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Class AB with Regulated Power Supply...Class A-like?
Class AB with Regulated Power Supply...Class A-like?
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Old 21st January 2013, 11:13 AM   #21
Jay is offline Jay  Indonesia
Join Date: Feb 2003
Location: Jakarta
Originally Posted by chicks View Post
Dynaco used a regulated supply in their class AB Stereo 120 amp. Did they just not know at the time how to get good enough PSRR, requiring the regulation?
Many "good sounding" vintage amps sound good simply because their input stage is regulated.

There are ways to increase amplifier's PSSR, but almost all the added "complexity" have consequences to the overall performance of the amp itself. Imo, improving regulation is always a good option sound-wise, but usually not money-wise.
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Old 21st January 2013, 11:20 AM   #22
sreten is offline sreten  United Kingdom
Join Date: Nov 2003
Location: Brighton UK
Originally Posted by chicks View Post
Dynaco used a regulated supply in their class AB Stereo 120 amp. Did they just
not know at the time how to get good enough PSRR, requiring the regulation?

Like the Quad 303 its a single rail design and both use a regulated supply.

Part of the design philosophy is no SOA stuff in the power amplifiers
and foldback current limiting in the supply. For this reason with
tougher loads the Quad 303 is ideal for bi-amping, probably
also the case for the ST120.


rgds, sreten.
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Old 21st January 2013, 11:21 AM   #23
analog_sa is offline analog_sa  Europe
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Location: Cascais
Originally Posted by triplej View Post
...Dosent the benefit come from regulating the supply for the driver stage back? i.e there is no benefit from a regulated supply for the output stage unless the amp is run into clipping?

Maybe in theory. I once had a Naim with a regulated supply. Did quite a few things very well and clearly sounded different to any similar amp without a regulator. The regulator, btw was more complex than the amp itself, but even so wasn't sufficiently, imo, powerful.

As for regulating the driver and input stages, i've done it on a number of amps, some with very high psrr. In every case there has been a substantial increase in perceived sound quality.
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Old 21st January 2013, 12:08 PM   #24
Bonsai is offline Bonsai  Europe
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Originally Posted by Boscoe View Post
I really don't know why everyone raves on about class A and it's linearity, I've never seen a class A amp good in distortion they are usually terrible. Class AB is superior in every way unless you want a heater.
I've built exemplars of both recently.

Class A sounds fantastic.
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Old 21st January 2013, 11:05 PM   #25
LineSource is offline LineSource  United States
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Location: SiliconValley
Has any built or researched a boost power supply as used on the Sanders Magtech amp?

Sanders Sound Systems is well known for their ESL speakers and ESL optimized amplifier. Sanders Sound also produces the $5000 Magtech amplifier with 500 watts of ClassAB power optimized for the demands of planar magnetic speakers. Each Magtech amp channel uses 10-pairs of ThermalTrak output transistors.

The amp has excellent specís (< 0.004% Thd), very clean packaging thanks to large PCBs, and an interesting boost voltage power supply. The Magtech power supply adds a modest voltage boost supply that rides on top of the traditional capacitor filter main supply. I think this is the interesting part of the design.

The Magtech uses two power supplies as you would in an up regulator. The low voltage one is called the "ride" supply. It is exactly like the power supply in a conventional, unregulated amplifier. The second power supply is the "boost" power supply. It has a higher voltage and current rating than the ride supply and can add massively more power to the ride supply when needed. The ride supply voltage is set for "easy" operation under optimum conditions. Under these conditions, only the ride supply drives the amplifier circuitry and the boost supply is just on standby.

When significant power is required, the rail voltages will start to fall. This is detected by the power supply's monitoring circuitry, which then switches on the MOS coupling(switch) transistors to connect the boost supply to the ride supply. The additional power provided by the boost supply prevents the rail voltage from falling, thereby regulating it.

The key to efficient operation lies in the way that the coupling transistors are operated. First, they are either fully switched on or fully turned off. Secondly, digital control circuitry is used to monitor the rectifiers' wave form and cause the transistors to switch states (either on or off) at the exact point where the DC pulses cross the zero voltage point. This is important because even though transistors change states very quickly, they do not do so instantaneously. So there is some resistance during the change of state. This is the same problem that causes switching power supplies to be less than perfectly efficient. If the transistors changed state while the power supply voltage was applied to them, there would be waste heat generated. By only allowing state changes at the zero voltage points, there is no waste heat. The digital control circuitry constantly monitors the pulsating waves from the regulator and the rail voltages. It will then make a decision to turn the coupling transistors on or off at each zero point to add as many or few pulses as required to hold the voltage constant.

Under heavy load, the coupling transistors would remain on (possibly even continually), letting most or all of the pulses through. Under light load, they would only be switched on occasionally to let a few pulses through. The regulator has a maximum resolution of 120 pulses per second, each pulse has to charge up a very large bank of capacitors. Doing so takes time and much current. Therefore, even though each pulse has a lot of current and energy, it can only make a very small change in the capacitor bank's voltage. By adding more or less pulses as needed, the regulator can maintain a stable voltage to within 0.2 volts. By comparison, without the regulator, the power supply's voltage would vary by more than 50 volts. Which would you prefer?
In some ways, the Magtech's power supply is like a switcher in that its transistors are either on or off. But there is no specific oscillation involved as in a switcher. Also, it is relatively simple and operates at low frequencies, so there is no noise or radio frequency interference.

Power 500W RMS per channel 8 ohm, 900W RMS per channel 4ohm
Bandwidth DC through 100kHz
Class of Operation Class AB
Slew Rate 500 Volts/microsecond
Input voltage required for full output 2 Volts
Input Impedance 50kohm (both balanced and unbalanced)
Gain 26dB
Noise More than 110dB below rated output
Damping Factor Greater than 600 into an 8 ohm load
THD Less than 0.004%, 20 Hz - 20 KHz
IMD Less than 0.003%, 20 Hz - 20 KHz
Voltage Voltage is user selectable for use world-wide.
Dimensions 17" wide x 5.5" tall x 16" deep (43cm x 14cm x 40.6cm)
Weight 55 pounds (25 Kgs)
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File Type: jpg SandersMagtech.jpg (179.0 KB, 164 views)
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