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I've just done some preliminary measurements on a DAC that has a switchmode supply and the audio measurements are astonishly good. I can't help feeling that adding $$ linear PSUs without knowing what exacvtly it might improve is just stabbing in the dark (while burning money).
True, but I'm not sure I see how a liberal return policy justifies taking leave of reason either. $2000 for an add-on supply to a device that has umpteen internal supplies (some of them most likely switch mode) derived from that seems, well, kinda crazy.
Features Ultralow RMS Noise: 0.8μVRMS (10Hz to 100kHz) Ultralow Spot Noise: 2nV/√Hz at 10kHz Ultrahigh PSRR: 76dB at 1MHz Output Current: 500mA Wide Input Voltage Range: 1.8V to 20V Single Capacitor Improves Noise and PSRR 100μA SET Pin Current: ±1% Initial Accuracy
Low noise, high PSRRA constant-current source feeds a zener diode as a stable voltage reference. A low-pass filter (with corner frequency of 1.6Hz) prevents zener noise from being introduced into the error amplifier. This is an effective yet lower-cost alternative to expensive voltage reference ICs. The low-pass filter also provides a soft-start characteristic.The output noise (unloaded) is less than 10µV at ±30VDC output (measured using a Tangent LNMP (low-noise measurement preamplifier) and a Fluke 187 50000-count DMM in ACmV mode). The output noise is even less when the output voltage is lower. This is much better than the noise of an IC regulator based PSU tested under identical conditions.The error amplifier is a discrete implementation of an opamp with a high open-loop gain of 102.5dB. The voltage supply to the error amplifier is isolated with capacitance multipliers to boost its PSRR (power supply rejection ratio). This greatly improves the line regulation performance of the PSU.A long-tailed pair differential amplifier with current mirror and constant current source forms the first stage of the error amplifier. The second stage is the voltage amplification stage (VAS), also with constant current source load. The 3rd stage is comprised of the power MOSFET output devices configured as a source follower.
The +/-15V linear regulators are fed by a dual switcher, working at a high frequency, so as to minimize radiated and conducted noise. Until recently this would not have been an option, but the linear post-regulators I selected have a PSRR of over 60dB at more than 1MHz! Absolutely incredible and exacty what is needed to clench what little ripple remains from the switcher. The 6/5/3.3 V source uses another dedicated high-frequeny switcher. It works in constant current mode into a relatively high decoupling cap and has low residual ripple.I also put a lot of effort in designing the PCB to avoid excess radiation; very low impedance ground- and current paths, short traces, decoupling caps right at the source, etcetera, etcetera. More time went into that than into the circuit itself...
[Big IMG] https://linearaudio.nl/sites/linearaudio.net/files/SilentSwitcherNoise.pngThe +/-15V outputs of the linear post-regulators have 0.2uV (neg) and 0.02uV (pos) noise at 1kHz, and broadband audio band noise level of less than 40uV (neg) and 7.5uV (pos). That is impressive by any standard. (The baseline for the test amplifier is also shown).
Note also freedom of low-frequency noise below 1 kHz, indicating exceptionally clean supply (external wall-wart is switching by the way).
Schematic descriptionThe regulator has all-discrete topology. No Operational Amplifiers are used. This allows complete design control over all operating points and parameters for superior performance. Low noise, high PSRR.There is a bridge rectifier block on the input which is filtered with a massive electrolytic capacitor on the output. This greatly reduce the ripple charge voltage and hence helps for better output filtering.The error tracking stage is based on a differential amplifier built with dual transistors chip which ensures the excellent thermal stability and equality of its parameters.The differential amplifier is cascoded which greatly reduce the input noise influence over the error (differential) amplifier and Erly effect.A Zener diode is used as a voltage reference. It is fed by a Constant Current Source attached to the output of the regulator which dramatically minimise the noise and ripple through the Zener (and hence at the input of error amplifier). The High-frequency Zener noise is additionally filtered by a Low-pass filter between Zener diode and error amplifier.The output stage consists of two power-Darlington bipolar transistors working in Common-Emitter mode. This allows the regulator to work as a LDO (Low-DropOut) with smaller difference between Input and output voltages which diminishes the thermal dissipation over the regulation transistors.At the input of the regulation transistors is placed trigger-type overcurrent protection network which engages at a given output current and turns-off them. Once the protection is engaged, it can stay in this mode for an unlimited period of time which prevents the system from doing any harm even in a human absence. In order to returns to normal operational mode the regulator must be turned OFF for a while (to discharce the electrolytic capacitors) and then turned ON again.And since SAFETY is a thing we care about a lot, a thermal switch is added in parallel with the overcurrent protection. This way even if the current protection is not engaged (which is almost impossible) the output regulation transistors will be turned OFF when their heatsink’s temperature reach 85°C.
