K & K Audio - Lundahl Transformers, audio DIY kits and more

The differential class-A triode parafeed output stage is a further refinement of the circuit, now in its fourth generation, first used in the excellent Raleigh Audio Line Stage detailed in the November 2004 issue of AudioXpressThis output stage uses a transformer-coupled output in a bridged parallel feed configuration that was used by Western Electric over 60 years ago. However, it employs a cascode connected depletion mode MOSFET constant current source (CCS) that is more effective than anything available to Western Electric engineers. The output transformers are the superb amorphous core Lundahl LL1674 units. The amplifying device is the Russian 6N6P dual triode. Dual high-voltage Minimal Reactance Power Supplies are integrated onto the Active Output Stage board, so only a power transformer (LL1683 or equivalent) is required to provide a complete vacuum tube output stage solution.

For DAC applications, current-to-voltage conversion is accomplished using high quality low value tantalum resistors. In addition to being a superb RAKK dac Mark III output stage, this kit can accommodate line-level inputs. The line-level input option is provided simply by adding LL1676 input transformers (not included) to locations on the Active Output circuit board.

Advanced minimal reactance power supply circuitry is incorporated into the Active Output Stage PC board, so only a power transformer is required to complete the picture. The LL1683 is an excellent choice, but the basic power transformer requirements are 500vCT @ 120mA and 6.3v @ 1.5A. 115VAC or 230VAC mains wiring is accommodated by the LL1683. To support this power supply, you need to separately provide an input power connector with fuse, and a power switch.

The kit is simple enough for anyone with soldering experience and the knowledge to safely handle the high voltages (~200VDC) present at several points on the energized circuit board. All of the parts necessary to complete the printed circuit board assembly are included, as shown in the photograph. Of course, the constructor must provide an enclosure, jacks, an input selection switch, volume potentiometer, and other minor parts. The Active Output Stage provides an interface for a volume control. Either a standard 10K to 50K stereo audio potentiometer or K&K Audio's shunt attenuator can be used to control the volume.

An optional volume control (not included) has also been accommodated (see Stepped Shunt Attenuator below).

The kit uses all top quality parts, among them: Lundahl transformers, Cardas film capacitors and tantalum resistors.

The Active Output Stage kit is offered in two levels. The levels differ in the amount of parts provided versus the parts that you must procure on your own. The Level 1 kit provides the minimum parts, the Level 2 all of the parts needed.

Included:

Circuit boards
Assembly manual (on-line)
Schematic diagrams
Parts list

Your involvement:

Order the parts from five suppliers
Cost of additional parts needed - approximately $540 (as specified)
Assemble the kit

Active Output Stage Level 2 Kit for Mk II owners without Mk II trade-in: $849

Included:

All parts, including circuit boards and two LL1674 transformers
Assembly manual (on-line)
Schematic diagram

Your involvement: Assemble the kit.

Fully assembled and tested Active Output Stage for Mk II owners with Mk II trade-in: $849

Fully assembled and tested Active Output Stage for Mk II owners without Mk II trade-in: $999

The evolution of the Differential Line Stage designed by Dave Davenport at Raleigh Audio Raleigh Audio has continued, with the aim of providing ever better sound, yielding a true high-end audio product that can be constructed by someone with some previous electronic kit assembly experience. The product has been renamed the Extreme Line Stage Kit to reflect its progress toward the ultimate in line stage circuits.

This Line Stage is fully balanced from input to output. It will accommodate both balanced and unbalanced inputs and outputs due to the use of a Lundahl input transformer phase splitter and Lundahl output transformer. It consists of the new "double-decker" Active Output Stage Mk III that accommodates both the input transformers and the active transformer-coupled output stage.The analog input is a passive phase-splitting stage that uses an LL1676 in each channel to accommodate both balanced and unbalanced sources and provide two equal signals of opposite polarity for the output stage. There is also provision for two pairs of high quality I/V resistors that can be added on the Active Output Stage board to convert the analog current signal coming from the RAKK dac Mk III into a voltage for the Active Output Stage. A high-quality switching relay is used to select between the RAKK dac input and the analog input.

