Fisher BA 4500 Brochure

Page 1

FISHER
BA4500

Professional-Type
Power Amplifier

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All stages pure complementary symmetrical SEPP OCL circuitry delivers
a full 300W RMS output power at 8 ohms, both channels driven from 20

to 20,000Hz with only 0.02% T.H.D. This advanced circuitry suppresses
noise so completely that the S/ N ratio of this unit is an amazing 105 dB.
The super-high SUV/p s slew rate also ensures superior handling of
rapid signal changes, thereby suppressing transient intermodulation

distortion.

SF-2610

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Our Goal: Dynamism but Virtually No Transient Intermodulation Distortion.

The ideal circuitry

Designed to eradicate transient inter-
modulation distortion, the BA 4500
employs state-of-the-art circuitry in all
stages of its power amplifier circuitry.
This all stages direct coupled SEPP OCL
circuitry includes a first-stage differential
amp, second-stage emitter follower, a
two-stage direct coupled Darlington pre-
drive circuit, a triple-push pull output
stage, 21 transistors and 2 diodes per
channeL

All Stages Pure Complementary
Symmetrical Push-Pull Circuit

To eliminate shock noise during switching
and ensure stable voltage at the output
terminal, the PNP-NPN transistors of all
stages are arranged in a purely sym-
metrical design. In this complementary
structure, Class A Operation is employed
in the stages up to the 2-stage direct
coupled Darlington pre-drive circuit.
while Class B Operation is used in the
drive and triple push-pull output stage.

Extra First-Stage Dynamic Range

Most amps are planned in terms of
output, with the result that the first-stage
differential amp is designed for high gain.
not large dynamic range. In the BA 4500.
however, the complementary push-pull
differential structure of the first stage
guarantees more than enough gain, so
that, even though they lower gain slightly.
high-emitter resistor and local negative a

I feedback loops can be employed to
extend the dynamic range. This design.
coupled with 1mA operating current and
the advance phase corrector incorporated
in this stage, ensure a dynamic range
that easily extends into the ultra-high
frequency range.

Limiting Negative Feedback

In a first-stage differential amp with a
negative feedback loop, the actual input
voltageis equal to the difference between
input from the program source and negative
feedback. In areas of the frequency range
where input signals are weak, however.
feedback is proportionally lower. This, in
turn, means that the actual input voltage
increases in this range, generating a large
amount of transient intermodulation
distortion. For example. if the rated input
voltage of an amp is 1V and gain is 30dB.
when negative feedback is 50dB, a full
0.997V is returned and the actual first-

stage input voltage is 3mV. However, in
the ranges where bare performance drops
to 40dB, negative feedback drops to 10dB.
so only 700mV is returned and actual input
voltage becomes a whopping 300mV-a
hundred times what it should be. Even if
this phenomenon occurs in the ultra-sonic
range, superposed signals will be nullified,
generating 100% transient intermodulation
distortion. Thus the greater the negative
feedback employed, the greater the
difference in feedback volume in these
ranges and the higher the transient
intermodulation distortion. To improve
overall performance. then, it is vital to both
extend the dynamic range of the first
stage and to reduce negative feedback.

3 Dee NFB W/O NFB
A:
W NFB
b] Light NFB W/O NFB
A W NFB

Transient intermodulation distortion is a
transient characteristic that cannot be
detected in standard static characteristics.
which are frequency responses and
distortion measured by simple sine waves
etc. It is clear, however, from the waveform
of the collector output, that TID is not
suppressed by negative feedback in the
first stage. When the slew rate is small
(that is, when response to the input signal
is slow-in terms of microseconds). even
though the first stage receives input, the
stages beyond it do not operate until the
input signal finally reaches them. Accord-
ingly. the signal is not amplified, and no
negative feedback returns to the first
stage. This means that 100% TID is
generated in the first stage, and the more
negative feedbacks employed. the greater
the loss of expected negative input, so
therefore, the larger the enlarged input.

Super Slew Rate-BOV/us

The slew rate mentioned above is a
measure of the time that energy-storing
elements like condensers need to charge
up or to release electricity. This rate is
expressed in terms of V,Jusl' which
stands for the voltage that can be received
or released in a microsecond. In more

basic terms. this represents the slope of
an amps sound production and cut-off
rates. An amps required slew rate is
proportional to its power output and the
highest cut-off frequency of the frequency
response. The higher the slew rate, the
less chance of clipping distortion, even
when output is large and signals are
quick and ultra-high.

