Simple Circuits: Turn a Tube Radio Into an MP3 Amp

Want to give your MP3 player vintage tube sound? You can with the proper circuits, an antique radio, and a little know-how. In addition to generating amazing sound, the design will be an eye catcher in your home or office.

Here I present excerpts from Bill Reeve’s article, “Repurposing Antique Radios as Tube Amplifiers,” in which he provides vintage radio resources, simple circuit diagrams, and essential part info. He also covers the topics of external audio mixing and audio switching. The article appeared in the May 2012 edition of audioXpress magazine.

Manufactured from the 1930s through the 1960s, vacuum tube radios often contain high-quality audio amplifiers at the end of their RF signal chain. You can repurpose these radios into vintage, low-power tube amplifiers—without marring them in any way or detracting from their original charm and functionality as working analog radios.

Wood-cased radios have especially good sound quality, and the battery compartments in antique “portable” radios (like the Philco 48-360 or the Zenith Transoceanics) provide perfect locations for additional circuitry. When restored properly, large furniture-style radios that were built for “high fidelity” (like the late 1930s and early 1940s Philco console radios) can fill a room with rich beautiful sound.

Simple Circuits

The simple circuits described in this article perform two functions. They mix an external line-level stereo signal (typically from an MP3 player or computer) and reference it to the radio’s circuit. They also use the radio’s on/off knob to switch this external signal to the radio’s audio amplifier.

There is not one circuit that will work for every antique radio. (Original schematics for antique tube radios are available on the web www.justradios.com). But the circuits described here can be adapted to any radio topology. All the parts can be ordered from an electronics supplier like Digi-Key, and the circuit can be soldered on a prototyping printed circuit board (such as RadioShack P/N 276-168B).

External audio mixing

Figure 1 and Figure 2 show some examples of circuit schematics that mix the line-level stereo audio signals together (almost all tube radios are monophonic), while providing galvanic isolation from high voltages within the radio. Figure 1 shows an inexpensive solution suitable for most table-top radios.

Figure 1: An inexpensive circuit for mixing an MP3 player’s stereo audio signals safely into an antique radio. None of the component values are critical. (Source: B. Reeve, AX 5/12)

These radios have relatively small speakers that are unable to reproduce deep bass, so an inexpensive audio transformer (available from on-line distributors) does the job. I picked up a bucket of Tamura TY-300PR transformers for $0.50 each at an electronics surplus store, and similar transformers are commercially available. Alternatively, the Hammond 560G shown in Figure 2 is an expensive, highquality audio transformer suitable to high-fidelity radios (like the furniture-sized Philco consoles). A less expensive (and fine-sounding) alternative is the Hammond 148A.

Figure 2: A high-fidelity circuit for mixing external stereo audio signals safely into an antique radio. (Source: B. Reeve, AX 5/12)

I use Belden 9154 twisted, shielded audio cable for wiring internal to the radio, but twisted, 24-gauge wire will work well. An 8′ long audio cable with a 3.5-mm stereo jack on each end can be cut in half to make input cables for two radios, or you can use the cord from trashed ear-buds. You can route the audio cable out the back of the chassis. Photo 1 is a photograph of a 1948 Philco portable tube radio restored and used as an MP3 player amplifier.

Photo 1: A 1948 Philco portable tube radio restored and repurposed as an MP3 amplifier. (Source: B. Reeve, AX 5/12)

Audio switching using the radio’s on/off knob

After creating the mixed, radio-referenced signal, the next step is to build a circuit that switches the voltage driving the radio’s audio amplifier between its own internal broadcast and the external audio signal.

Figure 3 illustrates this audio routing control using the radio’s existing front panel power knob. Turn the radio on, and it behaves like the old analog radio it was designed to be (after the tubes warm up). However, if you turn the radio off, then on again within a few of seconds, the external audio signal is routed to the radio’s tube amplifier and speaker.

The circuit shown in Figure 3 uses a transformer to create the low voltage used by the switching circuit. There are many alternative power transformers available, and many methods of creating a transformerless power supply. Use your favorite….

The next photos (see Photo 2a and Photo 2b) show our additional circuit mounted in the lower (battery) compartment of a Zenith Transoceanic AM/shortwave receiver. Note the new high-voltage (B+) capacitors (part of the radio’s restoration) attached to a transformer housing with blue tie wraps.