This kit is suitable for anyone needing an excellent line stage. It features moderate gain, low output impedance, wide bandwidth, and extraordinary sound quality.

The Extreme Line Stage Kit is designed for flexible configuration. Up to six relay switched inputs can be accommodated with any mixture of balanced (XLR) and unbalanced (RCA) inputs. The base output configuration is two stereo inputs (RCA/unbalanced, XLR/balanced or combination of these) and one stereo output (RCA/unbalanced, XLR/balanced). Input and output options are priced separately below. Up to six stereo input pairs and two stereo output pairs can be accommodated on the back panel. Note that a mixture of XLR/balanced and RCA/unbalanced outputs can be chosen, but they are not isolated from each other, so you may not be able to use them simultaneously and preserve the balanced operation of amplifiers being driven from the XLR/balanced outputs.

The Extreme Line Stage Kit includes the chassis, Active Stage Mk III Kit, K&K Audio Shunt Attenuator, two stereo analog inputs (RCA or XLR), one stereo output (RCA or XLR),Lundahl power transformer, and all of the wire, fasteners, and miscellaneous hardware.

Those of us who are using differential tube preamp circuits like the Raleigh Audio active output stages or Lynn Olson Raven-style preamps have had to use conventional audio potentiometers or their discrete resistor analogues, like the DACT or Goldpoint controls to provide control of volume. You can review the choices for volume control implementation in these types of preamp circuits below.

We prefer the implementation with one variable element per channel, the so-called "U" or balanced shunt attenuator. It is simpler and cheaper and has the significant virtue of causing no change in common mode rejection as different volume settings are used.

So what is special about our implementation of the "U" attenuator control? A standard audio potentiometer or discrete resistor analogue has a resistance profile (called a "law") that is configured for standard series attenuator use, not shunt attenuator use. You will probably have to trust us on this, but the math is not the same. Hence, when you use a smallish value of series resistance and a large value audio pot to make a shunt attenuator that won't cost you very much lost gain in the maximum volume position, you will get a control where most of the volume attenuation capability is in the first third of the rotation from zero volume to maximum volume. That's not really what you want, obviously. To fix this situation you can use a series resistor value that is approximately the same value as the pot and now the shunt attenuator performs more reasonably, so that attenuation is spread more evenly over the rotation of the pot, but you lose a fair amount of gain at maximum volume setting. (approximately 10dB in the case where the pot is the same value as the series resistors). Compromises, compromises.....

To avoid these compromises, we have designed the resistor values used for the stepped control to provide equal dB volume intervals (1.5dB) with 5K series resistors, which result in a reasonable 10K input impedance at minimum volume setting. Of course, any kind of volume profile can be implemented with discrete shunt resistors, however this one has proven to be useful for me for the 2 years I've lived with such a control (implemented in my case with a much bulkier (and slightly inferior sounding) array of Holco resistors). To be fair, there is a compromise inherent in these controls, too; that is price. Using Seiden 32NEG switches specially imported by us for this purpose, these are not inexpensive, but are reliable, sound excellent, and have a wonderful "feel". In my experience, no audio potentiometer provides the sound quality that these stepped attenuators do.

Note that you can also utilize these attenuators in single ended circuits with a single 10K series resistor per channel without any other changes.

We auditioned several candidate resistors for these attenuators and selected Vishay parts for their sonic properties. Surface mount resistors have the advantage of making a compact 23 step attenuator possible. The finished attenuator is little bigger than the Seiden switch that it is based on. The SM attenuator is not a kit; it is supplied with all of the SM resistors already soldered in place. Each attenuator is also supplied with 4 (2 for each channel) 4K99 Mills MRA-5 resistors for the series resistors, unless you have purchased the Combo Input Stage Kit, which already comes equipped with the appropriate series resistors..

We are also offering a kit of parts including the Seiden switch and PC boards that will accommodate standard through-hole leaded resistors (like Holco H4, Caddock MK-132, naked Vishay or Kiwame 1w resistors; Rikenohm or Allen Bradley 2w resistors are too large) so that you can use your favorite resistors and concoct your own shunt attenuation profile. Obviously these are considerably larger than the SM resistor-based attenuators.