RELATIVES AU

RISQUE DE FEU, CHOC ÉLECTRIQUE,

OU BLESSURES AUX PERSONNES.

AVIS:

AFIN DE RÉDUIRE LES RISQUE DE CHOC

ÉLECTRIQUE, N'ENLEVEZ PAS LE COUVERT (OU

LE PANNEAU ARRIÈRE). NE CONTIENT AUCUNE

PIÈCE RÉPARABLE PAR L'UTILISATEUR.

CONSULTEZ UN TECHNICIEN

QUALIFIÉ POUR L'ENTRETIENT.

Veuillez lire le manuel:

Il contient des informations qui devraient êtres comprises
avant l'opération de votre appareil. Conservez S.V.P. ces
instructions pour consultations ultérieures

Emballage:

Conservez la boite au cas ou l'appareil devait être
retourner pour réparation.

Warning:

Attention: Lors de l'utilisation de produits électrique,
assurez-vous d'adhérer à des précautions de bases
incluant celle qui suivent:

Alimentation:

L'appareil ne doit être branché qu'à une source d'alimentation
correspondant au voltage spécifié dans le manuel ou tel qu'indiqué
sur l'appareil. Cet appareil est équipé d'une prise d'alimentation
polarisée. Ne pas utiliser cet appareil avec un cordon de
raccordement à moins qu'il soit possible d'insérer complètement
les trois lames. Des précautions doivent êtres prises afin d'eviter
que le système de mise à la terre de l'appareil ne soit désengagé.

Cordon d'alimentation:

Évitez d'endommager le cordon d'alimentation.

L'APPAREIL

si le cordon d'alimentation est endommagé.

Service:

Consultez un technicien qualifié pour l'entretien de votre appareil.

N'UTILISEZ PAS

Looking at the right channel, the balanced input, (XLR Jack) and unbalanced input (phone jack) are wired in

parallel to the input of a balanced operational amplifier, (U1). The gain of this stage is 0.8 (-1dB) balanced and

1.4 (3dB) unbalanced. Resistors R25, R24, R26 and R27 along with capacitors C23 and C24 form a radio
interference elimination filter.

Switch S2B selects a flat or bass boosted frequency response. The bass boost filter provides a 20Hz
high pass, high Q filter response with a +4dB peak at 55Hz. The filter consists of a tee network on the input
of U1A along with R17, R18, R19, C7 and C8. The gain is 1 (0dB) in the passband, (above 100Hz).

Operational amplifier U3A is a high pass shelving filter with a +2dB shelf above 20KHz. This filter provides
the pre-emphasis required to obtain a flat frequency response (to 20KHz) on the power amplifier output at full power.

The audio signal out of U3A passes through the volume control P1 and the desired level enters U5A through
pin 2. U5A is set for a gain of 7 (16.8dB) when the volume control is in the fully clockwise position. The AUDIOPRO
1200's defeatable limiter is built around U5. The CLM50 is a opto-resistive cell comprising of a led that shines on
a photocell. As the led in the CLM50 becomes brighter the resistance of the photocell decreases, placing more of
the audio signal on pin 3 (non inverting input) of U5A. This voltage gets subtracted from the signal on the inverting
input and less signal appears on the output of U5A. Transistors Q2 and Q3 along with the surrounding passive parts
provide the attack and release time constants of the limiting function along with the drive currents for the clip led
and CLM50 led. When the voltage amplifier output of the AUDIOPRO 1200 goes into clipping (the peak of the signal
comes within 2 volts of the DC power supply rail), pulses representing the clipped position of the signal appear
at RCLIP (across C7L on M1049) shown in figure #1. These pulses turn on transistor Q2, and Q2 provides
current pulses into clip led LD1. The RCLIP pulses also pass through R70 and D1 to change C42 and C37.
When the voltage across C42 reaches 0.5V then Q3 turns on providing a current into the led of the CLM50
limiting the audio signal at U5A. The charging (attack) and discharge (release) times of the limiter are 80mS
and 3.5 seconds respectively. Resistors R38 and R70 provide the charging paths while R39 provides the
discharging path. The limiter can be defeated by placing the limiter switch (SW1) in the out position which
disconnects Q3 and the charging/discharging circuitry from V+. The activity led circuit consists of Q8 and
the surrounding circuitry. The audio signal enters the activity led circuit through R52. R52 and C3 are a
differentiator that turns on Q8 whenever the audio signal increases in amplitude. A constant current flows
through R69 and when Q8 is off, the collector current flows through D7.

