Synthesizer Build part-58: VCA-1 by THOMAS HENRY.

 A very good working VCA and Mixer using the AS3080 IC. Very easy to build and with a Line Out option too.

Having built the VCF-1 and the X-4046 VCO designs by Thomas Henry I thought I might as well build the VCA-1 aswell. By coïncidence I needed a new VCA in my DIY synthesizer anyway because the transistor based VCA from way back in the beginning when I started this Modular journey is now starting to act up after working flawlessly for 3 years. It was about time for a new and updated VCA and one with a little more options too. This project is another one from the Birth of a Synth website where I got the others from too.

I built and tested this VCA to run on a dual 12V powersupply so perfect for Eurorack too. It's designed for +/-15V but either voltage will work fine. No component changes needed.

If you're new to DIY synthesizer building and you're not sure about the function of a VCA and what it's for, then please go back to project 10 on this website and read the first bit of that article. That will explain it for you. Then come back here and build this one ;)

This VCA is not only a Voltage Controlled Amplifier. It's also a 3 channel mixer and it has a Line Out option (all in mono though). I did put in 3 inputs in the layout but I only used one in my own module. I didn't have the room for 3 because I needed this VCA to replace my old one and that one is only 4 CM wide. You could easily add even more channels just by putting in more 100K resistors connected to pin 2 of the TL074, and having your inputs come in through those resistors.

This project was yet an other hole in one. It all worked flawlessly at the first try, just like the X-4046 VCO. I LOVE Thomas Henry designs. They just work so well.

Here's a look at the finished product on the test bench:

I made a few changes in the way I'm using this VCA as opposed to how Thomas Henry intended it to be used. TH had the ADSR coming in on an unattenuated input and an extra CV input with attenuation. I turned that around and I have given the ADSR a level control and the extra CV input has no level potmeter. I also put in an extra bi-coloured LED because I used a quad opamp instead of a dual plus a single opamp and just like with the State Variable Filter I had one opamp left over. I connected that to the ADSR input so we have a visual indication that the VCA is receiving an envelope signal and if that signal is positive or negative in Voltage and how strong that signal is, going by the brightness of the LED. Actually a very useful feature to have.

SCHEMATIC:

Here's the schematic I used to make the layout. As mentioned, I made some changes in the type of opamps I used. I also only used one input instead of three. As you can see it's a very simple circuit and it works very well with the AS3080. I used a TL074 for the opamps so the opamp pin numbering on the schematic will be different from the layout. For the diode you can also use a 1N4148 and the small caps C1 and C2 can be ceramic types.

LAYOUTS:

Below are the layouts I made for this project. As ever they are verified. I used them for my build and it again worked straight away, just like the VCO.  Again, the LED was an extra I put in myself and it's not in the schematic. It's fed by an opamp buffer connected to the ADSR input with a Bi-Coloured LED on the output and a 2K4 resistor as current limiter. It's a useful addition.

Wiring diagram:

Stripboard only view:

This layout was small enough to have a few strips left unused which gives up plenty of room to connect some L brackets to mount the stripboard to the faceplate.

Cuts and wirebridges (component side view). I did not bother to make a layout with just the cuts on it because they are easy enough to see on this combined view:

And here's the Bill of Materials. 

You can order AS3080 chips from Electric Druid. 

You can also order them from Thonk.

Testing was pretty straight forward. I connected my scope to the Max. output and put a VCO signal on the input (yellow line) and an LFO signal on the ADSR input (blue line) to serve as envelope signal. The scope showed a beautifully responsive VCA reaction as you can see on the screenshots below. 

When you turn the INITIAL potmeter past the 1 o'clock position the audio signal will reach its amplitude limit. In that case the VCA will also not shut completely. (The audio will not distort though) You can look at the Initial potmeter as the 'Gain' control on other VCA's. You use it to open the VCA so you have continuous sound without any keys being pressed on the keyboard. The further you turn it clockwise the louder the audio will be.

Here you can see the negative voltage rejection on the ADSR input. As soon as the blue line goes through the zero Volt line and higher, the audio will come on. The LED will shine blue when there's a negative voltage presented on the ADSR input and the VCA will just stay closed.

CALIBRATING THE VCA:

This is simple. There's only one trimmer and you use it to trim away any DC offset on the audio output. What you do is you connect an oscilloscope to the Max output. Set the AC/DC switch to DC. Then you put an audio signal on the input and an LFO signal on the ADSR input, like the screenshot's above. Then you turn the trimmer until the signal is centered around the zero Volt line. In other words, the zero Volt line goes straight through the middle. 

