Active Pickup ETI

8/20/2019 Active Pickup ETI

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Growing numbers of acoustic guitarsthese days are available with pickups

— usually internally mounted —

complete with volume and tone con-

trols The advantages of such pickups

are pretty obvious, the main one being lifting the

feedback ceiling which simple microphonic

amplification of acoustic guitars suffers from.

Perhaps the most famous acoustic guitars to use

internal pickups, and certainly those which give

among the best sounds, are those made by Ovation.

Ovation guitars have piezoelectric sensing tranducers

built into bridge saddles, coupled to an internal

preamplifier which gives required amplification and

equalization. These bridge sensing tranducers are

specially made by Ovation.This Active Pickup follows the Ovation principle,

using a straightforward, commonly available and

cheap piezoelectric transducer (unlike the Ovation

bridge), and coupling it with a purpose designed

preamplifier which allows the same amplification and

equalization controls. Also like the Ovation

preamplifier, the project is designed to be mounted

inside your guitar, with three holes (two for controls) .

one for jack socket) allowing volume and tone controls

to be adjusted externally and your guitar lead to be

unplugged when the guitar is not in use

Although the Active Pickup is designed to be

internally mou nted in a guitar there is no reason why

the preamplifier couldn’t be mounted in a separate

box, away from the guitar. This w ould be particularlysuitable if you don’t feel up to drilling a few holes into

your guitar, and I certainly wouldn’t advocte drilling

any holes in any valuab le instrument unless you really

knew what you’re doing. Mounted away from the

guitar, however, you will find the pickup lead itself is

susceptible to the usual knocks and vibrations which

unpreamplified contact pickups suffer from. If you're

brave enough and up to doing it, best position is

undoubted inside the guitar.

Guitars are not the only instrument with which

the Active Pickup can be used — it wili work with any

instrument which has a soundboard (such as a

mandolin, violin and so on) of any description. For

test purposes, the prototype was used with a guiros

(a South American scraper), a kazoo and a toytrumpet, working well each time.

Power is from a PP3sized 9 volt battery. T he life

of this battery is quite exceptional as the Active Pickup

has a minute current drain of around 2.5m A and will

continue to operate with a voltage down to as low as

5 volts. A good quality alkaline battery will power the

circuit for over 250 hours’ use — long enough to see

you through the world tour. Power is automatically

disconnected from the circuit when your guitar lead

is unplugged from the jack socket, so no on switch is

needed.

This low power consumption is because the

project uses a particular type of integrated circuit. The

circuit of the Active Pickup is shown in Figure 1, where

the single integrated circuit IC1 can be seen to formthe heart of the project This integrated circuit is an

operational amplifier acting as an equalized pre-

amplifier, buffering the output of a piezoelectric

transducer which picks up the sounds made by the

guitar strings. Output volume and tone are controlled

by potentiometers RV1 and RV2.

Construction

Construction of this project is not too difficult, because

the printed circuit board is designed to hold all

controls, directly mounted. This greatly reduces the

amount of wiring you have to under take and, as it’soften this wiring which causes problems in the first

place, it increases your projects chance of firsttime

success.

Build yourself an

active piezo

transducer pickup for your acoustic guitar. Keith Brindley shows you how.

A fairly logical order of assembly should befollowed w hen making up a printed circuit board. First,

get familiar with the layout of the board and where

all the components are mounted or connected to. A

component overlay, showing these and wiring details,

45ETI OCTOBER 1990



is given in Figure 2.

Make sure all holes in the blank circuit board are

the right sizes. C om pon ent holes are usually 1mm in

diameter. Howeve r, holes for connection pins of the

stereo Vi inch jack socket and two potentiometer

controls should be around 1.5mm in diameter.

No w you can start actual assembly. Pushfit the

circuit board pins at all offboard connection points.

There are two for the connections to the battery, and

two allowing connection to the piezoelectric trans-

ducer. Although wired connections can be made

directly to the circuit board, circuit board pins make

a neater and more easily undertaken job of connecting

to the board. Once pushfitted into the board, solder

in the pins to the co pper track.

It is conventional (though by no meansnecessary) to insert and solder all resistors first,

followed by capacitors, followed finally by semi-

conductors. This convention simply aids construction

because the components less likely to be damaged by

heat are soldered first. But as there is only one

semiconductor (IC1) and we recom mend use of an

integrated circuit socket to mount it. It doesn’t really

matter what order components are soldered in.

Insert and solder jack socket SK I, the two

potentiometers RV1 and RV2, and the integrated

circuit socket.

