Krell KSA 50 pcb - Page 843 - diyAudio
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Old 14th February 2013, 04:22 AM   #8421
henryve is offline henryve  South Africa
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BTW, you guys are talking about Rb, the base resistors on the output trannies. You may actually also use a jumper there. Rb is only needed if you have some instability. Does everything work if you disconnect the output boards? You do know that you can test the main + driver boards into an actual load? Once that is proven, connect the outputs and bias.
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Old 14th February 2013, 04:25 AM   #8422
john65b is offline john65b  United States
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Hey, just realized something you said....

Quote:
I am using a variac to get +-15V,
I think you will have to get the voltage rails higher - up to 30 - 35V - before you will be able to bias...
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Old 14th February 2013, 04:32 AM   #8423
henryve is offline henryve  South Africa
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Also, I see from the picture that you only take your feedback from the one output board. I think you must get that from both. Connect the output point from both output boards together and then take the FB from there.

PS: Sorry, I see you did. I missed that orange wire. Early morning and not quite awake.

Last edited by henryve; 14th February 2013 at 04:35 AM.
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Old 14th February 2013, 04:43 AM   #8424
henryve is offline henryve  South Africa
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My suggestion: Disconnect the wires to the output boards from the driver board. Then check to see that you have around 0mV on the FB point, -550mV to -600mV on the negative drive point and 550mV to 600mV on the positive drive point. Make sure that the values vary correctly when you adjust the offset pot and bias pot. You can connect a speaker between the FB point and Ground and listen to music at quite respectable levels.
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Old 14th February 2013, 04:10 PM   #8425
alibear is offline alibear  United Kingdom
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Hi all, I am building my KSA50 using a layout the same as the original KSA100 with two heatsink towers, with the PCB's attached to the towers. I am using Jan's PCB's and I have quite limited space between the filter capacitors and the heatsink towers making it very difficult to adjust offset and bias pots.
Will there be a problem if I remove the pots from the board and position them where they can be easily adjusted, this will probably mean that they are connected to the boards with about 100mm of wire.
Will this lead to any detrement in performance?
Thanks
Alan
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Old 15th February 2013, 04:43 AM   #8426
toufu is offline toufu  United States
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Quote:
Originally Posted by henryve View Post
My suggestion: Disconnect the wires to the output boards from the driver board. Then check to see that you have around 0mV on the FB point, -550mV to -600mV on the negative drive point and 550mV to 600mV on the positive drive point. Make sure that the values vary correctly when you adjust the offset pot and bias pot. You can connect a speaker between the FB point and Ground and listen to music at quite respectable levels.
Thanks for the help, disconnected the output boards, running at 1/4 rail around 10v using a variac, I am getting close to rail voltage off the FB, D+ and D- relative go ground (around 7-8v), is something seriesly wrong here? Did I fry a mosfet or one of the little transistors?
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Old 15th February 2013, 04:48 AM   #8427
john65b is offline john65b  United States
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Try raising the variac voltage...i think the rails need to be higher for amp to bias up
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Old 15th February 2013, 05:28 AM   #8428
henryve is offline henryve  South Africa
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Also make sure all of your ground connections are correct. I had a similar type of result the first time I powered the board up, just because my signal ground was not connected. Also, short the signal input to signal ground when you do this. And crank that variac up else you will get strange results as the bias voltages might not be high enough to switch them small critters on.
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Old 15th February 2013, 10:16 AM   #8429
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What he said.

IIRC, it doesn't settle down 'till about 18-20V. Yours may vary a bit depending on component tolerances.

edit: Thinking about it, I wouldn't, (and didn't) use a variac, just the normal PSU with either a light bulb tester or a couple of 100 Ohm resistors in the rails.
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Old 16th February 2013, 06:23 PM   #8430
alibear is offline alibear  United Kingdom
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Hi all, any thoughts about my post 8425 ?
Thanks
Alan
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