audio – Engineering Radio

Vinyl Revival

I am seeing more and more people getting into (or back into) vinyl recordings. This is somewhat heartening. My personal feeling is that good analog recordings offer a great way to enjoy music, particularly older music. The other nice thing; when I am holding a physical disk, it is mine. I bought it, I own it. No one can track it or delete it from my device.

Grooves in a vinyl disk seen through an electron microsope, courtesy of Eric Muzipov @microscopyspecialist

Vinyl is actually Polyvinyl Chloride or PVC. Columbia records switched from Shellac record disks to PVC around 1947. According the the RIAA, vinyl sales peaked in the US around 1973. It is possible to find new vinyl in a few places like Target, Walmart and Barnes & Noble. There is a large market for used vinyl recordings in local record stores like Dark Side Records.

Many years ago, I retrieved this Technics SL-1200 Mk II turntable from the trash heap. This particular turntable had been through the flood after Hurricane Irene (2011). It spent more than 48 hours completely submerged in some pretty dank water. Following that, it went into the barn for many years without being looked at.

At some point in the last few years, I decided that I wanted a turn table for my Hi-Fi system. I retrieved the Technics unit from the barn and began the rehab. Fortunately, both the service manual and user manual are easy to find on line.

I disassembled the entire unit and inspected all of the parts. As it turned out, it was fresh water and the damage did not look too bad. There is some pitting on the under side of the platter and some general corrosion on the strobe dots. It took a while and a good deal of patience and elbow grease to clean everything off. The electronics in this turn table are only for control of the direct drive motor. Audio passes through to the outboard pre-amp. There is one 450 uF 50 VDC capacitor in the power supply which looked good, so I left it alone.

I put it all back together, but realized quickly that it needed to be set up correctly. There are many videos on Youtube that show how to align this particular model turntable. A complete alignment is important because the stylus must meet the grove at the correct depth and angle in order for the playback to be accurate.

The mechanical setup consists of five main things;

The turn table must be completely level front to back and especially center post to stylus (the track of the tone arm)
The height of the tone arm (stylus angle in the groove)
The vertical tracking weight (proper frequency response, stylus and record wear)
The anti-skating (proper pressure on both sides of the record groove, correct amplitude)
Alignment of the cartridge in the cartridge head (stylus angle in the groove, proper left/right phasing, stereo separation and image)

These items are very easy to deal with on the SL-1200. It has feet that can be adjusted to level the unit. The leveling of the tone arm track is especially important to get right.

The height of the tone arm is set with the base arm ring. Some turntables do not have this adjustment, particularly consumer grade units. The tone arm should be level when the stylus is on the record.

The tracking weight is set by the tone arm balance weight. First, the tone arm is balanced so that it floats (in other words zero weight). The tracking weight depends on the cartridge and stylus being used. In this case, the cartridge is an Audio Technica AT-OC9XEN Moving Coil cartridge with an elliptical stylus. The tracking weight for that cartridge is 1.8 to 2.2 grams. I set mine to 2.0 grams.

Anti-skating is generally set for the same value as the tracking weight. However, this is sometimes too coarse. A test record with a blank band on it can help get this exact. It does not make a huge difference, but it is nice to confirm this anyway.

The alignment of the cartridge in the head shell is important, especially with an elliptical stylus. This is done using a Cartridge Alignment Protractor which can be downloaded for free from the vinylengine.com website

I purchased this piece of Schiit a few years ago to use with my vacuum tube amplifier. I have since switched to a Kenwood VR309 for my main listening setup. While the phone preamp in my Kenwood stereo is pretty good, I think this preamp sounds better, especially with the moving coil stylus.

Circa 1960 Beethoven Symphony No. 7 in A Major. Murray Hill Records, One Park Ave, New York, NY

So, how does it sound? Pretty darn good. I dusted off some of my high school record collection and enjoyed explaining to my SO why a particular song was good. I also found a whole stash of like new Classical vinyl at the local Habitat for Humanity Restore, which was more to her liking. I love those places, you never know what you will find.

Unbalanced to Balanced Audio

I pulled this out of the published posts and updated it.

