Velleman K4700 Speaker protector

Speaker Protection in Audio

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Speaker Protection – what you should know.

So yesterday evening you switched off your sound system and today it shows the dreaded “protection” warning.  You check the cabling for shorts, open circuits and tear the unit apart hoping to find the offending gremlin. Reassembly and it works, bingo – you didn’t find the problem – it must have been a loose wire.

To date I have not read one forum which actually points to the offender – humidity. Yes, use a hair dryer.

This is a fairly common problem with Sony amplifiers in Durban or possibly Sea Point. It will be a problem then anywhere where there is a high moisture content. Sometimes even man made.

Protecting the speaker

Speaker protectors are there to protect the speakers – not the amplifier. In protection mode the first thing we think of is the worst.  But truth be told protect mode could also mean that the amplifier has detected a  short. Turn the volume down always when powering off and on.  Short circuit detection will occur if there is drive to the load.

The protection circuit also detects power on and then only after a few seconds connects the speakers to prevent switch on thump. The reality is that amplifier components need to stabilise first before coupling the loudspeakers.

  • The protection circuit detects transformer power, either AC or DC filtered .
  • It also checks that there is no DC at the output terminals of a DC coupled amplifier (most amplifiers are DC coupled nowadays).
  • The protection circuit may check to see if there is too much current flowing through the load (shorted leads or speaker impedance too low).

In most protectors there is a time delay circuit using an electrolytic capacitor and possibly a resistor in series to build up a charge time to act as a delay.

If unplugged for a while:

  • Instead of charging at the “faulty” amplifier like a bull at the gate use the hair dryer method first with power lead unplugged. When switching on keep volume at minimum setting.

If it hasn’t been unplugged:

  • If it is playing and then goes into protection check your cabling for shorts.
  • If playing at high volume the protector kicks in but after reducing drive (volume control) the protection is removed your speakers may not have the correct impedance (too low).
  • If even at low volumes the protection kicks in it may be a shorted speaker cable.  Do not be surprised to know that this is a common enough problem. The reason why Speak-On is way better than bare terminal or binding posts.

If you do not have much in the line of repair gear which includes a soldering iron and multi-meter I would not recommend a DIY approach.  I am adding some case histories to explain this later.  Otherwise:

  • With no load check the DC output at the amplifier – this should be a few milli-volts.
  • Smell and Vision test – burnt components.
  • Transistors shorted – this would cover final stage and drivers, all resistors in the final and driver stage.
  • Isolation of faulty stage.

Is the protector itself faulty?

Electrolytic capacitors go faulty. They dry out and loose capacity, sometimes even shorting. Often a basic multi-meter won’t show a fault. You may need an ESR tester. Most techie types will replace without checking – they are cheap enough.

Velleman K4700 Speaker protector
Velleman K4700 Speaker protector

Capacitors situated near a heat source are always a source of problem. Use 105 deg types.

Protection ICs are very reliable but can sometimes go faulty. Get the datasheet and look at the block circuit first to understand the inner workings.

uPC1237 Speaker and Amplifier protector
uPC1237 Speaker and Amplifier protector – Unisonic Technologies (UTC)

Diving into an amplifier and replacing all the possible culprits is known as the “block change” method, possibly even advised in the technical manual.  It is also known as the Shotgun Method and will rarely teach you anything.

One of my pet hates is when an amplifier carries more safety and protection circuitry than the audio stages.  When an amplifier does decide to shuffle the mortal coil be prepared for two things:  blown transistors and DC at the output terminals to speaker. Fuses stop fires but rarely are fast enough to stop a transistor from blowing.

Bad design is one of the leading causes of premature failure next to abuse.

When the protector itself becomes a problem then I put it down to a shitty design.  The speaker protection circuits shown in Rod Elliott’s designs are all that is required – see under further reading.  Take heed of notes covering over sensitivity which is a plague all on it’s own. Relays clicking in and out while you are trying to listen to a loud track is frustrating.

Some case histories to think about loudspeaker protection

The biggest learning curve for me was trying to repair amplifiers without a schematic. You may think you are clever if you do get it right but quite often this invites a return. In the professional space you do need the service manual. 

I do believe, my opinion only, that modern circuits are over-engineered. Because we have a chip which can do this or that we’ll use it. This makes repairs often difficult for the DIY techie.  The agencies just replace the board. This especially covers processor driven equipment. For a first timer this can be daunting – often the owner of the faulty equipment may try to conduct his or her own repair and cause damage to other circuits. Nothing worse than having a simple repair going south caused by the slipping of a meter probe. Yes, I have done this, even seemingly intelligent repair personnel.

I’d always advise going with something like the Outlaw processor pre-amplifier and a separate totally analog power amplifier as opposed to an all in one unit.  Digital receivers often have the processor tied to the power amplifier and fault finding can be tedious because the error may not be linked to what you think is causing the problem.

In a nutshell let me explain about my biggest conquest and of course a concern related to this entire article. A marine radar set that when faulty told the user where the problem lay, through the PPI or display unit. Such a glorious piece of engineering, German to boot.  The first and only failure had with the unit was the video driver transistor failed. 

Further reading:

As always, Rod Elliott on Loudspeaker Protection

Next:  Are vintage amplifiers better than modern releases?









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