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Old 9th May 2012, 03:24 PM   #31
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@xslavic thanks for your reply! I don't think the rectifier is too slow for the job. These are rectifiers which can handle 35A (MB3510). Almost the same as used in the original KSA-100, which works great. There is no need for faster diodes like schottky's. Schottky's were never designed for large currents, so it gets pretty expensive and difficult to get schottky's which can handle the current this amplifier draws. The only advantage for schottky's which I see is the lower voltage drop, but that's not a problem (got about 2*49V DC per channel). This is the MKII by the way :-)

@Dean: thanks for your reply! I am curious if more people used the same devices.

@Purtle: I've bought them second hand from a guy who stipped two pretty big PSU's down. There were a lot BUX81's on the cooling tunnels. That is why I did not mount the MJ15003/4's direct to the PCB's and chose this configuration; there are a few holes to much. Now they are covered :-) I have posted some pictures in this topic of how the tunnel coolers were before I striped them.

Last edited by Kaplaars; 9th May 2012 at 03:29 PM.
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Old 12th May 2012, 02:54 PM   #32
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Had some spare time, which does not occur that often, so I have made some further progress :-)! I've mounted the speaker protection board and the softstart I've bought earlier:

Click the image to open in full size.

I bought the soft start via Ebay (Class A power delay soft-start temperature protection board 110V/220V | eBay). It is a nice soft start which has also two terminals for two thermostarts. The thermostats (TO-220 housing) were included. At 75 degrees Celsius the soft start cuts the power due to shorting of the thermostats. Maybe I buy thermostats which shorts at higher temperature, because 75 degrees is not that uncommon for class A amplifiers (the heatsinks of my 60W AudioAnalyse A9 reach almost that temperature and that amplifier is not even biassed full class A!).

I had some serious problems with the soft start, it is pretty crappy designed. It was not proper protected against switch debouncing, so it did not turn on or shut down right. The problem was caused by two small bad 1uF capacitors on the board, which seem to be bad quality. Changed them for Philips 1uF capacitors and the board works like it should work. But there is a but. They did a strange thing, they mounted a capacitor in series with the AC mains, which is later bypassed by a relay (together with the NTC's). Why did they do this? Is this to limit the current which flows during power on? Problem is that they mounted it in such a way that it continue bleeds about 55VAC. I changed the value of this capacitor to 22nF (X1, Y2 specified) so it bleeds less AC, but it is still about 5VAC. Is there a way to change this? I can't cut the capacitor out, because the relays than start to bounce. Sorry for the bad schematic I drew with paint, but what they did is basicly this:

Click the image to open in full size.

At power on relay 1 switches and a few seconds later relay 2 switches.

I have also another question. Some 'high-enders' bypass the electrolytic caps with 1 - 4.7uF capacitors. They place them paralel over every electrolytic capacitor. Is this to repress HF distortion from outside? Should I place them aswell? I have some nice 1uF WIMA capacitors, so to obtain them is not a problem.

Below is a picture of how I want to place all of the components (I know it is still a mess )

Click the image to open in full size.
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Old 12th May 2012, 04:52 PM   #33
AndrewT is offline AndrewT  Scotland
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Do NOT fit NTC in parallel.
If you need more dissipation capacity than one NTC can manage then series connect them.

I don't like the risk you are taking by using caps across the rectifier diodes. They may be MKS which have slightly higher esr than MKP.
I think the risk of ringing and/or oscillation is too high using caps alone instead of a true snubber, i.e. R+C
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Old 12th May 2012, 05:13 PM   #34
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Hi Andrew,

The little schematic I drew was not entirely correct, what they did on the PCB was two NTC's parallel and than set two of these sets in series. So in total there are 4 NTC. My main concern is the capacitor which is mounted in series witch the mains. I do not understand what the function of this is.

What would be the risk of the snubber capacitors across the bridge rectifiers? It is a very commonly used concept. An accuphase E204 for example uses the same concept. Altough this amplifier has ceramic capacitors across the bridge rectifier. The capacitors I use are MKT and rated for 100VAC which should be enough.
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Old 12th May 2012, 05:25 PM   #35
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Hi Andrew,

Why is it a bad idea for NTC's to be mounted in parallel?

I know the mains capacitor has to be X or Y rated. Like I posted earlier the capacitor that is in series with the mains is x1, y2 rated. But that is not really my question. I dont understand why this capacitor is placed in series with the mains. I also do not entirely understand what is wrong with the C's parallel to the bridge. Can you explain that to me?
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Old 12th May 2012, 05:37 PM   #36
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That is not a constructive reply Andrew :-( Made a typo, I meant parallel. Sorry, I am only human.

But you still have not answered my question(s).
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Old 12th May 2012, 06:02 PM   #37
CBS240 is offline CBS240  United States
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NTC - negative temperature coefficient. When temp rises resistance lowers. If they are in parallel the resistance may drop too low. The R-T curves will no longer apply as designed. For more Pd use them in series.

Using caps alone across the rectifier (dielectric type is important for this app) creates damping with no where for the damped charge to dissipate. It needs a resistor to absorb the stray energy from the switching noise of the diodes, IOW R-C filter. Using faster diodes increases switching noise. Some of the ultra-fast or Schottky rectifiers are not so suited for 50-60Hz, more for SMPS switching speeds.
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Old 12th May 2012, 06:20 PM   #38
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Hi CBS240,

Thanks for your explanation So actually the NTC design in the softstart is ok, because they placed two NTC's parallel and then two of this sets in series. The total resistance would than be equal to one lone NTC. For example 1 NTC is 100 ohms, two of them parallel would give 50 ohms, but if you set two of this parallel sets in series the sum is again 100 ohms. In this case setting NTC's parallel is no problem, or am I wrong?

Aha that is a good point, the absorbed energy in the capacitors has no where to go. So actually how I mounted them now is nog dangerous, but not really functional?
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Old 12th May 2012, 08:20 PM   #39
AVWERK is offline AVWERK  United States
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Kap
I would place the fans to blow each channel out the back instead of thru a long tunnel.
You will get about a 15 to 20 C difference between the in and out temps on these sinks the way you have them positioned.
I know this to be true since I done this on my Synder monoblocks. Even though it never did any harm, you really want consistant temps across the sink if at all possible and that would be a short heat sink with fan

I see you actually had the correct orientation in post 24

Regards
David

Last edited by AVWERK; 12th May 2012 at 08:26 PM. Reason: Post 24 addItion
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Old 12th May 2012, 08:54 PM   #40
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Hi David,

Thanks for your input! Hmm, that gives food for thought. My motivation behind mounting the tunnel coolers together and place them like post 32 was to get better air circularion since the air can flow directly in and out the cabinet through the tunnel.

Basicly there are two fans, one which sucks air from outside and blows air through the tunnel. At the end of the tunnel I mount another fan which sucks the air out of the tunnel and blows it out of the cabinet (not in the picture). That would fit just right from side to side. Therefore I have to drill two holes in the cabinet at each side, so air can flow freely. When I mounted the tunnel coolers as in post 24 there would be less circulation of air was the idea. On the other side, in the new 'mounted' configuration differences in temperature are indeed almost inevitable :-(

In the original Krell configuration the tunnel coolers were placed upright, but I find that the least best option. It would give two ugly holes on top and two at the bottom for optimal airflow.

I think that it is a good idea to experiment a bit with the best configuration before I drill large holes in the cabinet.
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