The Choccy Block Simple Ring Mod: A No-Soldering Diode and Transformer Ring Modulator Kit

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Picture of the Basic Ring Mod Schematic, Click to enlarge.



This is a very simple ring modulator circuit which will allow you to make interesting analogue sound effects using two audio sources and some kind of amplifier or recording device.  Almost everyone has access to a couple of mp3 players and some powered computer speakers nowadays, so this is quite an easy way to go about it.  One very familiar use for this old style of ring mod is that which produces the "Dalek" voice.  I will concentrate on that application in the text, but there are other effects you can produce which I'll discuss later.

Bill Of Materials

Electronic Parts Included in the Kit

Qty    Refs          Type              Value

2      L1,L2         Transformer       LT44
4      D1,D2,D3,D4   Schottky Diode    BAT48
3      J1,J2,J3      3.5mm Stereo Jack Leads

Miscellaneous

1                    10-way 3A Terminal Strip
20mm                 Plastic Sleeve

Link to PDF of Simple Ring Modulator Schematic
Link to PDF of Simple Ring Mod Bill Of Materials

Pre-Requisites

Tools

I supply a kit of all the electronic parts in the list above, or you can buy your own.  I've assumed that you have a small screwdriver for the terminal screws, some small wire cutters for trimming the component leads and some means of stripping the insulation from the wire.  A craft knife is good for that.  It is best to mount up the circuit in an enclosure or nail it down to some wood when it is all working, and the holes in the terminal strips make that easy to do. 

What do you need to know before you start?

This is a nice simple circuit which you can make pretty much just by looking at the final picture.  The components list has links to photographs of the components.  I've included a full electronic explanation later on.  If that doesn't make any sense, don't let it put you off just building the circuit.

How To Build It: Follow The Pictures and Schematic Diagram

Cut Up the Terminal Strip and Assemble the Diodes

Using a craft knife or scissors, cut the terminal strip into two pieces of two terminals and two pieces of three terminals.  The four diodes can then be assembled the correct way around by cutting the leads to about the right length and trapping the ends under the screws in the terminal blocks as shown.  I fold the leads of the diodes back a bit at the end to make them easier to trap properly under the terminal screws.  Don't tighten the screws too much at this stage as you may want to re-adjust the assembly. 



There is a small length of PVC insulation provided which can be put on the leads of one of the diagonally mounted diodes. This avoids it shorting against the other one.  You should now have something that looks like this:



Connect Up The Transformers

Next connect the two transformers in a similar way.  The transformer sides with the three wires go into the three-way terminal blocks with the diodes, and the side with the two wires connects into another two-way block. 



Make the final connections to the Input - Output Plugs

Next, strip the wires on the jack plug leads by carefully scoring round the outer PVC and then the inner core and baring the copper wire as shown.  Twist the bare wires and fold them back at the ends to make them easier to trap properly under the screws.  Then connect them up as shown, one to the two terminals on the left, another to the two terminals on the right, and the final lead to the two centre terminals in the middle.  I have used the white cored wire and this corresponds to the left channel.





That’s It, but Check and Secure

This is a pretty simple project and if you're sure everything is in the right place, you can screw down the terminals a bit tighter and make sure that everything is secure.

Starting Up and Initial Testing



I've labelled the three plugs A,B and C.  C is the output from this circuit and that needs to plug into some computer speakers, or a line input to an amplifier with some speakers or headphones.  I'm going to call C just the "output."

A is the input which has the voice signal fed into it.  This needs to connect to the headphone output or line output of some mp3 player which has the voice signal to be processed on it.  If you have a portable recording device with a microphone, you may well find that the signal from the microphone is amplified and played out of the headphones while recording.  In that case you can use that as a microphone amplifier and use the system live, speaking into the microphone.  I'm going to call A the "voice input."

B is the input which needs to be connected to another mp3 player or similar device which is going to play back a 30Hz sine wave signal.  There are links to mp3 and wav versions of 30Hz tones shown below.  If you have an electronic signal generator you can use that instead, and that will allow you to change the frequency of the tone in real-time.  I'm going to call B the the "tone input."

Be careful with the volume setting on the output amplifier at first and adjust it upwards only when everything seems to be working right.  Set the tone input and voice input to midway and when everything is set to playback you should hear the processed voice coming out of the speakers.  If you've set up a live microphone input you might get feedback if the speakers are set too loud.  Using a live mic is the most amusing. 

