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?
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?
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
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
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
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
Examples:
Sony's 'best':
http://www.audio-circuit.dk/Schematics/Sony_TA-NR1.pdf
http://www.audio-circuit.dk/Schematics/Sony_TA-N80ES.pdf
http://www.audio-circuit.dk/Schematics/Sony_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.
Sony's 'best':
http://www.audio-circuit.dk/Schematics/Sony_TA-NR1.pdf
http://www.audio-circuit.dk/Schematics/Sony_TA-N80ES.pdf
http://www.audio-circuit.dk/Schematics/Sony_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.
), Kenwood, Onkyo, etc
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
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 said:
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.
anatech said:
It is in fact single slope single breakpoint non-linear foldback SOA protection.
See Pg 22~28 of article i sent you.
anatech said:
One of Yamaha's 'high-end' amps:
http://www.audio-circuit.dk/Schematics/Yamaha M-60.pdf
See TR143, TR145 and D153 on page 7
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
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
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
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
Hi Mike,
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
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
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