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H.H.
Scott Type 260 Integrated Amplifier
The Stereomaster 260 was the first all-silicon, capacitively
coupled solid state amplifier to be introduced as a consumer
item by Scott. It had little in common with its predecessor,
the transformer coupled 4270 amplifier that was manufactured
in 1963, but much to do with the 344 receiver that was first
marketed in the fall of 1964. The 344 had no circuit boards
and resembled audion amplifiers in that respect, and was the
last audio component to be issued by Scott that had point to
point wiring. The 260 was the second application of the 344
circuit, and was an intermediate step in the evolution of an
amplifier circuit that became the penultimate design for higher
power Scott units until the introduction of complementary designs
in 1969. The final design was introduced in the 344A and the
348, in mid 1965.
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The 260 has a control panel that
includes a stereo selector switch that allows the listener
to reverse the stereo signal, or listen to a single channel.
Bass and treble controls are on separate clutches. This
permits separate channel adjustments.
The 260 has a tape head input.
This allows the Scott to be used with barebones tape decks
that do not have playback amps.
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The 260's power amplifier is a quasi-complementary design.
Its power supply delivers 70 volts d.c. to the uppermost
of two rails that sit atop the ground rail. The intermediate,
signal rail carries 35 v.d.c which is blocked at the
amplifier's output by a 2000 mfd electrolytic capacitor.
This is sufficient to permit the amplifier to dissipate
30 watts of sine wave energy into an electrodynamic
loudspeaker at 8 ohms with a 'theoretical' frequency
response that extends to twenty hertz.. A rear panel
mounted switch selects between 8-16 ohms and 4 ohms.
In the 4 ohm position, the amplifier's input sensitivity
is attenuated by reducing resistance in the negative
feedback loop.
In a quasi-complementary design,
the signal rail operating voltage is one half the value
of the d.c. supply to the high voltage rail. Two transistors
operate across the three rails, and the transistor that
carries current from the high to the intermediate rail
pulls its source of ground through the transistor that
carries current from the signal rail to ground. This
configuration swings an a.c. signal through the 70 volts
supplied with a crossover at 35 volts. The a.c. sine
wave approaches zero volts at the crossover. A higher
supply voltage permits a larger a.c. wave, and this
translates into more power.
Apart from the supply voltage,
the voltage ratings of the capacitors, and the wattage
ratings of the power handling resistors, all Scott quasi-complementary
designs resemble each other topologically.
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The above diagram shows a circuit arrangement
that is common in Scott amplifiers that employ a quasi-complementary
circuit. The diagram shows five horizontal lines. The second
from the top is the high voltage rail. The middle line is the
intermediate signal rail that is capacitor coupled to a loudspeaker.
The ground rail is the fourth. Below it is the signal path that
supplies negative feedback to the circuit.
Apart from the DC rating on the power
supply and coupling capacitors and the value and wattage rating
of the cathode resistors, the 260's circuit configuration closely
resembles a Scott amplifier that was manufactured in the late
1960's for use in consoles, compact stereos, and the 330T tuner.
A seven watt amplifier, the 214 has 2000 mfd coupling capacitors,
the same value that all Scott quasi-complementary amplifiers
use, rated for 20 v.d.c., and has the same characteristic sound
as the 260. Both can deliver power into a similar load impedance
up to their rated capacity, sounding much alike because they
are supposed to. The seven watt amp's schematic (which is a
close match to the 242 circuit that Scott first used in the
299F) will have much the same component values, that is to say,
of resistance and capacitance, in comparison to the 260.
Indeed, the seven watt amplifier has a lower noise floor than
the 260, but is still noisier than most audion amplifiers. This
permits the 214 to be used with loudspeakers that have ratings
of up to 94 dB with a quiet noise floor. An efficient loudspeaker
can end up being useless with many solid state amplifiers, for
all but the somewhat deaf, because it will reproduce the noise
floor of the amplifier with ruthless accuracy. Since most loudspeakers
can emit no more than 88 dB at one watt, this is not a problem
for most listeners. However, much of the criticism of solid
state can be aimed at noise floor hiss, scrape and rumble.
A quasi-complementary
amplifier will always be quieter than a complementary amplifier.
The higher the rail voltage(s),
the noisier the amplifier.
Scott
260 circuit stages
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PREAMP TONE
DRIVER
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Except for the output transistors,
all circuit components are on six PC boards. The magnetic
pre amplifier circuit boards are mounted in a shielded
compartment that also contains the input selector.
Carbon film resistors and paper
foil capacitors are used in this circuit.
