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G8MNY > TECH 07.05.25 17:16l 143 Lines 5568 Bytes #4 (0) @ WW
BID : 32130_GB7CIP
Subj: LS Protection
Path: JH4XSY<JE7YGF<LU4ECL<LU9DCE<GB7YEW<GB7CIP
Sent: 250507/0807Z @:GB7CIP.#32.GBR.EURO #:32130 [Caterham Surrey GBR]
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 ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ +
\__ \ /ｳ 5 second delay
ｳLSｳﾄﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄ/ ｳAmp ﾚﾄﾄﾁﾄﾄｿ PS
ｳ__ｳﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ\ ｳ ｳrelayｳ
/ \ｳ ﾀﾄﾄﾂﾄﾄﾙ
/ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ -
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
100 that stops heat damage to the load resistor. However corrosion is a
problem though, & the load resistor may need replacing every few years!
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ 500W
ｳ ﾚﾄﾄﾄﾄﾄﾄﾄﾄ Amp
ｳ ｳ ｳ ｳ
ｳ~ｳ~~~~ｳ~ｳ
ｳ ｳ ｳ ｳBoiling
ｳ 2x 15R ｳWater
ｳ 10W ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ
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ｴ .-. A typical single LS Mid range impedance is around 1.5x
30ｴ / \ .-ﾄ the DC resistance. Except inefficient
20ｴ ﾞ \ .-ﾄ~ long throw LS (low bass distortion)
12ｴ ﾝ 'ﾄ-._ .-ﾄ~ which may be only 1.1x DC resistance.
8ｴ ﾞ ~ﾄ-ﾄﾄ~~
5ｴ-ﾄ'
ﾀﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄ>
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
ｳLSｳﾄﾄﾄﾄoﾄoﾄﾄﾄﾄ
ｳ__ｳﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ
/
/
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 (DF)!
ﾚﾄﾄﾄﾄﾄR fuseﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ Typical Values
R LS ｳ R LS 5R R Fuse 0.2R
ｳ R amp R amp 0.8R (DF=10)
L Coil ｳ L LS 0.4mH
ｳ V amp
R Motion ｳ R Motion 3R R Cable 1靡
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄR cableﾄﾄﾄﾄﾄﾄﾄﾙ
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
ｳ\ /'\./ \./'\ /ｳ
ﾚﾄﾄﾄｳ '>ﾄﾄﾄﾄﾄﾄ L S ﾄﾄﾄﾄﾄﾄ<' ﾃﾄﾄﾄｿ
ｳ ｳ/' `\ｳ ｳ
/'\./ ﾚﾄﾄﾄﾄﾄﾄﾄﾄｿ /'\./ ｳ ｳ
______ｳ PHASE ﾃﾄﾄﾄﾄﾄﾄﾄﾙ ｳ
ｳINVERTERﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ \./'\
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 ____
10Aﾄﾄﾄo-oﾄﾄｴ ｳ
ｳ500Wﾃﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄ>
-110V ｳAmp ｳ 390 ｳA
10Aﾄﾄﾄo-oﾄﾄｴ____ｳ 5W _ｳ__ Costly
fuses ｳ ﾃﾄﾄﾄﾄ\//\ LS System
ｳ 100uｳ G ﾄﾂﾄﾄ
ｳ Bi === ｳA'
ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄ>
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 mean current (sensitive) one
will do!
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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