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Old 4th December 2007, 07:38 PM   #71
dinu is offline dinu  United States
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Wow, Glaciers move faster than I do.
Amazing how moving and getting married will slow things down. Well, now I have a proper shop and have picked this up again for the last piece of the puzzle. The front end works great after a couple of tweaks. For anyone using these boards please note that P3 and P2 are reversed. You have to start them in the full clockwise position. Lastly, there is an eronious connection in the board itself. If you look at my schematic, I mistakenly connected the junction of R3 and R27 to the -Drv pin. Problem!
I had to cut a little hole in the board between R23 and R24 to disconnect this trace and then repositioned R26 on the board and it all works.
Another thing to note is that, unlike the original boards, Z3 and Z4 do not have a separate connection to ground and you'll see a crossover notch at the output when setting up the front end if R81 is installed. I suggest putting off installation of either R81 or the zeners until you're ready to plug into the output stage. I set up the front end with the zeners installed and no R81. You can pick up the output from the R23/24 junction.
So all that aside, I bought a really nice chassis from Hifi 2000. I got tired of metal working and this is much nicer than anything I could have made even though I have an actual work bench now.
Finally, I need to get an output power transformer into this monster. I planned on 65v rails so the transformer of choice would have been a 45v. I found a gorgeous 2.4Kva unit unfortunately, the closest I can get to my planned 45V is about 54. That would give me 76v rails. Do any of you know a good way to drop about 10v? Maybe a high current regulator. Maybe I don't need to drop the voltage. Maybe the best choice would be to just drop the quiescent current a bit and call it a day.
Any suggestions?
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Old 5th January 2008, 04:53 AM   #72
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Dinu,



"I mistakenly connected the junction of R3 and R27 to the -Drv pin."

Is this the error?
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Old 5th January 2008, 04:59 AM   #73
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Would it be possible to show the modification you made to the board or maybe a picture. When I get around to building this, I want to make sure I get it right. Thank You.

John
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Old 7th January 2008, 12:58 PM   #74
dinu is offline dinu  United States
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Mega,
That's it. You have to cut a hole in the board as shown in the attached pic to disconnect the junction of R3R27 from -Drv. R26 is now in the wrong position so I just soldered it to the leads of R24 and R12 as shown.
VERY IMPORTANT. You can not install R81 in it's marked board position because now you are essentially connecting it from -drv to output. I plan to install R81 as a 3 position switch that has leads going to the board attached to resistor leads.
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Old 15th January 2008, 02:22 PM   #75
dinu is offline dinu  United States
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The whole amp is assembled and I tried connecting the output stage this weekend. Everything seems to work except that there is ALOT of gain. The amp clips (65v p-p) with only about 300mV of input. So, of course, there's alot of noise on the output. What am I doing wrong?
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Old 15th January 2008, 06:14 PM   #76
eLarson is offline eLarson  United States
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Are the feedback resistors (I can't remember their schematic designations) what you think they are?
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Old 15th January 2008, 06:23 PM   #77
dinu is offline dinu  United States
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http://i125.photobucket.com/albums/p...RSCHEMATIC.jpg
Here's the schematic again. I have tried R81 open and closed and the results are the same. I am wondering if the servo resistors R31 and 33 are having an effect on the output impedence. Also, I made R25 and R26 20K instead of 10K just to keep dissipation down a bit because I have 80v rails on the front end. Are they considered part of the output impedence?
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