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DACT
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Danish Audio ConnecT - FAQ. Read here the questions that others have asked before you. Thorough answers are included.
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General questions
Q I have read about a DACT CT100/CT101 combination in the magazine The Absolute Sound and I am very interested in it. I don't need the phonomodule but would like to know if I can use the preamplifier section with my Densen B-400 CD-player and my Bryston 4b amp. Furthermore I would like to know if the sound of my system will improve with the use of your product instead of my Mark Levinson ML-1 preamplifier. Lastly I would be grateful if you could inform me to what extent I need to assemble it myself (cabinet etc.).
A What we are currently offering are ready-made preamplifier modules (CT100 phono stage and CT101 line stage). However, you do have to build the modules into a cabinet yourself and power them by an external power supply. Alternatively, you may ask a trained technician to do the job for you.
You may use the C101 line stage between your CD-player and amplifier. You might want to consider to equip the CT101 with a high quality attenuator, for instance our CT2-10k-2.
We prefer to let others judge whether the DACT combination sounds better than specific ready-made preamplifiers.
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Attenuators
Q Hello, would you please advise if the attenuators in your product range have a mute position by completely disconnecting the input from the output? (Sort of like an "off" setting).
Also, when you nominate the impedance of a particular model, say a 10Kohm CT2, how does that change when you adjust volume. ie., does input impedance change?
By what amount does the output impedance change as you adjust the attenuator? (using say a 10kohm impedance design).
A Yes, the attenuator position all the way counterclockwise is "off", a complete muting.
All our attenuators are series attenuators. It means that the input resistance will change depending on the load. If there is no load on the attenuator output, the input resistance is always the nominal resistance. So, in case of no load (infinite), the input resistance will always be the nominal resistance (for example 10 kOhm), independent on the volume setting. This is often the case (or a very close approximation) in active designs, where the volume control may be followed by a high-input-impedance amplifier stage.
The other extreme situation is when the attenuator output is shorted. This makes a largely varying input resistance. In case the volume is turned fully up, the load is coupled directly to the input. So with a shorted output, the input resistance is 0 when the volume is turned fully clockwise. When the volume is turned all the way down, even with a shorted output, the input resistance is still 10kOhm.
All practical applications fall between the two above extremes. If you for instance have a 100kOhm load on a 10 kOhm attenuator, for most audio applications, the input resistance variation will not be significant as the volume setting is varied.
In case you load a 10k attenuator with a 10k resistor, the input resistance will vary between 5k (at max. volume setting) and 10k (0 volume).
Q What is the main differences between the CT1 and the CT2? They look different, and have different dimension, but which one is better sonically?
A CT1 is our original attenuator, which is still the one that has gained the most fame.
CT2 is a later design, where we, compared to CT1, obtained to design a couple of parts out.There are not really any drawbacks buying a CT2 instead of a CT1. The measured performance is the same, and we were never able to tell any difference in sound either. So we usually recommend CT2 as it is cheaper than CT1.
Q Basic questions here: What's the best way to connect the “antistatic” 1Mohm resistor that you recommend on the CT1, CT2 and CT3? And for balanced connections on the 4-deck CT1, I assume one would use decks 1 & 2 for channel 1, and decks 3 & 4 for channel 2, but it doesn't really matter, does it?
A If you mount the switch (CT1, CT2 or CT3) directly in a metal part that is grounded, you don't need the 1Mohm resistor at all.
Only in cases, where the switch metal housing is not connected to ground, is the resistor required. You just squeeze one of the resistor leads in under the attenuator nut before tightening the nut. The other resistor lead should be connected to signal ground.
On a 4-deck attenuator, you can really connect any channel to any deck. The separation between two decks next to each other is not infinitely large, but sufficiently large to be of no concern in practical audio applications.
Q I am highly interested in your CT1 mono/mono audio attenuator and know that you provide four kinds CT1 (10K, 20K, 50K and 100K). How do I choose suitable resistances of CT1 ? If we compare frequency bandwidth, I know 10K is better than 100K, but, anything else that I need to consider?
A In case you want to use our attenuators for upgrading existing equipment, and replace a normal volume control, we recommend that you use same CT1 resistance as you original volume control.
For passive preamplifiers, we usually recommend 10 kOhm values, as they are less sensitive to cable capacitances on the output.
By the way, CT1 is also available in 250k.