The slew rate for any amp stage is deter-
mined by the amount of current in the
preceeding stage and the input capacity
of the following stage. To raise the slew
rate. then, the amount of current in the
preceeding stage has to be increased
and/or the input capacity of the following
stage has to be decreased. By doing both
for each stage of the BA 4500, the
unprecended slew rate of GOV/us was
achieved.

Slew rate comparison

BA4500
V/us wow-sous

60
MakerA
125W-17V445

50*

4O 4
Maker C

250W-11Vils

30*

Slew Rate

20+

Maker 0

600W~16V/}15
101

o 100 200 300 400 500 600 ' (w;
Power

More on Negative Feedback

f an amp relies on negative feedback to
improve performance, it has to employ a
high-cut condenser called a phase corrector
to prevent oscillation. This, however.
lowers bare characteristics and slew rate
and increases actual input power, so
transient intermodulation distortion can
occur even at small input levels. Moreover.
most negative feedback loops are used on
units with poor bare characteristics, and
though feedback is ample in the mid-
range, it becomes low at the ends of the
frequency range. In this type of system,
distortion may be low at 'lkHz. but it tends
to be amplified in the range over 10kHz.
In the BA 4500, bare characteristics were
improved as far as possible. then a weak
negative feedback loop-only 25dB-was
attached, to ensure that the 20kHz range
would actually be better than the 1kHz
range at output up to rated power. This
means the power bandwidth is an
impressive '10Hz to 50kHz. This low

negative feedback and super-high slew

rate

also mean minimal TID, and almost

no total harmonic distortion.

(dB)

'60

'80

80

'lllD characteristic
(Analysis for components for each harmonic order)
Fundamental OdB ' tkHz

1W

0.01%

10W

100W

150W

ii. ..

50:18 9101'11'21'31'41'5
Harmonics Order

3

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Frequency response characteristics

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70
60 W10 NFB
50

40

WNFBtZSdBl

30

20

10

(HZ)

THD characteristics W NFB (ZSdB)

1 10 100 (W)

Ci

Power bandwidth characteristics

at 75W that! power)

100 1k 0 10
My

Second-Stage Emitter Follower

The second-stage emitter follower was
designed with a low impedance level to
decrease the load on the first-stage
differential amp while ensuring ample
drive for the 3rd stage. This design improves
the'first-stago S/N ratio and makes the
second-stage ideal for impedance
matching with the pre-drive stage. Its
slew rate. though determined by the
amount of current in the first-stage and
tie input capacity of the pre-drive stage.
is ideal, since there is no mirror capacity.
c ue to the emitter follower. The emitter
follower can also relatively freely select
trie amount of current. and allows an
ample flow of 3mA. which helps improve
tie slew rate of the third stage. This stage
also employs a local negative feedback
loop to improve bare characteristics.

High Speed Response Circuitry

The third stage, which corresponds to the
drive stage, normally is common emitter
complementary push-pull circuitry with
ample gain and local negative feedback.
A bias circuit stabilized by use of transistors
is also employed. Because the circuitry is
complementary, the two sides operate
together as load and constant current
circuit. This allows the elimination of the
boot strap circuit, which reduces output
in the lower ranges. Since it has to drive
and to deliver gain, this stage is susceptible
to distortion. However, by splitting high
resistance to this emitter in half and using
a 25dB local negative feedback loop on
the 55dB basic gain distortion is suppressed.
With 1kHz resistance, this stage also
employs a speed up condenser that
acts as a high speed response circuit.

This charge passes through the emitter
and releases energy in the opposite direc-
tion to make the slew rate even higher.

All stages up to and including this stage
employ Class A Operation. Following this
stage. a drive stage with the ample idling
current of 'IOmA is employed. Between
these stages. an advance phase corrector
is employed.

Advance Phase Corrector

If the phase of a signal is delayed. cut-off
frequency will be lower. whereas if it is
advanced, cut-off frequency will be higher.
The advance phase corrector incorpo-
rated between the Darlington pre-drive
stage and the drive stage of the BA 4500
operates like that employed in the first-
stage differential amp, improving bare
characteristics by emphasizing the ultra-
high frequency range in proportion to the
amount it is reduced in the drive stage.
This circuit acts as a real phase corrector.
unlike many other elements called by that
name. which usually refers to nothing
more than a condenser inserted between
base correcters. Such a circuit does not
correct phase, it delays phase and is a
detrimental structure that just acts as a
high filter. Moreover, its mirror capacity
becomes a monstrous input capacity 13
times that of the transistors.

Frequency msponse characteristics

(dB!

