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Old 11th August 2007, 04:55 PM   #1
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Default Amplifiers that not work protection circuit

Hi all


This is a big problem in some amplifiers, the protection circuit not work efficiently, "the dc protection fail" to avoid damage speakers, when the output transistors is shorted, or simple is not designed whit this in mind

I know some brands of prestige that blow speakers when this happen,
and know low price brands that work good

the propose is no talking bad, but when you have some much $$$$ in loudspeakers, I'm

if you have any models please post in this topic

this is one

BAD
1. Rockford P8002, and the majority's of models

Good low price = Boss AVA 650
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Old 12th August 2007, 04:34 AM   #2
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Dedicated DC offset protection is not very common in car audio amplifiers. Some manufacturers like Zed have used it extensively but otherwise, it's relatively rare. The most common way that the amplifiers avoid having DC on their outputs is through overcurrent protection.

When an output shorts, it will essentially short one rail to the speaker output. The DC servo (the negative feedback circuit) will drive the opposing transistor 'on' as hard as possible in an attempt to pull the output off of the rail. This triggers the overcurrent protection and the amp is shut down (including the power supply).

In Rockford's amplifiers, they have localized shutdown of the driver circuit but nothing to shut down the power supply. If an output shorts, the overcurrent protection will engage but it doesn't latch and doesn't shut down the supply so eventually the outputs fail if the user doesn't notice the problem. With one shorted output and the drive circuit shut down for the opposing transistors, the amp will produce DC on the outputs. This may have changed in their latest amplifiers but it's how the older amps operated.

Dedicated offset protection is the only foolproof way to prevent having excessive DC on the outputs. If, for example, you have an op-amp fail or a transistor before the differential amplifier fail, the amplifier will produce DC on the outputs (assuming DC coupling between those stages of the amplifier). The overcurrent protection circuit won't detect it because it's not causing excessive current flow.
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Old 13th August 2007, 06:07 PM   #3
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Hi pery
Thanks for your reply and explain this,

only sony, kenwood, jvc, and some rare brands have a Dedicated DC offset protection, but like you explain, if the output transistors is shorted the overcurrent circuit no detect it and all dc rail is present in the loudspeakers, any amp whit more than 28 volt in the rail is sufficient to blow the loudspeakers,

I have in my repair shop 3 amp whit only overcurrent and overtemp protection circuit that cost $1800.00 with 96 volt in the rails, and any time the predrivers is shorted the loudspeakers
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Old 13th August 2007, 09:37 PM   #4
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I lost a lot of good speakers to Rockfords I still like them though
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Old 13th August 2007, 09:39 PM   #5
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Don't these amps have rail fuses?
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Old 14th August 2007, 12:46 AM   #6
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Rockford hasn't used rail fuses in a very long time. I'd guess that they quit in ~1990.
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Old 14th August 2007, 06:49 AM   #7
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The 4-channel Audiovox amp I bought at Costco claimed to have fault protection, not that I've verified it or seen a schematic.

The Randall Vikan power amp that was published in Audio Amateur has a problem with its DC fault detection circuit; it'll work for + faults, but it'll be marginal with negative-going DC. I built the power supply portion to run a simpler amp circuit, and fortunately when the amp failed, it was + rail that shorted.

I'm not sure if Mr. Vikan got away with it in prototypes because of some borderline component specs, or testing only with faults at full rail voltages, or whether he borrowed part of the circuit from the "Speaker Saver" and overlooked the fact that in common-base mode the transistor has no current gain, and needs an additional stage of gain.
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Old 16th August 2007, 07:55 AM   #8
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Why do you think they don't use rail fuses? Because they think people will just replace them with foil and break the amp anyway?
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Old 16th August 2007, 05:03 PM   #9
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Fuses (as a protector in the aiudio section) typically work well once or twice. If the owner doesn't know that the amp has fuses, they may take it to the dealer to have it checked. After they find it has fuses, the owner may use the correct size fuse the first time they replace it but if it blows again, they will go up the the next size or wrap wire around the fuse holder (in the case of ATO/ATC fuses).

If the amp has to go back to the manufacturer under warranty and the only problem is blown fuses, it gets expensive for the manufacturer.

An amp needs to be built heavier (more power semiconductors) when using fuses because a fuse doesn't blow at precisely the rated current. If you use a 10 amp fuse, it will be able to pass 10 amp long term but if there is a fault, it will take significantly more current to blow it quickly enough to protect the output transistors. From experience, I know that a 20 amp fuse is too large to reliably protect three parallel output transistors rated at 25 amps each.

Fuses typically take up a lot of board space and are relatively expensive.

If you're a DIYer building an amplifier and don't want to go through the process of designing a protection circuit, fuses are a good choice. For amplifiers that are going to be sold to the general public, electronic protection (transistors, diodes, capacitors...) is better in virtually every way compared to fuses.
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Old 19th August 2007, 03:19 AM   #10
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I thought the reason they went away from internal fuses was to make it more difficult for the consumer to repair and save money versus getting it repaired at an authorized rockford facility or just buying a new more "improved" version from rockford. Kind of like the way they "improved" the thermal conduction of the transistors to the heatsink with their nice bonded transistor rails that mysteriously make it almost impossible to repair without destroying the mosfets. I don't know, those are two of the main reasons I've moved away from fosgate, they have great technology, but it doesn't seem to improve their reliablility, just makes more performance easier for them to produce in larger numbers.
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