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 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). and audioXpress are Elektor International Media publications.   

A Workspace for Radio & Metrology Projects

Ralph Berres, a television technician in Germany, created an exemplary design space in his house for working on projects relating to his two main technical interests: amateur radio and metrology (the science of measurement). He even builds his own measurement equipment for his bench.

Ralph Berres built this workspace for his radio and metrology projects

“I am a licensed radio amateur with the call sign DF6WU… My hobby is high-frequency and low-frequency metrology,” Berres wrote in his submission.

Amateur radio is popular among Circuit Cellar readers. Countless electrical engineers and technical DIYers I’ve met or worked with during the past few years are amateur radio operators. Some got involved in radio during childhood. Others obtained radio licenses more recently. For instance, Rebecca Yang of chronicled the process in late 2011. Check it out: and

Do you want to share images of your workspace, hackspace, or “circuit cellar” with the world? Click here to email us your images and workspace info.


Project Spotlight: Electronics + Wood Fab Speakers

MIT graduate student David Mellis is interested in how designers are combining high-tech parts like microcontrollers with low-tech materials in clever ways. Yesterday, I pointed everyone to Mellis’s inspiring 3-D Printed Mouse project. Now let’s look at another creative design—Fab Speakers.

Whether you’re a microcontroller fanatic, professional engineer, audiophile, musician, or all of the aforementioned, this open-source Fab Speakers project will surely inspire you to customize your own. I’d love to see how others tackle a similar DIY project!

Fab Speakers (Source: D. Mellis)

Mellis writes:

These portable speakers are made from laser-cut wood, fabric, veneer, and electronics. They are powered by three AAA batteries and compatible with any standard audio jack (e.g. on an iPhone, iPod, or laptop).

The speakers are an experiment in open-source hardware applied to consumer electronics. By making their original design files freely available online, in a way that’s easy for others to modify, I hope to encourage people to make and modify them. In particular, I’d love to see changes or additions that I didn’t think about and to have those changes shared publicly for others to use or continue to modify. The speakers have been designed to be relatively simple and cheap in the hopes of facilitating their production by others …

Use 6mm (1/4″) plywood. For the veneer, 1 9/16″ edging backed with an iron-on adhesive is ideal (like this one from Rockler), but anything should work if you cut it to that width. Pick whatever fabric you like. For the electronic components, see the bill-of-materials above. You’ll also need two-conductor speaker wire, available at Radio Shack… There’s also a wall-mounted, oval-shaped variation on the design. It uses the same circuit board, but combines both speakers into a single unit that can hang on a nail or screw in the wall. You’ll want to replace the batteries with a 5V power supply (included in the bill of materials); just cut off the connector and solder the wires directly into the + and – holes for the battery holder. You’ll also want to omit the power switch and just solder together the holes where it would have gone.

The design's battery holder (Source: D. Mellis)

Mellis gave me permission to write about the projects and post some of the photos from his website.

Click here to check out all the files for this project.

Hollow-State Amps & Frequency Response

“Glass audio” has been growing in popularity among average audio enthusiasts for the past decade. Music-loving consumers worldwide enjoy the look and sound (i.e., the “warmth”) of tube amps, and innovative companies are creating demand by selling systems featuring tubes, iPod/MP3 hookups, and futuristic-looking enclosures. I suspect hybrid modern/retro designs will continue to gain popularity.

Many serious audiophiles enjoy incorporating glass tubes in their custom audio designs to create the sounds and audio system aesthetics to match their tastes. If you’re a DIYer of this sort, you’ll benefit from knowing how amps work and understanding topics such as frequency responses. In the April 2012 issue of audioXpress, columnist Richard Honeycutt details just that in his article titled “The Frequency Response of Hollow-State Amplifiers.”

Below is an excerpt from Honeycutt’s article. Click the link at the bottom of this post to read the entire article.

Early electronic devices were intended mainly for speech amplification and reproduction. By the 1930s, however, musical program material gained importance, and an extended frequency response became a commercial necessity. This emphasis grew until, in the 1950s and 1960s, the Harmon Kardon Citation audio amplifier claimed frequency response from 1 to 100,000 Hz flat within a decibel or better. Although today, other performance metrics have surpassed frequency response in advertising emphasis—in part because wide, flat frequency response is now easier to obtain with modern circuitry—frequency response remains a very important parameter …

Just which factors determine the low- and high-frequency limitations of vacuum tube amplifiers? In order to examine these factors, we need to discuss a bit of electric circuit theory. If a voltage source—AC or DC, it doesn’t matter—is connected to a resistance, the resulting current is given by Ohm’s Law: I = V/R. If the voltage source is of the AC variety, and the resistor is replaced by a capacitor or inductor, the current is given by: I = V/X where X is the reactance of the capacitor or inductor. Reactance limits current flow by means of temporary energy storage: capacitive reactance XC does so via the electric field, and inductive reactance XL stores energy in the magnetic field.

Figure 1 - The values of reactance provided by a 0.1-μF capacitor and a 254-mH inductor, for a frequency range of 10 to 30,000 Hz (Source: R. Honeycutt, AX April 2012)

Figure 1 shows the values of reactance provided by a 0.1 μF capacitor and a 254 mH inductor, for a frequency range of 10 to 30,000 Hz. Notice that capacitive reactance decreases with frequency; whereas, inductive reactance increases as frequency increases.

Click here to read the entire article.

audioXpress is an Elektor group publication.



Weekly Elektor Wrap Up: Preamplifier 2012, Pico C, & a Webshop Hunt

It’s time for our Friday Elektor wrap up. Our Elektor colleagues were hard at work during this first week of April. Here’s quick review.

Elektor Preamplifier 2012: The Sound of Silence

Elektor has a 40-year history of high-end audio (tube and solid state) coverage: projects, books, circuit boards, and even DVDs. The latest project is the Preamplifier 2012, which was designed by renowned audio specialist Douglas Self, with Elektor audio staffer Ton Giesberts doing the board designs and testing on Elektor’s $50,000 audio precision analyzer! It achieves incredibly low noise figures using low impedance design techniques throughout, but still based on an affordable and easy-to-find opamp: the NE5532. The Preamplifier 2012’s most notable characteristics are its ultra low noise MC/MD section (get out your vinyl records) and the remarkably low-value pots in the Baxandall tone control (like 1-kΩ).Douglas Self and Elektor Audio Labs already stunned the audio community with their NE5532 Op-amplifier a while ago with 32 NE5532 op-amps basically paralleled on a board producing 10 W of extremely high-quality sound. Simply put: they know what they’re doing!You can read about the seven-board design in the April 2012 edition. In fact, why not follow the series?

Part 1:

Part 2:

Part 3: currently in editing for June 2012 edition.

NE5532 Opamplifier:

Pico C Webinar Announcement

Elektor announced this week that it will run a new webinar via element14 on the Elektor Pico C meter, which was featured in the April 2011 editions. The Pico C meter can measure small capacitances. In February 2012 the device was upgraded with new firmware.

According to an Elektor news item, UK-based author/designer Jon Drury will run the webinar slated for Thursday, April 19, 2012. He’ll cover a unique way of giving the original instrument a much wider range while also extending its functionality, all with new software and practically no changes to the existing Pico C hardware. Microcontroller fans, including AVR enthusiasts, can also learn how to adapt the software for different calibration capacitors. Elektor staffers are reporting that Jon may also give a sneak preview of his PicoLO oscilloscope and Pico DDS generator.  You can register at element14.

“E” Hunt!

In other news, Elektor is challenging you to find hidden Easter eggs in its webshop. Find eggs, get a discount. Click here to get started.