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β24 - thoughts about safety and amp control

beta24

β24 - thoughts about safety and amp control

Postby mpa2 » May 22nd, 2018, 1:42 pm

Hi,

after my last "adventure" (https://www.amb.org/forum/enclosure-for-beta24-t3573-60.html) I'm concerned about the safety of my β24 build, which basically follows the reference build. I'm sure, this thoughts are not new, but I think it's worth while talking about it.

Things I'd like to avoid are (in order of severity): fire, speaker damage, severe amp damage. My current build is not immune against any of these mishaps.

1) Main fuse. I use 6.3A slow one. A 4A one blows on power on because of the high transformer inrush current. But even a 4A fuse would probably withstand a secondary short, so in case of a problem with rectifiers, or OPS failure (MOSFETS short), we will have a nice firework, and it will probably continue until the transformer overheats and we have a primary short. The fire fighters will probably have arrived at this time...
I guess, a 3-3.5A fuse (at 230 V) would bring some safety, but then we definitely need a soft start circuit.

2) Power rail fuses. Ti's explanation, why he doesn't like them are easily comprehensible. Still in my opinion an overcurrent protection is desirable and necessary. A fast acting circuit on power rails, which could utilize a 10 mOhm resistor for current measurement, could switch the amp off and disconnect speakers in case of failure and thus limit the damage and nevertheless might have little to no influence on sound quality.

3) DC protection. No need to generally discuss it, especially in a DC coupled amp. There are a lot of circuits and kits available, but I haven't found a specialized solution for a balanced amp so far. One could use four "normal" boards instead, but it is not really elegant. The way to disconnect speakers (relay vs. "SSR" vs ?) is of course to be discussed...

4) Me and some other users have/had problems with the stability of Epsion24, and the 12V trigger circuit I use is more a workaround...

Provided that these 4 points are not absolute crap :), a following device would be desirable: an amplifier control board, which would incorporate the function of epsilon24 (front switch control/temperature protection), and additionally provide a soft start functionality, rail overcurrent and speaker dc protection as well as 12V trigger input. User dsolodov presented his Attiny85 based power control board here; it doesn't offer all above-mentioned functionality, but it is probably the way to go...

Would like to hear the opinions of the specialists on this topic :)
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Re: β24 - thoughts about safety and amp control

Postby amb » May 22nd, 2018, 10:43 pm

Like you described, to implement protection in all those scenarios require quite a bit of extra circuitry. Some of them may be detrimental to the performance of the amp. It's mostly a matter of how far you want to go, there is no right or wrong answer. My observation is that most failures occur during assembly and test. Once the amp is built and stable, securely bolted together in a sturdy chassis, it tends to remain stable over the long haul. I don't have any of those protection circuitries in the reference build, and it has been working flawlessly for 10 years now.
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Re: β24 - thoughts about safety and amp control

Postby mpa2 » May 23rd, 2018, 1:29 am

Well, for me the building time was free of mishaps, the failure occurred when the amp was in place, working for several months without any problems. So yes, I would go far :)
You never know, who attemps to build one of your projects. One could expect that only very experienced guys would give b24 a try, but you can't be certain (look at me, for example :). Someone could think: "it has a fuse, so it is safe". And it is not, as mentioned above. It's one thing to burn a woofer, it's another thing to set a house on fire. So, in my opinion, at least a soft start and some kind of rail overcurrent protection is a must have.
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Re: β24 - thoughts about safety and amp control

Postby BesPav » May 23rd, 2018, 6:08 am

mpa2 wrote:Someone could think: "it has a fuse, so it is safe".


Surely, no.
Here in Russia peoples are little more sophisticated because 220 V mains is much more sensible to human body parts than 110 V.
So each understand - you must have all the knowledge/understanding and skills/equipment BEFORE, not AFTER...
:)

Just a practice from our grandfathers...
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Re: β24 - thoughts about safety and amp control

Postby Pieterh » May 23rd, 2018, 7:49 am

I use a 5A/230V slow blow fuse for the main xformer. It might work for yours as well. I know it doesnt' abate your raised concerns much.

About the e24. I dont have your exact implemention of the e24 in my mind but i remember that it differs from the reference, while this reference implementation is working fine here. You only have a higher VA xformer compared to reference and a trigger circuit right ?
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Re: β24 - thoughts about safety and amp control

Postby mpa2 » May 23rd, 2018, 8:01 am

BesPav wrote:Here in Russia peoples are little more sophisticated because 220 V mains is much more sensible to human body parts than 110 V.

Just a practice from our grandfathers...


Yeah, sure, I know :) I lived in Russia till I was 15 years old. Now I combine the practice from russian grandfathers with german thoroughness :)

BesPav, what do you think about my first post?
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Re: β24 - thoughts about safety and amp control

Postby BesPav » May 23rd, 2018, 9:21 am

mpa2 wrote:BesPav, what do you think about my first post?


Divide your wanted functions to simple and reliable devices.

Use any liked softstart, say simple RC-circuit with relay for your trafo, connect mains through ~20-50R 10Wt for 5 sec, then short resistance by relay.

Use rated or little lower slo-blo fuses at mains.

Use mains filter like Epcos (say with IEC inlet).

Use 30 mA circuit breaker (10 mA better, but pricey) and rated protective automat at your home mains.

Use speaker protection from there:
https://www.lynxaudio.net/app/download/ ... x_PA21.pdf
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Re: β24 - thoughts about safety and amp control

Postby mpa2 » May 23rd, 2018, 11:11 am

Pieterh wrote:
About the e24. I dont have your exact implemention of the e24 in my mind but i remember that it differs from the reference, while this reference implementation is working fine here. You only have a higher VA xformer compared to reference and a trigger circuit right ?


I think we use the same main transformer, but I use two EMRs, and not EMR+SSD.
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Re: β24 - thoughts about safety and amp control

Postby mpa2 » May 23rd, 2018, 11:53 pm

BesPav wrote:
Use speaker protection from there:
https://www.lynxaudio.net/app/download/ ... x_PA21.pdf


The circuits look interesting, and even include soft start functionality. The guy writes about "adding some capacitance" between amp outputs and ground. Would Beta like this?
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Re: β24 - thoughts about safety and amp control

Postby amb » May 24th, 2018, 12:05 am

mpa2 wrote:...
The guy writes about "adding some capacitance" between amp outputs and ground. Would Beta like this?

I don't know how much capacitance we're talking about here (the article is all in Russian which I can't read). But I would not recommend it, at least not without some resistance too (a.k.a. zobel). β24 has fully-differential outputs, so a zobel should be in parallel with the load (across the hot and cold outputs, not to ground).
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