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Old 27th February 2005, 10:17 PM   #981
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K-Amps, Thanks for doing those calcs! Both you and Stuart have been a big help and you've definately saved this set of output devices! You are right BUT I was ALSO running this thing at +/-43 volt rails, not the +/- 36 volt rails that you thought I was. Could you crunch that one again for me? Jeez, I think was in the KSA-100 realm there for a while but I have now lowered my rails to the normal +/- 36 volts. Your calcs are just about right on the money too! I am measuring 1.85 amps for 400 mv across the .68 ohm emiter resistors. Sink temp is down and device temp is down under 50 deg c. Now I feel better about this whole thing..... BTW: At the normal 630 mv and 36 volt rails as per the old Krell schematic from that other thread I was drawing slightly over 2.8 amps per rail! So is this new 400 mv setting because it is split up between three devices per side and not the normal two as was used?

Thanks!

Mark
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Old 27th February 2005, 10:23 PM   #982
K-amps is offline K-amps  United States
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Here u go Mark with 43 rails and also with regular KSA-50 (per your numbers).. Sorry guys about so much data being posted...

Class-A output Calculation: Krell KSA-50 Mk. II(43 rails and 520mV)

Inputs
Number of output devices: 3 Pairs
Voltage rails (per rail): 43 volts
Emitter resistance (per device): 0.68 ohms
Bias voltage per Emmiter resistor: 520 mv
Idle bias per device 764.7 mA
Speaker ohms 8 ohms

Results
Idle bias per device: 0.765 Amps
Total Amplifier bias (per rail)** 2.29 Amps
Total Amplifier bias (both rails) 4.59 Amps
Total Dissipation (per channel) 197.3 Watts
Dissipation per device pair at idle 32.9 watts
Class-A output: Peak 168.4 Watts peak
Class-A output: RMS 84.2 Watts RMS
Efficiency 42.68 %

Here with 400mV

Class-A output Calculation: Krell KSA-50 Mk. II

Inputs
Number of output devices: 3 Pairs
Voltage rails (per rail): 36 volts
Emitter resistance (per device): 0.68 ohms
Bias voltage per Emmiter resistor: 400 mv
Idle bias per device 588.2 mA
Speaker ohms 8 ohms

Results
Idle bias per device: 0.588 Amps
Total Amplifier bias (per rail)** 1.76 Amps
Total Amplifier bias (both rails) 3.53 Amps
Total Dissipation (per channel) 127.1 Watts
Dissipation per device pair at idle 21.2 watts
Class-A output: Peak 99.7 Watts peak
Class-A output: RMS 49.8 Watts RMS
Efficiency 39.22 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 73.70 Watts RMS


NORMAL KSA-50 Per Mark's numbers:


Class-A output Calculation: Krell KSA-50 Mk. II

Inputs
Number of output devices: 2 Pairs
Voltage rails (per rail): 36 volts
Emitter resistance (per device): 0.68 ohms
Bias voltage per Emmiter resistor: 630 mv
Idle bias per device 926.5 mA
Speaker ohms 8 ohms

Results
Idle bias per device: 0.926 Amps
Total Amplifier bias (per rail)** 1.85 Amps
Total Amplifier bias (both rails) 3.71 Amps
Total Dissipation (per channel) 133.4 Watts
Dissipation per device pair at idle 33.4 watts
Class-A output: Peak 109.9 Watts peak
Class-A output: RMS 54.9 Watts RMS
Efficiency 41.18 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 73.70 Watts RMS
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Old 27th February 2005, 10:30 PM   #983
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WOW! That was really fast! And I think these calcs will help others as well.... sorry I'm not an engineer type. I'm set up right now like your second set of calcs and the slo-speed fan has no problem keeping the sinks at about a reasonable 45 deg c. temperature. Seeing those calcs has made things very clear to me now. The amp also sounds very, very good set up this way....

Thanks again!

Mark

P.S. another thing I have noticed is that the driver stage temperature and the output stage as measured on the device cases appear to track each other really well showing that the bias transistor mounted on the driver stage hs works really well.
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Old 27th February 2005, 10:59 PM   #984
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Default output device current sharing...

...is pretty good, basically because we are using relatively large emitter resistors.

But if you keep total idle current the same and add more outputs, the voltage on each emitter resistor decreases proportionally, ie twice as many transistors gives half the voltage...

If it gets too low, like below 0.2v, the adjustment will become twitchy and the sharing between unmatched transistors will degrade. At that point you'd probably want to increase the value of the emitter resistors to the next value, ie from 0.5 to 0.68, or from 0.68 to 1.0, perhaps up to 1.2 or 1.5 if you are seriously increasing the number of outputs... like 8 or more pairs.

IIRC krell used 1ohm on the ksa100, not sure what the 200 had, but it would be interesting to find out, they must have had a lot of transistors in that beast...

Stuart
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Old 27th February 2005, 11:44 PM   #985
K-amps is offline K-amps  United States
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I also had a KSA-250 that I sold a couple of months ago ;-) It had 1 ohm emitters on all 12 pairs of metal cans. I measured 111mV across the 1ohm emmiters at idle... according to my calcs thats about 28.4 watts pure class-A.... umm it was supposed to be 250 watts class-A.... I guess they used an auto bias scheme.

Mark not a worry, I have a spread sheet and all I do is plug in 3-4 values

cheers!

BTW, Stuart is the expert here, I just try to be a good understudy as my business major did not teach me how to make amps ;-).

PPS: Of the 3 Krells I owned (KSA-100mk.ii, KSA-250 and a KAV-250) I like te KAV-250 THE best by far.... I cannot find the sound wanting in anyway... yes I know it is class-AB but still, the front end is pretty good. Not selling my KAV-250.
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Old 28th February 2005, 12:48 AM   #986
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Default Re: output device current sharing...

Quote:
Originally posted by Stuart Easson
[BIIRC krell used 1ohm on the ksa100, not sure what the 200 had, but it would be interesting to find out, they must have had a lot of transistors in that beast...
[/B]
One specimen of KMA200 : 2 ohm emiter devices. 8 devices per rail (16 in total). Rail voltage 67.5V. Bias 3.75A .
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Old 28th February 2005, 12:53 AM   #987
K-amps is offline K-amps  United States
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Thats 224 watts pure class-A (937 mV across the 2ohm ER) , none of that trick class-A as in later models.

Dissipation at idle over 500 watts.

I wonder what the unloaded rails were, guessing 3.6kVA trafo or so.
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Old 28th February 2005, 06:07 AM   #988
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Default Transformers

One last thing - I've found that big transformers (when loaded with lots of current demand from class-A output stage) tend to buzz. This is mechanical hum, and comes from saturated core (there's always little bit of residual DC in the mains).
I had to put in the mains DC filter (large electrolytic caps back to back in series with primary side, with safety diodes across). Leave some room in your boxes. I had to scramble things in mine because I made everything too tight.

And don't forget the inrush supressor. You will need it - trust me.

Bratislav
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Old 28th February 2005, 06:38 AM   #989
tmblack is offline tmblack  United Kingdom
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hi Bratislav,

What specifications are needed for the capacitors? They must heat up with all that current flowing.

Tom
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Old 28th February 2005, 07:23 AM   #990
NUTTTR is offline NUTTTR  Australia
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Default Inrush...

So - what is everyone using? I was going to not worry about it - as i have 800va toroids, 136,000mfd on each channel, so i'll just have 3 switches... switch on all at once it would trip the breaker, i'm sure!
Hmmm...
Time for more testing
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