SOA Protection by Relay.

[mikeks]

Further examples of manufacturers who routinely use this approach include Marantz, Denon, Yamaha, Rotel, NAD, Harman Kardon (not Hardon Karmon ), Kenwood, Onkyo, etc

[mikeks]

Originally posted by 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

Hi Chris,

The problem is some of these amps, eg Sony's TANR1, cost up to $5000 second hand....

No excuse methinks.

[anatech]

Hi Mike,
I like those relays. I've seen much worse used!

Release time is a function of spring tension and residual magnetism in the pole piece. They state 15 mS as a maximum, so I'll believe them and take 15 mS as the opening time.

Current interruption is also a function of current flow and inductance. I've seen a few relay contacts melted together. No interruption occurred in those cases. If the arc is AC, then it may self extinguish unless enough inductance and current exist to keep the arc going. In this case you can consider it a DC arc.

With the schematics you showed, detect time is simply the propagation delay through the protect chip. There are no time constants being applied. Poor, misused relay contacts! You would think that they would mute the input signal at the same time to save the contacts. All bets are off in a fault condition.

I still don't think they are concerned with SOA issues. This is more short circuit damage control. The cows have already left as they close the gate. If may may agree with you that this is a poor practice. I disagree with you only on the point that this is any form of SOA protection.

-Chris

[anatech]

Hi Mike,

The problem is some of these amps, eg Sony's TANR1, cost up to $5000 second hand

You have my full support. There is no excuse for that in an amp that costs this much.

I think we can agree that there has been no attempt at SOA protection at all. Inexcusable.

-Chris

[mikeks]

Originally posted by anatech
They state 15 mS as a maximum, so I'll believe them and take 15 mS as the opening time.Chris

I'll take your word for it, Chris, but, as i am sure you've observed, i know of no power BJT that would survive 15mS at nominal operating case temperatures on a dead short.

Originally posted by anatech

I still don't think they are concerned with SOA issues......I disagree with you only on the point that this is any form of SOA protection.

-Chris

It is in fact single slope single breakpoint non-linear foldback SOA protection.

See Pg 22~28 of article i sent you.

[mikeks]

Originally posted by anatech
Yamaha? That would be new. I've never seen them do that......
-Chris

One of Yamaha's 'high-end' amps:

http://www.audio-circuit.dk/Schemati...aha%20M-60.pdf

See TR143, TR145 and D153 on page 7

[anatech]

Hi Mike,
I interpreted that as Yamaha's attempt at DC offset detection. The schematic is chopped up and a bit hard to follow. Looking at an M-40 schematic, I can see a DC offset function, so I have no idea what they were thinking. I can tell you that often the current detect transistor would be damaged in a burn out, the switch transistor may go leaky.

I was doing warranty for Yamaha at this time. The thought was that if you had DC offset, you had excessive current. Hard to argue that.

I had brought this issue up with Yamaha Canada and that's what they told me. Their amplifiers were blowing up pretty badly at the time. This was just after the thermal fuse fiasco.

I've always felt that this was a little irresponsible of them.

So I guess you could call this single slope detection, very sloppily done if you ask me. I still have trouble calling this any kind of SOA protection. They really ought to be limiting the drive signal, or muting the input. Whatever

-Chris

[Netlist]

Like Peavey. Their DDT protection works flawless and at the output they use the famous Triac protection in many series. Drawback is often a broken Triac to be replaced.
In pro-series amps I consider this a good solution.

/Hugo

[anatech]

Hi Hugo,
like that option with the provision that it be used for DC offset or oscillation only. The triac pretty much guaranties blown outputs. In some cases huge expanses of copper trace as well.

A triac is cheaper than speakers. Make the protection independent for each channel so you don't end up with two blown channels when one was at fault.

This is definetly not SOA protection!

-Chris

[anatech]

Hi Mike,

See Pg 22~28 of article i sent you.

I'll re-read the article. Not doubting your word, but relay disconnect on an over current trip isn't how I would interpret SOA protection. I guess if you focus on the detection part, you are correct.

Pretty low quality engineering if you ask me. The knowledge is out there and they have made a choice not to do things that way.



-Chris