Speaker Protection Board Input Voltage Handling

[jamesdean]

Hey everyone,

This might not be the right forum for this but I'm installing it on my solid state amp so I figured why not.

I purchased this cheap speaker protection kit from ebay a week ago:

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

It just dawned on me that it might not be able to handle the full volume output voltage from the amplifier.

I've no schematic for the board. I was wondering if anyone here had experience of could tell me what the input handling range on this guy is.

I know my rail voltage is +/- 85V on this particular amp.

If the unit cant handle that voltage, can i replace the transistors/capacitors with higher rated units and make it able to handle?


Thanks!

Kris

[AndrewT]

Do you have a schematic?

How many inputs does the circuit have?

Does each input have a different set of limits for correct and reliable operation?

Buyer is responsible for shipping and proof of delivery on all returns.

are you willing to accept this condition?

If you accept that condition and then discover that it does not suit your use, then you will have to spend more money.

[Mooly]

The relays look far to small to handle a fault condition from such an amp as yours. They have to survive breaking the connection under a real fault condition and also under high drive (high volume). The relays say 10A DC.

Don't want to sound negative about it, just being realistic. For a switch on delay and played at normal volumes they should be fine. If an output transistor goes short in the amp then you could probably expect the relay contacts to weld together as they "try" to open.

There is a thread in this forum using solid state (FET) switches that I can confirm work very well. Depends on your DIY skills. You could probably add such devices to this circuit you have but you would need to understand and know how to implement it.

[jamesdean]

Did you actually read the post before responding? All of your questions were answered if you would have read the post and viewed the link....

I'm not particularly concerned with returning it as it wasn't a hugely expensive kit. I most likely wouldn't bother if that case occurs.

[Mooly]

Just to add... your ebay circuit looks to need a separate low voltage supply to power it. It looks intended to be used with a small auxilliary transformer. Probably something around 12 to 18 volts AC to feed it with. That's not a problem if you know how to generate a low voltage DC rail from your -/+85 volts although its a lot to drop dissipation wise for a linear reg suppling a few 10's of milliamps.

[jamesdean]

Mooly,

Without this board the amp naturally has no other protections so as it stands this is better than nothing. I didnt expect amazing things from this unit as it was only $9.99.

My only concern (as of before your fault condition concern) was if the circuitry could handle the AC voltage coming out of the amp.

EDIT: You are correct, It needs some AC. I just picked up a transformer from radioshack than try to drop the 85 down.

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[PRR]

Resistors attached to the board INputs seem to be 15K 1/4W (someone check my eyes).

This will long-term take 60V steady or 90V sine-peak.

> +/- 85V on this particular amp.

Assuming you won't KEEP it in shut-down, it seems ample.

> relays look far to small to handle a fault ....10A

But relays such as this are widely used in heaters and microwave ovens which cycle many-Amp loads thousands of times; how often do we expect a dead-short fault?

I think it is a fine 10-buck gizmo.

Are our speakers worth more than $10?

I'd probably use it at home, where ANY shut-down would be instantly investigated.

Probably not in a 42-amplifier PA rack where shutdowns may go unnoticed or be re-set repeatedly.

[Mooly]

Originally Posted by jamesdean

Mooly,


My only concern (as of before your fault condition concern) was if the circuitry could handle the AC voltage coming out of the amp.

It will handle the voltage as the "input" to this kind of circuit is an integrator.

[Mooly]

Originally Posted by PRR

Resistors attached to the board INputs seem to be 15K 1/4W (someone check my eyes).

This will long-term take 60V steady or 90V sine-peak.

> +/- 85V on this particular amp.

Assuming you won't KEEP it in shut-down, it seems ample.

> relays look far to small to handle a fault ....10A

But relays such as this are widely used in heaters and microwave ovens which cycle many-Amp loads thousands of times; how often do we expect a dead-short fault?

I think it is a fine 10-buck gizmo.

Are our speakers worth more than $10?

I'd probably use it at home, where ANY shut-down would be instantly investigated.

Probably not in a 42-amplifier PA rack where shutdowns may go unnoticed or be re-set repeatedly.

The problem is that a large speaker presents itself as an inductance at DC.

Switching a resistive load at 110 or 240 vac doesn't generate an arc as the contacts open in the same way that even 12 or 24 volts dc would into an inductive load.

How often do you get a short... it happens when you see how many ask for help with blown up amps on here.

[RJM1]

Just a thought, couldn’t you place a 130V MOV across the relay contacts to suppress the back EMF when the relay opens?