Ashly Power Amplifiers FET-200 User manual

ASHLY
MOS-FET POWER AMPLIFIERS
OPERATING INSTRUCTIONS
ASHLY AUDIO INC

TABLE OF CONTENTS
Page
!• Introduction 2
2. Unpacking, Installation
3. Input & Output Connections, Stereo Operation 5
4. Mono Operation 7
5. Bridging Operation 7
6. Meter Display g
7. Thermal Status Indicator g
3. Specifications/Block Diagram g
9. Definition of Terms 9
1

INTRODUCTION
The Ashiy tEX-200 and FET-500 represent an entirely new breed of power
amplifiers which are destined to replace conventional amps in virtually every
application. Far from being just'another variation on well worn designs, these
amplifiers have been engineered from the ground up to take full advantage of
au exciting new semiconductor technology which has recently become
availabie_the Power MOS-FET (metal-oxide-semiconductor, field-effect-
ttausistor). Combining many of the desireable operating characteristics of
vacuum tubes with the efficiency of transistors, the MOS-FET offers superior
audio fidelity and introduces an entirely new level of field reliability.
Ashiy power amplifiers put all of these features together into a carefully
thought out, roadworthy, user-oriented package which will provide years of
maintenance—free service.
WHY MOS-FET’3?
coavtntiouai power amplifier design as typically produced a circuit which
consists of two parts; 1) the amplifier itself, and 2) a collection of
protective circuits which keep the amplifier from destroying itself and the
speakers it is driving. Some of the more common protective circuits include
VI iimitiug, ciiermai feedback, detectors that monitor DC at the output
terminals, short circuit protection and turn-on delay circuits that keep the
speatcers from jumping out of their cabinets when power is applied. The result
has typically been a compromise in audio performance, greatly compounded
electronic and mechanical complexity, and amplifiers which are intolerant of
less than ideal operating environments.
The basic weakness of these amplifiers has always been the power output
transistors themselves, which are prone to failure as a result of "thermal
runaway . This breakdown occurs when the amplifier is delivering a lot of
power and begins to heat up. Increasing temperature produces an increase in
coiltator current in the output transistors, which in turn produces more heat.
If this cycle is unchecked, the transistor will be destroyed.
ProDaoiy tiic greatest advantage of power MOS-FET's for professional audio use
is their complete immunity to thermal runaway. Not only does this remove the
primary failure mechanism of the conventional amplifier and eliminate the need
for elaborate thermal protection circuits, it also allows the design engineer
to use a mucii nigher quiescent current in the amplifier's output stage,
thereby insuring that the output devices are always operating in their most
lineal region. This results in consistently lower distortion.
In addition, the simpler input drive requirements of the MOS-FET devices makes
possible a clean, concise amplifier design. The resulting reduction in
component count and internal wiring provides for a compact design and enhanced
reliability.
THE ASHLY DESIGN
Our goal in designing the FET—200 and FET—500 was to produce amplifiers that
combine the sonic excellence of a high-end stereo amp with the ruggedness and
stamina necessary in pro audio.
2

ELECTRONIC
Both amplifiers are fully complementary, push-pull type with totally discreet,
high voltage, wide bandwidth electronics. This approach inherently assures
low noise, low distortion, and excellent transient response. Inputs are
bringing, active balanced (transformerless), equipped with both 1/4" jacks and
XLii-type connectors of both sexes to facilitate linking several amplifiers
together. All input connections can, however, operate as balanced or
unbalanced. This is determined by the connector used in conjunction with the
input. Stereo, mono and bridging modes are user selectable by means of rear
panel switches, requiring no internal modifications or additional components.
All electronic components are conservatively rated and are mounted on a rugged
glass-epoxy circuit board. An efficient U-I laminated power transformer saves
space and minimizes magnetic flux leakage, permitting other low-level
equipment to be mounted above or below the amplifier with no danger of induced
hum. Large value, computer grade electrolytic capacitors assure long service.
Two lU-segment, 27 dB range LED meter displays provide an easily viewed
indication of power levels, and a separate LED indicates thermal overload.
HECiL\NICAL
ihe Ashly MOb-EST power amplifiers are packaged in a rugged, one-piece welded
16 gauge steel chassis with an anodized aluminum front panel and an easily
removed steel top cover. The chassis and ail internal components are
integrated into a single, rigid unit. The FET-500 requires 5 1/4" of rack
space and ttie FET-200 requires only 3 1/2". The power transformer is located
near the front of each amplifier, reducing mechanical stress on the chassis.
Each channel's electronics and heat sink are combined into a compact, plug in
module which can be removed from the amplifier in seconds. Hard wiring is at
an absolute minimum.
Large heat sinks and an aerodynamic internal design permit the use of an
ultra-quiet, slow speed fan which pulls air in through the front panel
directly across the heat sinks and power transformer, and out through the rear
panel. It is unnecessary to provide space or ventilation above or below the
amplifier, allowing for economical use of rack space.
The amplifier's output connectors are conveniently arranged on standard 3/4"
spacing tor quick interconnect via banana plugs, delecting the bridging output
involves only moving the banana plug to the two center (RED) connectors. Both
the AC line and the speaker terminals are fused protected. Fuses can be
removed instantly without tools. There are no internal fuses.
IN USE
The result: better sounding amplifiers, remarkably un—fussy about how you use
them. They will tolerate low impedance, highly reactive loads and hot,
crowded racks. They are highly immune to RF interference and will not be
damaged by intermittent short circuits—sustained shorts will simply blow the
output fuse. Due to their balanced circuit design, the use of delay circuits
and output relays are unnecessary. There is no turn-on thump and the volume
controls can remain up when power is applied and removed. Both amps will keep
on running right through a brown-out, even when line voltages fall to as low
as 20 volts!
3

