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Old 2nd November 2014, 07:20 PM   #1
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Default DC speaker protection

Hi, Could someone explain DC speaker protection to me?

Am I right in thinking its a delay to stop any popping when the amp is turned on. And with disengage a relay if the amplifier goes wrong and sends DC current to the speaker?


Whats best to use? An IC or breadboard a circuit?
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Old 2nd November 2014, 07:55 PM   #2
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Why would you want to breadboard it? - just build one, there are plenty of examples out there.

Mostly it's to prevent possible speaker damage, but anti-thump is an added bonus.
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Old 2nd November 2014, 08:02 PM   #3
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DC protection uses a relay to disconnect the speakers if any appreciable DC appears at the amplifier output. Typically that is in the range -/+ 1.5 to 3 volts. A major problem is that the relay must be able to break the current flow without welding the contacts together. That is made much much worse by the inductance of the bass speaker. If you try and break the current in an inductive you actually draw an arc as the break occurs, and a big inductance means a big arc, and that can easily weld small relay contacts together. Typically you are looking at a relay that can switch 30 amps DC (the DC rating part is important).

In the last few years solid state relays have become popular (I use them and would never go back to mechanical) but they are a little more complex to make (two power FETs and a photo-voltaic coupler to drive them)

Its easy to incorporate switch on delays and instant disconnect on power off to stop thumps and bangs but that a different issue to DC offset protection.

Its for this reason that I recommend AC coupled amps to begin with. Your speakers are always safe no matter what.
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Old 2nd November 2014, 08:32 PM   #4
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I want to bread board one because Im a new player, just trying to learn some basics.

Am fooling around with some really junky speakers that aren't worth anything.
The more I read, the more my heads hurting.
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Old 2nd November 2014, 09:55 PM   #5
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Level 1 speaker protection is just to put a normal blow fuse in the speaker hot line, bigger than the amp is ever supposed to put out. Many blown outputs are because the speaker wire got partially disconnected and shorted to case, or the speaker surround went bad, the wires scrubbed and a turn shorted, and too much current came out.
Take your wattage, figure I=sqrt(P/R) where p is power, r is lowest rated speaker impedance (after 198? usually 4 ohms, before1975 8 ohms) I is current rating. Then multiply current by 1.5 or 2 to avoid nuisance fuse blows on brief loud music passages. I use DC rated car fuses: the 32 v SFE fuses did blow when my rail went to 170 v due to a shorted output transistor.
If you want to see the FET voltage rail disconnect, look up RG Keen protection. If you want to see just speaker disconnect protection, look up Michael Bean protectioin. Rail protection takes two more photovoltaic FET drivers than speaker disconnect, but both take four FETS for a stereo amp.
Most of the speaker disconnect kits on e-bay, the relay looks too small to break a good sized arc. I gave up a relay project in the middle and went on to nFETs particularly when some 52 amp 200 V ones got down to <$2 each.
To see what the pros use, look at amp schematics. I've studied Peavey CS800x (which uses op amps) and CS800s (which uses a microprocessor) with current transformers to detect overcurrent, plus a DC detect network (15k ohm, 2.2 uf, 8 v diac to flashover current when "too much" DC occured .)
My speakers cost $600 new and the bass drivers are $100 with the freight, so I do intend to build one of these for my PV-1.3k amp. My main SS amp (ST120) has a single supply and a speaker output capacitor, which is fairly stupid proof. I melted the solder with PA use on the ST120, shorted out two output transistors, made a big fireball under the lectern and didn't hurt my speakers.
Look up the East Electronics G-amp or the John Ellis Basic 50W amp for beginners, for two of these 50W/channel designs. Apex AX6 or the MJR7 Mark IV are simpler 50 w amp designs with an output cap where a circuit board is published.
In many cases a capacitor protected speaker type amp is simpler to build than an effective protection circuit: Until you get over 100 w/ch.
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Last edited by indianajo; 2nd November 2014 at 09:58 PM.
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Old 3rd November 2014, 09:06 AM   #6
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Quote:
Originally Posted by Sounduser View Post
I want to bread board one because Im a new player, just trying to learn some basics.

Am fooling around with some really junky speakers that aren't worth anything.
The more I read, the more my heads hurting.
Have a look here:

Loudspeaker Protection and Muting
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Old 3rd November 2014, 03:37 PM   #7
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I designed my own PIC based DC protection circuit.
It simply looks at the amplifier output and looks for DC above 10 volts or below -10 volts for 500mS. If it sees this it opens a relay.
The PIC also holds off speaker relay for 4 seconds on power up until amp settles.
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Old 3rd November 2014, 03:40 PM   #8
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Quote:
Originally Posted by nigelwright7557 View Post
I designed my own PIC based DC protection circuit.
It simply looks at the amplifier output and looks for DC above 10 volts or below -10 volts for 500mS. If it sees this it opens a relay.
The PIC also holds off speaker relay for 4 seconds on power up until amp settles.