We have a later designed attenuator called CT2. CT2 is just as good as CT1, just a little cheaper. CT2 is available in 10, 20, 50, 100, 250 and 500kOhm.
Q I bought a 100kohm CT1 attenuator last week from one of your distributors. After I soldered it into my pre-amp, I got a problem. Maybe it's because the gain of my pre-amp is too high. Anyway, I can only put the attenuator on the 1st step. 2nd step will be too loud. Not to mention that there are another 21 steps. Is there any way I can fix that to lower down the gain.
A Your problem is not unusual, but still quite unpleasant.
If you have any way of reducing the gain of your power amp (for instance if it has a volume control), that would be one way to go.
Otherwise, you can try to add a resistor in series with the attenuator, on it's input side. To make a difference, the resistor needs to be quite large in your case. Probably 200 kOhm or more.
Another way is to add for instance an attenuation unit (may consist of one series and one parallel resistor) between the preamp and the power amp.
Q I have a Conrad Johnson PV12 preamp and want to install a DACT volume control. The pot is 100k. Which DACT do I need, CT1 or CT2
A The CT2 is a later design than CT1. They do the same in terms of sound and specifications. Therefore, as CT2 is smaller and cheaper, we recommend you to use CT2. A CT2-100k-2 should be the right choice for you.
Before ordering, I suggest that you make sure there is space in your amplifier for a CT2, preferable in the same place as where the original volume control was located.
Q I am looking at building a passive attenuator to connect my DAC to my amplifier.
My question is, what are the sonic differences between the CT1 and CT2? I would assume the CT2 would be more transparent, as there is one less set of contacts in the construction. Is this true?
A Principally you are right, CT2 should sound better than CT1. However, the difference is so small that we never managed to prove any differences, neither by measuring nor by listening.
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Passive volume controls
Q Hello there in Denmark!
I am going to build myself a preamplifier for a CD player and I have two possibilities. First one is to build a passive preamp using your 10kohm attenuator. Second one is to use your CT 101 preamplifier module with an attenuator.
I believe that the sound quality with passive pre will be good, ok. Then my question. Do you think that I can get real sonic advantages by using CT 101 with a CT1 attenuator? I don’t need any extra gain, just looking for best possible sound.
My other equipment are Rotel RCD-990 CD-player and Musical Fidelity A3cr power amp. The input impedance of the amplifier is 30 kohms.
A Your question with respect to passive vs. active preamps is an interesting one. I don't think we can offer you any definite answer.
For sure, if you have to run long cables between your preamp/volume control and the power amp, it is recommendable to use a buffer like CT101 after the volume control.
In other cases, when the cables are not very long, the results are different. We have had many customers sending us feedback telling how good sound they have obtained by using one of our attenuators as a passive preamp. See for instance the review at http://www.dact.com/html/ct1_in_etm_com.html
In other cases, it seems that the sound using a passive preamp lacks dynamics, especially in the bass.
Our experiences are that we most often prefer to buffer the volume control.
Q I'm interested in CT1. I have a problem to choose which kOhm is match my system. Does it depend on the power amp or the CD source? Does higher kOhm provide more attenuation of the signal ? I have a power amp Mark Levinson.No.334, no CD source yet, which attenuator kOhm should I choose for a passive preamp?
A Principally, the attenuator resistance depends on everything, CD-player, power amp and cables. In practice, the most critical parts are the cables. Always avoid long cables with high capacitance. That's why we always recommend using 10kOhm attenuators for passive volume controls. So we also do in your case.
Q I am right now driving my power amp direct from the output of my CD player using the player's volume control. I would like to fit a stereo attenuator to the output of the CD player. My question is which value is suitable ? The input impedance of my amp is 50 k ohms . Should I use the same value or the 10 k one as recommended in your web site ? Or should I use a 50k one that matches the amp input impedance at full volume? Right now I am in the middle of upgrading the the CD player power supply. Your technical expertise is much appreciated ...
A We recommend that you use a 10k attenuator. Usually, what you want is that the source impedance is as low as possible. In your case, the attenuator determines the output impedance of your CD player. Higher attenuator resistances will potentially reduce your treble level.
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CT100
Q What is the best configuration of the CT100 to go between my Van den Hul "THE FROG" and the ARC REF300 fully balanced mono blocks? With or without a preamp.
A CT100 can easily drive your ARs and the sound will be the best in that way. However, you will be missing a volume control. Therefore, you might consider adding an active or passive volume control between the two.
