av Hetsporren » 2004-09-10 07:30
Design of Semi Digital Amplifiers
When the world’s first fully digital amplifier, TacT Millennium M1 was introduced in 1998 it was hailed as a phenomenal technological achievement and a quantum leap in sound quality. Since then, TacT have introduced the S2150 and M2150 fully digital amplifiers with even more technological breakthroughs such as advanced digital crossovers, frequency control functions, time delay adjustments, etc.
Now the time has come to realize many of the advantages of digital amplification at a fraction of the cost. Compared to true digital amplifiers, the SDAI 2175 is a less complex design which relies on analog inputs to generate the digital switching at the output. The output stage operates by pulse-width-modulation (PWM) at a very high switching rate and constant voltage. This is similar to the operating principle of the TacT Millennium, but while the Millennium converts the digital information directly from the digital source, the SDAI 2175 utilizes analog inputs to generate PWM switching at the output. The H-bridge output stage of the SDA 2175 is virtually identical to the one used in TacT M1 and 2150, but before the output stage, the technology is very different. The input is analog only, and either single-ended or fully balanced connections can be used.
The power amplifier section is a fully balanced design – all the way to the output terminals. An input buffer stage limits the voltage to the PWM modulator, so that maximum power is limited at the input, not by clipping of the output stage. This results in much more benign clipping behavior than for most analog amplifiers. Short circuit protection is provided by a patented current sensing system, which is wholly isolated from the signal path. The system simply stops the next pulse from the PWM modulator. This allows us to completely eliminate fuses and relays in the high-power signal path.
The preamplifier section is designed without compromise. It uses the finest digitally controlled analog attenuator. The volume is controlled with 0.1 dB steps, and channel balanced is maintained with incredible precision at any level. Instrument-grade dual gold relays are used for input switching. To optimize signal to noise each individual input can be attenuated by 6 dB for high voltage sources. Each of the 7 inputs can be individually adjusted in level with 0.1 dB precision. The display can be programmed to show the name of the source. (SACD, TUNER etc.)
Benefits of the SDAI 2175 over conventional analog amplifiers:
# A well-designed PWM amplifier will have less tendency to ring than an analog amplifier, because in a PWM design voltage remains constant while the varying width of the pulses generates the sound.
# The design is inherently free from crossover distortion, one of the most perniciously audible types of distortion present in analog amplifiers.
# IM distortion is greatly reduced compared to most analog amplifiers.
# All loudspeakers produce current, which flows back to the amplifier (Back-EMF). The SDAI 2175 is completely immune to this current, while analog amplifiers can become unstable and generate severe distortion due to Back-EMF.
# All loudspeakers present a reactive load to amplifiers, because impedance varies with frequency. In analog amplifiers, this results in greatly increased distortion. SDAI 2175 is virtually immune to the effects of reactive loads.
# Analog amplifiers need relays or fuses in the signal path. Power relays in particular are subject to deterioration over time. SDAI 2175 has no fuses or relays in the high-power signal path.
SDA 2175 generates 75% less heat than a class AB amplifier of similar power. Heat is the greatest contributor to component aging. Capacitors in particular are prone to heat damage over time.
# Less heat means that SDAI 2175 will outlast analog amplifiers and produce much more consistent sound over its lifespan. Long-term reliability is far better than for analog amplifiers.
The signal path is extremely short. This makes the use of feedback more effective and less prone to ringing.
# Power supply noise rejection can be implemented more effectively than for analog amplifiers. This contributes to the extremely low level of noise. SDAI 2175 noise levels are 10-100 times lower than the typical analog amplifier.
# Due to substantially increased efficiency, the size and weight of components can be reduced.
Features
6 stereo phono (RCA) inputs + 1 stereo XLR balanced input User programmable 6 dB input attenuation for each input Max input voltage +- 5 volt Individual level and balance adjustment for each input with 0.1 dB steps 4 step Display dimmer with programmable time-out IR remote control RS 232 connection for software upgrades Channel balance +- 0.05 dB at any level down to -80 dB Pre-amp output balanced and un-balanced High Quality speaker terminals
The Sound
Put simply, the SDAI 2175 is very similar to the TacT Millennium and 2150 amplifiers. Transparency, depth of image and agility are simply overwhelming. The highs are sweet, yet accurate. Bass is tight and controlled, while retaining the warmth of real music. In other words, you could say that the treble resembles the best you can get from tubes, while the lows are like solid-state on steroids. The complete lack of noise is a further benefit.
