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first monoblock works ! :-))) - question for transformer
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ullus  
#1 Posted : Thursday, June 21, 2007 7:56:44 AM(UTC)
ullus

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hello,

1. recently I got my RevC parts and today I populated one of the boards. I worked in the garden under the parasol; the parts were all nicely labeled; (the wind blew some resitors away from the table, but happily nothing was lost. ;)

1a. I use a large enough aluminum heat sink which I ripped of a blown UPS. (some screw holes are just in the right place; I guess I can mount the 2nd channel to the same sink and do not need to cut it in two parts since the lm3886 chip is isolated. right?)

2. for first power up, I've followed the recommendation found in http://www.inginerie.pro...s/100w_rms_amplifier.htm
a link I indirectly found from a diy-forum which discussed RevC.

quote from that page:
"I suggest the use of a ballast resistor - a 60 to 100 watt light bulb in series with the AC mains. You get a bright flash when the caps charge, and then it goes (almost) out as the idling supply current reaches its nominal low value. The amplifier will then work normally at low volumes. ..."

3. I connected an old cheap speaker box for the initial test and powered-up with a 60W bulb, but without the lm318 installed. The bulb went bright and then went out, but the RevC's LED (which I ripped out of an old (pre-PS/2) PC keyboard, because it is not part of the kit) was green!

4. I'm using a 300W toroid secondary 2 * 25V. with this setup (a 60W bulb in series on the 220VAC side of the toroid), I checked the voltage at both elevated resistors; 23V each; seems ok. Voltage on the lm318 socket seemed ok to me too.

5. so I powered off; put lm318 into its socket; connected the output of a pre-amp to the RevC input; powered-up; the light bulb flashed and the LED went green again ... and in addition, there was music! great kit! (the 2nd channel, as well as darwin and joshua are waiting...)

one question remains however:
the label on my toroid says: 2 black wires for 220VAC primary; red/green and blue/jellow wire pairs each deliver 25V/6A.

I took one of each secondary wire arbitrarily and used it as centertap, going to PGND. The remaining two wires go to AC1 and AC2 respectively.

what puzzles me is that in another thread, concerning a different brand toroid, Brian wrote:

"you form a center tap by tying the two center secondaries together (RED+ORANGE), then provide three connections to each boards power connections (AC1: BLACK, AC2: YELLOW, PGND: combined RED+ORANGE)."

Does this mean, I need to know about the center of the two secondaries?
the secondaries are independent and don't have a center.
Since I don't have a scope, I'd be lost, if the rectifier bridge required equal phase for both secondaries!

I know the 300W is the lower limit for supplying two RevC channels. so in the meantime I bought another used 600W toroid with 2 * 24V; but guess what? its four secondary wires are all black!

p.s.: if you want, please add my location in your google client map for postal code 04340 in France. thanks!
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Brian Donegan  
#2 Posted : Thursday, June 21, 2007 8:32:19 AM(UTC)
Brian Donegan

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Quote:
1a. I use a large enough aluminum heat sink which I ripped of a blown UPS. (some screw holes are just in the right place; I guess I can mount the 2nd channel to the same sink and do not need to cut it in two parts since the lm3886 chip is isolated. right?)


Chips are insulated, so fine to share a heatsink, as long as it's big enough.


Quote:
he label on my toroid says: 2 black wires for 220VAC primary; red/green and blue/jellow wire pairs each deliver 25V/6A.


You do absolutely need to form the center tap with your transformer. Is there a diagram on your transformer, or just text indicating wire colors?

Here's a way to figure out which wires to join on your current transformer (disconnect from amp first). In between tests, be sure to remove power completely, and be very very careful!

First, try joining your GREEN and BLUE secondaries to form a center tap (CT) and power up
- Measure voltage between RED and CT, then YELLOW and CT. both should show 25VAC.
- Measure between Red and Yellow. Should show 50VAC. If not, swap BLUE for YELLOW in the CT and measure everything again.

For your other transformer, you must first seperate the two secondaries. Do this by measuring for continuity between the leads. Each secondary pair will have continuity, leads of different secondaries will not. Once you have that sorted out, you can use the above steps to find your center tap.

Again, be very very careful.

Edited by user Thursday, June 21, 2007 8:33:41 AM(UTC)  | Reason: Not specified
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ullus  
#3 Posted : Thursday, June 21, 2007 9:12:21 AM(UTC)
ullus

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Brian,

thanks for your quick reply. no, there is no diagram on the transformer.

just measured; as I had wired it, the voltage was almost zero VAC between the outer rails! (so the phases canceled each other out. I can nothing but wonder that it worked nevertheless in this setup! - but I hadn't yet tried heavy volume.)

I swapped the wires of one pair; between the outer rails I now get 53.2 to 53.9VAC; reconnected the transformer to the amp; and it still works ;) - with the difference that's now more efficiant (louder) than before at the same pre-amp volume level.

I understood the center-tap test procedure and will apply it also to the other transformer(s).

thank you again - and sorry for my stupid question.

best
ullus
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Brian Donegan  
#4 Posted : Thursday, June 21, 2007 9:32:54 AM(UTC)
Brian Donegan

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Not stupid at all. You now have a two wire center tap?
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ullus  
#5 Posted : Thursday, June 21, 2007 10:02:01 AM(UTC)
ullus

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> You now have a two wire center tap?

sorry, why did you put a "?".
I thought I confirmed that with my previous post.

Code:

|| II------------------------> AC 1
|| II
|| II-|
||    |-----------> center tap PGND
|| II-|
|| II
|| II------------------------> AC 2

I have 26VAC between CT and 1, as well as between CT and 2,
and 53VAC between 1 and 2.

Is there something else to do?
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Brian Donegan  
#6 Posted : Thursday, June 21, 2007 10:52:02 AM(UTC)
Brian Donegan

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Just didn't see it said, just being sure. I hope you enjoy the amp!
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ullus  
#7 Posted : Friday, June 22, 2007 7:42:57 AM(UTC)
ullus

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> I hope you enjoy the amp!

thank you!

just finished the assembly of the 2nd board and it works too.

I cut my big heat sink in two parts because two independent boards are easier to mount in the cabinet.

(2 boards on one sink would no longer be 2 monoblocks; maybe it could have been called "a double monoblock" - or "a mono doubleblock"? ;)
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Brian Donegan  
#8 Posted : Friday, June 22, 2007 8:20:16 AM(UTC)
Brian Donegan

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How about "double bock"? Cheers!
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ullus  
#9 Posted : Friday, June 22, 2007 10:02:59 AM(UTC)
ullus

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doppelbock ;) twinmonoblock, twinpower with a light bulb.
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