Photo 2a: The inside view of a Zenith Transoceanic AM/shortwave radio restored and augmented as an MP3 audio amplifier. b: This is an outside view of the repurposed Zenith Transoceanic AM/shortwave radio. (Source: B. Reeve, AX 5/12)

The added circuit board that performs the audio re-routing is mounting to a 0.125″ maple plywood base, using screws countersunk from underneath. The plywood is securely screwed to the inside base of the radio housing. Rubber grommets are added wherever cables pass through the radio’s steel frame.—Bill Reeve

Click here to view the entire article. The article is password protected. To access it, “ax” and the author’s last name (no spaces).

CircuitCellar.com and audioXpress are Elektor International Media publications.   

A Workspace Built for Precision Design

Brad Boegler is a do-it-yourselfer’s DIYer. His West Bloomfield, MI-based workspace is something to admire. It features a sturdy 8’ × 5’ workbench, a well-built machining bench, and dozens of handy tools that enable him to work on projects ranging from constructing a temperature-monitoring network to milling custom heatsinks. Simply put, it’s an appealing space for any innovator interested in DIY electronics and machining projects.

Photo 1: One of Boegler's Altera CPLD breakout boards is on the bench. He said he was "experimenting with some video generators in VHDL" when he took this picture. (Source: B. Boegler)

As I reviewed Boegler’s space, the same word kept popping into my mind: precision. Why? Let’s see.

Building a bench (or benches) for a workspace like Boegler’s takes a lot of precision measuring, cutting, fitting, and constructing. Check out the workbench in Photo 1. That’s no “Ikea hack.” The 8’ × 5’ bench fits a dual monitor setup, plenty of test/measurement equipment, a solder station, and more.

Boegler—who works as Linux sysadmin—described some of the equipment on this bench via email:

The left side of the bench is mostly RF equipment: there are two HP RF frequency generators, a VNA, and spectrum analyzer. The analog scope is a Tek 2246 and is one of my favorite scopes. Next to that is an HP 16500B logic analysis system and then a HP 54112D digital scope … The bench was custom made. I was not able to find any benches to my liking so I ended up building my own. It is 8′ wide by 5′ deep and constructed out of mostly 4×4s. It weighs a ton, but it has to be sturdy as a lot of this equipment is very heavy. I like very deep benches as I can push the equipment back far enough on it and still provide enough working space.

And don’t forget the power!

Those are various adjustable voltage current limiting power supplies, when working on projects needing various voltages you can never have too many supplies.

I’m sure everyone agrees that access to power supplies is key.

Photo 2: Boegler's workspace for machining (Source: B. Boegler)

On a separate bench (Photo 2) are Boegler’s milling machine and drill press, which are two tools intended for precision designing and machining. Boegler wrote:

The drill press is used almost daily, one of the best tools ever. I use the milling machine for custom shielded aluminum cases for RF boards, making special sized heatsinks, and it comes in handy for any special brackets I can make to hold boards or components.

I’m sure you’d agree that machining board cases and heatsinks requires a bit of exactitude.

Much like the bench in Photo 1, building the actual machine bench required precise measurements and cuts. Just look at its clean edges and sturdy frame. And don’t you like the shelf underneath? It’s a simple yet effective place for stowing frequently used tools.

On the topic of storage, check out Boegler’s wheeled shelf system. I like it and will consider something similar for my garage. (We all take wheels for granted until we’re in a pinch and need to move a heavy object. For instance, try moving a wheel-less six-shelf system full of parts in order to track down a screw that fell on the floor. Actually, don’t try that. It’s an accident waiting to happen.)

A wheeled shelf system for microcontrollers, op-amps, and parts of all sorts (Source: B. Boegler)

Lastly, check out the neatly labeled parts boxes. I see labels such as “Microcontrollers/DSP,” “Op-Amps,” “Serial Cables,” and more. Nice!

Share your space! Circuit Cellar is interested in finding as many workspaces as possible and sharing them with the world. Click here to submit photos and information about your workspace. Write “workspace” in the subject line of the email, and include info such as where you’re located (city, country), the projects you build in your space, your tech interests, your occupation, and more. If you have an interesting space, we might feature it on CircuitCellar.com!

Issue 263: Privately Funded Engineering

The public vs. private funding debate endures in the United States and Europe. Everything from energy generation (e.g., oil) to social welfare programs are debated daily by government committees, discussed in corporate board rooms, and argued over at lunch tables from Los Angeles to Brussels and beyond.

