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Old 2nd April 2015, 11:07 PM   #1
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Default Cap upgrade for Borbely DC100

DC100 amp that I built in 1991 from an Old Colony kit. A few years ago I changed out the main P/S caps after the hum coming through the speakers became audible from my listening chair. The original caps were 8 of the large screw terminal Sprague 10,000 uf. I used 8 Cornel Dublier 10,000 uf snap-in style on 2 filter PCB's. I did not bother yet to do the other 'lytic caps on the boards or the ones point-to-point soldered near the output devices. They are an unknown model from Rubycon 47 uf and 100 uf 100v. What would be a good replacement for those? And any chance someone has the schematic for this amp? I lost the original documentation.

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Old 3rd April 2015, 04:54 AM   #2
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I would recommend much much more capacitance, if you can swing it and your transformer is capable.

Also the ground should be at a single point, preferably at the actual electrical center of the ground connections of the cap bank - all grounds should go to that spot, or more properly to a suitably high current tab or solder point connected there.

In your original layout, I'd have turned the two cap banks 90 degrees and put the ground center between the two ground planes.

You can likely safely leave the caps near the output devices.

The amp will benefit from the replacement of the output zobel coil with one done up from
substantially heavier wire - this requires some "drilling and blasting" on the PCB, as you will not be able to fit it into the same two holes - one, not two. Number 10wire is a good bet. You can get away with a few less turns.

If you are unable to find the schematic online, I may be able to find one somewhere on a computer around here. The amp was an article in Audio Amateur, so that is one way to find it.
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Old 3rd April 2015, 08:15 AM   #3
AndrewT is offline AndrewT  Scotland
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4 10mF to each amp seems OK to me.
That gives +-20mF per channel and is good for bass down to 20Hz.

Bear,
why do you suggest
Quote:
I would recommend much much more capacitance,
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Old 3rd April 2015, 11:33 AM   #4
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Unless you want multi-hundred watts at low (2-4ohms) impedances, the values you are using now are more than adequate. Anyway, your transfo will limit the charge current, so increasing cap values beyond that makes not much sense. If you want to gild the lily, you might try to replace your rectifiers with ultrafast recovery ones,(NTE 597, BYW 29-200, etc.) and bypass the main filter caps with a few uF's worth of good quality film types (MKP or MKC).
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Old 3rd April 2015, 02:02 PM   #5
GaryB is offline GaryB  United States
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Quote:
Originally Posted by Heathkit View Post
And any chance someone has the schematic for
Here you go.
---Gary
Attached Images
File Type: jpg dc100schem.jpg (389.6 KB, 105 views)
File Type: jpg dc100_outputs.jpg (209.6 KB, 105 views)
File Type: jpg dc100parts.jpg (575.1 KB, 102 views)
File Type: jpg dc100boards.jpg (914.7 KB, 47 views)
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Old 3rd April 2015, 02:25 PM   #6
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Thank you all for looking at this.

Gary, much appreciated! I could not find them anywhere on the Internet.

Dragonweed, I'm now using MUR1520 diodes in the rectifier bridges, and a CRC P/S filter arangement:

Click the image to open in full size.

Bear, I was planning to up the capacitance to 4 x 33,000 uf Epcos since now I'm using 1000 VA transformer instead of 2 wimpy 225 VA torroids. So, no longer a dual mono setup, but either channel now has access to 1000VA instead of 225. It has more "authority" now.

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Old 3rd April 2015, 03:24 PM   #7
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If you want to increase the filter bank as you indicated, with THAT transfo, (isn't it a bit too close to your driver board? Hum, stray field?)) then you will likely need an inrush current limiter unless you want to reset your home's circuit breakers every time you turn it on....
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Old 3rd April 2015, 03:31 PM   #8
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Andrew, those caps are at the output devices...

...it is safe to leave them alone.

Andrew, you asked why more total capacitance?
It be more bettah.
Click the image to open in full size.
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Last edited by bear; 3rd April 2015 at 03:43 PM.
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Old 3rd April 2015, 07:03 PM   #9
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Quote:
Originally Posted by dragonweed View Post
If you want to increase the filter bank as you indicated, with THAT transfo, (isn't it a bit too close to your driver board? Hum, stray field?)) then you will likely need an inrush current limiter unless you want to reset your home's circuit breakers every time you turn it on....
I installed a soft-start board when I did the cap replacement. More recently I installed the bigger xformer and high-speed rectifiers. Yes, it is close to the left-channel board but I don't have a hum issue. There is a slight 120 Hz buzz with your ear about a foot from the tweeter. Probably would have to eliminate the rat's nest of wiring between the driver boards and output stages to lower the noise level further.

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Old 4th April 2015, 04:21 PM   #10
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Default Which amp next?

Quote:
Originally Posted by GaryB View Post
Here you go.
---Gary
Gary, I'm guessing you own or owned at one time a DC100. What do you have now and how does the Borbely compare?

I'm looking at building a Pass F5T, Honey Badger, DX Blame, or a Symsym. I haven't been able to decide yet which one to go with.

Thanks!
Richard
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