If you've made this especially for Dalek impersonations, bear in mind that the gold coloured ones who are in charge have that deep gravelly voice, while the red footsoldiers tend to have the somewhat higher voices and can be easily panicked if used as a hatstand or when certain people are mentioned.  This circuit is very similar to the original device used in the 1960's, so having gone to the trouble of building up the hardware you can also claim some level of historic authenticity.

Picture of the sound sources and 'amplifier' that I used for the demo:



In the picture, the Nokia E6 is used to play the voice signal, the N900 is used to play the 30Hz tone signal and the output from the ring mod is plugged into the line input on the venerable Sony WMD6C pro Walkman.  I'm listening on the headphone output of the Walkman as it records.  The Nokias conform strictly to a slightly ridiculous EU limit on headphone output level, so they drive the circuit at a quite low level.  Here are the relevant audio files:

MP3 -3dB 30Hz Tone on Left and Right, 10 minutes, 44.1k sample rate
WAV -3dB 30Hz Tone on Left and Right, 10 seconds, 44.1k sample rate

MP3:  A voice demonstration before processing
MP3:  A voice demonstration after ring modulation

If I'd wanted to use a microphone live, I would have had to find something else to use as the output amplifier and could have used a microphone with the Walkman in record mode to provide a voice signal.

Other Things to Do

Left and Right

You might have noticed that we've stuck to using just the left channel of two separate audio sources.  This was just to make handling the audio files easier.  There's nothing to stop you putting the tone and voice signals into one stereo file if you know how to do that.  I will leave the details to the computer experts, but it's easy enough with some audio editing software.

Higher Tone Frequencies

You can turn up the tone frequency if you're using a signal generator to create interesting frequency shifting effects that way.  What happens is that two versions of the voice signal are created.  One is shifted up in frequency, and the other is shifted down.  The down shifted voice also reflects through zero Hz and is frequency inverted as the tone signal increases in frequency.  Feedback from the speakers can get very interesting if you have the live microphone set-up too.  If you have a long corridor and some long speaker cables, feedback from speakers to mic with different tone frequencies can get you into high-quality interesting Quatermass sound-effect territory.  Similarly you could use a short tape or digital delay with this in the feedback path.  Even if you use a digital delay, using an analogue processing stage like this introduces various forms of noise and distortion which can turn a boring digital effect into something much more interesting.

Reduced Tone Level

The operation of the diodes at normal drive levels has them either turned on or off most of the time.  This introduces more harmonics into the voice signal.  If you are trying to produce the frequency shifting effects already mentioned, these can be heard more clearly by reducing the tone level to a point where the diodes are operating more linearly.

Squaring or Frequency Doubler Operation

To Follow.  Basically, you feed the same audio signal into the audio and carrier inputs resulting in an interesting frequency doubling effect at the output.

More Detailed Electronic Discussion

(More To Follow.) 

Double balanced modulators like this were traditionally used in older radio circuits using radio frequency transformers to produce suppressed carrier AM signals.  If you look up circuits like that and compare it to this application, it can appear a bit confusing as the RF 'carrier' input would be connected our 'voice' input and the 'voice' input would be going into our 'tone' input.

You might have heard that it's a good idea to match the diodes so that they all have about the same Vf, the forward voltage drop, and that a low forward voltage drop is a good thing.  All the BAT48s that I've seen so far have been very closely matched straight out of the bag, and have a low Vf at the currents that we are using them at.  You can use other signal diodes if you want, and higher Vf non-Schottky silicon types like 1N4148 will probably sound a bit different at some drive levels.

FAQs

Q:  I can't hear anything when I play back the 30Hz tone mp3.
A:  30Hz is generally too low to hear.  Don't play it back too loud in media player or it might damage your speakers.

Q:  I would like a circuit that does all this on battery power with a microphone and speaker on one circuit board.  Is there one?
A:  There is, but it's not in kit form yet.  I'll be publishing that at a future date.  Meanwhile you can see the circuit here: Portable Dalek Voice Ring Modulator Circuit

Q: Why not use the red LT700 type audio output transformers like many others do?
A:  Because they really are intended for low impedance loudspeaker matching into 3.2 Ohms.  That's a bit tough to drive on the voice input side and produces quite a low signal voltage on the output.

Q:  Can't I do this on the computer much more easily?
A:  Yes, but it's not as interesting and it never sounds quite the same due to various secondary properties of the real circuit.

Should you have any problems or questions, my main email address is shown below.  This address has been the same since 1997 and unless I'm on holiday beyond mobile coverage, it is checked daily including the spam folder.



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26-AUG-2022: Removed references to my ebay shop, having been "binned-off" from that site for telling the truth about their business practices to the UK vice-president.