FIGURE #1 AP1200 Clip Signal

safe_v3.doc Version 3.5 Mar 98

PAGE 1

The stereo mono bridge switch (SW3) receives the audio signal from operational amplifier U5A (right
channel), and U5B (left channel). In stereo mode the left and right channels pass through the switch to the power
amplifier sections. In mono mode the left channel audio is sent to both power amplifier sections, and in bridge mode
the left channel audio is inverted by operational amplifier U2A and is sent to the right channel power amplifier
(RSIG) while the left channel audio is directly sent to the left channel power amplifier.

deliver through the "kill" signal line a positive current to turn Q37 on and turn Q36 off to turn off the relay
and disable the voltage amplifiers. When the temperature of the amplifier has been cooled down by the fan,
then the kill signal will disappear and the relay circuit will turn on the relay to resume normal operation. Anytime
the relay is in the "protect" mode (due to the abnormal states) then contact pin 8 of the relay will illuminate
LD5 (the protect LED on the front panel).

M1049 "VOLTAGE AMPLIFIERS AND SYSTEM CONTROL"

This board contains:

- Voltage amplifiers to drive the current amplifiers on the M1048 board.

- Driver circuitry for the amplifier disable relay (used during amplifier turn on, turn off, thermal shutdown and current
limiting).

- Low voltage power supplies.

Circuit Explanation:
Refer to the schematic of M1049 as the sections of the circuit are explained.

The audio signal enters the board from M1050 through connector MW4. The two channels are marked
"LSIG" and "RSIG" for left and right. The signals are to be considered as differential sources and therefore are marked
as LSIG, LREF, RSIG and RREF. Since the left channel has the same topology as the right channel we will only look
at the left channel.

The signal ("LSIG") at the terminal block (MW4) enters operational amplifier U1L through R1L and R2L.
Voltage Amplifier:

The Audiopro 1200 has the unique feature of delivering 600 Watts per channel into 2 or 4 ohms. A wire
and spade clip assembly on the power supply board (M1051) selects the tap on the primary of the power transformer
for 2 or 4 ohm mode. The power supply voltages for 4 ohm mode are higher than for 2 ohm mode. The power
supply voltages are listed below.

CIRCUIT DESCRIPTION

Here is a detailed circuit description of the voltage amplifier looking at the left channel amplifier shown

on schematic M1049.

The audio signal from M1050 enters M1049 as LSIG and LREF. LREF is a signal referenced ground
from board M1050. LSIG and LREF enter U1L (pin 2 & 3) as a differential signal. The output (pin 1) is connected
to the other half of U1 which is configured as a inverting amplifier with a gain of 1. The output of the two
halves of U1L differentially drives the differential amplifier consisting of Q3L and Q4L. As the audio signal
causes pin 1 of U1 to go positive, pin 7 of U1 is going negative. Pin 1 going positive results in a current flowing
through R14L and R15L. This turns Q4L on more increasing Q4L's collector current therefore decreasing
Q3L's collector current from the shared current source of Q1L, R20L, D3L and D4L. (With no audio signal
Q3L and Q4L share a 1mA constant current from Q1L). As Q4L's collector current increases the voltage
across R18L increases which through the base emitter junction of Q5L causes a greater voltage across R15L.
This results in a greater collector current in Q5L. Q5L's collector is connected to the collector of Q2L. Q2L
is configured as a constant current source like Q1L sourcing a current of approximately 3mA. (If Q5L's
collector current is constant then the voltage at the junction of the two collectors will swing towards Q5L).
Since Q5L provides a current from the positive rail, the voltage on the collector of Q5L will swing positive.

Transistor Q6L is an emitter follower that buffers the signal at the junction of the two collector to provide
a lower impedance output at test point 2. The voltage at TP2 will always be approximately 0.6V more positive
than the voltage at the junction of the two collectors because of the base emitter junction of Q6L.

QUIESCENT CONDITION:

This design is class A/B and therefore the output driver transistors must be forward biased to provide low
crossover distortion. In most class A/B designs, a diode chain or VBE multiplier is used to control the bias voltage
and provide a means of adjusting the bias. This design is different as there isn’t a diode chain or VBE multiplier.

Note: Voltages are measured with 120VAC on the power cord.
For the ease of circuit explanation, the amplifier is configured for 2 ohm mode. (factory preset).

The voltage amplifier amplifies the audio signal's voltage from 4.8 volts peak (at the output of U1L) to
approximently 97v peak which is required to drive the current amplifier board M1048. M1048 provides the
current required for the 97v peak signal to drive 600 watts into 2 ohms out of the binding posts.

