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eddog 17th October 2006 01:16 PM

Curious voltage difference between V+ and V-
Hi, I have a Brian GT 3875 amp that I have been running with a dual secondary torroid on my work bench for about 1 year. Well the torroid had to go into another project so I am having to use a center-tapped transformer of questionable design that I had laying around. I've changed the power supply as per the instructions(Jumpers between diodes as pictured in the assembly manual) and when no load(amp boards)are connected I get a good 19.5 both - and +. However when I connect the amp boards the V+ stays at 19.5 V but the V- dropes to - 16.5. The DC offset becomes high at .5V also. These same amp boards when connected with the dual secondary torroid had DC offsets less than 10mv. Suggestions appreciated.

boholm 17th October 2006 01:27 PM

The centertap of the transformer - did you put that to GND?

eddog 17th October 2006 01:50 PM

I have the centertap connected to the PG+ of the rectifier board which is also effectively PG- due to the jumper on the board. I then run wires from PG- of the rectifier board to the PG- connection of both amp boards. There is no reason to run the PG+ wires to the amp boards correct?(same potential as PG-)

AndrewT 18th October 2006 08:31 AM

sounds like you have inadvertently made another change.

Have another look and try to identify the changes before to after.

Nordic 18th October 2006 11:02 AM

Aren't the rectifier diodes set up diffirently on Brians boards depending on wheter you have a split secondary and when you have 2 seperate secondaries?

eddog 18th October 2006 01:34 PM

Yep, you remove diodes D2,D3,D6 and D7 then jumper between the remaining diodes to form a Bridge rectifier. I am pretty sure that I have it wired correctly. Would a leaky cap after the bridge allow some AC to get through. Maybe my V- side cap is not healthy.

jan.didden 18th October 2006 02:24 PM

Sounds like one diode is'n working and you have only half way rectification at the low level side. That would explain the drop when loaded.
Do you have a scope, to see that the ripple at the lower voltage side is higher & half-wave?

Jan Didden

eddog 18th October 2006 07:28 PM

Voltage issue repaired
Well the power supply was alright, I disconnected my source input and my voltage issue disappeared. Turns out my source has some DC on it's output as well one side had some small amout of AC also. I have put a 1uf cap on the signal inputs and the problem has gone away. Thanks for the suggestions they always cause me to put more effort into my problem solving.


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