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Old 5th December 2011, 04:34 PM   #8341
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by Mark Allen View Post
The drop will depend on the design and actual capacity of the transformer itself.When I built mine I got a hellofadeal on a 1 kva single primary dual center tapped secondary surplus tranny for $60USD. It barely has any drop at all... Just a couple of volts. Yes, it is overkill...

Mark
Yes... but it's right .... so 2 volt droop .....?
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Old 5th December 2011, 05:01 PM   #8342
AndrewT is offline AndrewT  Scotland
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No.
You are both not reading what I told you.

Set up a Push Pull ClassA amplifier with an output bias of 1A. The ClassA output limit is ~ 2Apk.

Measure the supply voltage at the amplifier at quiescent condition.
Now attach a load and draw sinewave power from the output. The supply voltage does not change until the output current exceeds that 2A ClassA limit.
All that changes is the ripple on the supply rails.
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Old 7th December 2011, 05:38 PM   #8343
alibear is offline alibear  United Kingdom
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Hi everyone,
DISASTER Finished both channels and started to check out, I am using Jan's boards. Set bias to 1.5 amps on each channel and set offset to 0v,all set up easily. With amplifier still powered up I connected a pre-amp (Quad 44 ) and CD player.Re-checked offset and all OK. Again with the amplifier still powered I connected my normal listening speakers (Spendor BC1's).
Both channels sounded super, there was no hum and all was well until I turned the amplifier off at the wall socket at which point both bass drivers nearly popped out the cabinets. Being really stupid I switched back on from the socket and again the bass drivers shot forward with a terrible thump.
One of my beloved BC1's bass driver cone is now out of centre, I'm really angry with myself for not using a set off cheap speakers first.
What have I done wrong, is there normally a thump when powering on or off?
The only thing I can think off is that the input ground was only connected to ground via the phono lead going to the pre-amp ( although this connection was not broken at any time.
Since this has happened I tried without speakers connected and the offset is at 0v, but when the power is turned off the offset goes to about 12volts.
Any suggestions (except telling me how stupid I am ) will be gratefully accepted.
Thanks
Alan
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Old 7th December 2011, 06:13 PM   #8344
AndrewT is offline AndrewT  Scotland
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The KSA100 incorporated a BIG relay to isolate the speaker output.
Did Krell also fit isolation in the KSA50?
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Old 7th December 2011, 06:27 PM   #8345
Mooly is offline Mooly  United Kingdom
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You have learnt a lesson the hard way unfortunately.

Many amps thump at power on and off, this one obviously more than is acceptable.

I have not followed the thread and 8000+posts it's to big to refer back through.

A KSA50 (AKSA !!! no no no )

How does your KSA 50 differ from a production one ? Is it missing relays as Andrew suspects ? Do you have massively oversized reservoir caps that hold a lot of energy ? Could the amp be "latching" or swinging to one rail as the rail voltage falls due to some difference in circuit detail ?
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Old 7th December 2011, 06:41 PM   #8346
a.wayne is offline a.wayne  United States
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Alibear could you post up info on your PSU, you might want to try 2 small filter caps before your bridge rectifiers, what size PSU caps ..?
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Old 7th December 2011, 07:46 PM   #8347
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Hello
Sorry to hear that.
When you test any amplifier never connect a good speaker for test!
Some amplifier after you turn it of the power the power transistor will open and let 10V or so out to the speaker coil. Even if when is on full power you don't get any offset.
Sometimes enough to destroy your driver.
I have a NAIM clone it does the same. I also burnt a driver about 15 years a go.
Also thermal runaway and so many things can go wrong.
Better just hook up a cheap speaker or just a driver if you don't have but never a expensive speaker.
Greetings Gabor
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Old 7th December 2011, 07:56 PM   #8348
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Hmm, the only time I had significant offset on power up/down was when I was playing with C102/3 to try and optimise the values. What have you used there Alan?
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Old 7th December 2011, 11:31 PM   #8349
john65b is online now john65b  United States
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Maybe a bad reservoir cap? I had a Forte Amp that had a nasty turn on / off thump, but sounded very nice.

Is 12V dc enough to kill a driver - melt a voicecoil??
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Old 8th December 2011, 06:42 AM   #8350
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by john65b View Post
Maybe a bad reservoir cap? I had a Forte Amp that had a nasty turn on / off thump, but sounded very nice.

Is 12V dc enough to kill a driver - melt a voicecoil??
If constant then yes. 12 volts across a 4 ohm driver, 3 amps/36 watts. I can solder with half that power.

If the voltage were due to "just" reservoir caps discharging and didn't go over 12 (and a DVM is no good to check that) then it is possible the large cone excursion could push a coil outside its limits. What happens after that depends on the physical construction of the speaker and whether the cone passes a point of no return.
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