That's all.

TIP: Instead of an envelope signal opening and closing the VCA, try using an audio signal. This VCA can work very fast and if you use an audio signal you get a sort of ring-modulation in combination with the signal you have on the audio input. Try experimenting with this.

PICTURES:

Here are some pictures of the finished module:

So that's it for now. Three in a row this month and three very good Thomas Henry projects for you to sink your teeth in.

If you have any questions or remarks please comment below or in the special Facebook Group for this website.

Synthesizer Build part-29: VCA (Yusynth design).

A good working and easy to build VCA for use as extra VCA or as the endstage and line out for your modular set-up. The VCA as presented in this article is specifically meant for audio, not CV however if you leave out the input electrolytic caps you can use it for Control Voltages.

This module is a great solution for all your VCA needs. I needed a new VCA module for the second stage of my synthesizer and I wanted to upgrade from the first one I built, although it functions fine I hasten to add. This VCA is a more luxurious version of the first one you could say. And this one doesn't invert the signal like the first one did.

The function of a VCA:

Before I get ahead of myself, let me explain what a VCA does for all the people who are new to DIY synth building. A VCA or Voltage Controlled Amplifier, is used to stop your synthesizer from continuously making sound and to only produce sound when you press a key on your keyboard. So it's like a volume knob that is only opened up if it receives a signal from the Envelope Generator. Now the higher the voltage from the envelope generator (or ADSR)  is, the louder the output of the VCA will be. VCA's can of course be used for other things, like in drum machines, as a sort of Gate, to let through short pulses of noise to create percussion sounds but this is not what we're going to be dealing with here. This VCA is primary used as the last stage in your synthesizer from where the audio goes to the HiFi amplifier and the speakers. But the output must be further attenuated to make it suitable for the line in of a HiFi amplifier. In fact the term 'Amplifier' in VCA is a bit misleading. It should be Voltage Controlled Attenuator because when the VCA is fully open the output signal will have the same strength as the input signal. It has not been amplified.

So to sum up: The VCA lets through audio when it receives a signal from the Envelope Generator and it stops audio from passing through when there's no envelope signal present on the ADSR input of the VCA. Opening up the 'Gain' control will enable you to bypass the Envelope Generator Input and output sound/audio without pressing any key on the keyboard and therefor without any Envelope Signal being present but normally Gain is set to zero.

I hope that's clear but you can always ask me things in the comments if you need more info.

Here's a cool video showing the how VCA's and ADSR's work together. ---- CLICK HERE ---

The VCA is capable of handling input signals of + and - 10V and outputs them at the same level if the audio and ADSR potmeters are fully opened up and if you use an Envelope Generator that outputs an envelope of  +10Vmax. This VCA is perfect therefore to pair with the Yusynth 7555 ADSR from article 24.
If you need to output the signal to a HiFi audio amplifier or mixing desk then you must use extra attenuation. You can easily do this with a resistor voltage divider that divides the amplitude of the audio to one tenth of the input amplitude. If you then turn the audio and ADSR potmeter back, the signal will be low enough to go into the Line-In of a HiFi amplifier or mixer. There's an extra additional layout below to show you how you can do that.
As to running this VCA on a dual 12 Volt power supply; I don't think there will be any problems with that. After all the opamps and transistors all work fine on 12V so I think it's just a matter adjusting the trimmers. You could have an issue where one of the trimmers is turned all the way to its limit because of the lower voltage but I have no data on this so I can't be sure. The first VCA I describe in chapter 10 is purpose built for +/-12V so you can always build that one if you're not sure. I've build 3 of those so far and they work very well. As you can see on the layout below, this design has one extra potmeter compared to the earlier VCA I built, and that's the audio level control potmeter on the input, which is always a good thing to have.

The two BC547 transistors Q1 and Q2 must be a matched pair. It's enough to just stick them in the Hfe meter of your multimeter and select a pair that have the same Hfe value. When you measure them let the transistors cool a little after you touched them because their values will change with temperature.

Here's the verified stripboard layout. Wiring diagram:

(Last revised: 23-April-2025: cosmetic changes to the layout.)

Stripboard only. 

Here's the layout with just the cuts and wirebridges. Mark the cuts on the component side with a black Sharpie or Edding marker and then stick a pin through the marked holes and mark them again on the copper side. Then cut the copper at the marked positions with a hand held 6- or 7mm dril bit.

