Simplifying multiple user interfaces for a pre-amplifier: LULO

Now it is clear that the 8th note attenuator (as a passive pre-amplifier) will use a single linear potentiometer for control. In the last post I already listed the benefits of potentiometers over rotary encoders or simple tactile buttons in this application.

For a reminder, a potentiometer gives us a specific value if we turn it to a specific position. This is why we like it and this is what we call a natural behaviour.

The attenuator has an option for IR remote controlling. In fact, for control you get a potentiometer reading, and an IR receiver. You can omit any of those literally.

Option 1: use only remote control

You may ask why would anybody do that but I know of at least one headphone amplifier that has this only user interface, the Trilogy Audio 933.

Option 2: use only the potentiometer

For my own headphone amplifier I will follow this route. With headphones on my head I would not run to anywhere. Next to my chair or sofa the amplifier is always within reach, and I like to turn that big shiny knob.

Option 3: use both the potentiometer and the remote

For my DIY power amplifier I will use both. It is not like a headphone amplifier and it need to be controllable from moderate distances.

There are some things I think worth to mention in connection with option 3.

Usually a remote controlled amplifier has either digital controls (buttons, rotary encoder, etc.) or a motorized potentiometer.

In a traditional amplifier a motorized potentiometer follows the remote control, so basically you only control the motor with the remote. The potentiometer shows you the actual volume setting. Not using a motorized potentiometer somehow the volume need to be tracked when using the remote.

If it has digital control, like a rotary encoder, then usually it has a volume setting display, numbers, or sliders. Without this you would not know what percent of the volume it has. Because the knob actually don't give you a clue what is the setting

So these are the currently known options to have full control either from a remote or touching a volume knob.

But what if the potentiometer is not motorized and the amplifier neither has rotary encoder or display. Because the 8th note attenuator does not have those. Instead it has a volume lock function, called shortly voLULOck or LULO.

LULO:

This feature of the 8th note stepped attenuator takes care of switching between the remote and the potentiometer control.

If you use the remote to change the volume, the actual volume may be lower or higher than the position of your potentiometer (because it does not follow the remote). What LULO does is "unlocking" the volume knob when this happens. To "lock" your knob again, you have to turn it to the actual volume level.

But it would not be perfect like this. You may not know if the desired position to "lock" the knob is clockwise or counter-clockwise.

Because of this it only "unlocks" in one of the possible two cases.

Case 1:

You use the remote to turn up volume, higher than the knob setting. Next time you touch the volume knob, it sets the volume. No "unlocking". Remember if you move the potentiometer it sets the volume thus it may end up in sudden decrease in volume. But it does not harm you. You can turn it up again to wherever you like.

Case 2:

You use the remote to turn down volume, lower than the knob setting.

Now the "unlock" happens.

Without unlocking the knob, any change of the potentiometer would end up in immediate increase of the volume. Without some protection this incident could damage of your hearing or even your equipment.

To prevent this, the LULO "unlocks" the volume knob. It will not work if you touch it. To "lock" again your potentiometer you have to turn the knob to as low as the current volume. So now you know direction is counter-clockwise. If you reach the current volume setting, the knob will be "locked" again.
This sounds weird first but trust me, it is quite intuitive after you try it.

The trick is, if you touch the knob and nothing happens, you know LULO kicked in to unlock the knob to prevent volume jump, so the knob need to be turned down, and suddenly it "locks". I am using this for almost a year now and I like it.

So that is voLULOck, that protects you and your equipment, with deciding whether the knob should be in control after using the remote control.

Beginning of the 8th Note attenuator

At this time when the attenuator is ready I finally have time to make a lists of things I wanted and maybe have some thoughts on the resuls ts somewhere. And this is the place.

The idea of the attenuator turns back to 2012 summer when I decided to build a headphone listening system from scratch. The one thing I learned before that I would need a passive attenuator, the best I can make.
But I already had some rules back then:

It need to be relay based stepped attenuator

I believe this is the ultimate passive attenuator. My research on passive attenuators proved me a high-end DIY system must have a stepped attenuator and the most advanced of all is the relay based logarithmic resistor ladder attenuator.