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The 260 magnetic phono stage uses three
transistors. The first two are connected in cascade, while the
third, a pnp, acts as a follower. Equalization is inserted between
the first and the second transistor using negative feedback.
The output can be attenuated in three stages to permit level
setting to match other inputs.
There is no separate unequalised microphone
input. Scott added a microphone input to the 260B, manufactured
from 67353 to 70044. By 1968, most of the Scott receiver line
had microphone inputs. Matched with the late '60s magnetic circuit
first employed in the 342C, a Scott receiver so equipped provided
a level of performance with microphones that has yet to be surpassed
by modern solid state designs.
The tone stack features a Baxandall
circuit that was first introduced in the 4270. Scott used the
same circuit in all of their solid state equipment. The solid
state Baxendell circuit is a near equivalent to the circuit
that Scott used in its tube equipment from 1961 on.
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its side: SCOTT
LOOK
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Scott
manufactured its electronics products with electrolytic
aluminum between 1955 and 1973. Aluminum conducts
heat effectively, an attribute that permits the
chassis to serve as a heat sink.
This attribute was used by Scott to great advantage
in its lower power amplifiers. The 260 and the
260B and the higher powered receivers use heat
sinks. The 260B's heat sink is mounted on the
rear panel, and is smaller than the sink used
in the 260, which is capable of dissipating the
150 watts of thermal energy that the output transistors
produce at their least efficient point of operation.
Aluminum
does not rust, so Scott units that were manufactured
forty years ago can look as good as they did when
new.
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Capabilities
The Scott 260 amplifier outperforms the
299T amplifier series (342/382 receivers),
but sounds no better than any of the lower priced Scott receivers
that are based on the 242B design, including the 299F and the
342B/382B. Indeed, the simplest design of all, the 241, sounds
better than the 260 when used with the Quads, and supplies an
additional 3 dB of headroom over the very quiet 214 without
a noise floor penalty.
Scott introduced its first complementary
circuit in the 342C. In some ways remarkable, it employed an
inductive circuit to protect a loudspeaker from any d.c. that
can load the output when the amplifier becomes unstable, or
as is more often the case, on power up, when the amplifier may
not be functioning optimally. This is a problem with complementary
designs that is absent in a quasi-complementary, capacitively
coupled amplifier, and in any amplifier that employs output
transformers.
The 260 is a capable amplifier from 30
hertz on up... While it reproduces low frequencies sufficiently
well to allow me to hear them in reasonable proportion to other
frequencies, I have listened to amplifiers that do so with more
authority. Since many tube amplifiers that were manufactured
around the early Sixties were seriously deficient below 50 Hz,
the 260 must have been interpreted as a step forward. It was
characterized as such by the audio press at the time:
There is no question, however that the
260 does not measure well, when compared with power amplifiers
that can deliver a power bandwidth that is close to 0 dB deviation
at 20 Hz. The 260 is down 3 dB at 20 Hz
Frequency response performance tends
to be a specification that amplifier manufacturers quoted before
audio became mystical. Here it should be noted that the 260
fares quite well, being 3 dB down at 10 Hz, 1 dB down at 20
Hz, and flat to 100 kHz. Frequency response is not a good indicator
of amplifier performance with a loudspeaker connected, with
the exception of controlled impedance loudspeaker, or one that
offers the amplifier a load that varies between a range of high
impedances. An easy load is more readily handled, variances
and all, than a difficult, low impedance load with variances
that will drive an amplifier into periods of instability.
One such loudspeaker is the ELS, which
presents a high impedance at low frequencies, but drops to around
one ohm in the upper mid frequencies. The 260, and all of its
quasi-complementary brethren, is effective driving the Quads.
It handles the Quad's impedance variation with no difficulty,
benefits from the rising curve at low frequencies, and handles
the higher frequency impedance drop with good success. The 260
won't sound as refined as an audion from 100 Hz up, but it won't
lose itself to the inductive saturation that an undersized output
transformer will manifest when presented with a difficult load.
One thing that is worth mentioning is
that the 260, like many other Scott products, has a versatile
control center that can be used in much the same manner as the
210 series of amplifiers (minus the dynaural circuit). For the
archivist who works with RIAA, NARTB,
or ORTHO, the unit supplies very accurate curves. Complete control
functionality can be obtained by tapping the loudspeaker terminal
through a resistive load of 1kohm, terminated to ground through
a 600 ohm resistor. Or, a single resistor of 220 ohms can be
used, with no termination to ground. The signal that passes
through the resistor will be attenuated to a level that matches
the input sensitivity of a typical line, which is 2 v.a.c.