This is the schematic from which I made the layout. In my build I have the 10µF electrolytic capacitors on the input as is shown in the schematic below. That means this VCA is an AC version meaning it can not handle signals from a Low Frequency Oscillator (LFO). If you need a VCA capable of handling very low frequency signals (a DC version) then leave out the 10µF caps on the input but for normal audio use LEAVE THEM IN! In my own build I also added a 10µF cap on the output (+ towards the opamp) as an extra failsafe against DC offset voltages on the output but you don't have to replicate that. (It's not in the schematic but it's in the layout as extra option.) Alternatively you can put these capacitors on a switch in series with the input(s) and then you can use them when you need them.

Bill of Materials. The logarithmic potmeters are noted as linear types in the BOM. It hardly makes any difference it's just that a logarithmic taper sounds more natural to the ear but you can use whichever you prefer.

LINE OUT:

If you want to use the VCA to feed a HiFi amplifier then you can use a little voltage divider network to further attenuate the audio output level to make it suitable for line out levels which are usually around 100mV to maximum 1V. I made a little extra layout to show you how you can add such a voltage divider to this stripboard. I used a 1M resistor and a 100K trimmer potmeter to divide the output voltage by at least a factor of 10. You can set the initial output level with the trimmer and then further adjust the level with the ADSR and Audio Level potmeters on the panel.

These extra components are not in the Bill of Materials!

Calibrating the VCA:

Before we start, do all the measurements on the 'Normal Audio Output', not the AC one. I added the AC output as an afterthought to block any DC offset voltages from coming through but normally you can use the the Normal Output. So connect your probe there for calibrating the circuit.

With trimmer A2 you set the initial bias voltage on the base of transistor Q3. This influences the working of the Gain potmeter and you should set it in such a way that with the Gain potmeter all the way closed the signal is just muted. If you turn Gain up the last played note will then become audible without pressing any keys on the keyboard. So Gain is normally closed.
I found that the audio signal initially starts at 10V and then drops to about 6V. To counteract this you need to open up the Gain potmeter a little and then trim again so the signal is muted with Gain slightly open, This will stop the voltage drop of the signal and keep it at full power all the time. You will see this soon enough if you start testing it and connect this circuit to a scope. It's easy to counteract and it is in itself not a real problem because you hardly hear the voltage drop but you know, I strive for perfection ^___^
Trimmer A1 is the Balance trimmer. You set it so that the part of the signal that is above the zero Volt line has the same level as the part below the zero Volt line. In other words, you set it so that the signal has the same amplitude in both the positive and the negative part of the wave. This is best done using a triangle or sinewave on the input, together with an oscilloscope.
For trimmer A3 I advise to use a multi turn one. With this trimmer you trim away any DC Offset voltage on the audio output. Again you absolutely need a scope to do this but a cheap 20 dollar one will do nicely here.
The circuit has a LED that indicates the presence of an audio signal on the input. It's a sort of one LED VU meter and the brightness varies with the strength of the audio signal. If you turn the audio input level potmeter up, the LED becomes brighter.

Here are some pictures of the finished product. If you look carefully you can see the electrolytic capacitor on the audio output. This is the AC audio output option to prevent any DC offset voltage on the audio output signal. If you are going to use this VCA with very low frequency signals, like from an LFO, you need to use the DC output option: (If you're a beginner then don't worry about that, use the AC output.)

Normally the AC input is used for all audio signals and the DC input is used for Control Voltages.

You can see that I put a little white stripe alongside the counter clockwise first 20° of the throw of the Gain potmeter, to indicate to where you can turn the potmeter with the audio staying muted. If you turn it past the white, the signal becomes audible and normally the Gain should be slightly open but with in the white area.

The day after finishing this module I built two extra VCA's using the old layout and they work fine too. I wanted a few extra VCA's available but there was no need for the luxurious model. I'm going to use that as the end stage VCA in my second stage, with the output going to a HiFi mixer (at Line Out level) and use the other two VCA's for use in different patches.

Here's some pictures of the double model. I also posted these in the first VCA article:

Finally I want to leave you with this very informative video by Moritz Klein about the ins and outs of differential transistor VCA's. He explains why the transistors need to be matched and goes into the technical details of how a circuit like this functions:

TIP: Instead of using a CV signal to open and close the VCA why not try an other audio signal? That way the VCA will act as a (sort of) ring modulator. It's actually AM or Amplitude Modulation. Change the frequency to the lower/bass areas and experiment with modulating the VCO. You can get some weird sounds going that way.