This manual will explain the proper installation and use of Ashly amplifiers
and their controls. Please read it carefully and refer to it in case of
difficulty.
UNPACKING
As part of our system of quality control, every Ashly product is carefully
inspected before leaving the factory to ensure flawless appearance. After un¬
packing please inspect for any physical damage. iave the shipping carton and
all packing materials, as they were carefully designed to reduce to a minimuiu
the possibility of transportation damage suould the unit again require packiu,;
and shipping. In the event that damage has occurred, immediately notify your
dealer so chat a written claim to cover the aamages can be initiated.
THE RIGHT TO ANY CLAIM AGAINST A PUBLIC CARRIER CAN BE FORFEITED IF THE
CARRIER IS NOT NOTIFIED PROMPTLY AND IF THE SHIPPING CARTON AND PACKING
MATERIALS ARE NOT AVAILABLE FOR INSPECTION BY THE CARRIER. SAVE ALL PACKING
MATERIALS UNTIL THE CLAIM HAS BEEN SETTLED.
INSTALLATION
Ashly MOS-FET amplifiers are designed to mount in a standard ly" equipiacnf
rack. Since forced-air cooling is employed, it is not necessary to leave any
ventilation space above or below the amplifier. This permits efficient use of
rack space and multiple stacking of amplifiers. The only precaution to
observe is to leave the front panel air inlets and rear panel fan outlet
unobstructed.
The use of four front panel screws is recommended to evenly distribute the
stress on the panel. The amplifier chassis is sufficiently strong to support
itself by the front panel alone in permanent installations or where the rack
is moved only occasionally. For amplifiers which will spend much of their
time in transit, it is suggested that some method of rear support be used.
Connect all amplifiers to a 3-wire grounded outlet supplying 120 Volts 50-60Hz
Maximum power consumption is 600 watts. Maximum current required is 5 amps.
4

INPUT AND OUTPUT CONNECTIONS
STEREO OPERATION
Connections for stereo operation are shown in figure 4 below.
R L
V- OUTPUTS ^
MIXING
CONSOLE
+
+
Figure 4 Stereo Operation
INPUTS
The input of Asnly amplifiers is a balanced bridging type, with an input
impedance of lOk ohms. Use of the balanced input provides superior hum and
noise rejection while eliminating ground loops. The input may also be used in
a single-ended, unbalanced mode by simply using unbalanced input connectors.
Each channel provides a choice of either 1/4" jacks or XLR input connectors.
All four connectors are in parallel. Since only one of the four connectors is
used as an input at any one time, the others may be used to jumper other
amplifiers to the same source.
Signal connections are as follows:
The (+) or in-phase connection is on the tip of the 1/4" stereo phone jacks,
and is on pin 3 of the XLR connectors. The (-) or out-of-phase connection is
on the ring of the phone jacks and on pin 2 of the XLR's. Chassis ground and
shield is on the sleeve of the 1/4” jacks and pin 1 of the XLR's.
5

When feeding the amplifier from an unbalanced source, use either an XLk
connector or a standard stereo 1/4" phone plug and connect the signal "hot"
connection to XLR pin 3 or to the phone plug tip. Connect signal ground to
the XLR's pins 1 and 2 or to the phone plug’s ring and sleeve. The latter uiay
also be accomplished by simply inserting a standard 1/4" mono phone plug,
which automatically connects ring and sleeve together.
In the special case where the amplifier is physically mounted in the same rack
with the signal source that is feeding it, you may wish to use an alternate
connection system to avoid ground loops; connect the signal hot to XLR pin 3
or to the phone plug tip. Connect signal ground to XLR pin 2 or to the phone
plug ring. Leave the XLR’s pin 1 and the jack sleeve unconnected.
SPEAKER OUTPUTS
The speaker output connections are combination banana jacks/binding posts
spaced on standard 3/4" centers. Speaker connections may be accomplished
either by inserting banana plugs or by feeding bare stranded wire into tho
binding posts.
Figure 6

