BK251

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High-performance audio op-amp with 0,5-nV/√Hz ultra-low noise discrete input stage

The BK251 is a discrete input stage operational amplifier, specifically designed as a replacement for traditional integrated op-amps in ultra-low noise, high gain audio circuits, including MC cartridge phono preamplifier.

The particular combination of a discrete input stage and an integrated op-amp based output stage allows to get advantages from both technologies, providing exceptional dynamic performance and an incredibly low input noise, lower than any commonly used integrated audio op-amp.

Output noise of BK251 (blue) is significantly lower than the output noise of the state-of-the-art integrated op-amp LT1028 (yellow). (Measurement conditions: Av = 1000; Rf = 1 Ω; Bw ≈ 1,6 kHz.)

An exceptionally low input noise voltage of only 0,5 nV/√Hz with a very low 1/f noise corner frequency is obtained thanks to an accurate selection from the state of the art of ultra-low noise BJTs. In addition, an extremely fast topology ensures a very low distortion and an excellent load driving capability.

The BK251 module is pin-to-pin compatible with common integrate single audio op-amps. Power supplies must be in the range from ±12 V to ±18 V (the suggested voltage is ±15 V).

The project

The BK251 op-amp input stage consists of an ultra-low noise differential pair polarized by a constant current source. Each bipolar junction transistor used in this stage is carefully selected from the state of the art of low noise transistors.

Simplified schematic of BK251 op-amp. An ultra-low noise discrete differential pair drives a high-performance integrated op-amp. Bias current nulling circuit is not shown for simplicity.

The input pair generates a couple of differential signals, that drive the output op-amp. This topology is extremely fast and this ensures the low distortion and load driving capability required in low noise audio amplifiers designed to work with low impedance networks.

In addition, the BK251 operational amplifier includes a frequency compensation network specifically design to ensure the circuit stability without degrading its performance. Also an input bias current nulling circuit, useful to reduce the output offset, is included.

The assembly

The BK251 module is assembled on a 22 mm × 17 mm double face printed circuit board, manufactured through an high-resolution photolitographic process.

BK251’s mechanical drawings. All measure are in millimeters.

In order to get the best electronic performance, the tracks lenght has been minimized by using a combination of high quality surface mount and trough hole components, mounted on both sides of the board.

Pinout of the BK251 module. The printed reference sign on the top indicates the proper orientation.

The same pinout of standard single op-amps is used, so BK251 can be mounted on the same socket used for DIP-8 integrated circuits. A mark on the top side indicates the correct orientation of the module.

Important notice: similarly as any integrated op-amp, this module must be properly inserted to the docket. Any error could result in damages to the module and the connected devices. For this reason, a careful mounting of this module is highly recommended. The author disclaims any liability deriving by the using of this module.

Application notes

The BK251 op-amp is designed for the use in low noise and high gain amplifiers, especially when they are interfaced with low impedance sources. These specifications are typically encountered in high-performance phono preamplifiers such as the T-Phonum MKII, in particular when an moving coil cartridge is used.

In fact, while moving coil cartridges produce low output signals, their low internal impedance makes them substantially noiseless. For this reason, the use of a high gain and low noise input stage is crucial to maximize the signal to noise ratio and obtain the best performance from the system. The use of a fast circuit configuration also allows to keep an extended frequency response even at high gains and a low distortion over the entire audio band.

On the other hand, moving magnet cartridges, which have a higher internal impedance, are tipically much noisier than moving coil cartridges, resulting in a limitation of the noise performance of the system. However, this issue is partly compensated by the higher output signal of these cartridges, thus common op-amps usually provide acceptable noise performance.

Stability

In order to ensure the stability of the circuit, many calculations, simulations and measurements have been done to properly compensate the circuit. The compensation network has been specifically design to provide the best performance in high-gain circuits such as phono preamplifiers.

Therefore, it is mandatory that the BK251 is used in a circuit with a gain greater than 20 dB, equivalent to a voltage amplification greater than 10. This condition is normally encountered in preamplifiers for which the BK251 is designed.

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