0

'4

6

12

-16

20

24

28

t 10 100 100k 1M
10k IHzI

300W Output Stage
With Class A Operation at Low
Output Levels

The output stage of the BA 4500 is pure
complementary push-pull circuitry
employing a Darlington emitter follower
with Pc100W power transistors. Output
power flows through protective / muting
circuitry. and reaches a massive 150W
+150W, both channels driven at 8 ohms.
20-20,000Hz. With its large idling
current of 200mA, this stage uses Class A

Operation for levels up to 1W. This Class
A push-pull circuitry allows no change in
distortion when output level is low. Thus.
low-level output is truly impressive.

The emitter resistor which. as in the first
stage. also serves as a protector against
overload. keeps the balance of the median
electric potential neutral. The all stages
push-pull structure means that output
voltage remains stable within iIOmV at
all temperatures and despite line voltage
fluctuation of up to i20%.

Three Power Supplies

Three types of power supply circuitry are
used in the BA 4500. The i55V power
supply for the output stage sends enough
energy from a 600VA large cut core trans-
former (exciting current of less than
100mA) to a high-capacity bridge rectifier
diode. then smooths them out with two
40.000 HF - 70WV high-capacity condensers.
The iBOV power supply for the pre-drive
stage is a load voltage detector type
constant voltage circuit that rectifies
using 2 independent windings. The third
type of power supply is the i30V power
supply for elements like the sub-sonic
filter and protector circuit. 9 diodes and 6
transistors are incorporated in the power
supplies of the BA 4500.

The continuous output power of 150W

+ "150W at 8 ohms with both channels
driven, which is ensured by the ultra-
large power supply delivered by the
BOOVA large cut core transformer and
40.000uF - 70WV condensers, gives a
feeling of infinite scale. It also provides
peak actual music power of an extra
200W. The BA 4500 is truly a power
amp-totally free of restraint or clipping.

The Inaudible Sub-Sonic Filter

The sub-sonic filter of the BA 4500 has a
cut-off frequency of '16Hz (18dB/oct).
Instead of a conventional emitter follower.
it incorporates a PNP-NPN 2-stage direct
coupled design. 3 transistors per channel
are used. It is an active filter using an
amp with OdB gain ensured by 100%
negative feedback. This minimizes distor-
tion increase during large-scale amplifica~
tion. Its distortion-free output voltage is
over 15V RMS. When turned on, the
frequency range outside its control sounds
virtually as if there were no filter. And
when turned off. the circuit is completely
bypassed.

Frequency response characteristics (leB Oct)

(dB
0

-1O

-20

T 1 10 too 1k t er)

'IHD characteristics vs. output voltage
mo

002

001

01 i 10 (V)

THD characteristics vs. frequency
(9%)

01

002

001

10 100 1k 10k 20'! (Hz)

7 Types of Protective Circuitry

The BA 4500 provides 5 types of protective
circuits for the amp itself and 2 types for
the speaker system. To protect the amp. a
relay effectively turns the amp off if the
unit is turned on when the load is shorted
out. If the load short circuits during use.
the power line fuse will blow. Moreover.
if the fuse doesn't blow, or during the time
it takes to blow out, the current limiter
makes sure that the transistor will not
(S/B) breakdown. or the current overload
detector switches the relay off. If speaker
impedance is less than 2 ohms. a special
detector circuit shuts the relay off.

To protect the speakers. a circuit turns
the relay off if the DC voltage at the
output terminal changes more than iZV.
The power supply muting circuit also
keeps the relay off for 4 seconds after the
amp has been turned on.

The power limiter can be switched
between 4 settings: off, 50W, 100W and
150W. When the selected power is
exceeded and clipping occurs. the
clipping indicator lights up. The meter
range can also be set at 4 levels: off. 0dB.
-10dB and ZOdB.

Superior Bare Characteristics

The BA 4500 uses the best materials. and
each stage employs a local negative feed-
back 100p to tremendously improve the
bare frequency and distortion character-
istics of each stage. This means bare
characteristics that barely require overall
negative feedback. The Class A Operation
used in the stages up to and including
the pre-drive stage means that these bare
distortion characteristics reach the limits
of measurement. The remaining Class B
Operation stages mean just minimal
distortion caused by a slight imbalance of
the actual characteristics of PNP-NPN
transistors in the push-pull circuitry. To
suppress this, minimal overall negative
feedback of 25dB is employed. This
means surprisingly low distortion. in
addition to high power without TID.
thanks to the wide dynamic range of the
first stage, the low overall negative feed-
back and the mammoth SUV/us slew rate.
The result is high fidelity in every sense
of the word.

'lHD characteristics W"0 NFB

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05

02

0.1

O 1 10 100 (W)

lndication error of volume control

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DEVIATION

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