Big problem with my Krell AMP, need help please.

Florian · « **on:** 16 Feb 2005, 10:53 pm »

I opened the unit up to take a picture of the inside. I just removed the 4 top screws and the cover. Took pictures and put it all back together. I reinstalled him and everything worked perfect. About 1 hr laer, i disconnected all the stuff and i changed tubes in my pre amp. After hooking it all back up, the Krell doesnt turn on.

I press the button, and it doesnt move at all. The unit is still warm, but wont turn on. Any ideas?

nhtran · « **Reply #1 on:** 16 Feb 2005, 10:57 pm »

Krell amps shut down when using tube pre-amps. The output of a tube pre-amp contains DC which will trigger the Krell's protection circuit. You need to send the amp to Krell for modification if you want to use a tube pre.

Florian · « **Reply #2 on:** 16 Feb 2005, 11:00 pm »

The tube pre has worked flawlessly everyday for the past 3 weeks with it. I wouldnt understand that.

PS: The unit will not turn on even when sitting alone with only the powercord connected.

rosconey · « **Reply #3 on:** 16 Feb 2005, 11:02 pm »

might be a tube then

put the old ones back in

Florian · « **Reply #4 on:** 16 Feb 2005, 11:03 pm »

I tried that already.

It wont even turn on if sitting alone with only the powercord to it. In the back is something like a fuse, i dont know how to get it out though.

rosconey · « **Reply #5 on:** 16 Feb 2005, 11:08 pm »

buy a boat it looks like you already have the anchor

sorry ive been wanting to use that for a long time.

call the mfg would be my next move

Florian · « **Reply #6 on:** 16 Feb 2005, 11:16 pm »

whats mfg?

Marbles · « **Reply #7 on:** 16 Feb 2005, 11:18 pm »

Quote from: Florian

whats mfg?

Krell

nhtran · « **Reply #8 on:** 16 Feb 2005, 11:28 pm »

Ok, one more time....YOU CANNOT USE A TUBE PRE-AMP W/O MODIFYING THE KRELL FIRST!

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1103939290&openmine&zzOldpet&4&5

ted_b · « **Reply #9 on:** 16 Feb 2005, 11:51 pm »

Quote from: nhtran

Ok, one more time....YOU CANNOT USE A TUBE PRE-AMP W/O MODIFYING THE KRELL FIRST!

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1103939290&openmine&zzOldpet&4&5

I have a tubed preamp (Ray Samuels Stealth II) and a Krell KSA 100 and it works flawlessly. Has for over a year. And I've owned the Krell for 17 yrs, and it's not been "modified".

Please quit yelling.

Ted

Florian · « **Reply #10 on:** 16 Feb 2005, 11:51 pm »

Then why did it work for 3 weeks ? why wont the unit turn on by itsself?

John Casler · « **Reply #11 on:** 17 Feb 2005, 01:02 am »

My money says its the fuse, just above the EIC.

Florian · « **Reply #12 on:** 17 Feb 2005, 01:19 am »

How can i take that out ? Here is a pic of something i found in it. There is no way i did that tough, and my engenieering buddy says that it looks intentionally. I dont know....

See the small circuit brake next to the white resistor?

tkp · « **Reply #13 on:** 17 Feb 2005, 05:30 am »

Florian,

You need to take a picture of the surrounding circuitry so people can tell where in the circuit the trace brake occur. It appeared to be an over heat connection due to high current surge which cause the copper trace to melt. This is a lot of current. In this case, either the big resistor would burn or the trace so it looks like the trace gave in first.

There is nothing wrong with using tube preamp with any solid state amps. However, one needs to realize a few things.

a) Pretty much all tube preamp has DC blocking capacitors at the output. With a plate couple output, the output cap will have to block very high DC level voltage (in the range of 100VDC to 300VDC). As a result, some high level leakage DC current might occur. This will potentially create a DC voltage at the input of the amp. and cause the amp to put out some DC at the output. However, this should not be a problem with amp which has a DC gain of 0 db (ie... a DC blocking capacitor in its feedback path).

