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Old 2nd February 2006, 12:44 AM   #1
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Default P1.7 questions

What wattage do resistors R-44, 55, 59, and 70 need to be. I'd like to place metal films there but am limited to 1/2 watt in size at the local surplus place...... Thanks for any help on this.....

Mark
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Old 2nd February 2006, 01:01 AM   #2
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Even 1/4W is enough, acc to 33*(0.7/33)^2 and 22*(0.7/22)^2.

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Old 2nd February 2006, 02:59 AM   #3
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Are heatsinks needed on the devices?
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Old 2nd February 2006, 05:00 AM   #4
kmj is offline kmj  Sweden
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Mark, could you post a link to the servicemanual or partslist if you got one?

About the heatsinks, the pictures of other 1.7-projects I've seen has used some smaller heatsinks. That's all I know
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Old 2nd February 2006, 05:48 AM   #5
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As far as I see, the Vds of Q15 is about 30V and the bias is with about 30mA so that power dissipation = 30 x 0.03 = 1W.

I would summarize power dissipation as follows:

Q15 and Q18: 1W each
Q16, Q17, Q19 and Q20: close to 1W each

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Old 2nd February 2006, 06:44 AM   #6
Nisbeth is offline Nisbeth  Denmark
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One more question: From looking at the schematic, it seems that both the input and output caps could be a lot smaller than the recommended 10uF/30uF. I've tried calculating the input and output impedances but I'm not sure I have it right . What's the minimum value for these caps?


/U.
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Old 2nd February 2006, 11:52 PM   #7
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So I assume that 1 watt is ok without sinks on these devices. From photos of others it looks like some use them and some do not.

BTW: Thanks for all the help here!

Mark
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Old 3rd February 2006, 01:35 AM   #8
MikeW is offline MikeW  United States
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Default Screw up

I did screw up on some of the resister sizes, sorry. Better to be to big that not big enough. If you let that pre-amp run for a while you'll see why there are heatsinks on the MOSFETs.

It was only my second pre-amp project. The next one will be better.
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Old 3rd February 2006, 04:39 AM   #9
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Mike,

Really... I'm not complaining? I'm very grateful that you sent it. There is not a thing wrong with the boards or anything to complain about at all! If you had more boards I'd love to have several to build up for other people. I decided to take this one to full balanced operation. Had yet to run my 2s in balanced mode at all..... In order to do this there was some re-work that had to be done on both boards. And at the same time I made all the resistors MF's and changed a few others to coencede with the factory version. The ones that I installed were basically the same wattage as yours were...... Regarding the sinks... Some have them and some don't... which has me a bit confused....... So whats up here? Would seem to me that these devices can dissipate a watt free air... or can't they..... I left the sinks on the regulators. If they all need to be on a sink I would consider taking the time to machine a bar of alumnium to thermally connect them all together.

BTW: you ought to hear it in balanced mode

Thanks!

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