I have been working on updating some wiring at one of our client’s transmitter sites. I noticed that an off air monitor feed was going back to the studio on a Barix box, which is fine. It was being fed from a balanced output of a DA to the unbalanced input on the Barix box. This being at the transmitter site, was susceptible to RF noise. I decided to make a passive audio BALUN.

Balanced to unbalanced audio converter using 10K to 600 ohm transformer

In any case, there are several ways to go from balanced to unbalanced without too much difficulty. The first way is to wire the shield and Lo together on the unbalanced connector. This works well with older, transformer input/output gear, so long as the unbalanced cables are kept relatively short.

simple balanced to unbalanced audio connection

Most modern professional audio equipment has active balanced input/output interfaces, in which case the above circuit will unbalance the audio and decrease the CMRR (Common Mode Rejection Ratio), increasing the chance of noise, buzz, and so on getting into the audio. In this case, the CMRR is about 30 dB at 60 Hz. Also, newer equipment with active balanced input/output, particularly some brands of sound cards will not like to have the Lo side grounded. In a few instances, this can actually damage the equipment.

A Henry Match Box or something similar can be used. I have found, however, the active components in such devices can sometimes fail, creating hum, distortion, buzz, or no audio at all. Well-designed and manufactured passive components (transformers and resistors) will provide excellent performance with little chance of failure. There are several methods of using transformers to go from balanced to unbalanced or vice versa.

Balanced to unbalanced audio using 1:1 transformer

Using a 600:600 ohm transformer is the most common. Unbalanced audio impedance of consumer-grade electronics can vary anywhere from 270 to 470 ohms or more. The 10,000-ohm resistor provides constant loading regardless of what the unbalanced impedance. In this configuration, CMMR (Common-Mode Rejection Ratio) will be 55 dB at 60 Hz, but gradually decreases to about 30 dB for frequencies above 1 KHz.

Balanced to unbalanced audio using a 4:1 transformer

A 600:10,000 ohm transformer will give better performance, as the CMMR will be 120 dB at 60 Hz and 80 dB at 3 KHz, remaining high across the entire audio bandwidth. The line balancing will be far better for the high-impedance load. This circuit will have about 12dB attenuation, so plan accordingly.

For best results, use high-quality transformers like Jensen, UTC, or even WE 111C (although they are huge) can be used. I have found several places where these transformers can be scrounged, DATS cards on the old 7300 series Scientific Atlanta satellite receivers, old modules from PRE consoles, etc. A simple audio BALUN can be constructed for little cost or effort and sound a whole lot better than doing it the wrong way.

A brief list, there are other types/manufacturers that will work also:

Ratio	Jensen	Hammond	UTC	Edcor
1:1 (600:600)	JT11E series	804, 560G	A20, A21, A43	PC600/600
4:1 (10K:600)	JT10K series	560N	A35	PC10K/600

Keep all unbalanced cable runs as short as possible. In stereo circuits, phasing is critically important, so pay attention to how the balanced transformer windings are connected.

As for cost; I purchased the Edcor PC10K/600 transformer on eBay for $20.00 and the Hammond 1590B Enclosure was about $9.00. The audio jacks and resistor were in the parts drawer. It took about 20 minutes to layout the holes, drill, mount the audio jacks, and solder the jumper wires. I used a tie-base, wire tie, and some Gorilla glue to hold the transformer down. I used a 1/4 inch TRS jack because the enclosure was a little bit too small for an XLR jack. If a stereo pair needed be converted, it would require two of everything.

Overall, I fun project. The old Simpson 260 is still accurate!

Checking the accuracy of a Simpson 260 on audio frequencies.

GPS controlled clock based on Raspberry Pi

This is an adjunct to my previous post on creating a Stratum 1 NTP time server with a Raspberry Pi. I finished this project about a year ago, and I have to say it has been running flawlessly since then. I thought that perhaps the inexpensive GPS module designed to work with drones might not hold up. But it has.

Wouldn’t it be nice to use this time source, not just to set hardware clocks but also display the time in varous places? Yes, yes it would.