MONO OPERATION
With the MONO switch depressed, the Channel 1 input will feed both sides of
the amp. Volume for both sides is controlled with the channel 1 LEVEL control.
IN
BRIDGING OPERATION
Ashly amplifiers are in ttie bridging mode when both the MONO switch and
BRIDGING switch are depressed. In this mode, the channel one input feeds both
sides of the amp, but the phase of the channel two amplifier is inverted 180
degrees, so that the two amplifiers' outputs are out of phase. The speaker
connection is then made to the two RED output Jacks, as shown in Figure 8.
The black output jacks are not used.
7

METERS
The meters on the front panel respond to the peak output voltage of the
amplifier as measured at the speaker terminals. A ten segment, three color
LED display covers a twenty-seven decibel range.
the LED’s on one channel light up and stay on with only a small
input signal, a blown speaker fuse or faulty speaker wiring is indicated.
THERMAL STATUS INDICATOR
In the event that the amplifier overneats, it will automatically shut down and
the green THERMAL STATUS LED will turn off. The fan will continue to run, and^
vdien the heat sink temperature returns to normal the green THERMAL STATUS LED
will come back on and the amplifier will turn itself back on.
It is extremely unlikely that any combination of musical program material and
load conditions will activate the thermal shutoff mechanism in normal use.
Should overheating occur, check to be sure that the air flow in front of and
behind the amplifier has not been blocked.
Note^ that there is a very generous safety margin between the temperature
required to activate the thermal cutouts and temperatures which could damage
components in the amplifier.
SPECIFICATIONS:
FET-200
RATED POWER 100 Wotts/channel
Both channels driven into
8 ohms
160 Wofts/chonnel
Both channels driven into
4 ohms
320 Watts into 8 ohms bridged
mono
FET-500
RATED POWER 250Wcitts/channel
Both chonneis driven into
8 ohms
400Watts/channel
Both channels driven into
4 ohms
SOOWotts into 8 ohms bridged
mono
FREQUENCY RESPONSE
HARMONIC DISTORTION
IM DISTORTION (SMPTE)
DAMPING FACTOR
±.5db lOHz-SOKHz
< .05% 20Hz-20kHz. 100 watts,
8 Ohms
< .01% 100 watts, 8 ohms
> 100 20Hz-20kHz
SLEW RATE: 50V/ fisec
RISE TIME 2 fisec 10%-90%
FREQUENCY RESPONSE ±.5db 10Hz-50kHz
HARMONIC DISTORTION
IM DISTORTION (SMPTE)
DAMPING FACTOR
SLEW RATE:
RISE TIME
< .05% 20Hz-20kHz, 100 watts,
8 Ohms
< .01% 100 watts, 6 ohms
> 100 20Hz-20kHz
50V/ fisec
2 fisec 10%-90%
8

DEFINITION OF TERMS AS USED IN THIS MANUAL
ACTIVE
ectronic circuits which use devices such as transistors and
c rcuits, and which are capable of voltage and power gain as
OSS. xrcuits using only resistors, capacitors, transformers,
referred to as passive.
integrated
well as
etc., are
AMPLITUDE
voltage level of a signal. May be measured in volts or decibels,
era y corresponds to the volume or intensity of an audio signal.
BALAfJCED
^ circuit arrangement in which two conductors are designated as
signa ~)» and the third is a shield and chassis ground. The
sigua ines are of opposite polarity at any given moment, and are of
equal potential with respect to ground, balanced input amplifiers are
use on a Ashly SC series products^to improve hum and noise rejection,
umpering signal minus (—) to ground provides an unbalanced input.
BREATHING
A usually undesirable fluctuation of background noise
compressor action. (Also called "Pumping.”)
resulting
from
BUTTERWORTH
f particular filter response shape. The response is
ssentially flat within the pass-band, is 3dB down at the cutoff
frequency, and continues to attenuate at a constant slope. Also called a
maxima y t at or critically damped” filter shape, it is very popular
for crossovers and shelving filters.
CENTER FREQUENCY
The frequency (or
parametric equalizer
boost/cut control has
pitch) at which a filter is most effective. In a
» it refers to the frequency where a particular
maximum effect.
COMPRESSOR
An amplifier wliich reduces
predetermined "threshold."
its gain as its input is increased
beyond a
DAMPING
A force which opposes the tendency of a system to oscillate,
dB
A unit by wnich audio levels can be COMPARED. Often thoroughly
misun erstood are the concepts that decibels represent the level of a
signal compared to some reference level (15 dB cut means a certain level
ess an a previous level the absolute level of the signal need not
decibels are a logarithmic unit. Some handy numbers
to remember when dealing with decibels;
+3 dB - Double Power +10 dB = lOX Power
+6 dB - Double Amplitude, +20 dB = lOX Amplitude, lOOX Power
Quadruple Power
9