So that's an other one done. One more and I'll have published 30 synthesizer related projects.

Okay, if you have any comments or questions please put them in the comments below or post them in the Facebook Group for this website. 

Synthesizer Build part-10: THE VCA or VOLTAGE CONTROLLED AMPLIFIER.

A very simple, old school, transistor based VCA circuit that's tried and tested. Easy to build and small enough for Eurorack. You do need an oscilloscope to calibrate it though.  This module runs on +/-12V as standard so a very Eurorack friendly design.

EDIT April 2024: There's now also a new VCA project available that's also a 3 channel mixer and has a line out option. Please go to Project 58 to check that out. 

Please read the whole text before building this project. 

WHAT'S A VCA FOR?

The VCA is nothing more than a voltage controlled volume knob. It lets audio pass through when you press a note on the keyboard and it shuts it down after you let go of the key. (So actually the term Voltage Controlled Amplifier is a bit misleading because it does not amplify the signal, it attenuates it.) The opening and closing of the VCA is done using signals coming from the Envelope Generator or ADSR or from an LFO depending on what you use it for. Don't mistake a VCA for an Audio Amplifier. You can not hang speakers on the end of this circuit. A VCA is used to make sure your synthesizer only produces sound when you press a key on the keyboard. The keyboard produces a Gate signal that is high as long as the key is pressed down and a 1 Volt per Octave signal that tells the VCO which note to play. That Gate signal then triggers the Envelope Generator and the output from the Envelope Generator goes into the VCA together with the audio output from the VCO and as the VCA detects the Envelope signal it opens up and lets the audio pass through with the volume or amplitude depending on how high the envelope signal is in voltage. The output of the VCA must later be attenuated to audio line level if you want to feed it into a HiFi amplifier. So the VCA is just a link in the synthesizer chain.

In the picture below you see a block diagram to show you the position and function of a VCA in a synthesizer. There seems to be a lot of confusion about this with people new to (modular) synthesizers. The VCA is the white triangle marked 'Amplifier'. I does say "Output to amp/speaker" but they mean that the VCA outputs a line level signal going into a normal HiFi power amplifier. There is no line level output in my VCA layout but it's easily added with a voltage divider.

(Image taken from 'The Complete Synthesizer' PDF Book)

I used a very simple design for the VCA which I again found on the Yusynth website.
Here's the schematic:

If you want to experiment with this circuit then here is a link to a Falstad simulation I made:  --- CLICK HERE ---

This is an old design and there are some updated versions out there but this is tried and tested and has served me well for over 3 years now. It's very simple and it works very well except that in my build the signal came out inverted. This isn't really an issue because it's an audio signal and they sound the same whether inverted or not but my Obsessive Compulsive Disorderly mind wants it coming out the same way it came in so I added a little opamp inverter to the output to set this straight. Afterall a VCA can also be used for Control Voltages like signals from an LFO and in that case polarity is important. 

I put a 10µF Electrolytic Capacitor on the output (plus connected to output VCA) because I noticed a 240mV DC offset voltage on the output which I couldn't trim away with the potmeters. This may be due to a slight difference in resistance between the two 10K resistors connected to opamp U1b. Make sure you measure those and use two that have the same value resistance. In fact all resistors with the same value should be matched because this circuit depends on symmetry!! The transistor pair must be matched too.

Use an oscilloscope to set the trimmer potmeters. You should be able to measure a DC voltage (before the 10µF cap I mentioned earlier) and, with trimmer R18, trim away as much DC voltage on the output as you can and with the other trimmer R14 you can trim the balance of the signal. You set it so the positive part of the wave has the same amplitude as the negative part of the wave, with the zero volt line being the dividing line. If you want to use this VCA for Control Voltages then you can't put a capacitor on the output because that will block low frequency signals, which it sees as DC.
The output level may be a bit lower than the input level, even if the ADSR potmeter is fully opened up. If that's the case and you want to correct that then you can change the gain of the output buffer opamp. This is something I added myself and is not included in the schematic but only in the layout. If you change the 150K resistor between pins 1 and 2 of IC-2 for a 470K resistor, you should get 3 times gain! That should bring the level back to input level. You can experiment with this yourself. If you put a 500 KOhm potmeter between pins 1 and 2 you can control the gain of the opamp with a knob on the panel. Just a thought ;) Make sure you give it a different name though otherwise you'll have two gain potmeters on your panel :D 