Build Quality
The chassis of the SDAI 2175 is made entirely from CNC machined aluminum. Aluminum is non-magnetic, while still capable of shielding against high frequency RF interference. Even the bolt holding the mains transformer is made from non-magnetic stainless steel. The bottom panel is machined from a solid 4 mm aluminum sheet. The transformer is a Toroid-Holmgren design, which is particularly resistant to mechanical hum. All PCB’s are highest quality grounded 2-layer or 4-layer glass-fiber boards.
Specifikationer:
Parameter Value Note
Balanced input connectors 3 pin XLR, gold-plated. Case=Gnd, Pin1=Gnd,Pin2=Hot(+), Pin3=Cold(-)
Balanced input impedance 10Kohm AC-coupled.
Balanced input CMRR 40dB 20Hz-20KHz.
Unbalanced input connectors RCA (phono) jack, gold-plated. Case=Gnd, Tip=Hot(+)
Unbalanced input impedance 100Kohm AC-coupled
Max. input voltage ±15Vp AC, 0-20V DC.
Input sensitivity 2V 200W/8ohms
Voltage gain 26dB
Output connectors 4 insulated binding posts, gold-plated. Will accept bare wire ends of up to 5 mm diameter.
Output power, 8ohms 2*200W 1KHz, 1% THD+N
Output power, 4ohms 2*375W 1KHz, 1% THD+N
Nominal load impedance 4ohms-8ohms It is safe to operate the amplifier with no load.
Frequency response 0.3Hz-33KHz -3dB points, 8ohms load.
Frequency response -0dB/+0.2dB 20Hz-20KHz, 8ohms load
Frequency response -0.2dB/+0dB 20Hz-20KHz, 4ohms load
Output impedance 0.035ohms 20Hz-1KHz
Output impedance 0.4ohms 20KHz
THD+N, 1W/8ohms 0.004% A-wgt.
THD+N, 1W/4ohms 0.006% A-wgt.
THD+N, 100W/8ohms 0.01% A-wgt.
THD+N, 180W/8ohms 0.07% A-wgt.
THD+N, 275W/4ohms 0.07% A-wgt.
S/N ratio 113dB A-wgt. ref 200W/8ohms.
Channel separation 84dB 1KHz, 200W/8ohms.
Peak output current ±40A
Output common mode voltage 35V DC Ref. Ground.
The amplifier can not be used in bridged mono mode.
Output DC voltage ±5 mV
All audio measurements, except frequency response, are measured with a 20KHz low-pass filter in accordance with AES-17.
Protection
Parameter Value Note
Grounding Mains earth, chassis and audio ground are connected internally.
Output short circuit current ±40A
Output DC voltage ±5V @ <0.1Hz
Over temperature All heatsinks and mains-transformer.
Mains
Parameter Value Note
Mains input connector IEC 320 cold type Mains lead supplied.
Mains voltage range 100-120V AC, 50–60Hz 115V version
Mains voltage range 200-240V AC, 50-60Hz 230V version
Internal mains fuse 8 Amp 115V version
Internal mains fuse 4 Amp 230V version
Power consumption 1.5 W STANDBY mode.
Power consumption 35 W OPERATE mode, no output.
Power consumption 116 W 2*37.5W/4ohms.
Power consumption 820 W 2*300W/4ohms.
Trigger
Parameter Value Note
TRIGGER IN connector 3.5mm (1/8") mono jack Case=Gnd, Tip=Input
TRIGGER BYPASS connector 3.5mm (1/8") mono jack Case=Gnd, Tip=Output
TRIGGER IN impedance 10Kohm
TRIGGER IN sensitivity 4V DC or 4Vp AC Positive DC voltage on Tip. AC > 50Hz
Max. input voltage ±25Vp
Mechanical
Parameter Value Note
Width 450mm (17.72")
Depth 340mm (13.39") Excluding loudspeaker connectors.
Depth 361mm (14.21") Including loudspeaker connectors.
Height 88.8mm (3.5") Excluding feet.
Height 100.6mm (3.96") Including feet.
Net weight 15.5Kg (34lb.)
Shipping weight 17.5Kg (39lb.)
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! (... bredvid Korsssun V8i (A*´s precccious 
!))
