Raleigh Audio Active Output Stage for Analog Input
This 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 AudioXpress. The circuitry is fully balanced from input to output, accommodating both balanced and unbalanced inputs and outputs.
The input section of the Active Output Stage provides a passive phase-splitting circuit that uses a Lundahl LL1676 transformer in each channel to accommodate both balanced and unbalanced sources and provides two equal signals of opposite polarity for the output section.
The output section of the Active Output Stage provides a transformer-coupled circuit 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 amplifying device is the robust Russian 6N30P (6η30π) dual triode, one per channel. The output transformers are the superb amorphous core Lundahl LL1674.
Advanced Minimal Reactance Power Supply circuitry is incorporated into the Active Output Stage PC board, so only a Lundahl LL1683 power transformer is required to complete the picture.
The circuitry employs all top quality parts, among them: Lundahl transformers, Cardas film capacitors and tantalum resistors.
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 Active Output Stage, as shown in the photograph, is assembled and tested. Of course, the line stage constructor must provide an enclosure, power transformer, jacks, an input selection switch, volume potentiometer, and other minor parts.
Features, requirements, and specifications:
Raleigh Audio Active Output Stage for analog input (assembled and tested): $1595
Optional Stepped Attenuator: $199
Raleigh Audio Extreme Preamplifier
The evolution of the Differential Line Stage designed by Dave Davenport at Raleigh Audio has continued, with the aim of providing ever better sound, yielding a true high-end audio line-level preamplifier. The Extreme Preamplifier consists of an attractive enclosure containing two sections; an analog input selection stage and Active Output Stage (described above.) The analog input selection stage provides the circuitry to interface with either single-ended RCA jacks or balanced XLR jacks and relays to connect the selected input to the Active Output Stage.
The Extreme Preamplifier is designed for flexible configuration. Up to six relay switched inputs can be accommodated with any mixture of balanced (XLR) and unbalanced (RCA) inputs. 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.
Extreme Preamplifier is suitable for anyone needing a versatile, high quality line-level preamplifier. It features moderate gain, low output impedance, wide bandwidth, and extraordinary sound quality.
Call or e-mail to get a price quote for your custom configuration.
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 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.
23 step attenuator parts kit for your favorite standard through-hole resistors (includes switch, PC boards, and instructions, but no resistors) $129