Proper KSA-50 bias setting???

[Mark Allen]

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]

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

 

[Mark Allen]

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]

.

 

[Mark Allen]

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]

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]

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]

...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.

 

[tonimxp]

krell KSA50 3 out devices

Hi guys this is my current situation.

Class-A output Calculation:

Inputs
Number of output devices: 3 Pairs
Voltage rails (per rail): 48 volts
Emitter resistance (per device): 0,5 ohms
Bias voltage per Emmiter resistor: 300 mv
Idle bias per device 600,0 mA
Speaker ohms 8 ohms

Results
Idle bias per device: 0,600 Amps
Total Amplifier bias (per rail)** 1,80 Amps
Total Amplifier bias (both rails) 3,60 Amps
Total Dissipation (per channel) 172,8 Watts
Dissipation per device pair at idle 28,8 watts
Class-A output: Peak 103,7 Watts peak
Class-A output: RMS 51,8 Watts RMS
Efficiency 30,00 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 115,19 Watts RMS

Max Class-AB factor (100%=ideal load, Krell/565's = 91%, 555=70% regulation of rails at max power) 80 %
** A.K.A total bias.

Strangely on my CRC PSU filter I have 0.130mV on 0.1R which means 1.3A and not 1.8A.
In addition, I also measured the temperature on the heatsink and it is 46 ° with 20 ° of ambient temperature after 1 hour and 15 minutes from switching on.
Do you think everything is ok?
Any advice is welcome

 

[Penki76]

Hi
I honestly not understand that calculation. Why if I put lower speaker impedance getting lower power , should be opposite. Any explanation?
Best regards

 

[llwhtt]

All of the calculations are for an 8 Ohm load. You will get more power with 4 Ohms but once the current exceeds the level set for 8 Ohms it will move out of Class A.

Craig

 

[Penki76]

My speakers are 6 ohm . My amp have 25 watts on 8 ohms , 50 watts on 4 ohms . It have 2 pairs of power transistors and I cannot set it to 620 mA due to temperature . Best is around 570 mA. Then how to calculate?

 

[analog_sa]

My speakers are 6 ohm . My amp have 25 watts on 8 ohms , 50 watts on 4 ohms . It have 2 pairs of power transistors and I cannot set it to 620 mA due to temperature . Best is around 570 mA. Then how to calculate?

570mA per transistor?

15.6W RMS/6ohms

 

[K-amps]

My speakers are 6 ohm . My amp have 25 watts on 8 ohms , 50 watts on 4 ohms . It have 2 pairs of power transistors and I cannot set it to 620 mA due to temperature . Best is around 570 mA. Then how to calculate?

The spreadsheet works as long as you input the correct impedance number. The thing you could be struggling with is what while lower impedance, gives more power, this power is usually in the Class-AB realm. For pure class-A, you will need more bias to maintain class A into lower loads.

 

[Penki76]

Thank you guys for the answers
Hope you all well .
Been douing some changes since outbreak and i did some improvements in my ksa 50.
Change ransformer for 2x 500VA 2x 30VAC each, which gives 42 VDC per rail.
2x rectifiers per chanel with 2x 47000 uf caps
new case with 400 x 200 x 5 cm radiators almost 4kg per side

now i can set bias
900 mA per device
this is over 151 watts in heat
and 51.8 watts rms
heat sinks temps are 55 C to 70 C depends on ambient temps

I have boards from ebay HM2S 50watts class a , they nothing special but that sounds amazing with my setup.

Only I wonder is it needed that big bias , because found in web factory sometimes set bias lower bias and making as a 16 watts amps