b) Most well tube design preamp will have muting relay at the output which will ground the capacitor for some amount of time after turning on to prevent DC current surge through the amp. However, some tube preamp might not have this feature hence a huge amount of DC current surge csould occur upon turning the preamp on with the amp already on and might result in burning signal traces as shown in your picture. The way to resolve this is to turn the tube preamp on for some amount of time so the DC blocking cap could be fully charged before turning on the amp.

c) Some amp actually implement a relay at its input to allow the internal circuitry to stable prior to letting the preamp output connecting to the amp input circuitry. However, this might create a condition where the tube preamp rely on the input resistance of the amp to charge its DC blocking capacitor. With the relay disengage inside the amp, the DC blocking cap does not have any path to charge. As a result, as soon as the relay inside the amp closes, the DC blocking cap at the output of the preamp charges and cause high inrush DC current. The way to solve this problem is to put a high resistive load to ground at the output of the tube preamp (ie... 500Kohms to ground) to provide a charging path for the capacitor.

In any case, you should measure the output of the amp for any DC voltage (with nothing connecting to the amp) and probably should contact Krell for further advise.

Good Luck,
TKP

lonewolfny42 · « **Reply #14 on:** 17 Feb 2005, 05:43 am »

Link for Krell Service.... http://www.krellonline.com/html/c_service.html .

nhtran · « **Reply #15 on:** 17 Feb 2005, 03:10 pm »

Quote from: ted_b

Quote from: nhtran
Ok, one more time....YOU CANNOT USE A TUBE PRE-AMP W/O MODIFYING THE KRELL FIRST!

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1103939290&openmine&zzOldpet&4&5

I have a tubed preamp (Ray Samuels Stealth II) and a Krell KSA 100 and it works flawlessly. Has for over a year. And I've owned the Krell for 17 yrs, and it's not been "modified".

Please quit yelling.

Ted

That's because your preamp has DC blocking caps. If you use a tube preamp w/ a Krell and run into problems, that's the first place you should look.

woodsyi · « **Reply #16 on:** 17 Feb 2005, 03:35 pm »

I wondered why there was a caution on my pre that stated, "FIRST ON LAST OFF". I just leave the pre on the whole time unless I am sure I won't be listening for a few days.

tkp · « **Reply #17 on:** 17 Feb 2005, 08:00 pm »

Even with power on muting ciruitry (ie... a relay that connect the end of the ouput cap to ground for a short time to help the cap charges up quickly), audio manufacturers prefer tube preamp to be turned on first to prevent any potential DC charging. One thing I found a bit disturbing is that some tube preamp do not have a loading resistor at the output to provide a guaranty path for the DC coupling cap to charge with or without being connecting to the amp. This has the benefit of if there is intermittent connection between the amp and the tube preamp, the cap would not get in and out of charging and dischargin cycle. The output loading resistor should be a large value (ie...500+ Kohms) to prevent additional loading on the weak output stage of plate couple output preamp.

Having DC coupling cap at the output of the tube preamp does not guaranty that the tube preamp will work with all amp. Some amp will shutdown if connecting to a particular tube preamp because one and/or all of the followings:

a) How much DC leakage occurs from the cap and what is the resistance of the amp input impedance. For example, if the cap leakage is 10 uA at and the input impedance of the amp is 100Kohms then the DC voltage at the input of the amp would be at 0.00001 A x 100000 ohms = 1V. Is this a huge problem. The answer varries from "it depends" to "yes".

b) If the amp has servo DC circuit to detect excessive DC at the input then the condition in a) will cause the amp to not power up.

c) If the amp is DC couple and it has a DC gain of 0db (unity gain) then it will put out 1V dc at its output. The tweeter should be okay since passive crossover for tweeter typically has a capacitor in the signal. The woofer might or might not be okay but most large woofer can take a volt without any problem

e) If the amp is DC couple and not DC unity gain then you can kiss all your woofers/tweeter goodbye and probably your amp too

.

Florian · « **Reply #18 on:** 18 Feb 2005, 08:14 pm »

i got the Krell fixed a local electronics shop for 300$.

I got ripped off, but what the hell the thing works perfect and i would have paid 300$ alone in shipping to Krell.

woodsyi · « **Reply #19 on:** 18 Feb 2005, 08:31 pm »

Is that like 30 Euros?