You've reached the new home for Jay Rose's “dplay.com” articles and tutorials!

Playing a Mac through a standard telephone line.

Also works with other equipment.
This document was first posted here in 1993 (yes, I was a web pioneer). Some of the computer references are obsolete, but phone wiring hasn't changed and the circuit will still work.
So this guy asked me how to get digitized Macintosh sounds into a modem, so he can play them over the phone line during a conversation. Lions roaring when he calls customer service, that sort of thing...

The simple answer is, it can't be done. Most modems don't have the necessary digital-analog-converter (DAC) circuit. Voicemail units are another story.

But every Mac has a DAC built-in! It's what beeps, boings, and insults you with Talking Moose. You just need to connect it to your phone.

Some time ago, I posted a circuit that would let you connect any Mac to almost any phone circuit. Since then, I've been e-mailed almost two letters of thanks. But some folks asked for more:

For the answer to most of these questions, read on...


Stereo? Louder?

Original Macs (most compact models, from the 128 up through the SE series) had a low-current audio output jack: it worked well with an amplifier, but didn't have enough oomph [technical term] to drive headphones. So the first circuit I devised just matched that output to the phone system. It wasn't particularly loud, but it worked with any Mac.

Newer Macs have a more powerful output jack. So I modified the circuit to use that power to boost the voltage going to the phone. At the same time, I doubled it so you could use it to pick up both sides of stereo sounds (they'll be mono on the other end of the phone line, of course), or to connect your phone to a stereo amp or tape player.

Which circuit fits your Mac? You can tell by plugging a pair of headphones into the back. If they play in stereo with full volume, use Circuit II. If they sound kind of wimpy, use Circuit I... and dont leave the phones plugged in.

If you've got some other brand of computer, or want to hook your phone to a stereo or other gadget with a headphone jack, one of these circuits will probably still work. But don't ask me for installation advice.


CIRCUIT I

Read this even if you're building the other version. Some of the descriptions apply to both.

You'll want a circuit like this:

I've included Radio Shack stock numbers, but you should be able to find equivalent items at any electronic store or hackers closet.

PLUG - 1/8" mini plug to fit your Mac. Compact Macs and older Mac II models with mono output can use a 2-conductor plug, #274-286.

SWITCH - Any single-pole, double-throw switch. #275-635 is a good possibility, or you can use knife switches, three-way wall switches, or block switches from the model train you abandoned when you bought a computer.

TRANSFORMER - This is the key to the whole thing. It keeps the Mac and The Phone Company from interfering with each other. If you don't use one, you'll probably disable your phone service.

Any low-power 1:1 audio transformer should work. You can rip one out of that 300-baud modem youve been meaning to toss, or get a #273-1374 for under four bucks.

(If you get the Radio Shack transformer, cut the Scotch tape wrapped around it to free up the four wires. The wire colors should match the diagram.)

CAPACITOR - This isn't necessary for transmitting sound, but it lets you hang up your phone. Otherwise, the phone system will think the transformer is an active telephone.

Get a film or ceramic capacitor rated at least .1 uF at 75 volts. Slightly higher ratings aren't a problem. #272-1053 (.1 uF / 200 volts) is a good choice. Electrolytic capacitors won't work.

SPEAKER (optional) - Using the sound output of your Mac disables its internal speaker. Pull the plug out to turn the speaker back on. Or wire in a substitute, #40-246 (or any other small low-power speaker, like the one from that 300-baud modem). It won't be as loud as the internal speaker, which could be a good thing.

You'll also need some hookup wire, to extend the components and reach your telephone. Use bell wire, #278-1509, or anything else in your basement or on Radio Shack's shelf.


CIRCUIT II

Build one of these:

The parts are the same, except the transformers are audio-output stepdown ones, used backwards. Radio Shack #273-1380 does the job nicely. (You'll need two of them, but they're cheap.)

The plug should be a stereo ring-tip-sleeve one: #274-284 matches newer Macs and most Walkpersons; use #274-139 for most home stereo amps. Or rip the cord off a pair of non-functioning headphones.

