Custom Systems

[paulguy]

Nice, I was thinking the same with adding the relays. Thanks for the explanation. Some of the alarm wiring can be very tricky. Got to draw it out on paper before I attempt anything. What do you think about the VG800? Pretty self explanitory with a lot of options. Should be easy since I have an aftermarket already installed. Looking to run multi conductor cable to all of the locations to tap and terminated in the center concole under the bucket. Going to remove the factory amp for mounting the alarm equipment. In the center console storage will be the keyed valet switch. My daughter and I finished the polished aluminum center console control panel and head unit/EQ face over the weekend. I came out to a mirror finish like the amp rack, getting excited. All I got left is dynamat/ensolite, running the wires, mounting equipment, termination and a RTA tune.

 

I'm not familiar with that part#(vg800).

Good to hear it's coming together.

[highvoltamps]

I'm not familiar with that part#(vg800).

Good to hear it's coming together.

 

 

The Scytek VG800 manual I posted for you.

[paulguy]

The Scytek VG800 manual I posted for you.

 

Is it a Scytek VG8000?

Sorry, I got confused. The last alarm manual you posted was the astra 777, which we where testing at my work, they're made by a guy who used to work at compustar, so they're very close to the same design. They seemed to work good.

 

I did one of those camera alarms once...

This is from 2006

[highvoltamps]

Nice, what do you think about the alarm? I hope it will be as good or better than the DEI Automate installed now..

[paulguy]

Nice, what do you think about the alarm? I hope it will be as good or better than the DEI Automate installed now..

 

Should be alright.

[highvoltamps]

Hope so. Off the alarms, I came across this from the IASCA rule book. Thought you might like. Going to fuse my alternator at 250 amps after reading this.

 

A fuse is required to carry its rated load continuously, 135% of rated load for 1 hour,

and double its rated load for 30 seconds. So, if a 500 amp fuse is used and the

cable shorts to ground, it must generate 1,000 amps of current through the short to

blow the fuse quickly, which isn't likely. Using too large a fuse is potentially very

dangerous!

 

 

THE CREST FACTOR

Now that we know the basics of a fuse, how does it apply to use with audio

equipment? To understand how it applies, we need to know about the Crest Factor.

The Crest factor is simply the ratio of the peak signal level to the average signal

level. An un-clipped sine wave has a crest factor of 3 dB (i.e. the peak level is double

the power of the average level). Pink noise has a crest factor of 6 dB (the peak level

is 4 times the power of the average level). For every 3 dB increase in the crest

factor, the power level is divided by 2.

A crest factor of 10 dB means the peak power is 10 times the average power. 20 dB

yields a peak power of 100 times the average. And a 30 dB crest factor yields a

peak power 1,000 times the average!

Most music has a crest factor between 10 and 20 dB. Some of the newer popular

recordings unfortunately have very low crest factors, often less than 10 dB. Music

with a very low crest factor is not dynamic, has little impact, and typically sounds

loud all the time.

Music that is very well recorded will have crest factors above 15 dB, and sometimes

well into the 20+ dB range. (The James Newton Howard and Friends recording from

Sheffield Labs has a 30 dB crest factor).

Now, amplifiers are rated at an RMS power output, which is simply the average

output when reproducing an un-distorted sine wave. Since a sine wave has a crest

factor of 3 dB, a 1,000 watt RMS rated amplifier will reproduce 2,000 watt peaks

cleanly.

Now, let's take this 1,000 watt amplifier being driven to its maximum un-distorted

output level of 2,000 watts with a music track having a crest factor of 20 dB. 2,000

watts divided by 100 = 20 watts average power output. If the crest factor is dropped

to 10 dB, then the average power output will be 200 watts. And if really poorly

recorded music is used with a crest factor of only 6 dB, the average power output

 

 

would be 500 watts. So as you can see, the actual power output of an amplifier

reproducing music is nowhere near the theoretical maximum output. It may be as

high as 1/2 the rated output with really poor recordings, but will likely be much less

than this.

NOTE: This does not apply to SPL competition, where amplifiers are typically driven

to their maximum output with sine waves, and are often driven into substantial

clipping as well.

