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r
R 8 - R 6 ( o r R 7 )
R 6 ( o r R 7 )
O n b o t h a m p l i f i e r s t h e g a i n c a n b e i n c r e a s e d b y f i t t i n g a n e x t e r n a l
resistor across pins 4 I7 : 4-l on the 240 or 1 k8 on the 260 will double
the gain. The resistor can be decreased further if more gain is
r e q u i r e d , b u t w i l l n o t n o r m a l l y b e l e s s t h a n a b o u t 1 0 Q o n t h e 2 4 0
o r 1 8 0
o n t h e 2 6 0 ( g i v i n g a m a x i m u m g a i n o f a r o u n d 5 0 0 ) .
D 1 0
C o m m o n f a u l t s
I n a n y p o w e r a m p l i f i e r t h e d e v i c e s m o s t l i k e l y t o f a i l a r e t h o s e w h i c h
d o m o s t w o r k - i . e . t h e o u t p u t t r a n s i s t o r s T r l 1 t 1 2 . l f o n e o r b o t h
f a i l t h e a m p l i f i e r a n d p o w e r s u p p l y m a y o v e r h e a t a n d i t w i l l b e
o b v i o u s t h a t a s e v e r e f a i l u r e h a s o c c u r e d .
The protection on 240 and 260 will make this sort of failure
u n c o m m o n - e x c e p t w h e r e h e a t s i n k i n g i s i n a d e q u a t e .
O v e r h e a t i n g d u r i n g n o r m a l u s e m a y s i m p l y i n d i c a t e t h a t h e a t s i n k i n g
i s i n a d e q u a t e ( s e e s e c t i o n D 6 . 4 . ) b u t i t c a n a l s o i n d i c a t e i n s t a b i l i t y ,
i n w h i c h c a s e i t m a y w e l l o c c u r o n o n e c h a n n e l o n l y . I n s t a b i l i t y i s
c a u s e d b y e x c e s s i v e l e a d l e n g t h s o r l a y o u t s w h i c h p l a c e o u t p u t s a n d
i n p u t s t o o c l o s e t o g e t h e r
DA Z4O
D A l
T e c h n i c a l D e s c r i p t i o n
T h e i n p u t s i g n a l i s a p p l i e d t o T r 1 ' s b a s e , w h i c h c o m p a r e s i t w i t h p a r t
o f t h e o u t p u t a n d a m p l i f i e s t h e d i f f e r e n c e . T r 2 m a t c h e s T r 1 ' s o u t p u t
t o T r 3 . a n d o r o v i d e s a l i m i t e d m a x i m u m a m o u n t o f d r i v e t o T r 3 .
On positive-going half cycles Tr3 drives the output transistor f rJ
d i r e c t l y a n d i f t h e c u r r e n t t h r o u g h T r 7 i s e x c e s s i v e a v o l t a g e i s
d e v e l o p e d a c r o s s R 1 6 , w h i c h t u r n s T r 5 o n r e m o v i n g d r i v e f r o m T r 3
a n d t h e n c e p r o t e c t i n g t h e o u t p U t .
On negative half cycles Tr8 is driven by phase inverter Tr4. Excess
c u r r e n t o n t h i s h a l f c y c l e i s d e t e c t e d b y R 1 7 , c a u s l n g T 1 6 t o c o n d u c t
a n d r e d u c e t h e d r i v e t o T r 4 .
DA4
240 performance
S i z e
S u p p l y v o l t a g e
O u i e s c e n t c u r r e n t
P o w e r o u t p u t
Distortion
I n p u t i m p e d a n c e
5 5 x B 0 x 2 5 m m
t r 5 5 0 . c .
1 5 m A t y p i c a l ( t 5 m A ) a t 3 5 v .
1 2 w r m s 8 ! l a t 3 5 v .
20w rms 4!lat 35v.
0 . 1 % a t 1 0 w , 8 0 , l K H z .
9 0 k f / + 1 0 % .
DA 9.1 Amplif ier for crystal pick-up
DB 2 260 circuit
A
A
R 1
3 9 K
A
A
A
R 2 0
0n1
A
T R 3
c 2
1 0 n
R 2
3 9 K
C 1
1 0
T R 6
A
T R 1 2
R 2 1
1 0
R 5
t , 7 0
A
t K f
1 0 0 K
R 6
3 9 K
C 5
2 P 2
D 5
R 1 1
D 6
A
z 5
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