If you are interested in the functions of the transistor pair and the other components in this circuit then here's a link to a blog post that explains it in great detail:  -- CLICK HERE --

LAYOUTS:

Here's the layout I made for standard 24x55 hole stripboard using only 24x39 holes. Like I said before, I added a signal inverter in the shape of a second TL072. Only one of the two opamps in the chip is used, the other one is properly connected to ground. Make sure the Gain potmeter is linear. In fact use linear types for both pots. You can also use other values like 47K or 100K because one is an input level control and the other a voltage devider switched between 12V and ground so the values have no influence on the working of this circuit.
The layout below is verified and absolutely faultless. I guarantee it. It has been used successfully by many people now. All potmeters seen from the back.

Stripboard only:

Here's the Bill of Materials. It says to use logarithmic potmeters but you can also use linear types. In fact for the Gain pot it's best to use a linear one:

EDIT 24-April-2025

I made a new smaller version of the VCA without any extra's. It's just what you see on the schematic drawing. This will be handy if you need a few of these behind one panel or if you're building for Eurorack. I haven't built this layout so I can't say it's verified but I double checked the connections with the schematic and I can't see any mistakes. If you do please contact me immediately.

I did switch the 10K connections to the opamp U1b in the schematic. The left BC547 (Q1) is connected to the inverting input (-) and the right BC547 (Q2) is connected to the non inverting input (+). I can not imagine that will make any difference. It doesn't in the Falstad simulator anyway.

Here's that layout, wiring:

Stripboard only:

Connect the 3 ground positions of the eurorack powerheader together by flowing extra solder to bridge the gaps and short them together. 

Here's a picture of the finished VCA installed in my synthesizer:

The Level control is for the ADSR input signal and determins the volume of the audio signal. The Gain controls the quietness of the VCA when no keys are pressed on the keyboard and it should normally be set to zero. If you turn it up, the last note you played will become audible.
You must use an oscilloscope to test the signals and trim the offset and signal levels. Put it in DC mode when testing. 

In the picture above you see a yellow patch cable connected to the audio output. That is actually my oscilloscope probe, so I can see the output signal on my scope screen, and the audio output is connected internally, behind the panels, to the Line-Out and Effects Unit to the left. From there the audio signal goes to a stereo RCA output on the back of my synthesizer and from there to the Line-In of my HiFi Audio Amplifier.

Below is a picture of the double VCA that I built on April 25th and 26th of 2020 from the same schematic and layout. These VCA's work like a charm! They are so handy to have, I use them a lot in patches as a sort of Gate to let audio through when there's a signal on the ADSR input. When the ADSR potmeter is turned fully open (clockwise) the output signal will have the same strength as the input signal. Of course this is also influenced by the type of control voltage you feed it. 

With the double VCA I wired up the inputs in such a way that when there is no connection made to input 2, that input gets the same signal as is present on input 1. (Input 2 is normalled to input 1 as it is officially called.) So with one input you get two outputs. As soon as a patch cable is connected to input 2 that connection with input 1 is broken (by the socket switch of input 2) and the VCA's work as two independent VCA's.

A look behind the panel:

Eurorack module:

EDIT 25-Januari-2022: I'm now building a Eurorack system and I needed a VCA that I can use until I buy some ready made VCA's so I took one board out of the Double VCA I built earlier and made a Eurorack sized panel for it. This worked perfectly and I just transplanted everything to the new panel and it worked fine. Here's a look at the new module. I sawed off the extra copper strips that weren't in use (where the M3 bolt went through) which left me with a 52mm deep print which I glued directly to the back of the panel next to the potmeters and sockets which are all on the right side of the panel.

Here are some pictures of the Eurorack panel:

I made an extra output on this VCA that has 1/10th the amplitude of the normal output. I though that might be handy to use as a line level output but I'm not sure if I'll need it or use it. It just consists of a 100K resistor going from the output to the 1/10th output and a 10K resistor from the 1/10th output to ground to complete the voltage divider.

Troubleshooting:
If, after building this VCA, you have trouble with noise, especially at low volume levels, then you most probably need to replace the opamp you're using. I've had people contact me about this and it turned out the opamp was the cause. An other cause of excessive noise has been bad soldering joints. So make sure your solder connections are good. 

Like I said, I built 3 of these and they all work fine and are absolutely quiet.

