Hi,
two 1983 Krell KMA-200 monoblock power amplifiers are driving a pair of Apogee Scintilla 1 Ohm speakers, all these years without any problem.
Now, since about six months, one of the Krell's is humming (the other is still dead silent). The hum of the offending amplifier is disturbing in a quiet environment.
The hum is entirely mechanical, it comes from the power transformer.
The hum is there immediately when switching on the amp. The amp is not connected to anything, only the mains. Also, a DC blocker does not change anything.
So I am thinking of replacing the power transformer.
I will have to order a (silent) custom made transformer, the Krell original is no more available as far as I know, and I am not sure about the required VA of this transformer.
I measured the following voltages and currents:
2 x 58,6 VAC (power output stage)
65-0-65 VAC (driver stage)
12,6 VAC (power indicator)
Bias current over the emitter resistors:
0,35A x 16 power transistors = 5,6A bias current. Power supply is +/- 58,6V.
So, the DC power with no signal is about 728VA?
This is alle measured with no load and no signal.
Since this is a real class A amplifier I suppose with no signal 728 VA is the maximum power consumption.
Am I right about this?
Krell claimed class A power output is 1600W in a 1 Ohm load.
What should be the minimal VA rating of this power transformer?
Does anyone knows the original specs of the Krell KMA-200 power transformer?
Any idea's or advice is highly appreciated!
two 1983 Krell KMA-200 monoblock power amplifiers are driving a pair of Apogee Scintilla 1 Ohm speakers, all these years without any problem.
Now, since about six months, one of the Krell's is humming (the other is still dead silent). The hum of the offending amplifier is disturbing in a quiet environment.
The hum is entirely mechanical, it comes from the power transformer.
The hum is there immediately when switching on the amp. The amp is not connected to anything, only the mains. Also, a DC blocker does not change anything.
So I am thinking of replacing the power transformer.
I will have to order a (silent) custom made transformer, the Krell original is no more available as far as I know, and I am not sure about the required VA of this transformer.
I measured the following voltages and currents:
2 x 58,6 VAC (power output stage)
65-0-65 VAC (driver stage)
12,6 VAC (power indicator)
Bias current over the emitter resistors:
0,35A x 16 power transistors = 5,6A bias current. Power supply is +/- 58,6V.
So, the DC power with no signal is about 728VA?
This is alle measured with no load and no signal.
Since this is a real class A amplifier I suppose with no signal 728 VA is the maximum power consumption.
Am I right about this?
Krell claimed class A power output is 1600W in a 1 Ohm load.
What should be the minimal VA rating of this power transformer?
Does anyone knows the original specs of the Krell KMA-200 power transformer?
Any idea's or advice is highly appreciated!
Hopefully this will help
https://elektrotanya.com/cgi-bin/do...ile=krell_kma-160_400_ksa-80_200_info_sch.pdf
https://elektrotanya.com/cgi-bin/do...ile=krell_kma-160_400_ksa-80_200_info_sch.pdf
Hi 968driver,
Of course anything can break. However, tranformers are usually very robust. In my experience there are a couple of things that can get a transformer to hum mechanically. One thing is small amount of DC on the mains. However, since only one of your amplifiers does this I think that might not be the cause.
The second is the amount of current drawn from the transformer. Do you think it's possible that the quiescent current of this one mono-block could have driftet due to some ageing in one of the components related to this? The measurements you qouted is that compared between both mono blocks?
Just a thought.
Cheers,
Mogens
Of course anything can break. However, tranformers are usually very robust. In my experience there are a couple of things that can get a transformer to hum mechanically. One thing is small amount of DC on the mains. However, since only one of your amplifiers does this I think that might not be the cause.
The second is the amount of current drawn from the transformer. Do you think it's possible that the quiescent current of this one mono-block could have driftet due to some ageing in one of the components related to this? The measurements you qouted is that compared between both mono blocks?
Just a thought.
Cheers,
Mogens
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I think current draw is O.K., since it does not run hotter compared with the other one.
A DC blocker changes nothing, so DC on the mains can be ruled out.
I think some windings have become lose and vibrate, causing the transformer humming.
A DC blocker changes nothing, so DC on the mains can be ruled out.
I think some windings have become lose and vibrate, causing the transformer humming.
Hi 968driver,
You might be right. It's just that potted transformers as I remember these Krell amplifiers use are very sturdy. But as said. Everything can break.
Hope you find a solution to these great amplifiers.
Cheers,
Mogens
You might be right. It's just that potted transformers as I remember these Krell amplifiers use are very sturdy. But as said. Everything can break.
Hope you find a solution to these great amplifiers.
Cheers,
Mogens
Replacing the transformer with anything different will change the sound. If you really have to take such a drastic measure rather change both channels.
Have you examined possible mechanical issues? Screws getting lose, something magnetic vibrating in the vicinity? Have you considered a damping sheet under the transformer?
Have you examined possible mechanical issues? Screws getting lose, something magnetic vibrating in the vicinity? Have you considered a damping sheet under the transformer?
This very big and heavy potted torodial transformer is mounted on four silent blocks. The rubbers are still elastic. Nothing in the vicinity that vibrates or can cause the humming.
Using a screwdriver as a stethoscope clearly identifies the transfomer humming.
If I have the transformer specs, I can have a suitable transformer made.
Using a screwdriver as a stethoscope clearly identifies the transfomer humming.
If I have the transformer specs, I can have a suitable transformer made.
I've had a few toroidals go mechanically noisy on me.
A decent transformer place should be able to test and or dissect the original and work out what it is current and voltage wise.
A decent transformer place should be able to test and or dissect the original and work out what it is current and voltage wise.