Q Hi, I've heard your DACT CT100 phono stage, and it is a very good one. However, I'm looking only for a MC Step-up transformer. Do you have such a product, and similarly as flexible as the phono stage ? Or can the phono stage be modified to be a step-up ?
A Our CT100 phono stage is actually so flexible that it also works with MC cartridges. There are so many gain and input impedance settings, and CT100 is so sensitive, that practically all MC cartridges can be connected directly to CT100 without any kind of step-up transformer.
Q I' ve just seen your Internet site, and I' m interested in your CT100 phono preamp. In particular I' m interested in possibilities to have a balanced output, cause I must link to a turntable with an amp in a different room. Is the output really balanced?
What' s the maximum path (in meters) available for resolve this problem?
A Yes, CT100 has a 100% truly balanced output signal option.
I am not able to give you an exact measure for how long cables you can run in balanced mode, but I am sure it is several meters. This is not only because CT100 has a balanced option but also because CT100 has a very low output impedance, that can drive capacitive loads from long cables.
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CT101
Q What is best resistor value for a DACT attenuator when using with the CT101 preamplifier?
A We generally recommend the lower attenuator resistances, so 10 or 20kOhm is what we recommend. For more information, please download this PDF file http://www.dact.com/CT101_Instruction_Manual.PDF.
Q What are your thoughts (or test results if possible) on using the CT101 buffer IN FRONT of the attenuator?
A You will not gain so much by adding a CT101 in front of (before) the attenuator. More important with a CT101 after. Of course, using a CT101 in front gives you the option to decide for any input impedance that you want. And the input impedance will be completely independent of the attenuator position and load.
If you have both a CT101 before and after the attenuator, there are certain advantages not related to the attenuator: Using two CT101s give you a possibility of reaching 24dB of gain. Or, if you need only 12 dB, you may set each of the CT101s to 6dB, obtaining better spec's than if one was set at 12dB.
Q I noticed that the CT101 has an input impedance of 100 Mohm and output impedance of 0.1 ohm. What is the effect of different volume attenuator resistances when using this device in the signal path?
A CT101 has so high input resistance that it does not load the attenuator at all. So for that matter, the result is the same regardless of which attenuator resistance you use.
However, we generally find that lower attenuator resistances sound better.
Q you were completely right: This combination ((CT1/CT101) is simply great! I've now even managed to install it directly into the CD player using the available PSU after having initial problems which I cannot explain why. In the first approach I thought I did everything right but it did not work. In the second approach last weekend I did all the same way and now it works!? And the compactness of the design is wonderful. You can place the PCB almost everywhere in the housing and therefor keep the signal path incredibly short. Am I right in assuming that it is better to install it in the CD player than in the Amplifier? I think the signal from the CD to the Preamp is more sensitive than the one from the preamp to the amp.
A There is no doubt that the CT101/attenuator should be mounted in the CD-player rather than further forward in the chain. That's the way you make sure the interconnecting cables have as little influence on the sound as possible. In your setup, we suggest not to use any other preamp at all. That would only make the sound worse.
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Power supplies
Q Can switching regulators be used for CT100 and CT101? I want to make a compact preamplifier with high quality.
A Yes, switching regulators can be used. Just try to make sure of good filtering of the switching frequency and to make sure the power supply has low output impedance up to at least 100kHz.
Q Just bought your DACT RIAA preamp, with one CT-pow1, a few days ago...works fine!
I want to make two power supplies for it...Do I need regulated power supplies?
Or is a transformer (2x15V), rectifier bridge, buffer elco's ok???
Any other interesting ideas?
A The sound of the CT100 phono stage generally improves with better power supplies. "Better" is in the usual sense of power supply virtues for audio circuits. Key specifications are for instance:
1. Low output impedance, also at extremely high frequencies.
2. Low noise
3. Good current reserve with potential of quick delivery
We don't think a good audio power supply is necessarily regulated. The idea you have may work very well.
We know from our own tests, that CT100 sounds superb operated by batteries, although this may not be such a practical solution.
However, please make sure to have good filtering, so the 50 Hz (100Hz) mains frequency is very well suppressed. Also, it is very important that you use filtering capacitors with good high frequency characteristics (= low ESR at for instance 100kHz, and low series inductance). There are some electrolytics, that are designed specially with these characteristics. That's for instance what we use in the CT-pow1, that you already have, which by the way does not have active regulation.
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