with max UK of 254 you will get ~42Vacafter subtracting a single bridge rectifier you get <=59.1Vdc on the caps. 63V is OK.
You are more likely to use a dual bridge on your dual secondary and that will bring you down another half voltage on no load.
Don't add 10%. Use worst case values and ensure the caps are rated for that worst case voltage, or other worst case scenarios.
A KSA50 with a 2pair 250W each output stage running on +-50Vdc is going to be running mighty hot.
50W of ClassA requires Ib (total) of 1.9A.
Your 4devices will have to dissipate 190W.
Whereas +-34Vdc and the same 1.9A of bias requires a dissipation for the 4devices of 130W.
I would be tempted to use a 3pair of 250W To3 devices for the higher supply voltage. If you go to 200W or 230W plastic packaged To264 devices, then 4pair are probably required.
50W of ClassA requires Ib (total) of 1.9A.
Your 4devices will have to dissipate 190W.
Whereas +-34Vdc and the same 1.9A of bias requires a dissipation for the 4devices of 130W.
I would be tempted to use a 3pair of 250W To3 devices for the higher supply voltage. If you go to 200W or 230W plastic packaged To264 devices, then 4pair are probably required.
Andrew, KSA50/KSA50mk2 (after '83) is under biased. Quiescent current is only cca 500mA per device. Checked on several amps. The only thing I do not know whether this is a later factory tweaking or original setting (not first owners) 🙄
if your data is correct then the low biased KSA50mk2 is only 16W ClassA amplifier.
That hardly fits with the specification we expect of the ClassA range of Krells.
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My ksa-50 (with +/-47V rails) is underbiased as well (550mA per device iirc)
Last time it was adjusted, it was done by a krell-approved service-center.
The air exiting the cooling-tunnel is ~70degrees Celcius.
The outputs are blistering hot under these conditions,
and i'm not even slightly tempted to see what would happen if i would bias it to full ClassA.
Last time it was adjusted, it was done by a krell-approved service-center.
The air exiting the cooling-tunnel is ~70degrees Celcius.
The outputs are blistering hot under these conditions,
and i'm not even slightly tempted to see what would happen if i would bias it to full ClassA.
Hi,
if you want to build a KSA50 Klone that is 50W ClassA then you cannot operate with less than 1.9A of total output device bias.
If you choose to run at high supply rail voltage, eg. +-47Vdc to +-50Vdc, then a 2pair output will not survive very long.
Is it a ClassA amplifier you want to build?
Decide.
if you want to build a KSA50 Klone that is 50W ClassA then you cannot operate with less than 1.9A of total output device bias.
If you choose to run at high supply rail voltage, eg. +-47Vdc to +-50Vdc, then a 2pair output will not survive very long.
Is it a ClassA amplifier you want to build?
Decide.
Try LCRC on the power supply.
The Inductor in front will get you .9VAC instead of 1.414VAC, so for a trany with 40VAC on Secondary, you will have 36VDC...perfect for the pair of outputs.
Class A amp (with steady high-ish bias to satifsfy L-crit) should lend itself well for a LCRC power supply, no?
LCRC filtered Power supplies are supposedly cleaner anyway...
The Inductor in front will get you .9VAC instead of 1.414VAC, so for a trany with 40VAC on Secondary, you will have 36VDC...perfect for the pair of outputs.
Class A amp (with steady high-ish bias to satifsfy L-crit) should lend itself well for a LCRC power supply, no?
LCRC filtered Power supplies are supposedly cleaner anyway...
16W Class A 😕
I don't want just only 16W class A power, thats sounds, well, limp.
So I have to lower the voltage and increase the output bias to 2A?
Or add another output pair and have even more class A power ?
The heatsink tunnel only has space for a total of eight output devices, TO-3 cases.
I want to use the original avel lindberg trafos and the original heatsink, so I'll need to lower the voltage. I have the following choices :
1) Use an inductor in front of the first cap
2) Use a 12V 120VA transformer to buck the voltage down by 12V
I already have a pair of suitable 120VA 12V trafos, so I'll do that.
It'll also mean I can use 50V caps 😎
I wonder why Krell decided to up the voltage and under biased the later ones ???
Regards
fs
Firestorm,
KSA50 is modestly rated as an 50W amplifier to maintain doubling output power at lower impedances:
50W/8ohm
100W/4ohm
200W/2ohm
400W/1ohm
However, the output power at 8 ohm is far greater that its name suggests.
Those who have played with bias adjustment said that there was no significant difference in sound between the lower and higher bias setting.
Maybe this is the answer to your question...more power
Regards
KSA50 is modestly rated as an 50W amplifier to maintain doubling output power at lower impedances:
50W/8ohm
100W/4ohm
200W/2ohm
400W/1ohm
However, the output power at 8 ohm is far greater that its name suggests.
Those who have played with bias adjustment said that there was no significant difference in sound between the lower and higher bias setting.
Maybe this is the answer to your question...more power
Regards
Neychi,
I'm sure there will be plenty of power, whether its biased at 1A or 2A.
I would want it to stay in class a for as long as possible, so I would want to bias it high. My speakers are rated at 4R, Magneplanar SMGa. So even at 2A bias, I will get 25W class A power, so with the 1A per output pair, I will be looking at 8W class A ??? Sounds more like a 300B SET, than a mercan heavy duty class A amp.
Now whether it will actually have a negative effect on the SQ, I'm not so sure.
So should I lower the B+ and run higher bias or leave it be ?
