Dx Troyan, a 650 watts channel amplifier. - Page 21 - diyAudio
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Old 6th August 2010, 05:44 PM   #201
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Default If it is only technicall.... schematic, values and so on...i cannot see

reason to avoid make it public....but if there's personal stuff...then it is better to remove the personal, as this does not interest our forum...ain't me too.

regards,

Carlos
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Old 6th August 2010, 05:53 PM   #202
AndrewT is online now AndrewT  Scotland
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here is my PM to DX in it's entirety. I cannot understand why DX cannot read it.

VI limiter
Quote:
Do you think the V/I limiter is reasonable?
It is working and limiting....i could not find failures till this moment....do you have suggestion to make it better?...............................
Can you help please AndrewT?
Yes.
But, the component values in the VI limiter depend on
Rth s-a
Rth c-s
output device SOA
output device temperature de-rating
Ta
Tc
transformer voltage
transformer regulation
smoothing capacitance
output emitter resistor value
matching of output devices if more than 1pair
matching of Re if more than 1pair

I have an Excel spreadsheet with a VI limiter built in.
You will see that altering just one of the above variables can throw the VI limit locus way off from where the builder might want it.
Oh, that's the other variable.
Where does the builder want the VI locus compared to the DC and 100ms and 10ms SOA limits?

Do you want me to send you the spreadsheet? It is based on Bensen's FET SOA sheet with many modifications since he posted it years ago.
The model that can be built up in the spreadsheet turns out to be quite accurate for current and voltage into resistive loads and allows one to see the effect of reactive loads.
One part that I cannot model is transient currents into reactive loads. I have to guess at that bit and use experience and some actual testing.

Here is my philosophy on a VI limiter.

The limiter must pass unaltered, all valid audio signals to all valid speaker loads that are within the specifications of the amplifier.

It should hopefully protect against damage when the amplifier is abused.
If the VI limiter meets that specification then it cannot affect the audio quality of the signal passing through the amplifier.
regards
Andrew T.
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Old 6th August 2010, 10:15 PM   #203
anatech is offline anatech  Canada
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Hi Andrew,
Quote:
I cannot understand why DX cannot read it.
Please cut Carlos some slack. You know that English is not his first language.

As for "the amp will not work" type comments, they are not accurate. A more accurate way to get across your concerns would be that running with a single pair of outputs into a 4 ohm load isn't recommended. The amp will work, but it may fail when running at high level into a four ohm load. The amp will work after all. It just may fail.

For years I've seen amplifier repairs that shouldn't have survived turn-on. I've learned that electronic parts can sometimes put up with all kinds of abuse. So while running parts beyond their safe operating area should be discouraged, the parts may just survive in spite of what the spec sheets say. Remember, the manufacturer states that they will not guarantee the parts will perform beyond the limits they give. That doesn't mean that they will not survive.

Does that help?

-Chris
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Old 6th August 2010, 10:19 PM   #204
PMA is offline PMA  Europe
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Nice to see you again, Chris.

IMO Andrew is TRYING to help.
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Old 6th August 2010, 11:07 PM   #205
AndrewT is online now AndrewT  Scotland
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Anatech,
have you looked at the circuit around the VAS?
It was posted just a short while ago that the 818 was only in simulation.
Don't you think that is funny, or was he just winding us up?
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Old 7th August 2010, 01:28 AM   #206
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Default Too much complicated AndrewT.... this looks more a terrorism than an explanation

I will do by myself..thank you anyway.

regards,

Carlos
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Old 7th August 2010, 03:22 AM   #207
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Quote:
Originally Posted by wg_ski View Post
Why? Either the peak junction temps don't really get that high (even in the s/b limited region), or the peak junction temps can be run higher than any 200C. In either case, the semiconductor makers are sandbagging their specs on the modern devices.
Hi! How's the amplifier building coming along?

I think you are right. Or more likely, it's both those factors combined. The thermal capacity of the transistors is NOT negligible. Temperature variation over a cycle isn't really that great, even at 20 Hz.

As for second breakdown issuses, the "dangerous" part of the dissipation pulses when driving reactive loads is about 10ms in length even at the lowest of frequencies and the duty cycle is low. In such a situation, transgressing the 100ms S/B a little is not going to be fatal. +-100V rail class A/B with 2SC5200 style devices sounds a bit excessive though. Most manufacturers seem to start using series output stages (like the Super Leach) above 80V or so.

