It is the K.I.S.S. method. Keep It Simple Stupid. Virtually nothing is more simple than a passive device to adjust the volume of your music reproduction system. In fact virtually every system has an audio attenuator in it! So why do you really need all those other parts in the signal's path to possibly add noise and distortion to the audio signal? Why, too, the added expense of of other parts when a simple less than $200 project can be considered by some as inherently better than a $4,000 active preamplifier? Is all perfect in the Land of Attenuators (L.A.)? Maybe... maybe not. And even if you have a great preamplifier now, how about a great way to possibly enhance it for only $200?
The virtues of passive attenuation has long been known. Since most source components like CD/DVD players and turntable preamplifier output two volts or more, they generally have enough power to "drive" most amplifiers. The "trick" is to simply keep your interconnect lengths as short as possible while keeping the output impedance of the source component at least a multiple of hundred smaller than the input impedance of your amplifier. To better clarify, if your CD player has an output of two volts at 200 ohms, the amplifier should have an input that maxes out at 2 volts with an input rating of 20,000 ohms. Since these type of rating are the norm, there is really very little to worry about here. Allen Isaksen of DACT replied to me in an e-mail "The passive preamp is also to be considered as a source - it is the source for the power amplifier. The source impedance when using a passive preamp is higher than you would normally like it to be. In your case, where you use a 10kOhm CT1, the source impedance (attenuator output impedance) will vary between 0 and 2.5kOhm depending on the volume setting, provided that the source in front of the attenuator has zero ohms output resistance. This is really far from ideal, but as long as the load impedance on the passive preamp is either purely resistive or at least the capacitive or inductive component of the load impedance is very small, it still works. In practice you will obtain very good results from using a passive preamplifier because the power amplifier normally has a purely (nearly) resistive input impedance. So all you have to consider is the capacitance of the cable connecting the passive preamplifier and the power amp. Therefore, you want to look for cables with low capacitance per meter and you also want to make them as short as possible. Based on the above it is obvious that 5kOhm attenuators (or even lower impedance's) would be better for passive preamps. Unfortunately, that is not practical because if the attenuator impedance is reduced to less than 10k, the load on the CD player output becomes too tough - many CD players cannot deliver the required current. In conclusion: 10kOhm attenuators are normally the best compromise for passive preamps."
In fact many of the new breed of highly praised integrated amplifiers use a simple passive attenuator circuitry for volume adjustment! Seen below is the Danish Audio ConnecT (DACT) CT1 10K-2 stereo audio attenuator with optional CT-knob1 stainless steel knob. Because this project is for a pure passive unit, the 10K rating was chosen for its low impedance. Higher impedance ratings are good for those replacing their active preamplifier's volume potentiometer.
The DACT CT1 uses extremely accurate surface mount devices (SMD) metal film resistors for 24-step attenuation. As seen to the right, the small blue and dark brown squares are the actual SMD metal film resistors. SMD devices are used in the military for their extremely high precision. In fact they are accurate within .05db! Because SMD are very small, the signal's path is about five times smaller than other typical non-SMD units. If there is one thing i have learnt over the years, it is that the shorter the signal path the better. DACT also used very close attention to the design and layout of the parts as they are mounted to the circuit board to insure very low series inductance and very low stray capacitance. Furthermore, all contacts are gold plated while the rotary switch itself in manufactured in Switzerland. DACT left no aspect to chance in their top-quality design. Because of the unit's accuracy, DACT claims the distortion of their units are an amazing .0001%!
Making your own passive attenuator box is actually quite easy. All that is needed are four female RCA plugs, some wire, solder, a 1 Meg Ohm series resister (ensures dissipation of any static charges), and a metal box. i won't get into the step by step building details here as these diagrams give virtually all the data needed. The 1 Meg Ohm resistor goes in series with the ground wires. Simply said, the entire project took me about 3 hours.
Once i had my unit built and tested, the first thing i noticed was that the music seemed to "time" better. To put it another way, the bass, midrange, and treble seemed to be better synchronized together. While not a huge difference, as a drummer for many year i tend to notice any change in timings of events within music reproduction. Some people say that passive volume takes away the drive from a music reproduction system. With the DACT CT1 in my system the opposite seemed to be true! It was also as if a veil was lifted from the music as well.
