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peniku8

Rob's Amp tests

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I investigated dielectric absorption quite a long time ago when I was 17 so my memory is a bit fuzzy.  I built a precision high dynamic range current mirror and monitored the charge/leakage current over the period of hours when the capacitor was connected to a voltage source with some series resistance.  I also had access to a volt meter with exceptionally high input impedance for monitoring the capacitor voltage.  I found that my capacitors fitted a model of a capacitor in parallel with another capacitor with a very high impedance in series with it that represented the long term leakage trend and the dielectric absorption. 

Static electricity can kill you if you use it to charge a large capacitor ;)

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I somehow feel like this is bad design or something isn't working as it should. The 4 transistors I replaced in the PSU, which all broke after about 2 minutes of 4KW output generate a humongous amount of heat even in idle.

The heatsink seen below reaches 80°C (176°F) in idle and SOMEHOW I measure over 1KV from heatsink to ground and when I touch it with the thermo sensor I see small sparks. What is going on here??

fWzrDr9.jpg

 

uNp7W4e.jpg

 

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What confused me even more was the seemingly random choice of output transistors, as two different models were actually used in the amp.

One output channel has an even distribution of both types, while the other one has 5 of one type and 9 of the other. Both are the NPN type.

https://www.onsemi.com/pub/Collateral/NJW0281-D.PDF

https://www.onsemi.com/pub/Collateral/NJW21193-D.PDF

Looks like the 21193 is a bit better. I'll solder two from the broken pcb to the working pcb to "upgrade" the FP-13000 to Fp-14000 on the output stage. It might make a slight difference into lower impedances. 14 vs 16 transistors is still a decent upgrade.

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Seems like this amp is haunted by Hoffmann's Law. I'm not sure why but the modifications I made (wich was adding the two missing resistors and transistors) broke the amp stage. The fuses melted so I removed the components again but it's still inoperational. Welp, now the amp doesn't work at all anymore, since both output stages are broken. I'm guessing that it's a relatively minor thing, but I'm not the one to track down a broken component and I'm fairly certain that getting it fixed will be more expensive than the 150$ for the replacement board.

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This is so annoying. I was just way too curious to see if I could improve the amp's performance by adding those components and now I caused 150$ damage again.

Well, heres more random info on the PSU: when both amp stages are disconnected, the PSU hold over 200V for an hour. I used a light bulb to quickly drain it. When it's down to 0.1V, it takes about 10 seconds to climb back to 2V. I immediately connected the 230V light bulb after I powered the amp off, when the PSU was still at around 350V from positive to negative. It was a 30W light bulb or smth, it lit up brightly for a second and the light faded completely after some 20 seconds. Very convenient way when you're working on this kind of stuff.

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The 1kv to ground sounds very weird as even if the heatsink was live on purpose you wouldn't expect 1kV to be generated anywhere in the amp.  Mix of output transistors is a very bad idea as they won't share current effectively.  On the plus side this is making me very certain I don't want one of these amps in my life :)

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21 minutes ago, kipman725 said:

The 1kv to ground sounds very weird as even if the heatsink was live on purpose you wouldn't expect 1kV to be generated anywhere in the amp.  Mix of output transistors is a very bad idea as they won't share current effectively.  On the plus side this is making me very certain I don't want one of these amps in my life :)

I ordered two replacement modules, will see if the current ones are the same. This amp is 7 years old after all.

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Quick update on the Sanway: I've recieved two replacement amp boards they sent me for 300$ including shipping. Amp is operational again.
The amp boards which I had were version 2, the new ones are version 10. They now also use the same type of transistors throughout.

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Rev 2 to Rev 10...Lot's of refinement along the way it seems. Good luck!

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This has all been quite interesting to follow.  I'm going to take a wild guess that the mixture of transistor types on the old board was "on purpose", albeit maybe as a last-minute hack or tweak for some other problem.  Hopefully the new revision is a lot more robust.

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15 minutes ago, SME said:

This has all been quite interesting to follow.  I'm going to take a wild guess that the mixture of transistor types on the old board was "on purpose", albeit maybe as a last-minute hack or tweak for some other problem.  Hopefully the new revision is a lot more robust.

Well the PSU blew up during testing. I only had to replace the amp boards because I damaged them by accident while trying to repair the PSU.

If the mix was on purpose, why use different amounts on both channels?

I also have the feeling that the amp gets hotter in idle now. So either the heat transfer from the transistors to the heat sink is better now or something else is the problem. I also reversed the fan mod I did and let the fans run full tilt now. I‘ll have a look at the idle power draw and see if it‘s higher than the ~150w it was before.

Either way, the bad gut feeling will now never subside when I bring that amp to a gig. I don‘t have any use for it at the moment anyways, so I might just sell it and call it a day.

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7 minutes ago, peniku8 said:

If the mix was on purpose, why use different amounts on both channels?

Re-work in QC?  I imagine that whatever is going on here, it's probably not good.  ;)

14 minutes ago, peniku8 said:

Either way, the bad gut feeling will now never subside when I bring that amp to a gig. I don‘t have any use for it at the moment anyways, so I might just sell it and call it a day.

I would agree unless you think you can develop your general knowledge/skills further by finding new ways to break it.  :)

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I measured the idle power draw and it looks like it went up to 160W (from 115). I can only measure in 0.1A steps, which is 23W, so the possible difference ranges from 23W to 69W. Not really a good sign, but it's not a huge amount either, so I'm not necessarily concerned about it. Their 4 channel amp draws almost 200W in idle. One reason for me to not switch the TSA1400 in the theater back out for the FP13000.

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On 6/15/2020 at 7:29 AM, kipman725 said:

https://en.wikipedia.org/wiki/Dielectric_absorption

From my tests the absorption of electrolytics can approach 50% (much more than Wikipedia claims).  Solved via bleed resistors.  I had a college who liked to leave 5kV film capacitors the size of a thermos flask without bleed resistors attached on his desk and around the lab when I worked at the University.

On the subject of amps I see that people who have experience with the Lab clones are reporting better reliability with CVR amplifiers:

https://www.cvr-audio.com/product/101.html

 

 

 

 

I've decided to order one of the cvr d-3002's to see whats up with them. I don't have the equipment to bench test but I can do some a/b listening tests vs my K10. Ill report back

 

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