Here's an oscilloscope picture of how the VCA lets through audio (the yellow waveform) only when it gets the Control Voltage from the ADSR (the blue line). You can see how the amplitude of the audio closely follows the amplitude of the CV from the Envelope Generator:

I had some eurorack layouts here before but I'm having problems with them so I took them out again. I won't have layouts here that are not verified.

Here's a link to a Falstad circuit simulation of this VCA showing how it works. 

-- CLICK HERE --

--- OR CLICK HERE ---

If you want to know more about the theory behind this VCA or in effect, how this VCA works then I have a link here to a great blog post that explains the workings of this very circuit.

https://kassu2000.blogspot.com/2015/10/vca.html

There's also a fantastic video by Moritz Klein which explains the technical details about the workings of a circuit like this and he also explains why the transistors need to be matched.

https://www.youtube.com/watch?v=yMrCCx6uqcE

Okay, that's it for now. Any questions? Put them in the comments below and I will answer them asap. You can also post questions in the Facebook Group for this website.

ELECTRO-MAGNETIC FIELD DETECTOR.

Here is an easy to build EM Field detector with 4 stage LED strength indication and which has a wide range of applications. This circuit will detect electromagnetic fields and also static electricity. It detects the mains hum on a 240 Volt (or 110V) wall socket or cable without having to touch the object. It is enormously sensitive to any changes of the EM field surrounding it so it could be used to detect lightning (proof is in the video below) or maybe even ghosts. (No video proof of that alas! At least not yet  ^__^ ). Please note: this circuit can not be used as a metal detector.

Here is the circuit (click on image to display full screen):

(Last revised: 02-june-2020: Changed 1M potmeter for 20 to 50K potmeter.)

Parts list:

Transistors:
8 x BC547 

Resistors:
1 x 680 Ohm 
4 x 470 Ohm
1 x 220 Ohm 
1 x 4K7 
1 x 3K3 
2 x 2K2 
1 x 100K
1 x 1M 

Potmeter:
1 x 20K or 50K potmeter (use either a trimpotmeter or a panel potmeter if you're building this into a case.) 

LEDs (3mm):
3 x green, 1 x yellow, 1 x red

Diode:
1 x 1N4148

Miscellaneous:
9V battery clip, 1 switch (SPDT Toggle Switch ON-ON), 1 Bullet conncector for antenna. (optional)

Before I go on with the rest of the explanation, here's a video showing this EMF Detector in action in a lightning storm. In the background audio you can hear the crackle of the lightning on an AM radio I had switched on, and you can see that the meter lights up as the radio crackles and lightning occurs. Sometimes it even detects the build up of the electric field in the air before lightning happens. I'm not influencing the meter in any way. I'm just holding it by the 9 volt battery underneath. Here it is:

I designed this because I always found it a shame that these "everything detectors" or 8 Million times amplifiers never had a strength indicator so you could actually see if and how it's working. So I tried combining two pre-existing circuits and see if I could make them work together and it turned out to work very well. The first of these circuits is this 4 LED signal strength indicator

and the second is this circuit which is the actual detector stage, consisting of the 8 million times amplifier.

You can easily build this on a piece of stripboard.
The circuit needs only 8 transistors (BC547 or 2N3904), 5 LEDs and 11 resistors. The extra (5th) LED is there simply to function as a on/off indicator and could be left out if you so wish. I used 3mm LEDs on this project but 5mm will work too. Don't use LEDs that draw a lot of current though like bright white LEDs or blue LEDs. The circuit is fed from a normal 9 Volt battery.

The sensitivity of the circuit can be changed with the 20K or 50K potmeter. If you're using it like me, without a case, you can use a trim-potmeter. If you're building this into a little case then use a panel potmeter for sensitivity. Make sure there's a grounding point when you build it into a case. Some connector from where you can ground it.

The circuit is very sensitive and it reacts to all sorts of things. If you hold this EMF Detector  near any mains cables it will instantly detect the voltage, I noticed that if you hold it near metal it will detect that too and even in an open space it will sometimes indicate a field even if there's nothing visible there but it's not a malfunction because it will keep indicating on the same spot in the room. 

This meter works best if it is grounded properly, either by connecting minus to a metal case in which you build the meter and then holding it in your hand  or by  grounding it to some metal item (do NOT connect to ground of mains power supply!!!)
Here are some pictures of the detector I build:

Enjoy building this awesome little "everything detector" ^____^ oh and hey, while you're here, please leave a comment! That'll be cool! :-)