Once you’ve completed main assembly of the

circuit board, you can insert the integrated circuit itself

into its socket. First, however, a word of warning. The

integrated circuit used in the project incorporatesC M O S transistors in its output circuit. CM O S devices

are susceptible to damage by static discharge in

careless handling, so take care when you insert the

integrated circuit. There are a number of precautions

you can take, like using an earthed mat to assemble

your project on; using a soldering iron with an earthed

tip; wearing a bracelet on your wrist wh ich is earthed

and so on. How eve r most devices won’t be so easily

damaged. Just make sure you don’t touch the pins

of the integrated circuit. Your integrated circuit should

be supplied in either a small plastic holder or pushed

into conduct ive foam. If it’s in a holder, you can push

it out onto your worksurface with the tip of a pencil.

Carefully pick it up between thu mb and forefinger at

each end, so you do not touch the pins. Now checkwhich way round it should go and insert into its socket

on the board

Testing

Your project is now in a position to test, but to do this

you n eed to connect the transducer and the battery,

and have an amplifier to hear the results with.

The piezoelectric transducer used is in fact

designed as a miniature loudspeakertype of output

device — the sort of thing which p roduces speech in

toys and dolls. In your Active Pickup, on the other

hand, it is used as an input device. It works perfectly

well in either mode, in the same way that a real

loudspeaker can also function as a sort of

microphone

Your transducer will probably be supplied with

a couple of short leads attached. These are, of course,

meant to be connections when in output mode. In

input mode, a screened lead must be used to avoid

picking up interference such as mains hum from

amplifiers and so on. So, you must unsolder the short

leads then solder on screened lead.

Be careful here! Piezo transducers of this sort are

easily damaged by heat. Hold your hot soldering iron

tip on the solder joint for only the minimum length

of time necessary first to unsolder the leads, then

resolder the screened lead. It’s best to let the

transducer cool down after each individual soldering

operation. Connect about a metre of screened leadbetween transducer and circuit board initially — this

is just for test purposes and you can reduce this length

to suit your guitar later. Figure 2 shows the method

of connecting screened and signal wires to the

transducer and board.

No w connect a battery clip to the board as shown

in the overlay, and connect a battery. Set both

potentiometer controls to midposition, and plug in

a guitar lead between your project’s jack socket and

your amplifier. Adjust your amplifier's volume level to

a suitable position.

Test your project simply by tapping the trans-

ducer gently. If the project works, you’ll hear this

directly through your amplifier. Now use masking

tape, tape the transducer to an acoustic guitar (on thescratchplate is a good place to start). Play the guitar

adjusting the controls to suit.

Once you’ve ascertained that your Active Pickup

works you should play around with the transducer,

moving it around the body of the guitar to find the best

spot. As with all contact types of pickup, you’ll find

that sound varies greatly with position. M y prototype

was tested using a conventional jumbostyle guitar

and a flatnecked 12stringed Spanish guitar and on

both, taping the transducer to the bridge proved a

good position. For the jumbo, another good position

was on the side of the body hips. For the 12string,

though, the best place was found to be around the

scratchplate area.

Onc e you ’ve found your optimum position, youneed to remove the guitar strings to allow you access

to the innards. This is where you need to drill your

guitar body to mount the project The best position

is on the upper shoulder of the body, so the controls

ETI OCTOBER 199046



are easy to adjust and the guitar lead plugs into the

socket mounted on the guitar back. So you need to

drill two holes for the potentiometers on the shoulder

and one for the socket on the back — a fair bit of

accurate measurement, estimation and good luck is

probably called for here!

Before final mounting of the circuit board into

your guitar, w ith two or three sticky pads m ount the

battery onto the back of the board. Also estimate the

length of screened lead you ’ll need between board and

transducer and cu t and resolder to suit. (Resolderingthe board end — not transducer end!) Mount the

circuit board in the guitar and hold it in place with

potentiometer and jack socket washers and nuts.

Next, fasten the transducer inside the guitar body

in a place internally corresponding to optimum

outside position. Instead of m asking tape this time,

though, use a sticky pad. Note that once yo u fix the

transducer on with a sticky pad, you ’ll probably never

get it off again in on e piece, so make sure it’s in the

right position. If the screened lead between the circuit

board and transducer is too long, you might find it

clatters against the inside of the guitar body — in which

case get a few selfadhesive cable ties to hold it dow n

permanently.Fasten on a couple of control knobs to the

potentiom eter spindles Finally, restring your new

electroacoustic guitar.