Op amp U1 is an inverting amplifier with a set gain of 1. The voltage amplifier consists of a dual operational amplifier
configured to differentially drive a transistor differential amplifier. The output of the differential amplifier is
single ended and is buffered by an emitter follower. The emitter follower output drives a self biasing
complementary mosfet pair which drives the output transistors on M1048.

SHUTDOWN CIRCUIT:

The last circuit on board M1049 is the shutdown relay and its associated drive circuitry. The relay circuit
has two possible operating states.

PAGE 2

Refer to simplified schematic #1. To bias the output devices, greater than 0.5V is needed from base to
emitter (or simplicity from base to amplifier output). Points A and B are at the same potential, so consider them
to be connected. If this is true then 2 times 0.5V from point 'C' to point 'D' must appear across R43L. There must
be some way of developing this voltage across R43L and there is using mosfet Q13L driver along with local
feedback.

Simplified schematic Fig. #1 shows the biasing circuit. The current needed to develop 2 times 0.5V across
R43L comes from the source of Q13L. When the amplifier is first turned on the current source (Q12L) turns
on Q13L and current flows through R43L developing a voltage. When this voltage approaches 2 times 0.5V
Q11L turns on and robs current from the gate of Q13L.

This causes Q13L to turn off until the reduced current flowing through Q13L maintains 2 times 0.5V
across R43L. Q11L will turn off slightly causing Q13L to increase its source current. The circuit reaches a
point of equilibrium with approximately 2 times 0.5V across R43L.

Because all output devices are not identical and base emitter voltages vary, some adjustment must be
available to slightly adjust the 2 times 0.5V across R43L.

A voltage divider is formed by R42L, Q10L, trim pot RT1L, Q9L and R35L. Transistors Q9L and Q10L
are connected as diodes. There is approximently 1.6 volts from the base of Q11L to the base of Q8L. As
RT1L is turned clockwise this voltage decreases turning Q8L and Q11L off slightly increasing the gate voltage

PAGE 3

SIMPLIFIED SCHEMATIC #1

on Q13L and Q14L. This increases the voltage across R43L thus increasing the voltage from base to base
of the output transistors (Q4L and Q9L), increasing the bias current through these transistors. The bias current
can be measured across test points 3 and 4 on board M1048.

As the output transistors heat up their base emitter voltages drop increasing the voltage across the emitter
resistors R9L and R18L. To maintain the same bias current, the voltage across R9L and R18L is fed back to
the bias circuit. A voltage increasing across R9L and R18L results in an increasing voltage across the bases
of Q8L and Q11L through R41L, Q10L, R38L and Q9L. Q8L and Q11L turn on more and lower the voltage
across the gates of Q13L and Q14L decreasing the bias on the output stage.

M1048 “CURRENT AMPLIFIER BOARD”

The current amplifier board (M1048) receives a high voltage audio signal from voltage amplifier board
(M1049) and provides the current drive necessary to drive speaker cabinets.

The current amplifier is a two tier complimentary output driver design controlled by a complimentary mos­fet stage.

CIRCUIT DESCRIPTION - REFER TO THE SIMPLIFIED SCHEMATIC #1 ON THE FOLLOWING PAGE

THE SECOND TIER:

The tier switching circuitry is mirrored image with circuitry connected to the positive power supply rail being
identical but of opposite polarity to the circuitry connected to the negative supply rail. For this reason we will look
at the positive side of the amplifier.

Refer to the simplified schematic Fig. #1 while reading the following text. One way of making an amplifier
more efficient is to vary the Power Supply Voltage on the collectors of the output transistors (Q4L & Q9L). The lower
the voltage from collector to emitter, the lower the device dissipation. During quiescent conditions, there is 47VDC
on the collectors of output transistors Q4L and Q9L. The peak AC voltage that can appear on the amplifier’s output
is approximately 66V peak. How can an output transistor deliver a 66V peak when its collector is only at 47VDC?
It can if its collector is pulled up to 85VDC as the output signal’s peak rises above 47VDC. (refer to Fig. #2).
The second tier voltage must remain above the amplifier’s output voltage by amount Vm. Therefore the circuitry
controlling the second tier voltage must increase the tier voltage before the amplifier’s output voltage reaches
47VDC. This leading voltage is necessary to compensate for time lag of the second tier circuit during fast rising
amplifier output signals.

