Urgent Warning - Feedback from System Setup
  About Cassette, Dat Machines and Computer
and
Ipods, Ipads, Iphones, Laptops, Tablet computers...... 

Urgent Warning about Tape Recorders

Print this message and post it where your sound equipment is.

Here is the bottom Line.....   Always have a separate Cassette machine or Dat machine or Ipods, Ipads, Iphones, computers and Laptops, for recording and a separate unit for playback. NEVER have a common Cassette machine or Dat machine for playback and recording or you will - that is YOU WILL DAMAGE YOUR SOUND SYSTEM.

This problem seems to never go away. Recently, there has been a rash of sound system loud speakers blowing up. This event either blows up the whole speaker system or it burns up the High Frequency driver in each speaker. The cause of this problem is simple to avoid, simple to cause and often miss diagnosed.

For all church sound systems - as stated in the HIS System Standard - there should be a separate device such as a tape or digital recorder to record the service and separate tape player/digital source for playback. If you need additional recorders, you can add a digital recorder or other tape deck. Just don't have the output stage of the recording lines looped back into your mixer for playback. If you do, you will create a feedback loop.

The feedback loop creates a square wave sound. That sound is equal and full power sound at all frequencies. That is - if you have a 200 watt amp, the resulting sound will make the amp put out the full power of 200 watts at 5000 hertz and 200 hertz and all other frequencies. Speakers - even those with high frequencies (HF) protection can not react fast enough to protect the HF driver and burns it out before a protection circuit can heat up enough to trigger the protection.

The feedback loop is created when the output of the mixer is feeding a recorder that also has the a signal from the same device looped back into the mixer channel. If the mixer channel is off, your OK. If the church gain is up, even just a little, you actually start regenerating the noise created by the recorder.

You see, all electronics make noise. When the noise is below 60dB, we ignore it. Good quality mixers have a noise floor as low as -128dB. The fact that their is noise means that if you loop the signal and amplify it, it will get louder. Any recorder device can amplify any signal from + 35 to 60dB. A Some digital Recorder can amplify any signal to a maximum of 90dB. If you set the tape or dat recording level to a low setting of +3dB, the noise will be amplified +3dB. Now, remember, sound travels through wire at the speed of electricity passing through wire, which is almost the speed of light. That mean, the signal can appear to hit maximum output instantly. If your recorder is set at +30 or higher, it will generate a full signal output almost instantly.

A square wave happens when a signal is so strong, that the peaks and valleys of a frequency wave are compressed or chopped off. It is sort of like what happens when you fill the cup of a funnel. At a given rate, the stream under the funnel provides a constant smooth flow. When you fill the cup faster than the rate that the funnel can release the liquid, the excess liquid spills over the top, follows the surface of the funnel down and joins the down stream. Now, the down stream becomes distorted with the excess liquid. The excess energy is the square wave.

In electronics, the excess energy has no place to go so it distorts the signal and tries to increase the limits of the circuits exposed to the signal. When a speaker system is attached to the circuit, it can not absorb the excess signal of a square wave and blows up the HF first and then LF if your not fast enough. If the amplifier to the sound system is off, you may find either the Left or Right input channel of you recorder no longer working.

Do Compressors/Limiters work?

Only for Bi amped or Tri amped systems where a Comp/limiter is on each signal path. If you are using a passive crossover, the limiter/compressor canít choose the frequency range to compress. Therefore if you limit at 200 watt amp to a maximum output of 150 watts, the HF can still get a signal sufficient enough to burn it out.

Do Light bulbs or Quartz lamps help?

Yes and no. The idea of the bulbs is to give you enough time to cut things back when you exceed a speakers limits. For most other application the bulbs work. For an instantaneous square wave, these method donít work.

What about Dual Tape Decks?

If your tape deck has only a single set of play and record connections, you can not loop the signal back as these unit are really just like a single cassette unit.

The only real solution is to have separate devices units. Digital devices are real cheap in cost. Make it a rule that recording devices and playback devices are to always be separate units. This is hard to do with an Iphone or laptop or any digital MP3 player that can record and playback.

If your church can afford replacing and repairing speaker on a regular basis, ignore this message. If your church is looking to cut expenses, for the cost of less than most speaker repairs, your church can afford a dedicated recorder and playback devices.

Options:

For the churches that still use cassettes, there are a few Double Dual tape decks that have two sets of record and play connections, then you can use these units as two separate tape deck. However, some of the units have a switch that links the two decks. If you hit that switch, you will have turned the unit into a single dual tape deck and formed a loop back. If you can guarantee that you and no one else will ever engage that linking switch, then you can use such a deck. But then, these double decks cost more than two high end single cassette units. The only reason for a double deck is when you have limited space for a second unit.

These days, you can get two used laptops that can do all of your MP3 recordings on one units and your MP3 playbacks on a separate unit for less than the cost of a mid quality Cassette player from the 1990's when Cassettes were king.

As stated above:

Always have a separate Ipods, Ipads, Iphones, Laptops, Cassette machine or Dat machine or Computer for recording and a separate Ipods, Ipads, Iphones, Laptops, unit for playback. NEVER have a common device or computer for playback and recording or you will - that is YOU WILL DAMAGE YOUR SOUND SYSTEM.




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