There also seems to be quite a bit of conservativeness in the temperature ratings. I was dissapointed that I couldn't blow up a 2SK176 (a TO3 lateral MOSFET rated 125W @ 25 deg C) with a power dissipation of 200W mounted to a heatsink kept at 90 degrees C! I gave up after about a minute. Tough little guys these transistors. Sure, a BJT isn't as internally themally stable, but instant destruction sure doesn't happen at 200 degrees C like most here seem to think. A Sanken note in a databook suggested it's more like 260-300 degrees C and that sounds about right... The LM3886 "SPIKE" protection activates at 250 deg C power transistor Tj if I remember correctly.
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Old 7th August 2010, 04:12 AM   #208
anatech is offline anatech  Canada
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Hi Pavel,
Thank you. It's really good to post a bit.

I agree Andrew is trying to help, but I'm afraid that our two combatants may be misinterpreting the language. My post was more to try and have Andrew and Carlos step back a bit and not pay so much attention to the details. From what I had read, Carlos has a valid argument in that he is running those amplifiers now. I merely suggested that the reason why this is so is because of "safety margins" built into transistors.

Hi Andrew,
No, I have not seen the Vas area. But I'll believe you that the transistors may be exposed to excessive C-E voltages. However my intent and post was more about about allowing some room in language. Note I did not say you were wrong. What I said was that it is entirely possible that Carlos has some of these running merrily away with self destructing right away. My only problem was the phrase "can't work" as opposed to the more correct idea that the amps can work that way. The fact that it is not recommended to run transistors (or any parts) beyond their maximum limits and they may fail in operation is another thing again. I think that this is what you were thinking when you posted, but the word "can't" is the wrong word. "Shouldn't" seems to more accurately reflect the situation.

Ahhh, when words get in the way ...

Quote:
It was posted just a short while ago that the 818 was only in simulation.
Post 818? What are you referring to there Andrew? I just sailed in after receiving a reported post.

-Chris
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Old 7th August 2010, 09:19 AM   #209
wahab is offline wahab  Algeria
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Quote:
Originally Posted by anatech View Post

Post 818? What are you referring to there Andrew? I just sailed in after receiving a reported post.

-Chris

A BC818 that is connected in a way, driver of the
darlington Vas , that it have to sustain a full rail voltage,
that is , 100V at the start , and currently 80V since the
PS voltage had been reduced.
Anyway, surely that he did use something else when
building this amp.
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Old 7th August 2010, 10:46 AM   #210
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Quote:
Originally Posted by wg_ski View Post
It doesn't surprise me a bit that it works - Uncle Charlie isn't taking any more liberties with SOA than QSC, Crown, and everyone who copies them haven't been taking for years and years and years. Less, actually. Look at some of those designs and you wonder why they don't just blow up the very first time the bass drops. Instead, the only thing that seems to kill them is long term heating.

Why? Either the peak junction temps don't really get that high (even in the s/b limited region), or the peak junction temps can be run higher than any 200C. In either case, the semiconductor makers are sandbagging their specs on the modern devices.
Correct, but the thing that makes all the difference is that QSC, Crown etc have sophisticated protection systems, while the Troyan only has an I/limiter which is not sufficient at these voltage levels.
Also note that a lot of really high-power amps have 8, 10 or even 12 pairs of output devices.

That is really the only argument here: make sure that you have enough devices to stay outside the danger zone, or put in a good protection system.
Sure the Troyan 'will play', will survive full power in a 2 ohms resistor - that's also what AndrewT found and clearly stated.

But hang a real speaker on it and the game changes.
Many people find that hard to believe because they are unfamiliar with these issues, but anyone who has used these types of amps and has been plagued by seemingly unexplainable random failures even when not playing full power knows.

And if you have an output stage failure in a 8-pair output it's a hell of a job to find the problem, let me tell you that. So if you build this amp, build it first with a single output pair and test it with a higher load resistor to make sure it is wired correctly and it will survive the full output voltage level. Then add devices pair by pair, and re-adjust the bias and recheck everytime. Amp building is fun, amp fault finding isn't. YMMV.

jd
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