PhilDick
pfm Member
Here's a bit more from the white paper about the a49. Arcams first class g stereo amp:
In a normal class AB amplifier there is an optimum bias current which minimises but does not eliminate variations in output
impedance with output current. It is these small but relatively sharp changes which occur at low levels near the crossover point,
between the NPN and PNP output transistors, which cause the insidious and audibly unpleasant phenomenon of crossover dis-
tortion. The brute force way to eliminate this is to run the output stages in class A, which requires a very high standing current of
several amps. This makes the output impedance virtually constant for output currents up to twice the chosen standing current but
dissipates huge amounts of power at all times; this is very inefficient, requires huge amounts of heatsinking and is definitely not
green! The A49’s output stage includes a proprietary error correction circuit that modulates the modest standing currents in the
output stage and ensures a near-constant output impedance for peak currents of up to about +/- 4 amps, corresponding to well over
50W in to 8 ohms. The A49 thus behaves exactly like a classical class A amplifier up to this power level in terms of performance
but without the heat penalty.
Detail here:
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.arcam.co.uk/ugc/tor/p49/Technical%20Notes/The_technologies_behind_the_A49_101014_A.pdf&ved=2ahUKEwjRyZT1wrbyAhVPhVwKHeYGB2YQFnoECAgQAQ&usg=AOvVaw2GacpcvhnUu4D1z13kVOvq
In a normal class AB amplifier there is an optimum bias current which minimises but does not eliminate variations in output
impedance with output current. It is these small but relatively sharp changes which occur at low levels near the crossover point,
between the NPN and PNP output transistors, which cause the insidious and audibly unpleasant phenomenon of crossover dis-
tortion. The brute force way to eliminate this is to run the output stages in class A, which requires a very high standing current of
several amps. This makes the output impedance virtually constant for output currents up to twice the chosen standing current but
dissipates huge amounts of power at all times; this is very inefficient, requires huge amounts of heatsinking and is definitely not
green! The A49’s output stage includes a proprietary error correction circuit that modulates the modest standing currents in the
output stage and ensures a near-constant output impedance for peak currents of up to about +/- 4 amps, corresponding to well over
50W in to 8 ohms. The A49 thus behaves exactly like a classical class A amplifier up to this power level in terms of performance
but without the heat penalty.
Detail here:
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.arcam.co.uk/ugc/tor/p49/Technical%20Notes/The_technologies_behind_the_A49_101014_A.pdf&ved=2ahUKEwjRyZT1wrbyAhVPhVwKHeYGB2YQFnoECAgQAQ&usg=AOvVaw2GacpcvhnUu4D1z13kVOvq
Class AB mode.'
