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Refining the Open Loop Diamond Buffer Headphone Driver (RJM Audio Sapphire 3.0)
Posted 31st January 2015 at 01:28 PM by rjm
Updated 18th March 2015 at 01:52 AM by rjm (add photo of finished amp)
Updated 18th March 2015 at 01:52 AM by rjm (add photo of finished amp)
A couple of years ago I built a standard op amp + diamond buffer headphone amplifier, called the Sapphire.
My original circuit (Sapphire 1.x) was the simple four transistor four resistor diamond buffer of the LH0002. Later small resistors (Sapphire 2.0) were added to the emitters of the driver transistors to boost the output bias current.
In this next go-round (Sapphire 3.0), I've replaced the emitter resistors with current sources. This provides a significant improvement in PSRR, over 20 dB in simulation. The output pair has been reinforced in a Sziklai configuration for lower distortion, and the primary output transistors five-way paralleled for improved thermal stability. The output impedance is 1~2 ohms, limited primarily by the output resistor.
It simulates to <-100 dB harmonics for 0 dB (1 V rms) output into 60 ohms. The total circuit standing current is less than 50 mA per channel.
LTSpice files below. R5,R6 on LTSpice circuit adjust the output bias currents and are set for 40 mA total through R13,R14. The operating points of the actual circuit ended up extremely close to the simulation, with minimal thermal drift.
The Sapphire 3.0 (photo, schematic shown) fronts the buffer with a noninverting OPA134.
My original circuit (Sapphire 1.x) was the simple four transistor four resistor diamond buffer of the LH0002. Later small resistors (Sapphire 2.0) were added to the emitters of the driver transistors to boost the output bias current.
In this next go-round (Sapphire 3.0), I've replaced the emitter resistors with current sources. This provides a significant improvement in PSRR, over 20 dB in simulation. The output pair has been reinforced in a Sziklai configuration for lower distortion, and the primary output transistors five-way paralleled for improved thermal stability. The output impedance is 1~2 ohms, limited primarily by the output resistor.
It simulates to <-100 dB harmonics for 0 dB (1 V rms) output into 60 ohms. The total circuit standing current is less than 50 mA per channel.
LTSpice files below. R5,R6 on LTSpice circuit adjust the output bias currents and are set for 40 mA total through R13,R14. The operating points of the actual circuit ended up extremely close to the simulation, with minimal thermal drift.
The Sapphire 3.0 (photo, schematic shown) fronts the buffer with a noninverting OPA134.
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