The Danish Audio ConnecT (DACT) CT1 Audio Attenuator is a compact, 24-step switched attenuator. It uses surface mount(SMT) resistors in a series configuration and is available in one (mono unbalanced), two (stereo unbalanced or mono balanced), and four (stereo balanced or quad unbalanced) deck configurations (Photo 1).
It offers standard controls with total resistances of 10, 20, 50 and 100kOhm. Custom versions will be considered. The nominal attenuation at each of the 24 steps is: 0, -2, -4, -6, -8, -10, -12, -14, -16, -18, -20, -22, -24, -26, -28, -30, -32, -34, -38, -42, -46, -50, -60 and -infinite dB.
The SMT resistors are soldered to a circuit board, which, in turn, is soldered directly to the switch deck pins. The layout is very compact, and is said to minimize inductance and stray capacitance. Both the circuit board and switch contacts are gold-plated. The switch source appears to be ELMA.
Figure 1 shows the outline dimensions of the two-deck version (also see Photo 2). The front of the switch has a locating tab that fits in the notch in the mounting- panel hole. The header pins allow you to make connections by hardwire, a header socket, soldering to a circuit board, or, I suppose, a wirewrap (Photo 3). The CT1 takes up little panel space and is suited for "slim-line" enclosures that can't accommodate larger switches or potentiometers. Danish Audio ConnecT's model CT2 (not tested) is slightly more compact.
Figure1. Two-deck CT1 attenuator outlines dimensions
The switch has a positive mechanical detent at each step. While the turning torque is light, the detents can be felt and heard. The 6mm-diameter shaft accommodates knobs with standard 0.25" shaft holes. If panel space is at a premium, you can use a relatively small-diameter knob and still turn the shaft without using excessive force.
Danish Audio ConnecT has a comprehensive data sheet for the CT1. Key electrical characteristics are listed in Table 1.
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You can find more specifications on the DACT website. My shop isn't equipped to confirm many of the electrical specifications. I checked attenuation accuracy, channel balance, frequency response, and channel separation with the outputs of a 20kOhm stereo CT1 feeding unity-gain buffers. This setup simulates the loading an attenuator might see in real life. The CT1 was mounted in an aluminum box with its inputs connected to the input jacks via shielded cable. The attenuator outputs were connected to the inputs of a pair of Borbely tape buffers1 by short, unshielded wires. The buffers isolate the attenuator from the loading effects of cables and test equipment and have a 1MOhm input resistance.
Measurements
Measurements were made with aFluke 8050A digital multimeter² using the Morrey version of a Heath IG-18 audio generator as a signal generator³. In addition to measuring the control resistance, I measured attenuation for each control step at frequencies of 1, 10, 20, 50, and 100kHz using the relative measurement capability of the 8050A. I then analyzed the measurements to extract attenuation, channel-balance, and frequency-response errors. The total resistance errors for the two channels were -0.1% and -0.075%, which meet the tight specifications.
For further information on attenuation, channel balance, and frequency-response errors, refer to http://www.dact.com.
At the specified level of attenuation accuracy, the loading of a 1MOhm resistor on a 20kOhm control is significant. Therefore, I compensated attenuation measurements for the buffer input resistance. Most of the attenuation errors at 1kHz were well within the ±0.05dB limit, specified at DC. Those that exceeded the limit I attribute to measurement error rather than a deficiency in the CT1. In any case, the audible result of attenuation error is an incorrect setting of the system output level. Step size and finite loading of the attenuator are likely to dominate the ability to establish the desired level, rather than the insignificant errors indicated by the measurements. The channel balance (tracking between the two decks) was well within the specified ±0.05dB at frequencies up to 50kHz. You should not experience any "image wander" when the control position is changed. The frequency-response errors at 10, 20, 50, and 100kHz are relative to the output at 1kHz and are inconsequential throughout the audible range. The larger errors at higher frequencies and high attenuation definitely result from the 8050A's measurement limitations rather than any deficiencies in the CT1.
Figure 2 
shows the separation, relative to the output of the driven channel, at -10dB attenuation with approximately 1.5V RMS out of the driven channel. The nondriven input was terminated with a 150Ohm resistor to simulate source output resistance. The least separation occurred around -10dB attenuation. Some experimentation indicated that application of shielding might improve the separation figures a couple of dB. Since the results were excellent without shielding, I didn't pursue it further. If you take care with the layout of the ancillary circuitry, the CT1 shouldn't audibly degrade separation.
