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Regulator wg Decibel Dungeona. Omówienie na podst. 10 pozycyjnego o wartości 10k, poniżej wartości elementów dla 24-ro pozycyjnego 15K i 59k
Ideally, we need a switch that will cope with both channels (mono volume controls are a pain in the backside!), that can offer around a minimum of 20 steps of attenuation. Switches of that specification tend to be fairly expensive. But what if we can get away with a smaller range of attenuation? When we make our own hi-fi, we can do this because we can tailor the attenuator to our system. A commercial attenuator could go in any system and therefore has to have a wide range of attenuation to work in any system.
I have found from experience that I can happily use a 12 way attenuator without problem. And fortunately, it is possible to build a 12 way (stereo) switch using a switch mechanism and wafers (the make-before-break type also called shorting).
In the above picture you can see the black plastic switch mechanism and the two blue switch wafers. The attenuator is a shunt type which means that there are only two resistors in the signal path at any setting. Other designs of attenuator can have up to thirteen resistors in the signal path. The trade-off with the shunt design is that impedance changes at each setting but in practice, this is not a problem if you use the correct value resistors.
I had found that my passive pre (which is just a pot in a pod) worked well with my buffered GC\'s so I decided to more-or-less use the same value resistors for the attenuator used in the active pre amp. Here is a list of those values.
Series resistor = 10K
Step Resistor Db cut
1 100 34
2 150 30
3 220 27
4 330 24
5 470 21
6 680 18
7 1K 15
8 1K5 12
9 2K7 9
10 5K1 6
11 12K 3
12 4M7 0
Note that these values and this range of attenuation works well if you use a unity gain buffer (no gain) and a gain in your power amp of around 20, as is the case with the basic Gainclone designs. If your system has more gain, in the pre amp and/or power amp sections, you may find that you need more attenuation than the 34 decibels of the above design. If you need more attenaution, contact me and I will send you a different set of values.
The construction of the attenuator is as follows:
* Assemble the wafers onto the switch mechanism.
The switch mechanism has an adjustable stop so make sure that it is set so that you have a full 12 positions available.
Make sure that you place the switch wafers the same way around and also ensure that the same position is selected on both wafers.
* Find the \'starting\' position of the wafers by turning the shaft anti-clockwise until it can not go any further, ie it comes up against the adjustable stop. Now, using your multimeter, check which tab on the wafer is connected to the input tab on the opposite face of the wafer. Mark this tab with a pen.
* I suggest cutting one leadout of all the resistors to about a quarter of an inch (6mm). Leave the other end intact for now.
* Solder the short end of the lowest value resistor (100R in this case) to the tab that you have just marked. The resistor should form a straight line with the centre of the wafer and the solder tag.
* Continue soldering all the resistors to the tabs of the first wafer in a clockwise direction (when looking from the shaft side of the wafer).
* Take a piece of copper wire (0.8mm to 1mm diameter) and remove any insulation. The wire should be about six inches (300 mm) in length.
* Find something circular about 2 inches (50 mm) in diameter that you can use as a former. Form the copper wire into a neat circle, trim the ends and solder them together.
* Place the attenuator into something like a vice (or a hole drilled into a piece of wood) so that the wafer is horizontal. Place the copper wire circle over the resistor ends so that it is centrally positioned in relation to the wafer. Carefully fold the leadout of one resistor over the copper wire making sure that you don\'t reposition the copper ring. Do the same with the resistor on the opposite side of the circle and then the two resistors between these two. And so on until the wire ring is securely held in place. Then trim the excess leadouts and solder all connections.
* Repeat the above steps for the second wafer.
* Solder the input resistors to the single tabs on the wafers together with the wires that will connect to the buffer input.
* Solder a wire to each wire ring. These wires will go to the ground connection of the buffer.
The attenuator is now completed. A slightly fiddly job but nothing that requires special tools or skills! Depending on your choice of case, you will need to either mount the switch in one side of the case, or make up a suitable bracket to hold it in position inside the case as I have done (made from acrylic sheet) in the above picture.
You can \'tune\' the sound of your stepped attenuator by your choice of resistors. I prefer to use some carbon film resistors for the series (input) resistors and 1% metal films for the shunt resistors. I have also used Welwyn RC55 types for the shunt positions.
TIP - If you use anything with a greater tolerance than 1%, I suggest that you sit down with a meter and measure the resistors to get the closest match as possible. Remember, keeping both channels as close as possible is very important!
Stepped Attenuator Specifications (24 way switch) shunt type.
Here are the specifications for two stepped attenuators. Both are designed around a 24 position rotary switch. Use a make-before-break type.
The maximum cut of around 54db is enough for the \'Decdun\' project system and should be enough for systems that do not have an excessive amount of gain. Most of the steps are about 1.5 db, with larger steps at the lower volume settings which you will seldom use. There is no mute position as it uses up another position on the switch and in the \'project\' system the selector switch provides a mute option anyway.
The resistor values quoted are available for the Welwyn RC55 and RC55C range. If you do want to use 1% metal films, choose the closest value. Because you have to buy larger quantities of the metal films, typically 100, it is often as economical to buy the better quality Welwyns.
The first design has an impedance which varies between 15 and 30K and the second an impedance which varies between 59 and 134K. I have found that both will work equally well in the \'project \' system.
Series resistor = 15K
Step Resistor Db cut
1 30R1 54
2 47R4 50.7
3 75 46.1
4 133 41.1
5 200 37.6
6 332 33.3
7 487 30.1
8 681 27.3
9 825 25.7
10 1K 24.1
11 1K21 22.5
12 1K4 21.4
13 1K69 19.9
14 2K15 18
15 2K49 16.9
16 3K16 15.2
17 3K83 13.8
18 4K87 12.2
19 5K9 11
20 7K5 9.5
21 10K 8
22 14K 6.3
23 19K6 4.9
24 30K6 3.5
Series resistor = 59K
Step Resistor Db cut
1 121 53.8
2 154 51.7
3 237 48
4 383 43.8
5 681 39.9
6 1K 35.6
7 1K47 32.3
8 2K05 29.5
9 2K61 27.5
10 3K32 25.5
11 4K02 23.9
12 4K54 22.9
13 5K9 20.8
14 6K81 19.7
15 8K25 18.2
16 10K 16.8
17 12K1 15.4
18 15K4 13.7
19 18K7 12.4
20 23K7 10.9
21 30K1 9.4
22 40K2 7.9
23 53K6 6.5
24 75K 5
być zrobione maksymalnie prosto, ale nie prościej"-A. Einstein
