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Old 17th January 2002, 11:39 PM   #11
AKSA is offline AKSA  Australia
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Default Output Stage Protection on Amplifiers

Hi Michel (and others interested!),

Thank you for your input on this vexing issue.

Protection is an important consideration for audio amps, particularly SOAR protection. Almost none of them are transparent sonically, however, particularly where series relays are concerned. It's like autos; we all want performance, but we must meet emissions standards, and we must have fuel economy. To some extent, these requirements are mutually exclusive. And so it is with SOAR protection...........

Why do we need it? It is required in the event the output goes DC to the rail and burns up the voice coils on direct coupled drivers, usually the woofer, which fortunately has a large voice coil and is in fact not too vulnerable in a large system. The midrange and treble, however, with passive crossovers, do not suffer in this scenario since they are DC blocked with a cap.

An output stage generally fails once in its life - if only every 20 years. The idea of taking out an expensive NHT1259 woofer is unthinkable. So how do we protect the woofer?

The output devices, in my view, are expendable. They are much cheaper than the driver. Someone has to lay down their life, after all, to protect the queen bee. So let's design the output stage so it is absolutely stable, never oscillates, and not worry too much about SOAR which almost invariably has major compressive effect on real music.

OK, so we can dispense with SOAR. Now, the failure mode. Let's select transistors which fail by gradation. I have designed the AKSA so the NPN output will go before the PNP on most loads. When this happens, the positive rail takes out the fuse, and the NPN device is now a short wire leading nowhere. However, the PNP is still alive, so the amplifier's global offset control remains effective. You might have 1 or 2 volts DC on the output, but the voice coil on the woofer is intact. None of my AKSAs has ever, to my knowledge, taken out a voice coil.

Overcurrent protection is important on speakers, and I recommend the polyphase switch, which is peanuts at RatShack and works with no audible signature I can detect. Let's use it.

Series relays on loudspeakers are anathaema. The only ones I have found which are inaudible are mercury relays, using the liquid metal and physical orientation to connect the signal. But they are slow, so not very practical. Perhaps the best solution is the shunt relay to ground. Here we set up a DC detection circuit on the output - very simple with a comparator or even a simple transistor/diode/cap circuit - and use it to trip a relay which effectively shorts the output to ground. Of course, a direct short will kill the output devices, so maybe a 1.5R resistor will offer the remaining intact device some small chance of survival. This short will, within about 200mS, blow the rail fuse of the intact device, removing all power from the output. The PNP device may even survive this trauma. The beauty of it is that the relay is open during normal operation, and thus has no sonic effect whatever.

This protection regime I would consider on some of my future designs. However, it is bulky, and quite expensive, and if the amp does not take out voice coils as it fails (and one can only rely on large numbers of amps being sold and used over a period of months to verify this is indeed the case), then even this approach is probably unnecessary. Testimony to this philosophy is the large number of high end amplifiers which use no output protection aside from fuses.

Thank you for bringing up this complex topic, Michel. I hope my take on this serves to explain the anatomy of output stage failure.

Cheers,

Hugh

Hugh R. Dean
www.printedelectronics.com
Melbourne AUSTRALIA
 
Old 17th January 2002, 11:57 PM   #12
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Michel,

I am well aware of that. If you consider the polyphase switch, in the absence of oscillation, it actually will prevent overcurrent through the output devices anyway. In effect, this is a de facto SOAR protection regime.

Perhaps you might care to comment constructively on the host of other considerations I brought up. This technology is like any other; dominated by compromise. Demonstrate your knowledge by identifying these compromises, and suggesting your solution. Criticism is easy; a superior solution could be of benefit to many people. This is the reason I have devoted considerable effort to answering your post; now the obligation rests on you. I have taken you seriously, Sir, please reciprocate.

Hugh
 
Old 18th January 2002, 12:35 AM   #13
AKSA is offline AKSA  Australia
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Hi Michel,

Thank you for your response; yes, I partly agree.

You say the scheme is only feasible if a suitable S.O.A regime is in place to protect the transistors; as you by now realise, I'm not too keen on SOAR protection because I have heard far too many of them. In truth, I account for SOAR this way: I choose 230V transistors and run them on no more than a 50V rail. This gives them a huge SOAR margin, something like 4A short termwith 50V across them. I specify 12A, which adds a further margin, since 49V/4R is little more than this rating. The polyswitch tends to protect overcurrent in any event. Then, the piece de resistance is how we consider these devices; Sir, they are expendable.

It is common for transistors to lay down their lives to protect the rail fuses, one of the little ironies of this game..... If you have an intact 4R voice coil, and you use an 8R resistor, the driver carries the bulk of the current still. No, you need to assume that the full rail voltage less output device and emitter resistor voltage will be applied to this resistor; let's assume 10A. This drops about 5V across the output device (assuming it is hard on) and about 5V across a 0R47 emitter resistor. With a 49V rail, that leaves 39V. This makes for a 3R9 resistor, which may still be a little large. But it will certainly blow the 7.5A fuse in short order, and with only 5V across the output device, it will most certainly survive.

Cheers,

Hugh

www.printedelectronics.com
 
Old 18th January 2002, 01:24 AM   #14
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Default Protection

Think I'll take Hugh's explanation and leave the relays for someone else.
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Old 18th January 2002, 02:11 AM   #15
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Hi Michel,

Actually, I cannot forget sdman. He is, in fact, a highly seasoned Electronics Engineer with huge experience in digital weapons control systems, and I'd just point out that in these forums - and perhaps email generally - nothing, but nothing, is as it appears. In this sense it is very like life itself; I like it a lot, not least because it focusses my thoughts into words which is no bad thing.