Listening Tests
For listening tests, I mounted the CT1 in an aluminum enclosure and directly wired it to gold-plated pairs of RCA-style input and output jacks. The buffers used for the instrumented testswere not used except for a brief listen. I used a 20kOhm Alps "black" pot, which was several years old, and my "passive preamp" for comparison. The Alps pot was configured the same as the CT1. The "passive preamp" has 32-step, dual-concentric series attenuator; a 20dB "mute" switch; and extensive input and tape-recorder selection switching. The dual-concentric control was built a number of years ago with 2% carbon-film resistors and Shallco silver contact switches. The "passive preamp" jacks are not gold-plated. I auditioned commercial CDs and master DATs of some of my own recordings. The system front end consisted of a Rotel RCD-955AX CD player or TASCAM DA-30 MkII DAT recorder, used as transports, feeding an Assemblage DAC 1.5. Since my DATs were recorded middle-side (M-S)with a crossed figure-8-pattern stereo microphone, the DAC fed a homebrewed M-S dematrixer only for the tapes. A partially "pooged" Hafler DH200 amplifier fed IMF Studio III-B speakers. While not state of the art, the system has a reasonably natural sound. Since it had the coarsest level of resolution, I first chose the listening level using the CT1. The other two controls were then set to give the same level out of the amplifier at 300Hz. Since the "passive preamp" was designed for the load of the amplifier and the CT1 wasn't, the output was 0.17dB lower with the "passive preamp" at the closest setting. The Alps pot wasn't competitive with either the CT1 or the "passive preamp". A DAT of Stravinsky's Pulcinella Suite provides a wide range of instrument combinations and colors. The Alps pot imparted a significantly colored, "hollow" sound to the ensemble, with no sense of "air". A CD of Maurice Durufle´s organ music (Hyperion CDA66368) has a great dynamic range, a variety of organ colors, and some wind and "room" sounds. In the first cut, there is a quiet section with a mixture adding brightness. With the Alps pot, I couldn't hear the fundamentals, and the mixture sounded pretty much of a hash. There wasn't much sense of the room or the wind noises at the end of the cut. I didn't invest more time with the Alps pot.
Natural Sounds
With the Stravinsky, the CT1 had very natural instrument sounds with a good sense of space. The harmonics were well integrated with the fundamentals. In comparison, the "passive preamp" had a somewhat darker sound, with less sense of the room. The sound was bound more to the speakers, and the harmonics were less well integrated. With the Durufle´ selection, there was clearer definition of the organ's voices, better sense of the space (and wind noises), and better integration of fundamentals and harmonics with the CT1.
A CD by the BBC Singers (BBC MM125) revealed some interesting contrasts. In Bach's Der Geist Hilft (BWV 226), the singers were clearer, and the continuo organ more distinct and rhythmic with the CT1. This cut has a lot of what I call "splatter echo", and I surmise the recording venue, St. Paul's Church, Knightsbridge, London, has hard surfaces. Anyway, the "splatters" were more distinct with the CT1 and more in keeping with my experience in similar acoustics.
In a piece by Max Reger, the CT1's sound was less dark and more natural. In the loud parts, the chorus sounded more intense and involved than with the "passive preamp”. In some songs by Mendelssohn, the micro dynamics were more evident and the choir less bound to the speakers when the CT1 was used.
In a recording of our church choir with organ and string orchestra, more individual singers were recognizable and there was better separation of voices and instruments through the CT1. I could go on but, you get the picture - the CT1 was simply clearer than the "passive preamp". This wasn't accomplished by the addition of artificial detail. The components of the sound were better integrated and closer to life.
In comparison, the "passive preamp" interjected a slight brownish haze that obscured detail. By the way, when the buffers were added to the equation, the sound was excessively bright and unattractive. It reemphasized the reasons I haven't had a line stage in my main system for 25 years.
With its clear, natural sound, superb specifications, compact size, and high quality standard, I highly recommend the CT1 for consideration when you need a volume control. After listening to it, I'm compelled to upgrade my system. Now, if they'd just provide a high-resolution means to adjust balance...
This review was initially published on Audio Electronics, Issue 1, 2000, p.p. 42-45, of Audio Electronics magazine.
We wish to thank Audio Electronics and Fred Gloeckler for the thorough review of our CT1 stepped audio attenuator. One detail to add is that the CT1 is now also available in 250kOhm and our CT2 attenuator is also available in 500kOhm. Since the review appeared, we also have changed our policy to include in our prices the shipping costs worldwide. The new prices (in US dollars) are: CT1 mono, $113; CT1 stereo, $154; CT1 balanced stereo, $284.50; CT2 stereo, $136; and CT2 A/V audio 6-channels (new item), $319.
We appreciate the care that Mr. Gloeckler has taken when considering the measurements and their validity. The CT1 matches most testequipment on many parameters, and it is of course important to evaluate the measurements' validity. Well done!
We have also concluded, like Mr. Gloeckler, that the sonic performance of a high-quality passive volume control is often better than most active preamplifiers. This may not come as a surprise, since all components add their own signatures to the sound, and only the finest preamplifiers are nearly "inaudible."
On the other hand, connecting, for example, a CD player directly to the power amplifier with only a passive volume control in between has its own limitations. Some important impedance issues need to be considered. At our web site there is an application note describing this topic. Both passive and active preamplifiers require a high-quality volume control and we are satisfied that Mr. Gloeckler's listening tests document that there are significant audible differences between standard volume controls and the more specialized controls.
ln his review, Fred Gloeckler caIls for a high-resolution balance control. We will not disappoint him. We are now working on a stepped balance control with very high resolution.
Allan Isaksen
Danish Audio ConnecT, Ltd.
Information originally from http://www.dact.com with permission
For Further information http://www.dact.com