2013. január 10., csütörtök

The love of headphones turned to DIY audio

Once I had a very good sounding headphone. I think it started all the love for headphones for me but only a few years later. It was a Philips, I bought in a warehouse. I walked trough the rows to buy a headphone but with my 50 pounds in my wallet I felt rich then, and wanted the most expensive one. Back then I had no clue such manufacturers existed like AKG or Grado. And guess what, I bought the most expensive headphones I could find in a shop for about 38 pounds and it was the Philips SBC HP 840!

That poor headphone died after many years together, and I felt so sad. I started to get know the names AKG, Sennheiser, Beyerdinamic, but after some search in the high-end audiophile realm I realized I am not feeling the same when checking my wallet as I felt a few years before.
What I wanted to say I had high expectations, even after a Philips headphone that soon turned out it was an exceptionally good pair of headphones!

I learned a lot from DIY audio forums and blogs. I learned that for a good pair of headphones I need good sources, and a headphone amplifier made of quality parts. Those things was not entirely available in my country so I searched trough American and far east sites to gather knowledge.

Finding some very good DIY pages, I thought I could build something myself that could be affordable, very high quality and not least fun to do. After this I was searching for a headphone amplifier kit I knew I would need for my next headphones. Even if I couldn't yet decide which headphones to buy.
In my study of amplifier circuits, power supplies, attenuators, cables and connectors, I found audio volume controlling the most interesting. It could be a good start to try something new.

That was the first high-end and yet unfinished digitally controlled volume control I started to design.

What was it all about? Once I read an article about an LDR based volume controlling method. The LDR is a photo-resistive component, which changes its resistance according to the light it gets. Some forum members found it very pleasing to listen trough that, some already declared it to the best sounding passive volume control.

Controlling this was not far from the blinking LED projects, as the easyest way to control the light intensity next to an LDR is controlling an LED.
The task is to make a resistor devider, both the serial and parallel part is controlled in inverse. This way it is theorically possible to make an electronic potentiometer. In real world it is not so perfect.
A potentiometer could attenuate from zero to some degree. But an LDR can not reach zero Ohm resistance at any state. Checking a manufacturer datasheet the resistance of the LDR could go down to 40-80 Ohm but no lower. It depends on the product, not every LDR is the same. I could find LDRs that went down to 17 Ohm! And that is a really good value when talking about LDRs.
Another aspects of using a potentiometer like device is channel matching or channel equality. We want at least two channels when talking about hi-fi. In a two gang or two channel potentiometer there are two almost identical resistive element. If we want to copy that we need similar working elements. And LDRs has no tight tolerance. You have to match them to build two voltage dividers for each channel. A few man offers matched LDRs for builders on the internet, you can buy them or match them yourself but that means you have to buy 10 or 20 to have a few pairs.

Long introduction... but what if you would not have to match them?!
Well I had the idea. Take a few individual LDRs, and measure the characteristics of them with a 24bit precision ADC.
This involves using some microcontrolled smart measuring tool. Using this information later this way each LDR could be driven to the same discrete resistance level. Take as many value as possible and store the needed settings for each LDR. Lets see how:
Each LDR has a driver to control the LED attached to it. At each setting from minimum to maximum the LDRs resistance value is read by an ADC. This data is stored in memory. Each LDR has a minimum resistance value. But if you select the highest, you have a value that every of the measured branch can have. This will be the lowest setting, so all data below this value can be omitted. Selecting the highest and the lowest value to the possible voltage dividers the rest in between can be divided into lest say 500 levels. For the 500 levels the data for setting the LDR-LED combo to that particular value must be stored in memory.
If it is working well, you get a pair of LDR-s that was not matched only at 2, 5, or 8 levels but matched at 500 levels! But this is not restricted for two pairs. With this technique you can match 8 of 8 LDRs and build a balanced volume controller. This was not done before. Will this be the ultimate LDR volume controller? We will see.
*Early prototype

Now the development of this device yet to be finished. It took a year and I had to learn a lot of things to build a prototype. And it is not finished yet. I had other things to do and put the plans away for a while. But this was something that made me learn many things. First of all, to not just think, but to make things happen. In the next few posts I will talk about this.

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