G rowing numbers of acoustic guitars these days are available with pickups — usually internally mounted — complete wit h volume and tone con- trols The ad vantages 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 famou s acoustic guitars to us e internal pickups, and certainly those which give amo ng the bes t sounds, are t hose mad e 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 sock et) 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 t here is no reason why the preamplifier couldn’t be mounted in a separate box , a way from the guita r. This w ould be particularly suitable if you don’t feel up to drilling a few holes into your guitar, and I certainly wouldn’t advocte drilling any holes i n 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 suscept ible to the usual knocks and vibrations which unpreamplified contact pickups suff er 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 Pick up can be used — it wi li work with any instrument which has a soundboard (such as a mandolin, violin and so on) of any description. For tes t purposes, the prototype was used with a guiros (a South American scraper), a kazoo and a toy trumpet, working well each time. Pow er is from a PP3sized 9 volt b atte ry. T he lif e of t his batter y is quite exceptional as the Active Pick up has a minute current drain of around 2.5m A and will continue to operate with a voltage down to as l ow as 5 vo lts. A good qu ality alkaline battery will pow er the circuit for over 250 hours’ use — long enough to see you through the world tour. Power is automatically disconnected from the circuit when you r guitar l ead 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 circ uit. The circuit of the Active Picku p is shown in Figure 1, where the single integrated circuit IC1 can be seen to form the 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 s trin gs. Outp ut volu me a nd 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, directl y moun ted. T his greatly reduces the amoun t of wiring you have to under take and, as it’s often this wiring which causes problems in the first place, it increases your projects chance of firsttime success. Bui ld your self an active piezo transducer pickup for your acoustic gu it ar . Keith Brindley showsyou h o w . A fairly logical order of assembly should be followed w hen making up a printed ci rcuit boa rd. Firs t, get familiar with the layout of the board and where all the comp onents are mounted or connected to. A component overlay, showing these and wiring details, 45 ETI OCTOBER 1990
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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 ,