PAGE 5

FIGURE #2 2ND TIER VOLTAGE

+85V

+47V

SPEAKER OUTPUT

-47V

-85V

peak output signal is about 31.5vp (47v-(12v-3.5v)) then Q1L will start to turn on the second tier. The second tier
voltage will remain about 11 volts (Vm) above the peak of the output signal to the point of clipping where this voltage
is reduced to about 4 volts. Zener ZD1L protects the gate source junction of Q1L and also provides a current path
through R6L for the “floating battery”.

The voltage between the ampli­fier's output and test point E is
approximately 12VDC derived
from the voltage drop across
ZD2L. We call this voltage the
“floating battery” because it floats
on top of the output audio signal
with point E always being 12VDC
greater than the peak of the
output signal. Point E drives the
gate of mos-fet Q1L. Q1L con­trols the transistors of the upper
tier. As Q1L turns on it’s source
foward biases the base of Q2L
and Q2L pulls the collector of
Q4L towards the 85 volt rail.
The gate to source voltage
needed to turn on Q1L is ap­proximately 3.5 volts. When the

Current Limit Protection Circuitry

FIGURE #3 AP1200 Current limiting into a shorted load

the firing of Q16L at low frequencies.

DC Protection

If a DC voltage greater than 8 volts appears on the output of the amplifier for more than 200 milliseconds
then triac Q12L will turn on holding the output at ground potential. MBS4992 is a device that turns on at either +
or - 8 volts DC.

To have an amplifier drive 1200 watts into practically any combination of speaker cabinets and know what
is a safe load and what is not is a very difficult task. An extensive amount of time was spent on the current limit circuitry
so that it may simulate the safe operating area of the output transistors (SOAR curve). No matter how reactive the
load may be the phase shift that it presents, along with it’s resistive component is used to set the output current limit
of the output transistor stage.

Refer to the schematic of board M1048 and M1049 while reading the following text. The current limit
circuitry is a mirrored image with circuitry connected to the positive power supply rail being identical (but opposite
polarity) to the circuitry connected to the negative power supply rail. For this reason we will look at the positive side
of the circuitry.

Transistor Q16L measures the peak current flowing through Q4L's emitter resistor R9L. The voltage
across R9L (as a result of the current flowing through it) is scaled down by R52L, R47L, R48L, R49L, D24L and
R50L- these parts make up the safe operating area along with the time constants of C17L, R46L, C18L and
R51L. Fig. #3 shows a waveform of the current that passes through R9L and R18L when the output of the
amplifier is shorted to ground. This can only be seen by using an oscilloscope to measure differentially across
R9L and R18L. The conditions of the measurement are contained on the diagram. During current limit when
Q16L turns on it reduces the voltage across R54L. R54L is in series with a 27V zener ZD5L. R54L is also in
parallel with the LED of a opto-coupler U3L. U3L's LED is normally kept on by the current flowing through
R45L, ZD5L and R55L. When current limiting occurs on the amplifier's output, Q16L turns on and reduces
the voltage across ZD5L and R54L to below 29V (27V zener + 2V LED). The current through ZD5L stops flowing
and lthe LED in U3L turns off. This turns off the normally saturated transistor in U3L, causing the "OVER_L"
voltage to increase positively as C7 charges. If current limiting continues then C7 will charge until Q6 turns
on, tripping the relay circuit. As soon as the relay is turned off the audio signal will be turned off at the voltage
amplifiers and will remain off for about 5 seconds before the relay turns on and allows the audio signal to
pass through the amplifier. If a current limit condition is still present then the whole cycle will occur again and
repeat until the load conditions on the amplifier's output are safe for the amplifier. When a safe load reappears
the amplfier will automatically reset and drive that load (the speaker cabinet). Subwoofers present large
inductive loads to the amplifier and are driven at low frequencies where the large current peaks must be
tolerated for short periods of time. To accomodate this type of loading C18L and R51L are used to retard

NOTE:
Everytime you replace blown

output transistors on a M1048
board test the DC protection
triac with the following circuit
.

Conditions of test:
A) Pass a 100Hz 25v peak

signal through the M1048
board under test with no load
connected to the amplifier out­put.

B) Connect points 1 and 2 as shown in the diagram. The amplifier should go into protect mode as the triac
( if working) shorted the output of the amplifier to ground, and the amplifier goes into current limit.

C) Disconnect the triac test circuit and allow the amplifier to complete it's protect cycle.
D) Reverse connections 1 to 2 and 2 to 1 and test again. The same results as in B) should be observed if the triac

is working.
Only test the triac for one protect cycle as prolonged testing will heat the triac to a high temperature.

TRIAC CIRCUIT UNDER TEST

9 VOLT BATTERY

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