Since most of my ideas are not original, I figured a quick internet search may shed some light on how to proceed. Keith, G6NHU did exactly this a year ago or so. His project can be viewed here: https://qso365.co.uk/2023/05/how-to-build-a-shack-clock-using-a-raspberry-pi-and-a-7-touch-display/

As I suspected, with a few more configuration steps, this NTP server can display the time on the native HDMI port as well as create a simple web page available on the LAN for any computer to access with a web browser. The web server is Lighttpd, which is a low CPU load, low memory demon, perfect for an older Raspberry Pi 3.

The display I chose is a 7-inch 16×9 non-touch purchased from Amazon for $33.99: https://www.amazon.com/gp/product/B0BGXB2Y67/ (not an affiliate link)

When I created the NTP server last year, I named it ntpserver. The unit runs headless (no keyboard or monitor) so I use ssh to get into the command line. Thus, the first command is:

ssh ntpserver@192.168.1.200

Once in, always do an update:

sudo apt-get update
sudo apt-get upgrade -y

The following programs need to be added to the Pi:

sudo apt install xorg openbox xserver-xorg xinit unclutter lighttpd -y

If the OS is off the shelf Raspian, then Chromium should already be installed, but if not, then add it:

sudo apt install chromium-browser -y

Once that is done, some things need to be configured. Using whatever text editor you like the xserver so that anyone can access it. Open the Xwrapper file:

sudo nano /etc/X11/Xwrapper.config

Then add line:

allowed_users=anybody

Exit and save. Next open the xserverrc file:

sudo nano /home/ntpserver/.xserverrc

Add the following:

#!/bin/sh
#Start the X server session with no power management so the display never sleeps
exec /usr/bin/X -s 0 -dpms -nolisten tcp "$@"

Exit and save. Next open the xsession file:

sudo nano /home/ntpserver/.xsession

Add the following:

#!/bin/sh
#Start Chromium at startup
chromium-browser --start-fullscreen --window-size=800,480 --disable-infobars --noerrdialogs --incognito --kiosk http://localhost

Exit and save. Note the display size can be configured to any screen resolution. This affects the HDMI port, not the web page. The 7-inch Raspberry Pi monitor that I purchased from Amazon has an 800 x 480 screen resolution. If you are using a different screen resolution, change as needed. Next open the clock.service file (it will be created when you save the file):

sudo nano /etc/systemd/system/clock.service

Add the following:

[Unit]
Description=Clock
After=network-online.target
DefaultDependencies=no

[Service]
User=clock
ExecStart=/usr/bin/startx
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

Exit and save.

The web page that will be displayed on the HDMI port as well as served to local hosts on the LAN is a java script page. It was originally developed by Nayuki: https://www.nayuki.io/page/full-screen-clock-javascript You can download whatever format you like from that site (I copied the page source) or you can download a 24-hour format from Keith’s site:

cd /var/www/html
sudo wget https://qsl.net/g6nhu/clock/index.html

The colors can be edited:

sudo nano /var/www/html/index.html

The background, foreground, and font type can be changed as desired.

Next start the clock service and reboot:

sudo systemctl enable clock
sudo systemctl start clock
sudo reboot now

Here is a quick video of the web page on my desktop computer. I have the GPS monitor from the ntpserver up and running in the left upper corner. That shows the GPS data going into the Raspberry Pi from the serial port along with some scratchy WWV audio. The actual clock sync is from the 1PPS output of the GPS module.

I could see this being used as an inexpensive master clock system somewhere. With an HDMI splitter (or a better name would be Distribution Amp), this could be sent to many locations.

Now all I need to do is figure out how to get a GPS synced 10 MHz output capable of driving multiple devices.

Audio Bit Rates and Formats

Occasional reader and blogger Robert has broken down all the information on audio bit rates and audio formats.

His work can be found here: Ultimate Guide to Audio Bit Rates and Formats

It is a good read, especially for those that use audio streaming as their main content distribution method.

Streaming-only stations used to be a big thing but have been supplanted by Spotify and Pandora. I am not a huge fan of either of those services but I do like to listen to podcasts.

Good audio should be near the top of the list for any content provider. Few things are more annoying than listening to an interesting podcast with low volume, background noise, or other technical defects.