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Workbench

Go Active!

Article from International Musician & Recording World, July 1986

Phil Walsh promotes an active life


These days you can't pick up a magazine or newspaper without reading an article urging you to eat certain foods, cut out stress, take up jogging and lead a more healthy and active life. Well, friends you will have noticed that IM&RW has been sadly lacking in this particular outlook.

Month after month we urge you to do decidedly unhealthy things like playing musical instruments, swilling alcoholic beverages and trying to get an appointment with an A&R person. But fear not! Ever conscious of modern trends I intend to right this serious wrong. Yes folks — Workbench goes active!

Get active and tone up your musical muscles.



Avid readers of the continuing Workbench saga will by now have an inkling of what I'm getting at. A normal guitar tone control circuit works by removing part of the signal. For example a bassier sound is obtained by filtering out the higher, treble frequencies. This type of passive tone control is fairly limited and can only cut, not boost, any part of the sound spectrum. A much more flexible type of tone control is the active sort which can either cut or boost one or more frequency ranges. They do this by using an amplifier circuit as part of the tone control so that only certain frequencies are amplified. In practice a pretty flexible tone control can be made by placing two tone filter circuits, one for bass and one for treble, in the negative feedback loop of an operational amplifier. In order to match the input impedance of the tone control to the output of the guitar pickups we also need to use a second operational amplifier stage, between the pickup and the circuit, to act as an impedance matcher. This is known as a unity gain impedance buffer. These days it is possible to buy the two operational amplifier stages in a single chip which simplifies the circuit layout.

DIY Active Tone Controls



You can build your own active tone controls for around about a fiver. It consists of a bass control, a treble control and a volume control. The circuit is driven from a nine volt, PP3 battery which, along with the circuit board and the three pots, must be accommodated in the body of the guitar. (See parts list at the end of this article.)

FIG 1. VEROBOARD LAYOUT
(Click image for higher resolution version)


Construction



Figure One shows the Veroboard layout for the project. This board carries all but three of the small components, C6, C7 and R7 being strung across the back of the tone pots to keep the board as small as possible.

I usually recommend the use of an integrated circuit holder when using ICs but this circuit is an exception in the interests of keeping the board height to a minimum.

1. With reference to Figure One, make the five cuts in the copper track by twisting a drill bit (about 4mm).

2. Fit the four wire links to the board and solder them.

3. Solder resistors R1 to R6 in place.

4. Solder capacitors C1 to C5 in place making sure that C2, C3 and C5 are the correct way round. The positive end of the electrolytic capacitors is indicated by an identification in the metal can.

5. Fit the integrated circuit in place ensuring that the spot or notch in the case is correctly placed. Quickly solder one of the pins in place, blowing to cool the joint. When that joint is thoroughly cool go on to the next and so on. Care should be taken to ensure that the chip does not get too hot as this can damage it.

6. With reference to Figure Two, solder C6 and C7 to the bass pot terminals.

7. Link up the pots and the battery clip to the board as shown, using short leads.

8. Having worked out the distance apart that the tone control pots will be when fitted to the guitar, solder R7 in place.

FIG 2. CIRCUIT BOARD HOOK UP DETAILS
(Click image for higher resolution version)


Testing



It is advisable to test the unit before fitting it into the guitar. The way you do this will depend on the existing wiring but as a general rule you should take leads from ground and the output of the pickup selector switch and connect these to the board. Wiring from the pickup selector to the existing tone control should be unsoldered as the tone control will drain some of the input signal from the active. The unit is switched on by pushing a MONO jack plug into the socket. Having waited a couple of seconds to allow the power rail capacitors to charge up, set the bass and treble controls to mid position which should give an unprocessed signal from the pickups (probably the first time you've ever heard the unencumbered guitar signal). Turning the bass pot clockwise will give up to 12dB boost to the bass frequencies, turning anticlockwise gives up to 12dB cut. Reset the bass pot to centre and repeat the tests with the treble pot's frequency range then altering the value of C1 will do this — make it smaller to operate the treble pot at a higher frequency.

Fitting into the guitar



Obviously I can't say too much about this as I don't know what guitar you've got! However there are a few general points.

1. Keep all the leads as short as you possibly can.

2. The input and output leads from the board (ie from the pickup switch and to the volume pot) should be coax, as should the lead from the volume pot to the output socket.

3. Whilst you've got the guitar open it is sensible to screen the compartment by lining it with aluminium foil and connecting this to a ground point. This foil should be insulated from the rest of the compartment using sticky backed plastic (shades of Blue Peter there — oh the joy of making an amplifier from washing up liquid bottles, toilet rolls and Copydex).

4. The battery should last about six months but you'll have to replace it sooner or later so make sure this can be done easily. One suggestion is the use of a commercial PP3 holder (eg RS Stock No. 508-116 or Verospeed No. 75-26271H) for which you should expect to pay around £1.80.

PARTS LIST

Resistors: all ⅛ watt
R1, R6 3.3Kohm orange — orange — red
R2, R3 100Kohm brown — black— yellow
R4, R5 33Kohm orange — orange — orange
R7 100Kohm brown — black— brown

Capacitors:
Cl 0.22μF polyester
C2 1μF 10 Volt tantalum bead
C3 2.2μF 10 Volt axial electrolytic
C4 0.022μF polyester
C5 4.7μF 10 Volt axial electrolytic
C6, C7 0.1μF polyester
NB: The voltage ratings given for C2, C3 and C5 are minimum ratings — anything with a higher working voltage will do as long as it will fit on to the board.

Potentiometers:
VR1, VR2 22Kohm linear
VR3 47Kohm linear

Integrated Circuit:
1458 Dual operational amplifier — 8 pin dil.

Miscellaneous:
0.1" Veroboard 12 copper strips of 20 holes length (approx 2" x 1¼")
PP3 battery clip
Skeleton stereo jack socket
Connecting wire


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Animal House

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The Hippest Way


Publisher: International Musician & Recording World - Cover Publications Ltd, Northern & Shell Ltd.

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International Musician - Jul 1986

Feature by Phil Walsh

Previous article in this issue:

> Animal House

Next article in this issue:

> The Hippest Way


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