popping noise in Krell CD player when power is cut off

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Krell KPS CD player with pop when powered off. I have replaced output relays & electrolytics in output section with no results. Krell doesn't release schematics but it's basically a 5v power on relay to output section. Output section consists of a ton of transistors & op amps,& many different types of capicators-styrenes ,mica,tantalum,teflon,poly & of course lytics. Any ideas would be appreciated.
 
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You need to determine the mode of operation of the relay. How does it mute ? by opening and breaking the signal path (poor design) or by being normally short across the output and going open to unmute (good design). Determine whether the coil needs to energised or not to mute and trace the winding back and see if you can figure how it works... without having it on the bench it's impossible to say or offer much else in the way of advice. Or have a circuit of course.
 
The relay will be a grounding shunt mute, rather than series contacts.
I would hazard a guess that the fault may be in the power supply for the output stage. You don't specify which KPS model, so can't be more specific, but the first thing I would look at is the discharge rate of the + & - output stage power supply lines. If one is taking longer to come down than the other, you would have the dc offset happen to make a pop at turn off before the muting relay is de-energised. The fault will likely be a lytic cap or two in the supply dying. You could alter the relay drive to turn off faster(difficult without schematic), but that would simply mask a developing problem elsewhere.

But the larger question is: Why do you EVER turn that player off? Those ridiculously long & overly-complex discrete output stages in Krell players, dacs & preamps notoriously take 24-48 hours to fully warm up to best sound at each turn on, so it makes no sense at all to ever shut one off.
 
The relay will be a grounding shunt mute, rather than series contacts.
I would hazard a guess that the fault may be in the power supply for the output stage.



Agree about the shunting mode - it is just silly to pass signal through relay contacts if it can so easily be avoided. Not so sure that the power supply is to blame though. Although i have no clue about Krell cd players i would imagine that instead of relying on capacitors they would rather look at the absence of AC voltage in the transformer secondaries as a mute trigger. Some Krell amps do check the line frequency in a pathetic way to control grey market product, so they obviously have experience with this.
 
The relay will be a grounding shunt mute, rather than series contacts.
I would hazard a guess that the fault may be in the power supply for the output stage. You don't specify which KPS model, so can't be more specific, but the first thing I would look at is the discharge rate of the + & - output stage power supply lines. If one is taking longer to come down than the other, you would have the dc offset happen to make a pop at turn off before the muting relay is de-energised. The fault will likely be a lytic cap or two in the supply dying. You could alter the relay drive to turn off faster(difficult without schematic), but that would simply mask a developing problem elsewhere.

But the larger question is: Why do you EVER turn that player off? Those ridiculously long & overly-complex discrete output stages in Krell players, dacs & preamps notoriously take 24-48 hours to fully warm up to best sound at each turn on, so it makes no sense at all to ever shut one off.

This is a KPS20i . I was restoring it for resale & didn't want any problem or return. I've replaced every electrolytic in the entire machine.I went all original with exception of caps in the main supply. VX series are no longer available so I used VR series which is supposed to directly replace these. I wanted to use muse but couldn't find high enough values in that series. I did notice a slight voltage difference in + & -18V supplies. -17.82 & +18.12. I haven't checked the discharge rate of the 2 sides yet though.
If you are familiar with these I do have a side question. Does the D/A stage need to be realigned after replacing the caps?
 
If it was all Nichicons to start with, a)there was no reason to replace them, honestly, unless one or more were clearly going bad, and b)they are so stable that going to new ones won't have altered anything as far as calibration goes. But I can't imagine needing to do msb/lsb adjusts from doing a cap change in any case.
Although I spent a lot of time with a KPS20i specifically, I can't recall ever shutting one off, at least while anything else was running, so I can't say for sure that a turn off pop is not completely normal. It may well be. If you check the dc voltage on the final output of both channels, while a cd is playing to make sure it's not muted, and you get 0V +/-50mV, then odds are that the pop is normal. If you have significant dc offset, then there is a faulty component in that channel's output stage, which, thanks to the complexity, could be a bear to track down. I don't imagine you'll find any dc worth noting, though.
 
Stephen- I just went to check output DC & the strangest thing happened. The output relays didn't cut on when I first powered up. I had to hard power cycle & cut on again for output relays to cut on. I'm not sure if this is related to my problem but have you ever seen this symptom before? BTW right channel had 2mv & left ch had 1.3mv DC.
 
No, have never seen that happen on any Krell cd before. I think you should perhaps very carefully check over all of the soldering on the power supply board(which I am 90% sure is where all of the relay control circuitry is), as well as the other boards. May be able to zero in on the locale if you get it to fail again & then poke & prod & tap things until the relays kick in.

As for schematics, which really would be helpful in this case, even though Krell would routinely authorise me to do warranty repairs on a per unit basis in the field, they never would release diagrams even to me. Their as tight with them as Proceed.
 
I traced the relay 5v cut on to a transistor on the DAC/out board then traced transistor cut on to front panel. I know it could loop back through the power board as you mentioned. These multi layered boards are a nightmare to trace. It does look like Krell would release schematics for a 15 year old product. Thanks for giving me an idea of what to look for. Just curious are there any simple mods for this unit to give it even better sound such as poly across caps supply or has Krell pretty much got every ounce of sound quality out of this already?
 
That player should sound a hell of a lot better than it does. However, I think it would require deleting just about everything after the dac chips, and building a new & simple discrete or one-stage opamp output stage. In any case, adding finesse to the power supply, i.e., soft recovery rectifiers, generous added film caps, can't do anything but help.
 
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