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Rev D Input Buffer & Op Amp Choice

NORD REV D INPUT
BUFFER BOARD

Our Nord One and Three MKII Hypex and Purifi Amplifiers use a new revised Nord REV D Input Buffer with on-board switchable Gain settings  12.4-21.9-27.0-30.2 dB. Now incorporating onboard high quality XLR and optional RCA sockets. Eliminating the need for input cable and reducing the signal path. Using Sparkos discrete Voltage regulators and choice of Op Amps, the Sonic Imagery 990 or the Sparkos Labs SS2590 (user Swappable) both run in full Class A for a richer tonally dense sound. Metal film resistors 1% and audiophile grade Wima Capacitors throughout the audio stage. We manufacture with 4% WBT-0804 Silver solder, tiny details accumulate into making all our amps so special.

READ ALL FEEDBACK

CLASS A WHERE IT MATTERS

Hypex and now Purifi are renowned as a wire with amplification reproducing what it’s fed with unfaltering accuracy and detail.

The Nord REV D feeds the Amp Modules with a musical, rich tonally dense signal from its Class A Op Amps resulting in an amplifier that keeps the benefits of the Hypex NCore but adds warmth and depth.  Hypex  and Purifi designed the NC500, 1ET400A, 1ET7040SA with a separate input section to allow manufactures to add their own house sound via the input buffer section. 

We went one step further by designing a board with the ability for you to change the sound of the amp using different Operational Amplifiers or Op Amps via an on board socket, we call it Op Amp rolling.

We start with discrete Voltage regulators from Sparkos Labs. These provide ultra low noise and accurate power with very fast transient responses feeding the NC500 , Purifi amp modules and the discrete Op Amps. Discrete Op Amps are used because they simply sound better. Their design and size allows the use of large capacitors and high current operation not possible in IC Op Amps due to size and heat dissipation requirements.

The board uses tight tolerance metal film resistors and audiophile Wima capacitors throughout the audio stages. A high quality gold plated sockets is provided for easy Op Amp changing. 

Which Op Amp?

Untitled design 5

SPARKOS 2590

The Sparkos 2590 produces a Rich and warm detailed Class A sound slightly warmer and softer than the 990 but the differences are much closer than previous op amps the 994 and SS3602. If your speakers are forward, bright choose the Sparkos.

The SS2590 :  After the success of the SS3601 / SS3602 discrete op amp we turned our attention to the discrete op amps that are found in studio gear and recording equipment such as the API2520 and Jensen 990 style devices.  Numerous devices that are compatible with these exist on the market, and after buying and evaluating a hand full of them we decided that we could do better.  Much better.

The first thing that we noticed upon evaluating some devices in this class was that most of them are manufactured with archaic, decades old through-hole components.  This is not so bad in and by its self, (aside from component lead inductance) but the problem with them is that most of these through-hole components (especially the transistors) are old, slow, and pale in comparison to modern devices.  The SS2590 discrete op amp makes use of the highest speed and highest gain transistors available.  We use 300MHz rated output transistors while everyone else is using devices in the 20 to 50 MHz region.

Two Pole Compensation :  We’re the only ones doing this.  Two Pole Compensation is the ultimate compensation technique for maximizing open loop gain within the audio bandwidth.  High open loop gain makes for a more precise amplifier, lower THD performance, and keeps the slew rates high, which makes for more natural attacks and transients.

​

We found the noise performance of the majority of the discrete op amps in this class to be abysmal; measuring in at several nv√Hz in most designs. Some of the better devices specified in the 1 to 1.5 nv√Hz range, but they all seemed to achieve this noise performance by putting shunt inductors across the RE resistors in the input stage which “shorts out” the RE resistors and eliminates their noise contribution.  While this technique yields lower noise, it does so at the expense of input stage linearity since the linearizing affect of the RE resistors are lost by the shorting inductors.   RE resistors in the input stage are a good thing, and will linearize the input stage by swamping the non-linear Rbb of the input stage transistors.   On the SS2590 discrete op amp, we opted to keep the input stage RE resistors in order to reap their associated linearity benefit, and we reduced the noise back down by paralleling the input transistors.  It’s the best of both worlds, with the only drawback being a slight increase in input stage complexity.

The DC precision of the devices that we evaluated left something to be desired as well.  We saw offsets in the double digit mV range with warm-up drifts of several mV more.  The SS2590 has a built in POT to allow precise offset adjustment and it has a warm up drift of less than 1mV.  Input bias current induced offsets are greatly reduced by using on board current sources to supply the input bias current for the device.  This keeps the SS2590 discrete op amp from pulling input bias current from the outside world and creating offsets against the resistances seen by the input pins.