Contrary to the notion that audio equipment
sounds better when it is warmed up, quasi-complementary designs
sound much the same whether they are cold or hot. The notion
that circuits need to heat up before electrons flow correctly
is a bit lost on me, although it is true that complementary
designs perform better when thermally stable.
The 260 is a class B amplifier that spends
much time delivering a small waveform in class A, and it runs
cool. The aluminum chassis and heat sink overkill don't help
much, I suppose, should one believe that all amplifiers should
run warm. Placing the amplifier in a walnut sleeve will doubtless
raise its operating temperature, but as I have discovered, not
by much.
Class B is well suited to quasi-complementary
circuits, as the waveform is developed across a single voltage
rail of say, 70 volts d.c. such as is found in the 260, instead
of 74 volts developed across two rails with values of +37 and
-37 v.d.c. such as is used in the 342C. Class B works best with
quasi-complementary because it is easier to control the signal
crossover point with one supply rail. In class B, an amplifier
passes the wave through the crossover point with no overlap.
Quasi-complementary offers a perfect mechanism to control the
voltage points on its supply and signal rails: the circuit itself.
This performance characteristic is critical to low distortion
at the crossover point.
Class A does not swing volts through
a zero volt point, but instead reproduces the waveform in its
contiguous form between minimum and maximum points, above zero
volts.. As is to be expected, a contiguous wave is the most
desirable, as it has no crossover distortion. Class AB attempts
to meet the two classes halfway. Some consider the different
operating classes to exist on a continuum, with what is called
AB2, close to class B, AB1 in turn to class A. In truth, there
is only class A, and then a series of compromises toward class
B. The Scott 260 is what I would call class AB2+, very close
to class B, but not quite.
Single
and dual supply rails
A single rail design is simpler to execute,
as voltage differences on the supply rail will be reflected
exactly at the crossover point. This is of some importance when
utilizing a class B circuit, since crossover distortions arise
when the voltage at the crossover point varies. This fluctuation
will feed into the main sound defrauder for any push-pull
circuit, namely switching distortion of the waveform at the
crossover point that, in one sense, gets more severe as the
operating class approaches class A. Perfectly balanced
with no overlap, a class B circuit can sound better than any
AB design, as it more accurately describes the waveform.
Dual rail, complementary designs can
exhibit deviations from one rail to the other, and such deviations
shift the crossover point. Supply capacity was consequently
increased tenfold to add greater stability to complementary
designs. Nevertheless, the design flaw exists, is apparent when
an amplifier is attempting to drive a demanding load, and makes
necessary the use of one sort of protection circuit or another.
Complementary designs are possible in class B, but will never
sound anything other than terrible.
If quasi-complementary designs are starting
to sound mystical, they should as the topology is part of solid
state audio's misty past. In audion parlance, they are single-ended
with a what can only be described as a follower bent.
If you are looking for some audio mystique,
a Scott 260 may be something for you to check out. The 260 is
readily available on eBay, and can often be had for between
twenty and fifty dollars, or what can only add up to an unbelievable
deal. The 260 is a decent amplifier that will outperform any
complementary design extant in just about every mid-fi audio
amplifier out there, regardless of price and claimed power output.
Although it will not reach down into 20 cycles in any
meaningful way, its coupling capacity of 2000 mfd. is sufficient
to pass such frequencies with zero dB of deviation. Some extra
supply capacity is in order.
What you do get for your bargain eBay auction price is a classic
quasi-complementary: a design that Scott still used in its 357B
and 636 when the company entered receivership in 1973, and used
very briefly in its post bankruptcy production run, the R3nS,
of 1974. IC based audio chips that are mostly fully complementary
were common by the late 1970's after fully complementary discrete
circuits started emerging for all circuit stages including the
magnetic circuit and the line circuit(s) in one design. The
Advent 300 is an
example of a receiver that supplies positive and negative voltage
rails for all circuits except the tuner, and the passive (but
additive) baxandall circuit.
My own preference in matters of amplification solid state is
quasi-complementary. I often find fully complementary circuits
to sound grainy and out of focus, the very uncomplimentary observations
that were made by the mystical audio press at the time. I find
no such complaint with the Scott 260, and with the caveat that
the circuit boards are phenolic and prone to rot (and you can
get the 260 quasi-experience with any other quasi-design by
Scott with the pinnacle being the 341, say, which comes with
the indestructible glass epoxy pcb. Further? Some claim that
phenolic boards sound better than epoxy boards. As we all know,
hard wiring sounds best. Find the 1964 344 and have it all,
with a state of the art tuner
(for 1964) thrown in as a bonus.
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