SOA Protection by Relay.

[mikeks]

Folks, a great many amps combine SOA protection with the DC protection system...using the relay to disconnect the speakers in the event of SOA violation, or DC overload...

The difficulty here is that the relay's drop-out time constant vitually always exceeds 10mS; the relay, therefore, cannot hope to open fast enough to protect the transistors, making this sort of arrangement largely cosmetic.

So, just why on earth do even respectable manufacturers such as Sony, Parasound, Audiolab,Yamaha etc persist with this faux pas?

[anatech]

Hi Mike,
What is the detect time for the circuit? It may exceed the relay disconnect time.

I can think of a couple reasons to use relays. They are better sonically than fuses. They never connect an amp with a defect. They allow the DC offset to settle down before connecting, and eliminate the turn off noises.

Personally, my vote is solidly for relays. I really do not trust my speakers to an amplifier with no protection. I will accept supply shut down as in a Carver M series.

Finally, they are cheap enough for a manufacturer to use. Do I trust power supply shut down from others? Maybe not. I know the Carver ones work.

What would you do, and what is the fault detection time?

-Chris

[mikeks]

SOA protection...NOT DC offset protection..

[mikeks]

.......while a relay might conceivably be used for DC offset fault protection of 'speakers, i fail to understand why some designers think that the relay also offers protection for the output transistors in the event of SOA violation.

[pinkmouse]

Originally posted by mikeks
[B...i fail to understand why some designers think that the relay also offers protection for the output transistors in the event of SOA violation. [/B]

What makes you think they do?

[mikeks]

Originally posted by pinkmouse
What makes you think they do?

All the aforementioned designs use a relay in series with the output to effect SOA protection.

[anatech]

Hi Mike,
I seriously doubt any manufacturer uses the output relay for SOA protection. It may kick out when the clipping is asymmetrical due to effective DC offset.

I've only seen clamps to limit current draw, some more advanced than others. That and an overtemp limit where I hope they mute the input signal rather than open the contacts on the output relay only.

I would be surprised and disappointed to see any manufacturer do as you say.

Yamaha? That would be new. I've never seen them do that. I think their idea of over current detection (to simplify DC offset detection) may be what you are seeing. If this is the case, I can see how it could be mistaken for SOA protection. One of the few amps I've seen do this is the Counterpoint SA-220. Michael designed an analog computer to calculate die temperature. Then basically disabled it!

Mike, could you post a section of a schematic that shows what you're looking at? I am reasonably sure I know what you are talking about now.

Thanks Mike,
-Chris

[mikeks]

Sony's 'best':

http://www.audio-circuit.dk/Schematics/Sony_TA-NR1.pdf

http://www.audio-circuit.dk/Schemati...y_TA-N80ES.pdf

http://www.audio-circuit.dk/Schemati...y_TA-E90ES.pdf


These are excellent designs in many respects, but, alas, are ruined by this relay SOA 'protection' arrangement.

Moreover, these designs compound the problem by detecting SOA conditions from only one polarity, which is a false economy at best as speech, for instance, is frequently offset to one polarity.

[mikeks]

...of a commonly-used relay with an opening time-constant of at least 15mS-when new! (unlikely in practice!)

[anatech]

Hi Mike,
It appears they went the cheap route. I can't even say it's SOA protection, just simply over current.

You gave them too much credit.

-Chris