Technical Notes Vof. 1, No. 5

Introduction:

In order to facilitate the specification of JBL systems exhibiting flat power response, all JBL passive dividing networks have been redesigned to include HF power response correction. There will be a need for some time for sound contractors and audio specialists to modify the older 3152A passive network as well as construct a plug-in .card for use in the 5234A active dividing network for use in flat power response systems. In this Technical Note, we will describe these modifications.

3152A Modification:

The 3152 network was designed a number of years ago for use wi th HF horns that beamed on axis and wi th L F horn-loaded reflex enclosures wi th sensit iv i t ies of the order of 103-to-107 dB, 1 watt at 1 meter. The network had suff ic ient HF attenuation to enable the HF sect ion to be padded down to match the LF sect ion, and of course the flat electr ical HF response of the network was offset by the on-axis rise in direct ivi ty index of the older-style radial horns and lenses.

When used wi th Bi-Radial HF horns and direct radiator ported LF systems, this network needs about two dB more HF mid-band attenuation to enable the HF and LF sect ions to be properly matched, and of course the flat HF output of the network provides no compensat ion for the HF driver's natural roll-off above 3 kHz.

The modi f icat ion we show here is simple and is done at the terminal board direct ly under the network's metal cover. Figure 1 shows the schematic of the 3152A network as it presently stands. Note that HF padding is accompl ished through voltage division as well as through autotransformer act ion of the tapped inductor. A metal strap is placed across any of five sets of three terminals in order to set a particular HF attenuat ion. Figure 2A shows the form this circuit takes when the terminals are strapped as shown in Figure 2B. The components required for this are shown in the Figure, and they can easily f i t into the space between the terminal board and the metal cover. Care should be taken that all leads are sleeved and all components wrapped in electr ical tape. Good engineering practice calls for lugs to be soldered to the component leads to ensure snug and secure contacts.

Figure 3 shows the result ing response of the modif ied 3152A network. Note that there is an appropriate HF mid-band loss of 13 dB, providing a good match between the 4508 LF system (100 dB, 1W, 1m sensitivity) and the 2360 Bi-Radial (at 113 dB, 1W, 1m sensitivity). The rise in HF output above 3 kHz provides an excellent match for the HF roll-off of the 2445 compression driver, and the net result is flat power response as well as flat axial response.

Field Network Modifications for Flat Power Response Applications

5234A Modification:

Figure 4 shows detai ls of how a blank 18-dB/octave card may be loaded to provide a 12-dB/octave plug-in card for either 500 or 800 Hz use. The 18-dB/octave card is required since it has the requisite number of spaces for the components.

Before wir ing the 51-5130 blank card as indicated in Figure 4, the user should refer to the sect ion in the Operating Instruct ions for the 5234A Active Dividing Network detai l ing the loading of these cards.

Figure 5 shows the response of the card loaded for 500 Hz crossover. Note that there is a mid-band dif ference of 15 dB. In actual appl icat ion, this dif ference wi l l be nearly offset by the mid-band dif ference in sensit ivi ty between the HF and LF parts of the system, and it is not ant icipated that there wi l l be any problems in division of gains and losses in system operation. The only dif ference the user wi l l note is that the HF level control on the 5234A wil l be operated nearly full on, as compared to normal use wi th stock crossover cards.

Finally, we present a simple circuit which may be used between the HF output of any electronic dividing network and the fo l lowing HF power ampli f ier to correct HF power response. The circuit is shown in Figure 6A, and its response is shown at B. The assumption is made that the output impedance of the dividing network is suitably low, and that the input impedance of the power amplifier is approximately 20k ohms.

2

3.6 mH

6 mH TAPPED INDICATOR

FIGURE 1. SCHEMATIC OF UNMODIFIED JBL 3152A NETWORK

LF O U T i o n

13.5/ iF 13.5 13.5 MF

INPUT

16.5 MF

OdB

- 2 d B

- 4 d B

- 6 d B

- 8 d B HF OUT

3

3f t

3f t

3ft

3 f t

3.6 mH

INPUT LF OUT

6.0 mH

HF OUT

A. SCHEMATIC OF M O D I F I E D NETWORK

JUMPER WIRE

10 WATT, WIRE-WOUND RESISTORS IN PARALLEL

B. VIEW WITH COVER REMOVED

4 FIGURE 2. MODIFIED 3152A NETWORK

13.5 13.5 10ft

13.5 juF

16.5 MF

1212 4.7 juF (200 VDC)

20ft (10 WATT)

18ft (10 WATT)

INP

UT

0 2

4 6

8

LF LF

HF

C

C

C

F R E Q U E N C Y

FIGURE 3. RESPONSE OF MODIFIED 3152A NETWORK

HF (16ft)

13 dB 10 dB LF (8ft)

RE

LATI

VE

RES

PON

SE (d

B)

5

6

6 CAPACITORS REQ 5 RESISTORS REQ

FIGURE 4. POWER RESPONSE CORRECTION FOR 5234A

820ft

3.9k H

VALUES

CROSSOVER FREQUENCY. C. R:

500 Hz 0.012 MF 18k (5%) 800 Hz 0.012 M F 15k (5%)

USE BLANK 18 dB/OCTAVE CARD (51-5130)

FIGURE 5. RESPONSE OF MODIFIED 51-5130 CARD (500 Hz CROSSOVER SHOWN)

6 dB/OCTAVE SLOPE

15 dB EQ FOR HF DRIVER

-12 dB/OCTAVE

12 dB/OCTAVE

RE

LAT

IVE

RES

PON

SE (

dB)

FREQUENCY

7

P R E A M P O U T (5(K2 O R L E S S )

=3.3 kHz f 2 = 22.3 kHz MID-BAND LOSS = 16 .6dB Ci = 0.0022 MF R-l =22k H R 2 = 4 . 7 k n (R2' = 3.8k H)

POWER AMPLIFIER (20k n TYPICAL)

f-l = 3 . 3 kHz f 2 = 10.4 kHz MID-BAND LOSS = 10 .0dB C i = 0 .0022 MF R ! = 2 2 k H R 2 = 8.2k a ( R 2 ' = 10.2k H )

PROFESSIONAL DIVISION

J B L Incorporated, 8 5 0 0 Ba lboa Bou levard , P.O. Box 2200 , Northr idge, Cal i fornia 91329 U.S.A.

U B L / h a r m a n i n t e r n a t i o n a l

2M 64297 6/84 Printed in U.S.A.

" 1 R 2

FREQUENCY

« 2 '

*2 *1

1 f i =

1 2ir R-j C-|

R2

MID-BAND LOSS = 20 log - — dB R-| + R 2 '

R 2 •20k p « ' = —

z R2+20k

' 2 - — 1

1 R T R 2 '

R-| + R 2 ' 1

FIGURE 6. A CIRCUIT FOR HF POWER CORRECTION