Raleigh Audio Class A Differential Line Stage Kit

Dave Davenport at Raleigh Audio has redesigned the Differential Line Stage to further improve the sound. While the last version was delightfully clean and dynamic, we felt that there was further work to be done to provide more robust bass and an even greater sense of musical dynamics and transparency. This is it!.

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 two kit "modules". These are the Combo Input Stage Kit and the Active Output Stage Kit. An illustration of the relationship of these kits is shown below. The Combo Input Stage Kit is a passive input 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. The Combo Input Stage is so named because it also allows the incorporation of the RAKK dac Mk II into the same enclosure as the Differential Line Stage, reducing cost and footprint in the process. Two pairs of high quality I/V resistors are added on the Combo Input PC board to convert the analog current signal coming from the RAKK dac as a current 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. The Combo Input Stage also provides an interface for a volume control. Either a standard 10K to 50K stereo audio potentiometer or K&K Audio's new shunt attenuator can be used to control the volume.

The Active Output Stage Kit is the Line Stage active gain stage with transformer-coupled output. The differential class-A triode parafeed 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 excellent-sounding, low impedance Russian 6N6P dual triode.

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 Combo Input Stage and Active Output Stage Kits use all top quality parts, among them: Cardas film capacitors, Mills wirewound resistors, Kiwame carbon film resistors, and Holco metal film resistors. See the linked pictures for details.

• Gain: 8dB
• Output impedance: 400 ohms
• Frequency response: 10Hz to 20KHz , -0.3dB, -0.5dB

The schematics of the Combo Input Stage and the Active Output Stage can be viewed via these links.

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 dimensions of the PC board are 6.1 inches by 9.7 inches. About 3.0 inches are required to clear the top of the 6N6P.

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 @ 100mA 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 Active Output Stage kit and Combo Input kit are 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. For the Active Output kit, the Level 1 kit also requires that you experimentally determine the value of the current setting resistor. Refer to the Active Output section of the Digital page for the contents of the Level 1 and Level 2 Active Output kits.

Active Line Stage Kit, Level 1: $349
Active Line Stage Kit, Level 2: $849
Assembled and Tested Active Output Stage: $999
Combo Input Stage Kit: Level 1 (PC board and Instructions only): $89
Combo Input Stage Kit: Level 2: $299
Cardas Quad Eutectic Solder (lead, tin, copper, silver), 100gm roll: $13

Stepped Shunt Attenuator

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 vol ume 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.

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