While i have a few different preamplifiers here (both solid-state and tubed), the tube unit i have here seems to give a more 3D soundstage at the expensive of ultimate clarity and imaging precision. Plus the tubed unit has a bit fuller lower frequency response as compared to the DACT CT1. Still, there were times i personally preferred the more dimensional tubes unit over the added clarity and seemingly more neutral DACT CT1. i found the DIY passive preamplifier to be so transparent that i was able to hear a greater amount of small inflections in music recordings that no other preamplifier here could resolve!
The CT1 is very chameleon-like as any great preamplifier should be. During monophonic recordings the image had depth when it was in the recording, yet the image was very solidly placed directly between the speakers. On modern stereo recordings like Chesky's O Magnum Mysterium (JG83), which is a great record with 3D-like ambience, the sound of the hall ambience came well into the room with a sense of height and depth only the best preamplifiers could challenge. This recording is very highly recommended as only the best systems will give you that full surround sound-type experience. While still a bit foreshortened in frontal depth and not as full in the midbass/lower midrange as compared to my beloved tweaked out Audio Note M2 silver-wired tubed preamplifier, the imaging with the CT1 was more pinpoint. As for which is "better", this could be considered as more of a preference than a "which is better or worse" in my humble opinion. Tonally, the CT1 seemed quite good indeed. Very neutral sounding vs. the slight midbass boost and either over sweetening or the boosting of the highs some people seem to prefer.
As for speed and attack, there was nothing i had on-hand that came even close to the CT1. This made sense as there really is not much to the CT1 and therefor less = more. King Records Sankyoku - Traditional Japanese Music (KICK 2007) is recorded using traditional Japanese instruments (shakuhachi, shamisen, and koto). Anyone who has had the pleasure to hear traditional Japanese music live knows the light-speed attack yet subtle finesse these instruments have. For it is only the very talented musician that can play such instruments within the musical structure correctly. This is where the CT1 is at the top of it's class! Nothing comes even close the the speed of the CT1.
When it came to deep bass, the CT1 was very balanced through the last few lowest octaves. While i prefer a bit more fullness in the midbass region (60Hz to 200Hz), one can not help but admit the balance of the music octave to octave. As for uppermost frequencies, there seemed to be a light sweetness yet impressively extended sound. The highly regarded Water Lily Acoustics CD A Meeting by the River (WLA-CS-29-CD) is a great example of upper frequency extension. In fact there were times with other preamplifier i felt there was a little extension missing within this recording. Well, with the CT1 as a passive preamplifier there seemed to be just the right amount of extension on the topmost octaves. Amazing how a simple passive volume control can make all the difference! Viva la difference!
DACT also makes attenuators which can be used to substitute for pre-existing volume potentiometers in various active preamplifier units. Word on the streets is that plastic film-type units add a veil to the music and also a thickening of the lower frequencies of the music. Is this the difference i am hearing with my beloved tubed preamplifier? Possibly. My friends who has substituted their stock potentiometers with a DACT all seem to feel that very good improvements in clarity and imaging were the main result. One of my friends swears his $2,000 DACT'ed preamplifier now rivals those in the $4,000 range! While i can not argue with him there, i will say that including labor costs the changing of a stock Alps, Noble, Burnes, etc. to the DACT should cost around $200 (including a technician's labor expense) and possibly achieve a performance boost worth multiples of dollars!
In the end there is no doubt in my mind the advantage of going purely passive volume adjusting with an audio attenuator. The main caveat is that interconnect wire lengths should be kept at a minimum while some attention is paid to the input and output impedances of your components as discussed earlier in this article. DACT also makes very high quality input selectors too by the way. Therefore a multi-input passive volume unit can easily be made if you so desire. In fact if you are very handy and have only one source component, you could potentially just drill the appropriate sized hole in the front of your amplifier and install the DACT CT1 directly on your amplifier! i can not stress enough that anyone whose preamplifier uses a film-type potentiometer could also achieve a healthy dose of added transparency and imaging simply by switching to a DACT audio attenuator. Though the signal path is only gold plated, in my humble opinion the DACT is worth it's weight in gold! Need i say more?
Danish Audio ConnecT A/S