Hello everyone. I'm new here.
Last month, I picked up my Krell KSA 100 Mk I from the Krell factory in CT. I had them go through it and bring it back to top operating condition. I traded a friend my KAV 250a for his KSA 100, and I knew that I had to more than likely invest in this upgrade. Krell went through it and replaced about $550 worth of parts and charged me for 4 hours of labor, another $500. Looking inside, I thought I was looking at a new amp. They did a great job. I know this is DIY site, but I have been reading too many horror stories of someone trying to work on a Krell (for years sometimes) on their own. Prior to bringing it in, the left side of the amp was noticeable hotter than the right, and I knew it was time for the upgrade. Now that I have it back, it sounds great, but I am very surprised by how cool it now runs. Very surprised. This amp was known to run hot. I am using Hales Revelation 3 speakers which need a good amp to move them. My original Krell KAV 250a would get hot pushing them as well as an earlier Belles amp I once owned. I have pushed the heck out of this newly refurbished KSA 100 and the thing doesn't even feel warm! I asked Krell about it and they stated that with the newer parts, they are able to get the amp to operate much cooler then before while still in full Class A mode. It sounds great with MUCH better imaging and bass then before it went away, but I can't get over the complete lack of heat! I'm baffled. When they turn the bias down, doesn't that take it out of Class A mode? I appreciate any and all comments to help me understand this. Thank you.
Last month, I picked up my Krell KSA 100 Mk I from the Krell factory in CT. I had them go through it and bring it back to top operating condition. I traded a friend my KAV 250a for his KSA 100, and I knew that I had to more than likely invest in this upgrade. Krell went through it and replaced about $550 worth of parts and charged me for 4 hours of labor, another $500. Looking inside, I thought I was looking at a new amp. They did a great job. I know this is DIY site, but I have been reading too many horror stories of someone trying to work on a Krell (for years sometimes) on their own. Prior to bringing it in, the left side of the amp was noticeable hotter than the right, and I knew it was time for the upgrade. Now that I have it back, it sounds great, but I am very surprised by how cool it now runs. Very surprised. This amp was known to run hot. I am using Hales Revelation 3 speakers which need a good amp to move them. My original Krell KAV 250a would get hot pushing them as well as an earlier Belles amp I once owned. I have pushed the heck out of this newly refurbished KSA 100 and the thing doesn't even feel warm! I asked Krell about it and they stated that with the newer parts, they are able to get the amp to operate much cooler then before while still in full Class A mode. It sounds great with MUCH better imaging and bass then before it went away, but I can't get over the complete lack of heat! I'm baffled. When they turn the bias down, doesn't that take it out of Class A mode? I appreciate any and all comments to help me understand this. Thank you.
Accroding to the schematic I have, there's about 35-40W of power dissipation per output device (8 per channel),
or a total of about 650W heat dissipated. Sounds about right for a 100W/ch class A amplifier.
Wish I had one running right now, it's -24F here.
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Thank you for your reply but I don't understand your answer. As I stated, my amp is completely cold to the touch, even after running it hard. The sound is fabulous, but I am lost as to why no heat after Krell upgraded it. Would I not feel some warmth with 650 Watts being dissipated?
Then the conclusion would be that it's been converted to a class AB amp by reducing the bias current.
Can you measure the line input current? Maybe you should contact Krell about this.
I think it is Krell that have some weird definition of Class A ? That the bias can be modulated by some kind of gating in "anticipation" of the signal. In other words by the accepted definition of the term, Class A it is not. That accepted definition of Class A and how that relates to single ended or push pull operation is well known.
I just got it back from Krell. They did the work. Yes, I did email them and stated the issue and they replied that it still is biased in Class A but can run cooler with the newer parts. I agree that something doesn't seem quite right with the "no heat" output yet still a Class A bias.
There is a maximum theoretical efficiency of about 50% at full output (regardless of the devices) for a class A push pull amplifier,
so you would expect at least 200W heat from a stereo 100W/channel unit at idle.