One particularly interesting discussion pertains to the role of the public and private sectors in space flight and exploration, which comprises fields such as aerospace design, embedded electronics, and robotics. In Circuit Cellar June 2012, Steve Ciarcia weighs in on this debate and makes a thought-provoking argument for the benefits of privately funded engineering endeavors. In “Google LUNAR X Prize” he writes:

This is certainly an exciting time to be an engineer. We have seen the success NASA has had with robotic exploration, especially on nearby planets such as Mars. Contrary to everything coming from NASA in the future, however, thanks to the advances in robotics and launch vehicles, “space” will soon become the province of private enterprise and not just government. Very soon, commercial space flight will become a reality.

The Google Lunar X PRIZE provides a focal point for these efforts. Google is offering a $20 million prize to the first team to complete a robotic mission to the moon. The basic goal is to put a lander on the surface of the moon, have it travel at least 500 m once it’s there, and send back high-definition pictures and video of what it finds. There’s a $5 million second prize, and also $5 million in bonus prizes for completing additional tasks such as landing near the site of a previous NASA mission, discovering water ice, traveling more than 5,000 m while on the surface, or surviving the 328-hour lunar night.

When the Lunar X PRIZE registration closed in December 2010, a global assortment of 33 separate teams had registered to compete. Seven of those teams have subsequently dropped out, but there are still 26 active teams, including 11 from the U.S. The first launch is expected sometime in 2013, and there’s plenty of time before the competition ends December 31, 2015. Some teams are even planning multiple launches to improve their chances of winning.

It’s interesting to browse through the team information and see the vast diversity in the approaches they’re taking. This is the part that is most exciting from an engineering point of view. Some teams are building their own launch systems, while others are planning to contract with existing government or commercial services, such as SpaceX. There’s a huge amount of variety among the landers, too: some will roll, some will walk, and some will fly across the moon in order to cover the required distance. Each one takes a different approach to dealing with the difficult terrain on the moon, and issues such as the raw temperature extremes between blazing sunlight and black space.

This sort of diversity is a powerful driver for future development. Each approach will have its strengths and weaknesses, and there will certainly be some spectacular failures. Subsequent missions will draw on the successful parts of each prior one. Contrast this to the approach NASA has tended to take of putting all its effort into a single design that had to succeed.

It’s also interesting to consider the economics of this sort of competition. The prize doesn’t really approach the full investment required to succeed. Indeed, Google is quite up front about the fact that it probably only covers about 40%, based on other recent high-tech competitions such as the Defense Advanced Research Projects Agency’s DARPA Grand Challenge and the Ansari X PRIZE. This means the teams need to raise most of their money in the private sector, which keeps them focused on technologies that are commercially viable.

I have long been a fan of “hard” science fiction, as typified by writers such as Larry Niven, Arthur C. Clarke, and Michael Crichton. To me, hard science fiction means you posit a minimal set of necessary technologies, such as faster than light (FTL) space travel or self-aware computers/robots, and then explore the implications of that universe without introducing new “magic” whenever your story gets stuck. In particular, Larry Niven’s “Known Space” universe—particularly in the near future—includes extensive exploration of the solar system by private entrepreneurs. With the type of competition fostered by the Google Lunar X PRIZE, I see those days as being just around the corner.

The competition among these teams, and the commercial companies that arise from them, will be good for society as a whole. For one thing, we’ll finally see the true cost of getting to space, as opposed to the massive amounts of money we’ve been pouring into NASA to achieve its goals. As a public agency, NASA has many operational constraints, and as a result, it tends to be ultra-conservative in terms of risk taking. Policies that dictate incorporating backups for the backups certainly makes a space mission more expensive than the alternative.

Despite these remarks, however, I don’t mean to sound overly negative about NASA at all. It has had many spectacular successes, starting with the Mercury, Gemini, and Apollo manned space programs, as well as robotic exploration of the solar system with the likes of Pioneer and Voyager, and more recently with the remarkable longevity of the Mars rovers, Spirit and Opportunity. There have been many beneficial spin-offs of the space program and we have all benefited in some way. We wouldn’t be where we are today without the U.S. space program. But the future is yet to be written. There are striking differences between a publicly run space program and the emerging free-market privately funded endeavors. We would do well to recognize the opportunities and the potential benefits.

Circuit Cellar 263 (June 2012) is now available on newsstands.

