Message 25817 in TECHNI.WW
From: G8MNY
Date: Mon, 13 Jul 26 06:44:00 Z
Newsgroups: TECHNI.WW
Subject: LS Protection
Message-ID: <64810_GB7CIP>
Path: N2MH4|GB7BED|GB7OSW|VE3CGR|OK2PEN|VK6HGR|GB7CIP
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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.
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 8R wire wound resistor in free air or in a jar of water. The water
huge cooling effect up to 25x that of air, & an upper temperature limit of
problem though, and the load resistor may need replacing every few years!
N.B. LS are not resistive and not 8R. A dummy load test may not show up some
faults, but a good way to soak test an amp without deafening yourself!
LS Z
30 100 300 1k 3k 10k Freq
LISTENING TEST
This is often used to assure an amp is OK, both for noise, stability and
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
/
/
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)!
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, and many amps can
deliver peak pulse powers 3-5 times their RMS rating!
BRIDGED AMPS
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 ____
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 and protecting the LS system. The triac only
has to handle the peak fuse current and 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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