Lowering the B+ means that I can use 50V caps <woohoo>, but I may just get 63V caps to allow me the choice.
I'm sure there will be plenty of power, whether its biased at 1A or 2A.
I would want it to stay in class a for as long as possible, so I would want to bias it high. My speakers are rated at 4R, Magneplanar SMGa. So even at 2A bias, I will get 25W class A power, so with the 1A per output pair, I will be looking at 8W class A ??? Sounds more like a 300B SET, than a mercan heavy duty class A amp.
Now whether it will actually have a negative effect on the SQ, I'm not so sure.
So should I lower the B+ and run higher bias or leave it be ?
Lowering the B+ means that I can use 50V caps <woohoo>, but I may just get 63V caps to allow me the choice.
Hi KV,
have you heard any of the high bias Krell amps ?
I'm trying to decide whether I should lower the B+ voltage and run higher bias.
Cheers
Firestorm,
I made a clone with a high bias and 36V DC rails, now I am doing another KSA50.2 with lower bias and 48V DC rails.
Wait and I let you know about the differences😀
Regards
I made a clone with a high bias and 36V DC rails, now I am doing another KSA50.2 with lower bias and 48V DC rails.
Wait and I let you know about the differences😀
Regards
Higher voltage, lower bias will get you a great sounding 150 WPC class AB Krell, but I would rather have a 50 WPC class A Krell instead. I bet neyichi will like the Class A much better.
I love my Krell clone.
I love my Krell clone.
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Lower bias, at 200mA and supply rails of +-55VDC, much more powerful and dynamic, sounds better.
Hi guy's,
I made a small resume of this thread, maybe it can help answer some questions?
Also, there are useful information here (maybe you guy's know already😕)
Regards,
Eric
diyAudio.com Wiki - projects by fanatics, for fanatics
I made a small resume of this thread, maybe it can help answer some questions?
Also, there are useful information here (maybe you guy's know already😕)
Regards,
Eric
diyAudio.com Wiki - projects by fanatics, for fanatics
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Nice work.
PS. the old Wiki is still available, don't need to used the archive.
Krell KSA-50 Building Wiki
PS. the old Wiki is still available, don't need to used the archive.
Krell KSA-50 Building Wiki
Hi,
No i havent heard any of them.
I can assure you that, even with a lowly 16w ClassA, limp is hardly a word that appropriately describes the sound.
Powerful, big soundstage with a generous bottom sounds more like it.
I dont know whether your proposed arrangement with 2 bucking trafos to lower the rails will work properly, or cripple the powersupply.
I strongly suggest to use at least 4 pairs of outputs (especially when using plastic devices) if you decide to go for a 'fully biased' ksa-50 with lower rails.
You still have to get rid of ~260 watts of heat instead of 206watts on the same heatsink, and running it blistering hot leaves little room for error.
Attached is an Excel-worksheet that was posted much earlier in this thread,
to help you calculate the amount of bias vs. ClassA power, generated heat etc.
Best regards,
Klaas
Hi Klaas
Thanks for your reply.
I would like to keep as close to the original KSA50 as possible. I have most of the parts from a real KSA-50mk2. With a mains ac of 243V, I measured the avel lindberg trafo of 2 x 40.3V. Various people including Neychi and Pierre have told me there have been various versions of the KSA-50 the earlier KSA50 with +-37V and the later versions with +-55V. The earlier versions had higher bias, approximately 2A versus 1A.
I plan to use MJ15003/4, 2 pairs per channel. They are TO-3 and are rated for 250W dissipation each. Of course that is with perfect heatsinks, so I will need to derate appropriately. The spreadsheet is very useful for this.
To be honest, I don't mind if I have more or less class a power, I would like to choose the one that sounds better. There seems to be differing opinions on this. My speakers are 4R impedance Magneplanar SMGa. I understand lower impedance load will mean that the amplifier will cross to class B a lot sooner. Also it is my understanding that when an AB class amplifier crosses, it produces more distortion than a class b amp. That is my rationale for optimising the class a power.
So do I go for high bias or low bias ? My gut feeling is to lower the rail voltage, either by bucking or CLC arrangement, I can adjust the value of the first C which will tune the voltage. I have used the bucking method successfully in the past, but not in a high current situation like this.
Regards
fs
Thanks for your reply.
I would like to keep as close to the original KSA50 as possible. I have most of the parts from a real KSA-50mk2. With a mains ac of 243V, I measured the avel lindberg trafo of 2 x 40.3V. Various people including Neychi and Pierre have told me there have been various versions of the KSA-50 the earlier KSA50 with +-37V and the later versions with +-55V. The earlier versions had higher bias, approximately 2A versus 1A.
I plan to use MJ15003/4, 2 pairs per channel. They are TO-3 and are rated for 250W dissipation each. Of course that is with perfect heatsinks, so I will need to derate appropriately. The spreadsheet is very useful for this.
To be honest, I don't mind if I have more or less class a power, I would like to choose the one that sounds better. There seems to be differing opinions on this. My speakers are 4R impedance Magneplanar SMGa. I understand lower impedance load will mean that the amplifier will cross to class B a lot sooner. Also it is my understanding that when an AB class amplifier crosses, it produces more distortion than a class b amp. That is my rationale for optimising the class a power.
So do I go for high bias or low bias ? My gut feeling is to lower the rail voltage, either by bucking or CLC arrangement, I can adjust the value of the first C which will tune the voltage. I have used the bucking method successfully in the past, but not in a high current situation like this.
Regards
fs