Issue 263: H2M & M2M Communication

Before I introduce this issue, I’d like to bring your attention to our recently redesigned website, CircuitCellar.com. It enables engineers and programmers around the world to communicate and share ideas via project articles, videos, and social media. The site’s features range from project posts (how-to articles, videos, and photos) to daily updates about products and industry news. We also run short write-ups on actual circuit cellars and workbenches in the well-received “Workspaces” section of the site. I encourage you to submit photos and info about your workspaces. Share your space with the design community!

Now let’s focus on this issue, which has articles on both human-to-machine (H2M) and machine-to-machine (M2M) communication. Topics as diverse as smart switch management and human motion-sensing systems are covered.

Kevin Gorga kicks off the issue with his “AC Tester” project (p. 14). It is an isolated variable voltage power source that includes an electronic circuit breaker for testing and debugging equipment. An mbed controller displays voltage and current, and it controls the breaker’s trip point and response time.

Circuit Cellar published 15 of Aubrey Kagan’s articles from 2000 to 2010. In an interview on page 24, Aubrey shares some of his engineering experiences from designing controllers for mines in Africa to helping create specs for the remote control arm on the International Space Station.

On page 28, Fergus Dixon presents a ’Net-connected controller for up to 50 smart switches for lighting systems. The controller’s RTC pulses a 24-V AC line once or twice to turn off the smart switches at the end of the day.

Final PCB with a surface-mount Microchip Technology ENC28J60 Ethernet chip (Source: F. Dixon, CC263)

Turn to page 36 if you’re interested in computer vision technology. Miguel Sánchez introduces depth camera technology, and describes how he used Microsoft’s Kinect in an interactive art project.

On page 44, columnist Bob Japenga starts an articles series on the subject of concurrency in embedded systems. In this article, he defines concurrency and covers some concurrency-related pitfalls.

Last month columnist George Novacek examined the topic of product testing and simulation. In this issue, he tackles a different yet equally important topic: diode ORing (p. 48).

On page 52, columnist Ed Nisley carefully explains MOSFET channel resistance. He describes power MOSFET operation and explores the challenges of using a MOSFET’s drain-to-source resistance as a current-sensing resistor.

A serious RF designer should have a sound understanding of frequency mixers. On page 58, columnist Robert Lacoste summarizes the basics of RF mixers and presents real-life applications.

A simple single-diode unbalanced mixer and its simulation done with Labcenter's Proteus. The RF and LO frequencies are 340 MHz and 300 MHz respectively. You can see on the output spectrum that these two frequencies are still visible, but as well as the difference 40 MHz and sum 640 MHz, among others. (Source: R. Lacoste, CC263)

Jeff Bachiochi wraps up the issue with an article about a DIY, MCU-based blood pressure cuff project (p. 68). He converted a manual blood pressure cuff into an automatic cuff by adding an air pump, a solenoid release valve, and a pressure sensor.

Circuit Cellar Issue 263 (June 2012) is now available on newsstands.

Electronics Engineering Crossword (Issue 263)

Across

1.     SPARKGAP—A space in an otherwise closed electric circuit [two words]

3.     FUZZYLOGIC—Began in 1965 with Lotfi Zadeh’s proposal of a certain type of set theory [two words]

6.     NULLTEST—Cancels a device’s signal input by negative feedback from its output [two words]

7.     COULOMB—One of these is equal to 6.28 x 1018 electrons

10.   EDDYCURRENT—A.k.a. Foucault currents [two words]

11.   KIRCHHOFF—Created a law of thermochemistry

13.   ANDGATE—Relies on truth and logic  [two words]

16.   HOLONYAK—Invented the LED while working at General Electric in the 1960s

17.   SOLIDSTATERELAY—Doesn’t rely on movement or contact to switch electric circuits [three words]

20.   ACCIRCUIT—A circuit with a current that goes one way and then another [two words]

Down

2.     ANNEAL—Run hot and cold

4.     OPTOISOLATOR—Changes electrical signals to light and back into electrical signals

5.     CHECKBIT—Adds a bit to make things even or odd [two words]

8.     MICROFARAD—1,000,000 pF

9.     THEVENIN—French engineer (1857–1926); augmented Ohm’s law

12.   KELVINSCALE—Temperature scale where 0° = absolute zero [two words]

14.   PHOTODIODE—Turns light into current or voltage

15.   ACORNTUBE—A small tube used at very high frequencies [two words]

18.   ERG—Causes 1 cm of movement

19.   ASCII—A character-encoding system used to represent text