HOW IT WORKS PARTS LIST

Th e p i e zoe l e c t r i c p i ck u p u se d i n ( h e p r o j e c t i s a c t u a l l y a n ou t p u t

t r a n sd u ce r , u se d in t oy s t o g ive sp e e c h o r m u s i c e f f e c t s . U se d a s a n

in p u t d e v i ce , h owe ve r , i t b e com e s a t y p e o f m i c r op h on e w i t h ve r y

h ig h im p e d a n ce a n d u n e q u a l f r e q u e n cy r e sp on se . So i t m u s t b e

coup led to a standa rd gu i ia r ampl i f ie r us ing a preampl i f ie r ; wh ich has

a co r r e sp on d in g l y h i g h i n p u t im p e d a n ce a n d i s c a p a b l e o f e q u a li z i ng

t h e s i g n a l t o g i ve a r e a son a b l y le ve l f r e q u e n c y r e sp on se ,

A l t h ou g h we ' ve sp e c i f i e d a p a r t i cu l a r t y p e o f p i e zoe l e c t r i c

t ransducer , the Act ive P ickup wi l l funct ion with many types of s imi la r

d e v i ce . Th e p r o t o t y p e wa s t e s t e d w i t h t w o o t h e r com m on l y a va i l ab l e

t y p e s , a l t h ou g h t h e sp e c i f i e d t y p e g a ve b e s t ove r a l l f r e q u e n cy

r e sp on se .

Th e i n t e g r a t e d c i r cu i t SC I f o r m s t h e a c t i ve com p on e n t i n t h ep r e a m p l i f i e r c i r cu i t , wh ose f r e q u e n cy r e sp on se i s sh a p e d b y t h e

com p on e n t s a r ou n d i t. Th e p r e a m p l i f ie r h a s a n i n p ut im p e d a n ce o f

1 0 0 k - h i gh e n o u g h t o m a t c h t h e h i g h o u t p u t im p e d a n c e o f t h e

pickup, wi tho ut be ing so h igh as to su f fe r f rom in te r fe rence and noise

p r ob l e m s , A sm a l l a m ou n t o f g a in i s d e s ig n e d i n t o t h e p r e a m p l if i e r.

Th is in tegra ted c i rcu i t has an extremely low current dra in, a round

2m A. and is thus idea l ly su i ted to ba t te ry-pow ered opera t ion . I t is a iso

ca p a b l e o f r u n n in g f rom a s i n g le su p p l y f e on l y on e b a t t e r y i s n e e d e d l

d o w n t o l e s s t h a n 5 V .

Th e p o t e n t i om e t e r RV 1 g i ve s a m e a su r e o f t on e con t ro l , a c t i ve l y

c u t t in g o r b o o s t i n g f r e q u e n c i e s a b o v e 7 5 0 H z . P o t e n t io m e t e r R V 2

con t r o l s ou t p u t vo lu m e l e ve l . O n / off sw i t ch in g i s u n d e r t a k e n a s t h e

outpu t jack p iug is inser ted in to j ack sock et SK I . Th e sock et is a ste reo

ve r s i on , on t o on e t e rm in a l o f w h i ch t h e b a t t e r y n e g a t i ve su p p l y t o r

e a r t h ) i s con n e c t e d . Th e c i r cu i t ' s e a r t h i s con n e c t e d t o a n ot h e rt e r m in a l I n se r t ion o f a m on o j a ck p lu g con n e c t s t h e se t w o e a r t h e d

t e r m in a l s a n d p ow e r i s a p p l i e d t o th e c i r cu i t

RESISTORS (all y.W5%l

8 1 . 2 3 , 5 100k

IU 4 7 k

R 6 39 R

R V 1 1M m in i a t u r e l og a n t h m ic p o t e n t i om e t e r

R V 2 10k m in i a tu r e l og a r i t h m i c p o t e n t i om e t e r

C A P A C I T O R S

C 1 ,3 ,8 4 * i7 1 6 V radia l e le c t r o ly t ic

C 2 ,7 4 7 u 16V rad ia l e le c t r o ly t ic

C 4 1n polyeste r

C 5 6 8 p p o ly s ty r e n e

C 6 3 3 n p o l y e s te r

S E M I C O N D U C T O R S

IC1 3 1 3 0

M I S C E L L A N E O U S

S K I s t e r e o ’/• in ch j a ck sock e t

P i e z o e l e c t ri c t r an s d u c e r - 4 1 m m d i a m e t e r , s p e e c h t r a n sd u c e r .

P r i n t e d c i r cu i t b oa r d , c i r cu i t b oa r d p in s .

8-pin dua l-in-l ine in tegra ted c i rcu i t socket .

P P 3-s i ze d b a t t e r y a n d c l i p,

S c r e e n e d l ea d .

S t i c k y p a d s .

BUYLINESM os t p a r t s a r e e a s il y ob t a i n a b l e f r om a n y com p on e n t ou t l e t.

P i e zoe l e c t r i c t r a n sd u ce r i s a va i l a b l e f r om Ma p l i n (p a r t n u m b e r

Y U 8 3 E ) . T h e c i r c u i t b o a r d is a v a il a b le f to m t h e E T I P C B s e r v i c e ,

ETI OCTOBER 1990 47

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Active Pickup ETI