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Old 22nd September 2006, 07:31 AM   #1
colinB is offline colinB  United Kingdom
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Default Newbie question regarding LM3875 chipamp Rev 3 PCB

Hi,

This is my first attempt to build a gainclone amp so I decided from the onset to buy a set of PCB's to ease the process. I recently purchased a set of LM3875 chipamp boards for a dual mono amp set-up (i.e. BrianGT LM3875 rev 3 PCB's) to kick things off.

Having now soldered all the components on the PCB's I've reached the stage of connecting the rectifier board to the transformer but I'm somewhat confused by the possible connection points.

My transformer has dual secondaries (22V) but there appears to be four possible connection positions on the Rev 3 rectifier board labelled AC1, AC1, AC2 & AC2. So what is the correct way to connect the 2 pairs of cables from my transformer secondary to the rectifier board? Do I just use one AC1 and AC2 and does it matter which?

Sorry if the answer seems obvious but as a newbie, I really don't want to trash the PCB or transformer at this stage of the project.

Any help would be greatly appreciated.

Thanks
Colin.
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Old 22nd September 2006, 08:02 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
can you confirm which pair of transformer wires are from each secondary winding?

Do not connect the amp to the rectifier board until you have checked the PSU output for voltage and polarity.

If so then wire one pair to the AC1s and the other pair to AC2.

If not then wire one pair to the AC1s. Is the voltage on the output OK? If not then swap one of the secondary wires from the spare two. Is the output voltage OK?
Now connect up the two spare secondaries to the AC2s.

check the polarity and voltage from both sides of the PSU.

power down and check the caps do not store any charge.

Connect up the amp to the PSU.

Connect the PSU primary feed from a mains light bulb. (in series with the live lead only) and power everything up.
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Old 22nd September 2006, 11:40 AM   #3
colinB is offline colinB  United Kingdom
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Andrew,

Thanks for your quick reply. I'm at work at the moment so can't answer any of your questions. Will do some testing as per your recommendations over the week-end - then will post results before making any connections to the rectifier board.

Regards
Colin.
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Old 26th September 2006, 09:48 PM   #4
colinB is offline colinB  United Kingdom
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Hi,

Checked AC output from transformer secondaries - measured ~ 24.5V between red/yellow cables and 24.5V between blue/grey cables.

Then connected transformer secondaries to PSU board and measured DC output after rectification.

Measured V+ and PGND+ = +34.5V (red from meter on V+ and black from meter on PGND+)

Measured V- and PGND- = -34.4V (red from meter on V- and black from meter on PGND-)

After power down caps on PSU discharge ok.

Are the DC voltages approx correct so I can safely go ahead and connect PSU to chip board now trashing LM3875 at this stage?

Thanks
Colin.
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Old 27th September 2006, 12:16 AM   #5
Vikash is offline Vikash  United Kingdom
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yup. sound fine.
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Old 27th September 2006, 08:08 AM   #6
Nordic is offline Nordic  South Africa
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yup yup... worst case it will release 'the smoke'
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Old 27th September 2006, 08:41 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
if you use the in series light bulb on the mains inlet there is almost no chance of releasing any smoke, even when there is a gross error in the wiring of the power supply or the amplfier.

The bulb dissipates the voltage if the current goes too high.

You can even wire the transformer primaries out of phase and the bulb protects the transformer from the effective dead short.
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