En passant, I might point out that a normally closed relay takes a long time to disengage, rivalling a fast blow fuse, in fact. Funny that.........

I think it is quite true that we are trying to prevent DC from hurting a loudspeaker and that the DC is coming from a shorted transistor (or other lesser problem earlier in the amp) which very likely happened because the speaker produced an inductive transient which destroyed the transistor. The speaker killed the transistor, and the transistor is now trying to kill the speaker.

Fair enough, I say. However, if the transistor fails open, no damage can be possibly done. If it fails short, as usually happens, then it will take out the fuse via the speaker and maybe the other transistor, which may or may not survive the impact. If it does, then only one fuse is blown and the VC is in jeopardy; if not, then both fuses are blown, the VC has maybe had a hard time for 50mS, and both fuses are blown.

You know, it's beginning to look to me as though a good polyphase switch on the speaker and a very fast blow fuse is all we need - but only if we take the attitude the transistors - worth maybe $US5 apiece - are expendable.

I say that the altar of good sonics justifies the occasional sacrifice of a good pair of output devices. Relays which are normally closed are slow anyway. Since the midrange and tweeter are automatically exempt from DC damage because of their blocking cap, and the woofer can be designed with a hulking VC which will handle 40 or 50 volts DC for at least a second or more, it seems unlikely we will do in our drivers this way. And at least we know the outputs are not virgin; this is not a complete and utter waste.........

Cheers,

Hugh

www.printedelectronics.com
 
Old 18th January 2002, 02:50 AM   #16
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Quote:
Originally posted by michel
I think we both forgot something here, and that is that the DC offset we are trying to protect our transducer from will almost invariably arise from a transistor failing short in the first instance? i am assuming of course that loudspeakers are not expendable. No, i reckon we will still need those relays in the supply rail, even if we decide that we can afford to shell out for power transistors ounce in a while....?
Well, the idea behind the output series relays is that if SOMETHING has to fail, it will be an output device and not a speaker. I can live with that; i've built a small board senses DC at the output and drops the relays quickly if something strange happens. A correctly placed fuse in the supply rail would blow after the transistor has been killed (this can happen extremely fast), thus, limiting the damage.
 
Old 18th January 2002, 04:10 AM   #17
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Michel,
Michel,

To continue; let's proceed with this interesting stuff.

The usual SOAR protection is a base/emitter strung across the output device emitter resistor, with a small diode beneath the emitter of the protecting transistor. Its collector is hooked to the base of the driver, and shunts signal to the output rail, removing drive, in the event of overcurrent through the emitter resistor on the output device.

Because of the 30mV or so switch-on zone of the protecting transistor, and the very peaky nature of music, quite a lot of the signal is cut off, causing short term clipping of the waveform. This has, naturally, a compressive effect, and because of the sharpness of the clip, fourier synthesis gives us a spray of harmonics which sound pretty bad.

I can't see any way around this unless the musical waveform is integrated to allow only short term musical bursts through, while shunting the long term high amplitude waveforms.

What is your solution, Michel? I believe you have one; though it is quite likely you might just reserve it tantalisingly for someone who pays, like 'Electronics World'.

And so, Sir, stand and deliver!! (grin).

Cheers,

Hugh
www.printedelectronics.com
Melbourne AUSTRALIA
 
Old 18th January 2002, 04:45 AM   #18
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Hi Michel,

Yes, thank you, very interested, email is: aksa1@optushome.com.au

Look forward to studying it in detail.

Cheers,

Hugh
 
Old 18th January 2002, 05:18 PM   #19
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Michel,

I built one of Hugh's 100 watter recently and was impressed with the sonics and how rugged it was. I was skeptical at first about all the accolades the AKSA was getting, especially with all the
advertising hype going on today but I am now convinced that it is a very good amp.
Don't get me wrong I'm not against relays and protection schemes but in the AKSA's case, I would leave it un-cumbered.
Being from the 60s and 70s era, the most reliable amp I ever
had was an Citation 12 which had no protection and it sounded well too. The Kenwood's,Sansui's,Technics that I owned were always blowing outputs,drivers and charred pc boards, all of them had elaborate protection schemes, even my Accuphase bit the dust occasionally.
Hugh is a nice fellow to correspond with and very helpful with any problems that might arise. Maybe he will let you look at the AKSA schematic which is an old topology similar to the Citation 12 but has been refined for top sonics.
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Old 18th January 2002, 05:20 PM   #20
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It's all been very interesting reading this far. One questions raises in my mind though. Why don't you consider using any of the specific IC's available for just this job. For example uPC1237 from NEC or HA12002 from Hitachi or TA7317 from Toshiba. As these are used in many commercial products they are not very expensive nor hard to get. If not elsewhere they are sold where you would otherwise look for replacement parts. I got a pait of TA7317 for about US5... + relay of course

These will all provide turn on delay (if needed), DC protection and also over current protection. The over current protection, if done with some sort of trigger schematic which checks the voltage drop on output stage emitter resistors, should not affect any of the amps functions and it would be obvious when the circuit launches the protection. So there would be no possibility that may be the circuit "compresses" the sound or whatever like that.

Is there any reason to believe that any voltage detection scheme on output resistors is by default bad? If there is, please give your comments and suggest which would be the least destructive to sound.

Ergo
 

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