Is there more information on this? Like how you're sreading the voltage? I'm more at hone with micros than day amplifiers
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Old 3rd November 2014, 03:42 PM   #9
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Quote:
Originally Posted by Sounduser View Post
Is there more information on this? Like how you're sreading the voltage? I'm more at hone with micros than day amplifiers
Here is the pic12f508 code I used.
;INTH0.ASM
;**********
;INTERNAL OSC, INTERNAL MCLR, POWER UP TIMER ON


LIST R=DEC
LIST P=PIC12F508
;INCLUDE REGISTER DEFINITIONS
INCLUDE P12F508.INC



;_MCLRE_ON EQU H'0FFF'
;_MCLRE_OFF EQU H'0FEF'

;_CP_ON EQU H'0FF7'
;_CP_OFF EQU H'0FFF'

;_WDT_ON EQU H'0FFF'
;_WDT_OFF EQU H'0FFB'

;_LP_OSC EQU H'0FFC'
;_XT_OSC EQU H'0FFD'
;_IntRC_OSC EQU H'0FFE'
;_ExtRC_OSC EQU H'0FFF'



__CONFIG _MCLRE_OFF & _WDT_OFF & _CP_OFF & _IntRC_OSC

INCLUDE MACRO.ASM

;ACCESSES REGISTER BANK BIT
#DEFINE RB0 STATUS,5
;
#DEFINE RL1 GPIO,5 ;SPEAKER RELAY
#DEFINE POSITIVE GPIO,0 ;POSITIVE GOING I/P
#DEFINE NEGATIVE GPIO,1 ;NEGATIVE GOING I/P


STATEA EQU 3

FIRSTA EQU 0
FIRSTB EQU 0

;***********************
FIRSTRAM EQU 7 ;JUST POINTER TO FIRST RAM LOCATION
DEL1 EQU 7
DEL2 EQU 8
DEL3 EQU 9
COUNTL EQU 10
COUNTH EQU 11


LASTRAM EQU 1FH
;;;;;;;;;;;;;;;;;;;;;
ORG 0
MOVLW FIRSTA
MOVWF GPIO

CLRWDT

MOVLW STATEA
TRIS GPIO

MOVLW 0C0H ;WEAK PULL UPS OFF & WAKE UP ON PIN CHANGE OFF
OPTION


;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; GOTO TEST




;POWER UP DELAY
;ALLOW AMP TO SETTLE FOR 3 SECONDS
CALL WAIT1SEC
CALL WAIT1SEC
CALL WAIT1SEC
BSF RL1 ;RELAY ON

MLOOP BTFSC POSITIVE
GOTO POSGOING

BTFSS NEGATIVE
GOTO NEGGOING

GOTO MLOOP

;DONT ALLOW POSITIVE TO KEEP GOING FOR MORE THAN 500MS
POSGOING MOVLW LOW 45000
MOVWF COUNTL
MOVLW HIGH 45000
MOVWF COUNTH
PLOOP BTFSS POSITIVE ;2
GOTO MLOOP ;POS GONE SO OK
DEC16 COUNTL ;4
MOVF COUNTL,W ;1
IORWF COUNTH,W ;1
BNZ PLOOP ;3 11 PER LOOP

BROKE1 BCF RL1
GOTO BROKE1
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;
;DONT ALLOW NEGaTIVE TO KEEP GOING FOR MORE THAN 500MS
NEGGOING MOVLW LOW 45000
MOVWF COUNTL
MOVLW HIGH 45000
MOVWF COUNTH
NLOOP BTFSC NEGATIVE
GOTO MLOOP ;NEG GONE SO OK
DEC16 COUNTL
MOVF COUNTL,W
IORWF COUNTH,W
BNZ NLOOP

BROKE2 BCF RL1
GOTO BROKE2




;********************
WAIT1MS MOVLW 1
GOTO MSS
MS250 MOVLW 250
MSS MOVWF DEL1
MS MOVLW 249
MOVWF DEL2
DD CLRWDT
DECFSZ DEL2,F
GOTO DD
DECFSZ DEL1,F
GOTO MS
RETLW 0
;***********************
WAIT1SEC
CALL MS250
CALL MS250
CALL MS250
GOTO MS250
;*************************
TEST
BSF RB0
MOVLW 0
MOVWF GPIO
BCF RB0

TEST1
CLRWDT
MOVLW 0
MOVWF GPIO
MOVLW 255
MOVWF GPIO
GOTO TEST1




END
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Old 3rd November 2014, 03:46 PM   #10
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Here is the circuit.

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