To put it another way, we did every thing on the SS2590 that we wanted to do on the SS3601 / SS3602 but were not able to due to its small size and more limited PCB space

 

Features:

  • 165 dB Open Loop Gain to 100 Hz

  • Two Pole Compensation

  • 10MHz Unity Gain Bandwidth

  • Class A Output Current Of +/- 32mA

  • Maximum Output Current Of +/- 250mA

  • 1.5nV√Hz noise in a 20KHz bandwidth

  • Less than 1mV Offset

  • On-Board Supply Decoupling Caps

  • Able to drive 75 ohm loads at full power

  • +/-9V to +/-24V supply voltage range

  • All BJT design with input bias current cancellation

  • Unity gain stable

  • Fully Discrete Design

Features:

• Ultra Low Total Harmonic Distortion, 0.00045 THD+N @1kHz
• Ultra Low Noise 0.89nV/rtHz
• High Current Output Drive (250mA into 75 ohms)
• +26dBu Output Levels (into 600 ohms)
• Standard Gain Block Footprint
• 120dB Open Loop Gain
• Operates over ±9V to ±24V supply rails
• Lower output offset voltage than existing counterparts
• Lower input leakage current than existing counterparts
• Particular emphasis on audio performance
• Designed, assembled and produced in the USA
• 3 Year Warranty

Applications:

• Low Impedance Line Amplifiers
• Active Filters and Equalizers
• Summing/Mixer Amplifiers
• High Performance Microphone Preamplifiers
• High Performance A/D and D/A front end Preamplifier

 

SONIC IMAGERY  990

sonic imagery labs nord acoustics

The 990 is slightly more open and accurate than the Sparkos 2590 but the difference is less than the previous Op Amps 994 and SS3602. And still has a warm and accurate Class A sound was our favorite but I just bought JBL 4367 speakers for the dem room which are quite forward, I now prefer the Sparkos, but its close either are superb.

The Model 990Enh-Ticha is a high performance discrete operational amplifier designed for professional audio application areas where ultra-low noise, low distortion and highly linear uncolored operation is required. It was designed as a high performance upgrade replacement for the Jensen JE990, Automated Processes Inc. API-2520, John Hardy Co. 990A-990C, FiveFish Studios DOA series, Seventh Circle Audio SC10, SC25, SC99, and Avedis Audio 1122 op-amp gain block. The pinouts conform to the 990 package, allowing direct replacement.

If the user is upgrading or replacing vintage or retro-clone gear, take note of the pin length required for your particular application. Older gear typically used modules with 0.480 to 0.510 inch long 0.040 pins. Sonic Imagery Labs offers this longer pin length variant at no additional charge. See the Model 990Enh-Ticha and 995FET-Ticha

The all-discrete SMT design is similar to the JE990 basic topology but has been completely redesigned to use an ultra-precision differential super-matched transistor pair specifically designed to meet the requirements of ultra-low noise, ultra-low THD, highly linear uncolored audio systems. In addition to the enhanced input stage, the 990Enh-Ticha uses high precision temperature stable power supply independent current sources. Supply independent current sources allow the bias to remain locked at the optimum operating point regardless of power supply voltage.

Dual matched pair temperature stable current mirrors, dual matched pair active current loads give the Model 990Enh it’s outstanding power supply rejection performance. The enhanced low distortion Class-A output driver stage can sink or source 250mA allowing this module to drive transformers easily.

Integrated power transistor heatsinks coupled to a anodized aluminum enclosure keeps the 990Enh-Ticha operating within a wide SOA and does not suffer from Beta droop when driving heavy iron or heavy loads. Each amplifier is fully tested and meets or exceeds published specifications.

Because of the 990Enh high current drive capability, supporting circuitry impedances can be scaled down within the application circuit. This can reduce the overall system noise, without increased distortion and provides higher headroom compliance performance.

Sonic Imagery Labs also can provide a variation of this model that can operate down to ±4.5V for low power low voltage applications. Contact us and ask about the Model 990LV-Enh-Ticha.

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SPARKOS SS78 SS79​

DISCRETE VOLTAGE REGULATORS

Ultra low noise with fast transient responses they sit at the heart of our Nord UP input board providing power to the NC500 and Op Amps. They deliver much of the low level listening ability, soundstage and high end organisation benifits. 

The noise specification of the discrete regulators are vastly superior to standard 78XX and 79XX devices. This is made possible by heavy filtering on the internal voltage reference which requires large capacitors that cannot be implemented on standard monolithic designs. Such luxuries are only possible with discrete designs.  

While noise specs are important, regulator performance is mostly about transient response and input rejection. Having a fully discrete error amplifier allows for extremely high input rejection and unique compensation techniques that cannot be implemented with standard monolithic IC error amplifiers 

I really noticed the benefit of these at low listening levels, detail and dynamics remain, while at high volumes the clarity stays at far higher levels.

Nord Acoustics Ltd

Unit 123 Pierce Way, Office 3
Gloucester
GL2 5YD

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