OpenBCM V1.06 (Linux)

Packet Radio Mailbox

ED1ZAC

[C.D.G. OpenBCM]

 Login: GUEST





  
G8MNY  > TECH     09.06.19 10:30l 141 Lines 5563 Bytes #2 (0) @ WW
BID : 7706_GB7CIP
Read: GUEST
Subj: LS Protection
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 190609/0827Z @:GB7CIP.#32.GBR.EURO #:7706 [Caterham Surrey GBR] $:7706_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                             (Updated Dec 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

When testing some high power audio amps, there is a risk of blowing up your
loud speakers, especially if they are not as well rated as the amp.

TURN ON THUMP
Some amplifiers use a relay to disconnect the LS lines until the amplifier
output has had time to settled down after turn on.

\         RL       旼컴컴컴컴컫컴컴컴컴컴 +
 \__        \      /     5 second delay
   냐컴컴켸 읕컴/ 쿌mp   旼컨컴       PS
 __냐컴컴컴컴컴컴\       퀁elay 
 /                 \      읕컫컴
/                  읕컴컴컴컴컨컴컴컴컴컴 -

This arrangement can be annoying! But it is possible to incorporate a DC fault
disabling system into it.

DUMMY LOAD
It is easy to build a high power dummy load for testing audio amps. This can be
just a large 8 wire wound resistor in free air or in a jar of water. The water
electrical conductivity is many times 8 & has no effect, but it does have a
huge cooling effect up to 25x that of air, & an upper temperature limit of
100C that stops heat damage to the load resistor. However corrosion is a
problem though, & the load resistor may need replacing every few years!

  旼컴컴컴컴컴컴 500W
      旼컴컴컴 Amp
      
~~~~~~
      쿍oiling
 2x 15R 쿥ater
  10W   
읕컴컴컴켸

N.B. LS are not resistive & not 8. A dummy load test may not show up some
faults, but a good way to soak test an amp without deafening yourself!

 LS Z
  50     .-. A typical single LS        Mid range impedance is around 1.5x
  30    /   \                     .-   the DC resistance. Except inefficient
  20        \                .-~      long throw LS (low bass distortion)
  12         '-._       .-~          which may be only 1.1x DC resistance.
   8              ~-컴~~
   5-' 
    읕컴쩡컴컫컴컴쩡컴컫컴컴쩡컴컫컴컴>
        30  100  300   1k   3k   10k Freq

LISTENING TEST
This is often used to assure an amp is OK, both for noise, stability &
distortion. You may need to protect your ears as well! But to protect your LS
use a series fuse! Many amp faults can give a DC output that will burn up a
voice coil in a few seconds, before you are aware of a fault!

\         Slow
 \___   blow fuse
  |  |컴컴o훟컴컴
  |__|컴컴컴컴컴
 /
/
                                             Fuse
LS POWER   LS Z    Pk V    RMS I     Pk I    Mean I
   4        4       5.7     1.0      1.4      0.7
   6        4       6.9     1.2      1.7      0.8
   20       4      12.6     2.2      3.2      1.5
   32       4      16.0     2.8      4.0      1.9
  100       4      28.3     5.0      7.1      3.3
  200       4      40.0     7.1     10.0      4.7
   8        8      11.3     1.0      1.4      0.7
   32       8      16.0     2.0      2.8      1.3
   50       8      20.0     2.5      3.5      1.7
  100       8      28.3     3.5      5.0      2.4
  200       8      40.0     5.0      7.1      3.3
  400       8      56.6     7.1     10.0      4.7
   8        15     11.0     0.7      1.0      0.5
   20       15     17.3     1.2      1.6      0.8
   32       15     21.9     1.5      2.1      1.0
   50       15     27.4     1.8      2.6      1.2
  100       15     38.7     2.6      3.7      1.7

Putting the mean rating of slow blow fuse in the LS line should give adequate
protection, without the risk of blowing with music at the rated power.

Adding the fraction of an Ohm of fuse in line will not affect the LS damping as
some may think, because the LS DC resistance is the main damping R not the amp
output Z or its damping factor!

旼컴컴R fuse컴컴컴컴컴컴커                   Typical Values
R LS                                  R LS     5      R Fuse 0.2
                       R amp                           R amp  0.8 (DF=10)
L Coil                                L LS   0.4mH
                       V amp
R Motion                              R Motion 3      R Cable 1
읕컴컴컴컴훀 cable컴컴컴켸

Note the LS peak Voltage has to be supplied by the amps PS rails less the
silicon loss, the raw DC available is often much higher, & many amps can
deliver peak pulse powers 3-5 times their RMS rating!

BRIDGED AMPS
                           \   /'\./     \./'\   /
                       旼컴 '>컴컴컴 L S 컴컴컴<' 쳐컴
                          /'                   `\   
/'\./ 旼컴컴컴커 /'\./                                
______  PHASE 쳐컴컴컴                               
      쿔NVERTER쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴
      읕컴컴컴켸 \./'\

Here a DC fault on 1 amp may not be apparent in use until the LS blows up!

CROWBAR
Commercial high power split rail amps sometimes use a crowbar to protect LS
systems from a DC amp fault. The circuit is quite simple & can be used across
the LS of a bridged amp too.

+110V        ____
 10A컴훟-o컴    
            500W쳐컴컫컴컴컫컴컴컴>
-110V       쿌mp    3k9    쿌
 10A컴훟-o컴____   5W    ___      Costly
      fuses          쳐컴\//\     LS System
                 100u  G 컫컴
                 Bi ===    쿌'
  컴컴컴컴컴컴좔컴컴컴좔컴컴좔컴컴컴>

The bidirectional capacitor will not charge up if only audio AC is present. If
any DC over 3V or high power frequencies below 20Hz are present the Traic will
be triggered, shorting out the Amp & protecting the LS system. The triac only
has to handle the peak fuse current & quite a low current (sensitive) one will
do!


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


Read previous mail | Read next mail


 17.06.2019 17:33:30lGo Back Go up