Krell's service manager communicated back to me the following on why my amp is running cool:
Hi Sam,
I checked with the tech, He advised me that the new transistors we installed are more efficient, These are the current transistors we use in all our products. This is why the unit is running cooler than before.
Raising the bias produces higher distortion levels, not recommended.
Thanks Ray
Raymond C Mutchler
Service Manager
Prior to that response, he also added this yesterday:
I looked at the traveler on this amp and the idle bias is 85 MV, Which I am pretty sure is correct for this vintage.
Hi Sam,
I checked with the tech, He advised me that the new transistors we installed are more efficient, These are the current transistors we use in all our products. This is why the unit is running cooler than before.
Raising the bias produces higher distortion levels, not recommended.
Thanks Ray
Raymond C Mutchler
Service Manager
An externally hosted image should be here but it was not working when we last tested it.
Prior to that response, he also added this yesterday:
I looked at the traveler on this amp and the idle bias is 85 MV, Which I am pretty sure is correct for this vintage.
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More efficient output devices would simply mean a reduction in required rail voltage for the same max output. This would mean a bit of a reduction in idle heat wasted. It would still run hot as a class-a amp though, and even if there was zero bias (class-b) when pushing it there should be some heat generated.
Very strange.
Very strange.
Ray from Krell also stated the following to me:
Usually if the bias is not set right the sound is deteriorated.
I can attest that the amp sounds fabulous, which is why I am so confused. How can it sound great with no heat output?
It would be trivial to check the bias current with a voltmeter over the source resistor(s). Hook up the meter with alligator clips while the power is off, of course. There's no sense in hurting yourself or shorting your amplifier.
It is also trivial to adjust the bias current in these by turning a screw.
It is also trivial to adjust the bias current in these by turning a screw.
Can you send detailed instructions or pictures using a KSA 100 amp? What setting should the bias be set to? I guess I'm just wondering though if the boys at Krell did do it correct. As I have stated, the sound is great so there's no complaining there. Just no heat. Doesn't make sense, does it? Maybe Krell has figured out a way to get high power and very little heat output. Bass is earth shaking at LOUD volume levels, so I have lost nothing there and the highs are smooth and grain free. I really can't see screwing around with it, but thought the someone here on this site might understand what's going on. I don't know if you noticed, but in one of my other posts above, I pasted a quote from Krell that the service manager Ray believes that the idle bias was/is set for 85mV, which he stated is correct for this vintage amp. I don't know what that number translates to, but some of you tech guys here can probably elaborate.
But what of the sound? Don't they live by "nothing sounds better than true class A"? Did they come up with a way to not have the switching distortion heard? I mean, that's why they originally designed the class A amp, because it sounded better than an AB amp. I can't imagine why they would purposely turn the amp into an AB design. What, they suddenly are concerned that the amp might not go another 25 years? When you think about, any failure of the amp (years from now of course) would benefit them with the hopeful purchase of a newer model. This is very confusing to me.Less heat, longer life
Well, I just had another reply from Krell as to whether or not my refurbished KSA 100 was operating in Class A or AB mode. Here is his reply:
Sam, The unit is running as it should be in Class A operation, The new transistors are much faster If you increase the bias now with the new components you will raise the distortion level, We always adjust the bias for the lowest possible distortion rating.
If it were an A/B design the bias would be adjusted to about 7 mv at idle rather than 85mv.
Ray
Raymond C Mutchler
Service Manager
203-298-4022 | 203-799-9954 ext 344
Fax 203-795-2287
rmutchler@krellonline.com
Sam, The unit is running as it should be in Class A operation, The new transistors are much faster If you increase the bias now with the new components you will raise the distortion level, We always adjust the bias for the lowest possible distortion rating.
If it were an A/B design the bias would be adjusted to about 7 mv at idle rather than 85mv.
Ray
Raymond C Mutchler
Service Manager
203-298-4022 | 203-799-9954 ext 344
Fax 203-795-2287
rmutchler@krellonline.com
An externally hosted image should be here but it was not working when we last tested it.
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