2013. március 2., szombat

The next level for volume controlling

The most important thing after the audio source is the volume control. Not only it is used to set the desired listening level on your stereo system but that is the first level where you can screw things up.

The most important requirements for a volume controller are these:
-transparent sound
-equality between channels
-many levels or steps of volume
-following logarithmic curve

At these days audio sources follows at least the red book standard, which is assuming that a 2V rms output is guaranteed. So when we are talking about volume control, it is basically attenuating that signal. Some of the options are already called attenuators or passive pre-amplifiers (which term is a bit misleading).

The well known variable resistors, namely potentiometers has the longest history in audio applications. It is basically THE audio attenuator.
What it does to form a variable voltage divider that follows the manufactured curve be it logarithmic or linear. What it does not well is doing it equally on more channels. The technology behind it does not match that precision we want in a high-end audio system. The channel equality is usually 10%.

A well known potentiometer is the ALPS RK27 - Blue Velvet:

 You may find it in many amplifiers and DIY-ers like it because of its better than average performance. The price is moderate, a few times more than a normal potentiometer, and it is available almost every local shops or at least on e-Bay for sure. Just be aware of the copies as there can be many fake ones out there.

For a good reason ALPS designed the ultimate potentiometer in their terms which is the RK50. It is a potentiometer monster which looks beautiful and the price matches luxury too.

Isn't it beautiful? So this is ALPS answer for those professional requirements.
It costs between $600 and $800.
If you asking me it is way too overpriced but I like the colour.

Now back to reality. If you want a better than potentiometer experience then discrete resistor networks come into considerations. The simplest attenuator would be a fixed voltage divider, but it is not variable which is a must, so we want at least a few fixed voltage divider. This device is called the stepped attenuator.

Many professional Hi-Fi builder agree that these are superior to any passive solutions before. Using precision resistors the tracking equality between products or channels can be easily 0,1%. Using resistors also allows for a selection of resistive materials to choose from. The stepped attenuator this way is highly configurable.

The most known manufacturers of these parts are DACT, and ELMA.

 Today most manufacturers make SMT version too which ensures shorter patch and easier building. Many stepped attenuators are available naked, only the body, and you choose and install the resistors you choose.

There are new manufacturers who make even advanced looking stepped attenuators like the one below. It is a Khozmo.

I really do not know if it is superior to the DACT or ELMA versions, so do not ask me. Because these are quite new to the market the longevity is not known yet.
These attenuators are superior but do not forget those mechanical elements. With time a stepped attenuator contacts will age. With care it works for ages.

The other thing need to be mentioned is the number of steps. On a potentiometer you have infinite numbers of settings. On these, you have 24 or 48 steps. With each step of -3dB you can have 72dB attenuation with 24 steps. This is all right but the 3dB change can be too much. You may find yourself turning it up and down because your preferred volume is a little bit lower or higher but you can not have that exact level. On a high-end system, the perfect volume setting is more important than in the kitchen radio. But the mechanical steppers are limited.

The similar solution is an electronic controlled stepped attenuator, which is basically a series of voltage dividers switched in a defined order. Imagine a  counter that switches an exact attenuation divider on and off with miniature relays. The amount of levels or steps are limited only with relay numbers.

Relays has an advantage over open mechanical switches. The contacts are insulated in a little case. Some relays supply gold plated over silver contacts and fast switching. Perfect for the task.

With lots of information about this design some very good articles are out there. The basics of  the design is available on Wikipedia.
For more professional explanation I can recommend the article from Jos van Eijndhoven. The idea of the Relay based stepped attenuator is the logarithmic resistor ladder with binary like switching of the relays or powers of two.

4 step of a log. resistor ladder with switches

If you do 2 to the power of n where n is 2,3,4...any number, you get the possible steps. The n number is also the number of the switches or relays you need to use. So if you have 5 or more relays the maximum number of steps are:
2 to the power of 5 = 32
2 to the power of 6 =64
2 to the power of 7 =128

You can go above 7 relays but above 100 steps you will notice it is so smooth you really do not need 8 relays and 256 steps. But it is possible.

Now you can have 0.5dB steps reaching a decent amount of attenuation. This is 6 times the 3dB you may have on a mechanical stepped attenuator. That means, you have 6 tiny change of volume in every step of a mechanical. It is wonderful!

The trick is in the controller which drives the relays. It is best to use a micro-controller and some digital circuits to do the job. The relays need to be switched according to the desired volume with a given order. So an input still needed to be processed and translated into relay configuration.
This input can be Up-Down buttons, a remote control, or a simple potentiometer which this time only gives the flavour but has no contact with the audio signal.

Next time I will introduce you my own relay based attenuator called the 8th Note stepped attenuator, and show the design rules that led me.

If You want to know more of the possibilities of the most advanced high-end electronic passive volume control solution you can get.

also visit the product page in my shop:

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