Specifications– Input voltage: AC 115V or AC230V (according to the destination country or user request)– Output voltage: DC 12V/19V/24V (user-defined values are also available upon request)– User selectable option for 8mm. aluminum front panel (black anodized)– Dimensions: 220x220x75 mm.– Transformer Power: 100VA– Weight: about 4.5kg– Output DC cable with barrel jack 5.5/2/5 mm. (OD/ID)– Possibility for different output connectors upon customer’s request– Built-in trigger-type overcurrent protection (in case of protection’s engagement the unit must be turned off for a while in order to recover its normal work)– Built-in trigger-type overheating protection preventing the unit from burning in case of serious malfunction– Common mode AC filter on the input of the AC supply voltage– Front panel LED
Schematic descriptionThe regulator has all-discrete topology. No Operational Amplifiers are used. This allows complete design control over all operating points and parameters for superior performance. Low noise, high PSRR.There is a bridge rectifier block on the input which is filtered with a massive electrolytic capacitor on the output. This greatly reduce the ripple charge voltage and hence helps for better output filtering...
MeasurementsMeasurements of DC and AC voltages were taken with Keithley 2015 Multimeter while the spectrum plots were drawn using RTX 6001 Audio Analyser. The obtained results are as follows:– Line regulation (AC input voltage between 216 – 235 V): 0.05%– Load regulation (DC output current between 0 – 4.5 A): 0.06%– Output AC ripple voltage when loaded with real load (Lenovo ThinkStation M92p): 4.2 mV RMS– Measured Power Supply Rejection Ratio (PSRR): -120 dBV for fundamental harmonic (100 Hz)
Technical highlightsAll-discrete topology.Single-pass, series regulator design.No IC (integrated circuits) are used. This allows complete design control over all operating points and parameters for superior performance.Low noise, high PSRRA constant-current source feeds a zener diode as a stable voltage reference. A low-pass filter (with a corner frequency of 1.6Hz) prevents zener noise from being introduced into the error amplifier. This is an effective yet lower-cost alternative to expensive voltage reference ICs. The low-pass filter also provides a soft-start characteristic.The output noise (unloaded) is less than 13µV at 24VDC output (measured using a Tangent LNMP (low-noise measurement preamplifier) and a Fluke 187 50000-count DMM in ACmV mode). The output noise is even less when the output voltage is lower. This is much better than the noise of an IC regulator based PSU tested under identical conditions.The error amplifier is a discrete implementation of an opamp with a high open-loop gain of 102.5dB. The voltage supply to the error amplifier is isolated with capacitance multipliers to boost its PSRR (power supply rejection ratio). This greatly improves the line regulation performance of the PSU.A long-tailed pair differential amplifier with current mirror and constant current source forms the first stage of the error amplifier. The second stage is the voltage amplification stage (VAS), also with constant current source load. The 3rd stage is comprised of the power MOSFET output devices configured as a source follower.High-current MOSFET pass transistorsTwo paralleled high-current, highly reliable MOSFETs (rated at 18A each) serve as the "pass" transistor.The high current rating provides a very high safety headroom against overcurrent damage.The use of paralleled MOSFETs divides the heat dissipation, simplifying thermal management. Onboard heatsinks can be used which would allow the this PSU to supply up to 1A continuous (with much higher peak currents). More sustained currents are possible by using larger, offboard heatsinks.The negative temperature coefficient of MOSFETs prevents damaging thermal-runaway conditions that may plague conventional BJT devices.No current-limiting.
The error amplifier is a discrete implementation of an opamp with a high open-loop gain of 102.5dB...
