The PDF Mod, Mk5e...
The Project Dalek Voice Modulator Mk5e is our very own voice conversion unit that allows you to speak like a Dalek. It is an ideal solution for those who want a cost-effective way to change their voice when they are inside their Dalek, operating it.
Here are some technical details of what is inside the 'PDF Mod Mk5e'...
The heart of the circuit is an LM13700 chip from Texas Instruments. We've made an attempt to make an affordable circuit that is as easy to construct as we could manage and does a good job reproducing the surprisingly broad range of effects used on the show over the years.
The circuit can be broken down into 6 sections:
- Power supply.
- Microphone pre-amp.
- Wein bridge oscillator.
- Four quadrant multiplier.
- Output buffer/mixer.
1. Power supply
This part is pretty simple. Audio circuits are usually designed with a split supply. That means a ground rail, and the +/- supply voltages. When operated from a single battery we need to create a reference at half the supply voltage to use as a stand-in for the ground rail. Then the battery positive and negative terminals can be thought of as the +/- supply voltages.
For this circuit, we used what is referred to as a virtual ground. It consists of a simple opAmp voltage follower. A resistor divider creates a reference at 1/2 the total supply voltage. This is fed to the + input of one channel of the opAmp. The opAmp output then “follows” this fixed reference at the output.
2. Microphone pre-amp
The circuit is designed for an electret microphone. Most head-worn mics you’ll find nowadays are electret, so this seemed like a logical choice. If you intend to use a dynamic microphone, the circuit can be modified to accommodate, but should not be used as-is.
R21 supplies “phantom power” to the microphone. C21 blocks the DC offset from the signal so only the voice gets into the amplifier. The opAmp is configured as a simple inverting amplifier with a gain of (R23+R24)/R22, so ranges from a minimum of 1 up to a maximum of 21.
D3 and D4 act as a crude noise gate. Basically, the inline back-to-back diodes block signals below ~0.5V from getting to the next stage. This should block light breathing and background noise from getting through. R25 limits the output current of the amp and the Green LEDs clip the signal introducing distortion on the voice. Similar circuits are used in solid-state electric guitar amplifiers.
You can use the green LEDs as a visual indication as to how much distortion is added to the voice. Turned down so there is no light from these LEDs will sound the cleanest (though there will still be some distortion from the noise gate). The higher the gain, the brighter the LEDs, the more distortion. How much is up to you.
3. Wein bridge oscillator
I won’t go into too much detail here. If you are interested in the theory a quick google search will turn up better descriptions than I can fit into this post. This one is a slight variation on a “Diode Stabilized Wein Bridge” in that we used LEDs instead of regular diodes. This was done for two reasons. First, so the amplitude will be sufficient for the next stage, and second, so you have a visual indication that the circuit is working.
There’s also a small resistor in series with the LEDs to soften the stabilization and reduce distortion. The frequency is determined by R11/R12 and C11/C12. Extra spaces are included to do some fine-tuning of the modulation frequency if desired. The frequency is fixed, not continuously variable like other ring modulators. This was done to keep it simple and cheap.
4. Four quadrant multiplier
This is the important part. “4 Quadrant” just means it can multiply two voltages, and both can be positive or negative. The chip we used for this is the LM13700 dual transconductance amplifier. The datasheet for this chip includes an example of a 4 quadrant multiplier that uses a single side of the chip.
This was the basis of the MartMod Mk3. Thanks go to Vince Martyn for graciously providing us with his circuit to get us started in our development. Though it works well, it requires very careful selection of the resistor Rm to get it balanced perfectly. The PDF Mod Mk5 is instead based on the 2 Quadrant multiplier from the datasheet.
It uses one half of the chip to amplitude modulate the voice signal, multiplying it by a sine wave that varies from 3->9V. 3->9V, all positive, hence the 2 quadrant designation. It uses the other half of the chip to multiply the voice signal by -6V (inverting configuration and fixed 6V in the modulation input).
Again since the modulation signal stays on one side it is considered a 2 quadrant multiplier. When the output from both sides are added together you get the same effect as a 4 Quadrant multiplier without needing to use odd resistor values. Here’s a picture illustrating what the circuit is actually doing, and that section of the PDF Mod Mk5 schematic.
This circuit uses a few resistors and diodes to create reference voltages of about +/- 0.5V from Vcc/2. Two halves of a dual comparator are then used to see if the voice signal is outside this range. If the signal goes above Vcc/2+0.5V one comparator output will go high, if it goes below Vcc/2-0.5V the other comparator output will go high. If it’s between the two both comparator outputs will stay low. The two outputs are tied to the gate of an nFET in such a way that either of them can turn it on. The nFET then switches the current to the lights.
The PDF Mod Mk5e is occasionally available as a pre-built unit, which is offered for sale, from time to time, via the Project Dalek Forum. However, the unit is firmly aimed at the DIY enthusiast. With that in mind, the 'Voice Modulator Assembly Manual' is available (via the Forum) for everyone to download and use to build their own unit. A comprehensive parts list is contained within the manual.
- The Voice Modulator Assembly Manual can be found here.
- The support topic for this unit can be found on the Project Dalek Forum, here.
- We also supply the printed circuit board required to assemble the unit. This is available to order, via the Project Dalek Forum, here.
- For those who would like to create/order their own PCB, the Gerber files have also been made available, in the Downloads section, here.
- We also provide files for those who wish to 3D print their own case/enclosure for this unit. These can be found here.