Krell KSA 50 pcb

[K-amps]

Originally posted by PMik
Luke
Just use 0.2 Ohm value if you prefere to do that

I prefere 0.1 or 0.15 Ohm emitter resistors,



AuroraB

True class A for 50watt/8Ohm demands 3.5 Amps idle current

A more "human" approach is to go for 2.5 amps idle

Crunched some numbers:

Inputs
Number of output devices: 2 Pairs
Voltage rails (per rail): 45 volts
Emitter resistance (per device): 0.5 ohms
Bias voltage per Emmiter resistor: 450 mv
Idle bias per device 900.0 mA
Speaker ohms 8 ohms

Results
Idle bias per device: 0.900 Amps
Total Amplifier bias (per rail)** 1.80 Amps
Total Amplifier bias (both rails) 3.60 Amps
Total Dissipation (per channel) 162.0 Watts
Dissipation per device pair at idle 40.5 watts
Class-A output: Peak 103.7 Watts peak
Class-A output: RMS 51.8 Watts RMS
Efficiency 32.00 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 115.16 Watts RMS

Or if you want to use 0.22 ohm ER:

Number of output devices: 2 Pairs
Voltage rails (per rail): 45 volts
Emitter resistance (per device): 0.22 ohms
Bias voltage per Emmiter resistor: 195 mv
Idle bias per device 886.4 mA
Speaker ohms 8 ohms


Results
Idle bias per device: 0.886 Amps
Total Amplifier bias (per rail)** 1.77 Amps
Total Amplifier bias (both rails) 3.55 Amps
Total Dissipation (per channel) 159.5 Watts
Dissipation per device pair at idle 39.9 watts
Class-A output: Peak 100.6 Watts peak
Class-A output: RMS 50.3 Watts RMS
Efficiency 31.52 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 115.16 Watts RMS
Max Class-AB factor (100%=ideal load, Krell/565's = 91%, 555=70%) 91 %

[bremen nacht]

Originally posted by K-amps



Paralelling 0.1 is 0.05 ohms not 0.4 ;-)




Remember, many people still think the KSA-50 is one of Krell's best sounding and one of Stereophile's top 10 power amps of all time. ;-)

I would stick with the stock ER's unless you want to play around.

Your arithmetic is correct. Read my post again (and the one I replied to) and you'll see that mine is too.

What sort of resistors are you getting, Luke?

[bremen nacht]

Slap me if this is off-topic, but would it be sensible to reduce the supply voltage and increase the bias to suit lower impedance speakers, or just lower the voltage to reduce dissipation/cost?

Would the driver circuitry be happy with, for example, 30V rails? Or would it make sense arranging separate supplies for driver and output (45V and [whatever])?

Cloning this well-regarded and hugely expensive amp is worth doing, for sure, and I take K-amp's point that it's too lovely to change, but how down-rateable or tweakable do y'all think it could be?

[K-amps]

Originally posted by bremen nacht
Slap me if this is off-topic, but would it be sensible to reduce the supply voltage and increase the bias to suit lower impedance speakers, or just lower the voltage to reduce dissipation/cost?

Would the driver circuitry be happy with, for example, 30V rails? Or would it make sense arranging separate supplies for driver and output (45V and [whatever])?

Cloning this well-regarded and hugely expensive amp is worth doing, for sure, and I take K-amp's point that it's too lovely to change, but how down-rateable or tweakable do y'all think it could be?

I would tweak it somewhat. Like I would use newer devices with more stable gain. (the Mj2119X are good choices) But I would not assume that my mods will make an improvement, if it does, Good if not I go backcken.

Also I would probably not reduce the rails too much, after all headroom is another thing Krells are known for. But to your point, reducing rail might make the sound a little less harsh maybe.

with 30v rails (I am assuming, fully loaded rails the are 30v not 30v unloaded) you will get a max of 50 watts with nada headroom. You will also need a huge Toroid. Just a few things to consider.

In the end it may depend on what parts you have scavenged.

[bremen nacht]

Originally posted by K-amps


with 30v rails (I am assuming, fully loaded rails the are 30v not 30v unloaded) you will get a max of 50 watts with nada headroom. You will also need a huge Toroid.

Not as huge as the one necessary for a clone, though!

[K-amps]

Originally posted by bremen nacht


Not as huge as the one necessary for a clone, though!

[Jozua]

Hi

My personal preference would be to design and build the amp as close to the original spec. If possible with better parts but definitely not as a downgraded version.


Jozua

[AuroraB]

30V supply is too low, - 50W into 8 ohms is 28,2V peak. 35 V is minimum in my opinion, but then of course idle dissipation increases as supply goes up.

K-amps... your calcs are OK, but the combined 1.8-2 A idle will flow through both halves... theoretical minimum idle will be 2,5A for 50W into 8 ohms in true class A, which makes my toroids just about good for a 50W amp, as the trafo is spec'ed at full load. Obviously, a larger trafo would make for a stiffer supply voltage, but at a cost,- price....
Maybe the frontand should be ragulated ?? any thoughts ??

ACD, - which design voltage was used for the driver section ??? Are the component values true to the original, except for the transistors ??

[bremen nacht]

Originally posted by AuroraB
30V supply is too low, - 50W into 8 ohms is 28,2V peak. 35 V is minimum in my opinion, but then of course idle dissipation increases as supply goes up.

K-amps... your calcs are OK, but the combined 1.8-2 A idle will flow through both halves... theoretical minimum idle will be 2,5A for 50W into 8 ohms in true class A, which makes my toroids just about good for a 50W amp, as the trafo is spec'ed at full load. Obviously, a larger trafo would make for a stiffer supply voltage, but at a cost,- price....
Maybe the frontand should be ragulated ?? any thoughts ??

ACD, - which design voltage was used for the driver section ??? Are the component values true to the original, except for the transistors ??

I wasn't hoping to get 50W out of a reduced supply voltage, rather to get similar performance but at a lower power.

By the way, there are a couple of inconsistencies in the schematic/layout:
-on the layout Q108 is labelled Q106
-notes on schematic state that Q107 and 108 must be fitted with suitable heatsink. This should read 'Q109 and Q110'. What thermal rating is required?

[AuroraB]

Ok, Bremen......see your point, but just lowering the supply will influnce the behaviour of the front end/driver section...depending in its design voltage.
Thats the reason for my last question.