It need to be small

Small size gives more options on layout of the enclosure later, and having the shortest possible tracks always has a benefit in audio. I also had plans with this exact size, as it matches the ODAC in dimensions. Small size means less manufacturing costs, and I want to stick to SMT parts.

It need to have very good resistors
In the selection of SMT resistors there aer many options, and many manufacturers. There are the Vishay foil resistors, but those would be rather overkill and expensive. I had to find a resistor which is available in SMT and it is already trusted in audio circuits. That was the Susumu RG series. There were other options I wanted to give a try but I needed a selection of resistor values in small quantities and this was not easy. As I wanted to source a few of the resistor sets I chose the Susumu.

It need to have two inputs for start

I am 99% sure I do not need more inputs. One needed for the main DAC and still one left if someone just want to plug in an Ipod. It is not hard to add inputs externally if it would really be needed.

It need to draw little power

Many relay based attenuator powers the relays all the time. The use of monostable relays does not allow power saving. This is why I chose bistable relays. These relays only need power to switch, and only for about 2ms of time. The driver circuit and logic is a bit more tricky though. But is was not a problem.

It need ... how many steps?

It took a while to find out how many steps are optimal and I found that 128 steps are the way to go. Having much more steps, for example 256, you will miss many steps when you turn a knob, because it will be so close together.

Imagine a volume knob, with any diameter. A knob usually can be turned from 0° to 270°. You probably can turn it exactly 1° and no more if you try hard but not likely. Most people will be happy to set only every second degree, and the real word example showed exactly the same. Also the lowest diameter knob you have, the less control you have on the small movements. Turning a 20mm knob slowly is much harder than turning a 50mm knob. With 128 steps I was almost unable to turn a 20mm knob only one step further... which was about 2°. Maybe with that small knob 64 steps would be more comfortable.
I like bigger volume knobs and I am sure many do. I tested 128 steps on a 48mm knob and I was more than happy with the result. So 128 steps are the way to go!

It need to have Potentiometer for control. It is a must!

Why we like potentiometers?
The advantage of using a potentiometer is if we turn it to a specific position we get a specific value. It feels natural. This is something You can only achieve with a potentiometer. And people like it. I like it too. Period.

How about alternative controls?

I do not think there is place for turn up and turn down buttons on a front plate. We are used to knobs for too long. The other similar solution some circuits use is a rotary encoder. It is kind of a digital potentiometer, but it has no end position and it may have multi turns until you reach the last volume level. That is annoying because every proper hi-fi potentiometer knob has a position mark! You may have hard time to find one without that. If it already has a position mark it would be silly if it would not tell you the volume, wouldn't it?

But there is nothing against implementing an infra-red remote control. Especially if I already use a micro-controller. Having some additional features on unused pins is good. The most known remote control protocol is RC5, so I used that. I noticed some implementations use a wast amount of calculation time to decode it. To prevent this I implemented a new method to decode the RC5 signals. This code is the most efficient RC5 decoder yet!

It need to have an onboard power regulator

The digital parts have so low power requirements an onboard regulator would not generate too much heat if any. I chose LM317 because I was already familiar with. I would never put a switching regulator (buck controller) near an audio signal.

It need to act like a good old volume control

A real solution need to be intuitive, and simple like a knob. I don't think it needs fancy displays either. So I did not design one but the controller may be supporting an external display circuit.

I think the above are essential probably to most people needs. When I designed the attenuator I faced some problems and had to find several solutions. But I always followed my rules above and I think it turned out very well.

There are some interesting subjects I will write about more like the new behaviour of the controls I implemented (now called LULO) to make the potentiometer and the remote control friends.

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.


EDIT:
also visit the product page in my shop:
http://www.8thnote.eu/attenuator_details.html