Recording Adapter

This gadget converts speaker outputs and line-level signals into a lower-level signal suitable for feeding into a computer soundcard or other recording device. It is specifically designed for transferring audio from an old tube-type wire recorder or record player to computer audio files, but can also receive signals from the line outputs of a mixing board or the speaker or headphone outputs of a low-power stereo or other solid-state amplifier. A stereo mini-jack is provided for connecting to a laptop or other computer, along with RCA jacks for connecting to other recording equipment. Only the RCA line input jacks remain stereo, the 1/4" speaker input jack is split to record mono. A 1/4" monitor speaker output jack is provided to monitor the speaker input with a level adjustment to permit raising or lowering the monitoring volume without significantly affecting the recording level.


The major parts to build the adapter were purchased at Radio Shack:

1     270-1805      6"x3"x2" project case
1 271-0265 25 ohm 3 watt rheostat
2 271-1720 5K-ohm audio taper potentiometers
1 274-322 four phono plug jack board
1 274-0255 1/4" mono phone jacks (package of two)
1 274-0249 1/8" stereo phone jacks (package of two)
1 271-0132 10 ohm 10 watt resistors (package of two)
1 42-2387A 1/8" stereo to 1/8" stereo cable, 6 foot long

The following extra parts were rounded up at my amp shop:

4     1K-ohm 1/2 watt "cheap" resistors
4 470 ohm 1/2 watt "cheap" resistors
1 4.7 ohm 2 watt "flameproof" resistor
3 salvaged knobs from a dead SWR amp
2 #6 sheet-metal screws
1 3/8" lockwasher (for monitor level control)
2 5/16" lockwashers (for L/R rec.lev. controls)
1 1/4" lockwasher (for 1/8" jack)
- wire, solder, silicone, etc

Tools used include a hacksaw and vise for cutting off the shafts of the 5KA controls, a dremel with a cutting wheel for making the rectangular cutout for the RCA jacks, a drill with 1/8", 1/4" and 3/8" bits (couldn't find a 5/16" bit so used a rat-tail file to enlarge 1/4" holes), a 1/2" bit for de-burring the holes, a triangle file for smoothing the rectangular cutout, wire cutter, wire stripper, 1/2" and 7/16" nutdrivers, and needle-nose plyers. Total assembly time including metalwork was less than 2 hours.

NOTE - the layout diagram is slightly wrong - the hole dimensions of the RCA jack board should be 2 9/16" (not 2 1/2"), however this resulted in a tighter fit so all is well (this is a record of what was build, not what it should have been). The holes on the jack board need to be slightly enlarged for the #6 sheet-metal screws.

NOTE - the 1/8" jack for the computer is wired tip=left and ring=right, not verified if this is correct yet, if not reverse the connections to the 1/8" jack if it matters.

The Radio Shack 1/4" jacks have bushings which are slightly less than the standard 3/8" so no lockwashers were used on them, just tighted extra tight. All grounds were wired together to avoid connection problems should the nuts become loose. The 10 ohm 10 watt resistors were stacked and siliconed in place to avoid movement and to provide better heat-sinking. The other resistors were wired point-to-point in the air with plenty of clearance to avoid shorting to other components.

Strictly speaking only the 1K-ohm resistors are needed to mix the speaker-level mono signal with the stereo line input signal. The 4.7 ohm and 470 ohm resistors were added to help avoid damage if speaker-level output signals are connected to the monitor or recording outputs but they can only go so far, values chosen to minimize chance of damage without adversely impacting the signal quality.


The primary purpose of this adapter is to dump the output of a vintage tube-type wire recorder or 78-rpm record player to audio files on a computer, which can then be transferred to CD tracks. For this application connect the speaker output of the signal source to the speaker input jack, and connect the 1/8" computer output to the line or microphone input of a laptop or other computer with a typical sound card. Typically using the speaker output jack of the signal source disables the existing speaker so for monitoring the signal connect a small speaker to the monitor output jack. Even with the monitor level up all the way up, the signal is attenuated by roughly 10db, the volume of the signal source should be turned up fairly hot but not distorting to avoid hum or noise and the monitor level control set to provide the desired monitoring volume.

On the laptop, install a program such as Audacity to record the audio (for Linux use Synaptic etc). Audacity requires the "Lame MP3 Encoder" package (or liblame0 or similar for Linux) to export to MP3 files so download and "install" that too by extracting the dll from the zip file and putting it in the Audacity program directory (C:\Program Files\Audacity, just to keep the files in one place). Important for Vista - while in the Audacity directory right-click the Audacity.exe file, click properties and in the compatibility section set to XP, otherwise it hangs when recording. Once installed and set up, run Audacity and check the settings - sample rate typically should be set to 44khz and for mono signal sources set to mono. MP3 export has to be set up by using the setup dialog to navigate to the lame_enc.dll file (for Linux navigate to where the file got installed to, on my Eee PC 701SD it was named in the /usr/lib directory, had to set the dialog to show all files to find it). Set Audacity so that it monitors the input level, set the signal source so that it's delivering a signal, and set the left and right record level controls to provide a good signal level, but safely under the maximum level. Typically set so peaks are between -6db and -10db to make sure they don't get too close to the 0 mark. Also, make sure the input level in the operating system's "mixer" is set high enough to avoid distorting the sound card's input from sending too much signal from the adapter, you should be able to go all the way into the "red" (0) by advancing the adapter's level controls, then back them off.

To make a transfer, cue the signal source so that it's ready to go, start recording in Audacity then start the signal source. When done stop the signal source and Audacity, extra space at the beginning and end of the track can be trimmed out. Export the track as MP3 to save. The full uncompressed track can be saved if desired but this consumes a huge amount of disk space so typically I don't save the "project", just export the MP3 file (but play it first before exiting to make sure the editing and export was done correctly). Free disk space is not recovered until the program is exited and restarted. Once all the MP3 files have been produced they can be lined up and burned to CD tracks using the appropriate software.

The adapter can also adapt line level signals from a PA amp or mixing board, connect the signal source to the line input RCA jacks on the adapter. These can be speaker-level signals from say an inexpensive stereo or cassette player but do not connect high-powered or tube amps to the RCA input jacks. A 1/4" or 1/8" stereo to dual RCA adapter cable can be used to use the headphone output of the signal source. There is very little or no monitor output when using the RCA input jacks, if monitoring is needed connect the monitor output jack to an amplifier to amplify what little monitor signal there is, or connect the RCA output jacks to a stereo amplifier. The RCA output jacks can be used to connect to another recording setup rather than using a computer.

Caution... if connecting both the RCA line inputs and outputs to the same system, to avoid feedback make sure the system's inputs from the adapter's outputs do not deliver signal back into the adapter's line inputs.

Last modified January 20, 2009
Terry Newton (