Proper KSA-50 bias setting???

Mark Allen · 22nd January 2011, 08:34 PM

With out wading through all 10 gazillion posts in the KSA-50 thread can someone tell me the proper bias setting for a KSA-50 with three sets of OP decvices? I had to repair mine due to a slight DC imbalance that existed... found a pre-driver that was way out of spec. and even the DC balance adjustment could not compensate for it. I replaced both predrivers with a matched pair and now it is spot on again. At the present I am measuring ~360mv across the emitter resistors on both channels. Is this correct, or good enough for now? I normally like to do bias by using the distortion analyzer but moine is not set up at the moment and although that works well for class AB amplifiers the distortion analyzer method may not work as well for this amp...

Thanks!
Mark

pinkmouse · 22nd January 2011, 09:03 PM

Mark, mine runs at 300mV, but I don't push the levels.

Mark Allen · 23rd January 2011, 12:03 AM

I donno Al, That does seem a tad on the low side. Krell originally set them to 620mv if you had 4 devices with .68 ohm resistors. I had the program K-Amps wrote to calculate it based on the number of OP devices. Ijust have to find it. We may both be running in class AB at the moment...

Mark

Zen Mod · 23rd January 2011, 12:08 AM

.

Mark Allen · 23rd January 2011, 01:08 AM

Thanks for that file Zen! Accorsing to the calculator I need to be at 400mv across the .68 ohm emitter resistors to get 50 watts class A out of this amp... Now... do I have .68 ohm resistirs in there? Guess I best take the cover back off and see...

3 Pairs
39 volts
0.68 ohms
400 mv
588.2 mA
8 ohms

0.588 Amps
1.76 Amps
3.53 Amps
137.6 Watts
22.9 watts
99.7 Watts peak
49.8 Watts RMS
36.20 %
76.04 Watts RMS
80 %

Nelson Pass · 23rd January 2011, 04:46 AM

50 watts into 8 is 100 watts peak.

I^2R=100 watts pk
I^2 = 12.5
I= 3.53A pk
I/2 = 1.75 A idle
1.76/3 = .586 A per device idle current
across 0.68 = .399V

looks like your just on it.

Mark Allen · 25th January 2011, 08:47 PM

Thanks Nelson for confirming that! I appreciate your input on this. I have at present set the bias to 360 mv since my heat sinking is limited on this amp. I have two 120 volt fans wired in series and this keep the sinks at a comfortable temperature. If I go up to 400mv I will have to re-wire the fans in parallel and put up with the noise. Perhaps some sort of thermostatically controled variable speed cooling system is in order for this...

Thanks all!

Mark

pinkmouse · 25th January 2011, 09:07 PM

Quote:

Originally Posted by Mark Allen

...Perhaps some sort of thermostatically controled variable speed cooling system is in order for this...

I had one of those.Then I realised that the fans run faster with quiet albums, (Class A - Doh!!), So now I just listen to noisy stuff with it.

22nd January 2011, 08:34 PM	#1
Mark Allen diyAudio Member Join Date: Jan 2008 Location: Bountiful, You-Taw	Proper KSA-50 bias setting??? With out wading through all 10 gazillion posts in the KSA-50 thread can someone tell me the proper bias setting for a KSA-50 with three sets of OP decvices? I had to repair mine due to a slight DC imbalance that existed... found a pre-driver that was way out of spec. and even the DC balance adjustment could not compensate for it. I replaced both predrivers with a matched pair and now it is spot on again. At the present I am measuring ~360mv across the emitter resistors on both channels. Is this correct, or good enough for now? I normally like to do bias by using the distortion analyzer but moine is not set up at the moment and although that works well for class AB amplifiers the distortion analyzer method may not work as well for this amp... Thanks! Mark

22nd January 2011, 09:03 PM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Mark, mine runs at 300mV, but I don't push the levels. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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23rd January 2011, 12:03 AM	#3
Mark Allen diyAudio Member Join Date: Jan 2008 Location: Bountiful, You-Taw	I donno Al, That does seem a tad on the low side. Krell originally set them to 620mv if you had 4 devices with .68 ohm resistors. I had the program K-Amps wrote to calculate it based on the number of OP devices. Ijust have to find it. We may both be running in class AB at the moment... Mark

23rd January 2011, 01:08 AM	#5
Mark Allen diyAudio Member Join Date: Jan 2008 Location: Bountiful, You-Taw	Thanks for that file Zen! Accorsing to the calculator I need to be at 400mv across the .68 ohm emitter resistors to get 50 watts class A out of this amp... Now... do I have .68 ohm resistirs in there? Guess I best take the cover back off and see... 3 Pairs 39 volts 0.68 ohms 400 mv 588.2 mA 8 ohms 0.588 Amps 1.76 Amps 3.53 Amps 137.6 Watts 22.9 watts 99.7 Watts peak 49.8 Watts RMS 36.20 % 76.04 Watts RMS 80 %

23rd January 2011, 04:46 AM	#6
Nelson Pass The one and only Join Date: Mar 2001	50 watts into 8 is 100 watts peak. I^2R=100 watts pk I^2 = 12.5 I= 3.53A pk I/2 = 1.75 A idle 1.76/3 = .586 A per device idle current across 0.68 = .399V looks like your just on it.

25th January 2011, 08:47 PM	#7
Mark Allen diyAudio Member Join Date: Jan 2008 Location: Bountiful, You-Taw	Thanks Nelson for confirming that! I appreciate your input on this. I have at present set the bias to 360 mv since my heat sinking is limited on this amp. I have two 120 volt fans wired in series and this keep the sinks at a comfortable temperature. If I go up to 400mv I will have to re-wire the fans in parallel and put up with the noise. Perhaps some sort of thermostatically controled variable speed cooling system is in order for this... Thanks all! Mark

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