The Portaphile NOK, aka “Nude OverKill”
The NOK is the latest amplifier from Portaphile and also our flagship. Webster defines flagship as “the finest, largest, or most important one of a series, network, or chain” and this is exactly what the NOK represents for us here at Portaphile. The NOK expands on what the highly regarded line of 627 line of amplifiers and improves upon this design in several key areas. Let’s begin with what’s inside the 627 line of amplifiers, which includes the Original 627, the MICRO and 627x.
- Three channel design with an active ground channel. Typical amplifiers only use two active channels, one for the left and right with the input and output grounds connected to each other. Three channel amplifiers implement a third active channel for the ground channel that keeps the input and output grounds separated with an extra opamp and buffers in between. This gives some of the benefits of a balanced design while remaining single ended.
- Voltage boosting circuitry. The power supply circuity in the 627 line takes the 3.7 volts of the internal Lithium ION battery and boosts this to over 18Vs p-p. This allows the opamps to work and sound better and provides enough voltage to drive high impedance headphones.
- Dual Buffers per channel for a total of 6 buffers inside each amplifier. Extra buffers allow for a low output impedance when driving difficult low impedance headphones. They provide extra current when needed by your headphones.
- High performance Audiophile Opamps, Texas Instrument’s OPA627. Each amplifier in the 627 line uses a version of this excellent audiophile opamp.
- High Performance Buffers. Each amplifier in the 627 line uses several high performance Burr Brown BUF634 or (NOK) Texas Instrument’s LME49600.
- High Performance Resistors. The resistors in the 627 line, excluding the NOK, uses Surface Mount Thin Film resistors that have a tolerance of 0.1%. Some amplifier manufacturers use lower spec 1%, 2% or even 5% tolerance resistors. The lower tolerance means that two identical value resistors, for instance resistors used between the L and R channels, will be within 0.1% of each other’s value resulting in greater channel matching.
Now how does the NOK improve upon this:
- The NOK uses the best resistors, in the amplification circuit, for audio applications, period: The Vishay Dale “Naked” Z-Foil. These were specifically designed for audio. Google them and you will find review after review of people improving the sound of their equipment by replacing just one resistor with one of these. They are that good.
- No potentiometer, just two Z-Foil resistors between your source and the amplifier stage. I believe the best potentiometer for audio applications is a stepped attenuator. Stepped attenuators reduce a traditional potentiometer’s construction down to different combinations of just two resistors. When you turn the dial on a stepped attenuator you are basically selecting two different resistors with each turn. Stepped attenuators are large though and too large for a portable amplifier. To implement this feature into the NOK I’ve designed a custom two stage stepped attenuator. You only have two selections for a set volume and then you control your volume from your source’s variable line-out. More and more sources, especially the higher end sources for which the NOK was designed for use some sort of variable line out. To get closer to the original signal, we’ve eliminated what really wasn’t needed anymore, the potentiometer.
- Cardas RCA inputs as well as a 3.5mm input. The NOK was designed to be paired with the best sources available and some of these source have RCA outputs.
- OPA627BM opamps instead of OPA627au opamps. The BM version is the “CAN” version of the OPA627. It is built to higher specs and is designed for extreme applications such as the NOK.
So, I present to you the NOK. The newest amplifier from Portaphile and as usual, built specifically for The Portable Audiophile who demands and expects nothing short of the best.
This is an expensive amplifier to build and I don’t have all the parts needed in stock all the the time. Please expect a 2 to 4 week build time after you’ve placed your order.