DAmp-10, 10 MHz distribution amplifier, or second life of old Ethernet hub

It is now possible to build or buy a precision 10~MHz clock source by using
either rubidium frequency standard or GPS disciplined OCXO. But such devices
normally have only single 50~Ohm output which makes it impossible to clock
multiple measurement appliances. The solution to this problem is to use
so-called distribution amplifier, which provides 3 or more galvanically isolated
outputs. Such devices are available on surplus market, but also can be very
easily built at home.

The circuit described below was constructed circa'2010 and has been proven to be stable and reliable.

One knob headphone amplifer

It is known fact, that design of headphone amplifier differs from the speaker amplifier due to notable difference in the required power. The other requirement is the ability to handle loads in range from 32 to 600 Ohms. There are tons of various designs available: tube based, BJTs or MOSFETs based, with or without global feedback, battery or mains powered, etc., etc.

I'm also a user of self made headphone amplifier for over twenty years now. During this time it was modified multiple times and moved to various enclosures. But recently, it was noticed, that electrical part wasn't modified at all for last two years, so I've decided to place it into new case in hope to make it more usable. The case was salvaged from the old SAT receiver kindly donated by a friend:

As one can see, it have (from left to right): power/standby indicator, four buttons, remote control eye, the phone jack, display and a big knob. Well, the knob isn't that big, but just because my cat stole the big black knob and I'm still trying to find it. The rear side have power connectors, mains switch and input connectors:

From outside, it looks like an ordinary device, but lets go inside:

Now, it is more interesting - seven(!) PCBs on the chassis and yet another two on the front panel. Obviously, this is not a minimalistic design at all. Lets take a closer look.

There are Russian saying, “Every theater begins with a hanger”, and each good amplifier starts with a good power supply. But wait, the PSU on the picture is a switched one! It is definitely will deserve admonition from some purists. In early incarnations, this amplifier had a transformer based PSU with linear post regulators. But later, it was replaced to a switched PSU with primary side regulation which allowed to dissipate much less heat in the linear post regulators. There was no difference noted in both, subjective and objective tests.

Power amplifier board located to the right of heatsinks of linear regulators. Each channel have four NE5532 amplifiers placed in parallel which gives a plenty of power to drive even low impedance loads. It have OP97 based integrators to maintain zero DC offset on the output. The small board nearby is a DC-protection.

The board near input jacks is a three channel input selector, the attenuator and CS3310-based volume control. Attenuator allows one to keep signal specifications within CS3310 limits. The whole board are controlled using I²C bus, the conversion is done by MCU (which is located on the bottom side).

The board in the bottom right corner is a bass/treble shelve tone control. This is the controversial  feature, but IMHO, this feature is invaluable because each headphones have its own frequency response and human ear also tends to change its sensitivity to high frequencies over the years. The board also digitally controlled over I²C bus.

Finally, the small board on top and near the PSU is the control unit, which handles all events from buttons, encoder and DC protection. It also puts information on display:

There are plans to implement remote control of amplifier, but currently it is not required because all controls can be reached by hands.

Thorough reader may notice that one channel of the input selector is not connected to anything, and there are two holes on the rear side without connectors. Well, this reserved for a DAC, which currently lives in its own case, but this is a different story.

When the trees were big...

Recently I've found an old board with pretty big chip soldered on it. The board also had large, but broken, liquid crystals display. The entire assembly looked like a electronic clock manufactured circa '83. The chip itself is a must to see:

Isn't it cute?

Tuning keyboard in Xubuntu (continued...)

In the previous post I've described a way to make Insert and Delete keys located on numeric keypad to act as plain Insert and Delete. Unfortutately, some programs have problems with arrow keys in combination with Shift key. Since I'm never use numpad for typing numbers, it is better (for me) to use the following ~/.Xmodmap file:

keycode 79 = Home
keycode 80 = Up
keycode 81 = Prior
keycode 83 = Left
keycode 84 = Begin
keycode 85 = Right
keycode 87 = End
keycode 88 = Down
keycode 89 = Next
keycode 90 = Insert
keycode 91 = Delete

It makes these keys indistinguishable from gray ones and makes them independent from NumLock.key state.

Tuning keyboard in Xubuntu

In recent years, Ubuntu became very popular desktop environment with large user base. But their new invention called Unity caused a bit of split. Some people try to stand it, some try to beat systems into a shape by recovering Gnome parts, and some, like me, switched to a different *ubuntu flavor. Xubuntu uses Xfce desktop environment which is very fast, compact and have almost zero bloat. This also means that average user will encounter some problems with fine tuning of some aspects. This time we'll look at keyboard setup.

Default keyboard setup of freshly installed Xubuntu wasn't acceptable at all:

• no language indicator;
• shift key have no effect on numeric keyboard;
• CapsLock key doesn't mapped to Control key.

The first item solved by adding "Keyboard Layouts" plugin to the appropriate panel. If it is missing, install xfce4-kxb-plugin.

As the old keyboard user, I'm use cursor movement, Insert and Del keys located on the numeric keyboard. By default, shift key do not allow to perform selection, copy or paste operations and merely enters corresponding numbers. Documentation suggests us to use /etc/default/keyboard file and modify XKBOPTIONS variable:

XKBOPTIONS="grp:ctrl_shift_toggle,grp_led:scroll,numpad:microsoft"

Execute

udevadm trigger --subsystem-match=input --action=change

command to apply changes. Everything works fine now. Try to reboot or logout and login again - test keys and now everything reverted back. This due to limitations in the "Keyboard Layouts" plugin, but thanks to it's author, things can be fixed. Logout once again, press Ctrl-Alt-F2 to reach console, login, locate ~/.config/xfce4/panel/xkb=plugin-*.rc file (* means that there can be any number) and add the following line to it:

never_modify_config=true

Switch back to the login prompt by pressing Alt-F7 and login into your session. Check keys - everything should work fine again.

Not all applications will recognize  Shift-Insert and Shift-Delete key combinations pressed on numeric keypad. This is due to the fact, that these keys have different keycodes. We can fix this by creating ~/.Xmodmap file with the following content:

keysym KP_Insert = Insert
keysym KP_Delete = Delete

it will be processed on subsequent login, to apply changes immediately run

xmodmap ~/.Xmodmap

And finally, lets make useless CapsLock key to act as Control key. To do this, open /etc/default/keyboard again and modify XKBOPTIONS line:

XKBOPTIONS="grp:ctrl_shift_toggle,grp_led:scroll,numpad:microsoft,ctrl:nocaps"

Apply changes as described above, and enjoy the easily reachable Control key.