With the crest factor considerations in mind, I would suggest the following:

First, a maximum allowable fuse value can be calculated based simply on adding up

the recommended fuse values for all the amplifiers, any larger than this would be

grounds for points deductions or disqualification. And quite honestly, this would be

severe overkill on the main system fuse anyway.

A more precise fuse rating can be calculated using the following method:

Determine the maximum peak power output of each amplifier by multiplying the

rated RMS power by a factor of two.

Allow for the efficiency of each amplifier. For class-A/B amplifiers, assume

60% efficient, and for switching or class-D amplifiers, assume 80%.

Assume supply voltage is 12 volts (This can be argued to be higher or lower,

depending on the system, but 12 volts is a good average)

Assume a worst case music crest factor of 6 dB, which means the average

output power is 1/4 of the peak output.

(continued next page)

Calculation Example:

A system has (3) 1,000 watt Class-D amps, (2) 500 watt class-A/B amps, and (2)

150 watt class-A/B amps.

The recommended fuse values for the amplifiers are as follows:

1,000 watt amplifiers - 100 amps each

500 watt amplifiers - 70 amps each

150 watt amplifiers - 25 amps each

The maximum allowable main system fuse size would be:

(100 amp X 3) + (75 amp X 2) + (25 amp X 2) = 500 amp

Now calculate the fuse value with peak outputs utilizing Crest factors:

The maximum peak outputs will be the RMS outputs multiplied by 2 to get the peak

output for each amplifier:

1,000 watts X 2 = 2,000 watts

500 watts X 2 = 1,000 watts

150 watts X 2 = 300 watts

Taking efficiency into account:

2,000 / 0.8 = 2,500 watts

1,000 / 0.6 = 1,667 watts

300 / 0.6 = 500 watts

 

 

So the total power draw for the amplifiers would be:

2,500 X 3 amplifiers = 7,500 watts

1,667 X 2 amplifiers = 3,334 watts

500 X 2 amplifiers = 1,000 watts

The total power required for all the amplifiers to be driven to maximum peak output

simultaneously would be 11,834 watts.

Now, take the 6 dB crest factor into account by simply dividing the above peak

output by 4:

11,834 / 4 = 2959 watts

Now divide this by 12 volts: 2959 / 12 = 247 amps

A 250 amp fuse would be a much more realistic value than the 500 amp fuse that

would be required by adding the recommended fuse values for all the amplifiers.

Notice that for a crest factor of 6 dB, the fuse value would be approximately 1/2 that

of the fuse calculated by adding the recommended fuses together. If the crest factor

is 9 dB, the fuse value would be 1/4, and for a crest factor of 15 dB, the fuse value

would be 1/16th.

I have tested this technique, and have never had a problem burning a main fuse

calculated simply by adding the ratings of all the amplifiers and dividing by two. This

is the same as going through the entire calculation for a crest factor of 6 dB.

I know going through the entire calculation is somewhat confusing, and may be

difficult to fully explain in the rule book, but this is how it works. And, using a fuse as

large as that calculated by simply adding up the recommended fuses values for each

amplifier will be overkill, and could even be dangerous.

 

 

 

Edited May 25, 2011 by highvoltamps (see edit history)

[paulguy]

That makes sense. I knew a fuse took double it's load before it trips, but i didn't know about the 'crest factor'. I usually do fuse my system a little on the small size, and they never blow.

[highvoltamps]

That makes sense. I knew a fuse took double it's load before it trips, but i didn't know about the 'crest factor'. I usually do fuse my system a little on the small size, and they never blow.

 

Not all fuses are like that, the slow blow would be that particular fuse. There are also current limiting, time delay and fast acting type fuses. I spoke with Stinger and the use slow blow on the anl type fuses.

Edited May 27, 2011 by highvoltamps (see edit history)

[highvoltamps]

Here's another part of the alarm I thought you might like. http://amazing1.com/ultra.htm Scroll down to the Phasor Pain and Shock Field. I'm planning to wire this into the siren output via relay and mount the speakers on the rear pillars.