I left the switch and speaker off the drawing of this circuit, but you can add them if you want. You can also leave out one transformer, if you're dealing with a mono circuit (but don't tie the tip and the ring of the plug together; that can harm your Mac).

The transformer color codes might change, so follow these simple rules:

  1. Note how one side of the transformer has two wires, and the other has three. The two-wire side connects to the Mac. The three-wire side faces the phone. The middle wire on the three-wire side doesnt connect to anything.
  2. Note how white is wired to white, and green is wired to green. Even if you use transformers with different colors, the two sides of the stereo circuit must match. Otherwise, stereo sounds will be mangled and mono ones will disappear entirely.

ADDING A HEADPHONE JACK

Somebody asked. So I added this circuit:


CONSTRUCTION

Both the Mac's audio output and the standard phone system are low-voltage. Special wiring techniques aren't needed.

I had this info up on the web for six years before Don Fredricks (donf@tns.net) rightly pointed out that when the phone is ringing, there's a low frequency 70 - 90 volt signal on the line. If you touch it, you'll feel it.

For safety's sake, make sure the phone connections are insulated.

And make sure nobody rings you up while you're touching the phone wires:

  • Leave another phone on the same line off the hook, or
  • Disconnect the phone line where it enters the premises (almost all phone installations have a plug and jack in the basement, or on the outside of the house, for this purpose), or
  • Use modular connectors and never touch the live wires yourself.

Solder or twist the wires together neatly, and insulate the connections with electrical tape (or even duct tape, Scotch Tape, or Band-Aids).

It doesn't matter which wires go where on the speaker or capacitor.

If you're using a stereo plug (circuit II), notice how the middle wire goes to the tip of the plug, the top wire goes to the ring partway up the plug, and the bottom wire goes to the plug's sleeve.

Follow the color codes, and -- if you're using a switch -- make sure the wire from the Mac goes to the switchs center connector.

Put the whole mess in a box to make it look pretty. I used part of an old intercom: the speaker and switch were already built in.


CONNECTION

The Mac end is easy. Plug it into the jack that has a speaker icon, like this one:

You have a few choices when it comes to the Telephone end.

  1. DIRECTLY TO THE PHONE JACK. Open up the jack on your wall (it's low voltage). You'll see at least one red and one green wire, connected to screw terminals. Connect a wire from the transformer to either of these screws, and from the capacitor to the other screw.
  2. DIRECTLY INTO THE PHONE. If you've got a standard, non-electronic, non-memory phone, open it up and trace the wires coming in from the back. They'll connect to two screw terminals marked L1 and L2. Connect the two wires from your circuit to these screws. It doesn't matter which goes where.
  3. JUST PLUG IT IN Get a phone cable like #279-391, with a standard modular plug on one end and four short wires coming out the other. Connect the transformer and capacitor to the red and green wires. Then plug the modular end into any standard phone jack or modular Y-connector.
  4. FOR MULTI-LINE PHONES If your office phones have thick cables and 1A2 multiline connectors (they look like SCSI plugs), get a #43-270 adapter. Then use method #3, above.
Digital phones -- found in some office buildings -- require a special jack. Look on the bottom of your office phone. If there's a Ringer Equivalent Number or REN stamped on it, it's NOT digital and you can use these methods. (Touch-Tone phones aren't necessarily digital.)

Office telecom managers can help you get the right jack, if they can keep from laughing uncontrollably when you describe what you want to do.


OPERATION

Make a phone call. Play a sound on the Mac.

If you don't hear the sound on your telephone handset, flip the switch.

Set the volume with the Sound Control Panel.


AUTHOR'S NOTE

The trademarks herein belong to companies with big law firms.

The circuit should work and be safe. I tested both versions (and have built many for friends) and never had any problems. However, strange things can happen to Macs, lightning can strike phone lines, and klutzes can burn themselves with soldering irons. So I don't guarantee anything.

This document is (c) 1993,95 Jay Rose


To tutorials index

To Digital Playroom main page

Mail to

jcrose@JayRose.com

**11