Beginners PSU bridge question, building external psu for power amp

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Hiya,
I am getting so close after sooo long :)
I am building an external psu for my Rega Maia power amp (for learning and solder practice)

(The original board has the transformers, then the connections from the transformers to the board, next in line are the diode bridges and then on to the smoothing caps, two bridges for the input side (I think) and two bridges for the power side)

I took out the original transformers, added them with new bridges and some new caps to the external psu box. (Wires will run from external to the original transformer connections on the circuit board).
External psu is completed and has dc output :)
I am now thinking about removing the original on board bridges that are build from 4 diodes, on the power part of the board it is easy (I hope I have it right) to follow the large traces and replace two of the diodes in each bridge with straight pieces of wire and leave the other two diodes removed. This should let the dc from the external psu go straight through...

However, on the input (it think) the diodes are tiny and am unsure of what goes where, am I okay to leave these bridges in, and just connect up anyway, the dc supply will just go through the on board bridge to the right place (polarity) yes?

Many thanks for your time and much appreciate any comments,
Cheers
DC
 
My advice would be to test it out in steps; starting with just the bridge and the A/C, use a "Dim Bulb Tester" if you can. Maybe to try the bridge out on a lower powered transformer first also when making changes beyond direct replacement.
Then when things are all going in the right direction, you can introduce the filter caps, and so on.
Don't ask how I have come to realize that more testing is better, and cheaper than more confidence.
 
Ah see I connected the transformers, bridges and caps, and fingers crossed I get dc - which I did get.. Maybe I should have read more to learn how to test more stages!!
Safer that way I agree :)

So if I have the new bridge in psu box and leave the old bridge in circuit too, so there are two bridges on one supply will it hurt?

Cheers
dc
 
Hiya,
I am getting so close after sooo long :)
I am building an external psu for my Rega Maia power amp (for learning and solder practice)

(The original board has the transformers, then the connections from the transformers to the board, next in line are the diode bridges and then on to the smoothing caps, two bridges for the input side (I think) and two bridges for the power side)

I took out the original transformers, added them with new bridges and some new caps to the external psu box. (Wires will run from external to the original transformer connections on the circuit board).
External psu is completed and has dc output :)
I am now thinking about removing the original on board bridges that are build from 4 diodes, on the power part of the board it is easy (I hope I have it right) to follow the large traces and replace two of the diodes in each bridge with straight pieces of wire and leave the other two diodes removed. This should let the dc from the external psu go straight through...

However, on the input (it think) the diodes are tiny and am unsure of what goes where, am I okay to leave these bridges in, and just connect up anyway, the dc supply will just go through the on board bridge to the right place (polarity) yes?

Many thanks for your time and much appreciate any comments,
Cheers
DC

Hi
A full wave bridge rectifier, works with four diodes. removing 2 diodes and replacing them with wire
connections as your text suggests is not a good idea. Likely you will end up with a fuse very quickly blowing
that's if a fuse is fitted, or a substantial transformer secondary A/C content on your supply,

The alternative is a half wave supply but it is connected differently. For audio circuits a full wave bridge
is used, as it lowers AC ripple.

**** Can you provide a diagram or an image of what you are doing please ***** .
Here is a tutorial on power supplies for amplifiers http://sound.westhost.com/project04.htm
Cheers / Chris
 
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Ok sorry, I am not explaining this very well, I shall try again please bear with me.

Original board has ac transformer inputs, these lead to a bridge, from the bridge to smoothing caps.

I have removed the transformers to an external case, put in this external case a new bridge, and new smoothing caps, I therefore have dc coming out of the external psu box.

Can I plug this new external dc supply, into the original on board ac transformer inputs? therefore supplying dc to the board, to go through the boards original bridge etc?

Hope sketch helps me explain ;-)
Thankyou! :)
DC
 

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Ok sorry, I am not explaining this very well, I shall try again please bear with me.

Original board has ac transformer inputs, these lead to a bridge, from the bridge to smoothing caps.

I have removed the transformers to an external case, put in this external case a new bridge, and new smoothing caps, I therefore have dc coming out of the external psu box.

Can I plug this new external dc supply, into the original on board ac transformer inputs? therefore supplying dc to the board, to go through the boards original bridge etc?

Hope sketch helps me explain ;-)
Thankyou! :)
DC

Hi
OK , but you need to explain what polarity of DC exits your
new external supply, that you are trying to couple, is it dual rail
meaning positive and negative, or is it single rail positive
or quite rare single rail negative.

Most common is the first dual rail.
I have attached a simple diagram showing
how you need just two diodes,
Note the fuses in each rail, :)
these are for initial testing, and
assuming your amplifier and external supply are fused,
can be then removed.Test with your amplifier at idle,
not playing music

You also need to observe how grounding is done
in your external supply, and your amplifier.

A proper schematic of each would greatly assist. !:)

Grounding is explained here:
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

This is all assuming voltages from your new
external supply are what your amplifier needs
and not exceeding what it requires.

VERY IMPORTANT connect polarities
of voltage from your external supply to
the amplifier polarities exactly.

You really need a multimeter to check
polarities before making any connections.

Cheers / Chris
 

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Last edited:
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Ok sorry, I am not explaining this very well, I shall try again please bear with me.

Original board has ac transformer inputs, these lead to a bridge, from the bridge to smoothing caps.

So that set up gives you DC.

I have removed the transformers to an external case, put in this external case a new bridge, and new smoothing caps, I therefore have dc coming out of the external psu box.

This set up also gives you DC.

Can I plug this new external dc supply, into the original on board ac transformer inputs? therefore supplying dc to the board, to go through the boards original bridge etc?

Hope sketch helps me explain ;-)
Thankyou! :)
DC

So you are saying the existing board has retained its bridge and reservoir caps.

Lets just be clear on the transformer... it is a two wire secondary that is used to generate a single rail DC supply on the main board. If so then yes, you can feed DC into the existing AC input of the bridge. The beauty of this is that the polarity doesn't matter, the bridge automatically corrects and outputs the correct polarity voltage. The disadvantage is that you now have two diode volt drops in the chain which slightly lower the DC supply by around 1.5 volts)

Another option is for you to connect the new DC supply across the existing bridges output terminals. This time polarity is important.
 
Many thanks for your replies :)
I have had my multimeter out and checked the board to I could remove the bridges and short them - thanks for the idea!

One pic is the original board with the bridges removed / shorted, other is of the diy psu - I hope there are no obvious balls ups :-o

Cheers
DC
 

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Yes can understand, picture somewhat ambiguous to anyone else ;-)

I have no bulb tester (as of yet) only a multimeter. I get 58.8Vdc and 58.7Vdc on the two smaller smoothing cap circuits and 51.2Vdc on the two larger smoothing cap circuits. Which I guess have to be right as they are the original transformers.

Unfortunately when I do plug the psu unit in to the original board I still get one fuse blow (one fuse/transformer) I was hoping that problem may have been solved with the new psu experiment, but, I think the problem must be on the main board somewhere which is way beyond my understanding so will pop it in to a professional to resolve that issue for me, and will also ask for my psu to be given a quick once over ;-) we don't want anything too unsafe with curious kids and cats around ;-)

Many Thanks for your assistance, much appreciated :)
 
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The voltages sound balanced and I guess nothing is smoking or getting hot and so yes, it sounds as though the PSU is at least basically OK.

A bulb tester is definitely the way forward because it would let you measure under the fault conditions and so get an idea of what was happening. If the main amp is faulty then that's going to be a whole different ball game in fault-finding and testing.

Hope you get it sorted :)
 
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