Why two bridges are better than one?

Andypairo · 13th October 2002, 07:58 AM

Somewhere else in this forum I read the suggestion to use two rectifier bridges instead of one, but i was wondering if there are other motivations apart from a (slightly) reduced output voltage and fewer thermal dissipation needed for each bridge..

I am building a Aleph 30 with a 500 VA 22V CT transformer (I know, 600> VA were better... but i had this one lying around), so I also need advice for lowering the rails voltage.

I was thinking about a coil between bridge and filtering caps, I guess about 1mH should be enough.. suggestions?

Thanks

andypairo

skaara · 13th October 2002, 10:00 AM

You should do it like this: rectifier-caps-choke-caps

kristijan-k · 13th October 2002, 10:23 AM

Hello Andypairo,

If you would use two diode bridges than it is better
idea to use transformer with two separate secondary
windings instead with the transformer that have
Center Tap.

Best regards,
Kristijan Kljucaric
http://web.vip.hr/pcb-design.vip

roddyama · 13th October 2002, 01:56 PM

Hi Andypairo,

I’m assuming you meant to say you have 44V CT. That would give you 22V to each bridge. A choke input PSU is going to give you too low a voltage for an Aleph 30. It calls for 25V rails and you’ll only have about 18V rails and less with higher loads.

I would go with Skaara on this one and run a PI filter PSU (CLC). If you use your 1mH (assuming low DC resistance), and add input and output caps like in Kristijan’s schematic, your rails will run from 23V to 26V (light load to high load) according to Duncans PSU Designer.

Hope this helps,
Rodd Yamas***a

Petter · 13th October 2002, 03:17 PM

Even though kristijan-k has come up with an excellent solution, the original question remains unanswered:

Why is 2 bridges better than one apart from the obvious (current capability, thermals etc.)

First of all, as kristijan-k has written, there is no gain unless you have two distinct secondary transformer windings. If you do, you can treat the two rails as separate entities and then connect their zero's

This allows you to take control of where large currents circulate, and not let that point be ground which is often by definition also "signal ground" and sensitive to noise induced from such currents.

If you have a center tap transformer there is no benefit from such an approach.

Herein lies the answer.

Petter

Nelson Pass · 13th October 2002, 08:20 PM

If you use one bridge with two secondaries and some
net DC current flows through the ground, there will be
a tendency toward saturation (and mechanical noise)
in the transformer if the two windings are not precisely
identical. Most of the time this is not a problem, but
when it is, two bridges prevents it.

Andypairo · 14th October 2002, 05:53 AM

Thanks for the answers, I have 2 split secondaries and also a vibration problem (but only without inserting chokes,now I'm using a single bridge), you probably solved this problem...

The trafo is a toroid, so the secondary windings should be very closely similar but I'll give this method a try.

Another question: the service manual indicates 30000 uF per rail (and channel), kristijan-k's schematic more than twice (with the add of a choke).
Is the latter common to both channels?

I'll download also Duncan's PSU designer (Thanks roddyama) and will make some simulations by myself...

Andypairo

Andypairo · 14th October 2002, 07:25 AM

Quote:

Originally posted by Petter
Even though kristijan-k has come up with an excellent solution, the original question remains unanswered:

Why is 2 bridges better than one apart from the obvious (current capability, thermals etc.)

....

Herein lies the answer.

Petter

You got it! That's what I wanted to know...

By the way, my caps are a connected in a array-style since I only found 6800 uF ones.
Like this (I wish I had a digital camera..):

------------ -Rail
CCCCC - Caps
------------ Gnd
CCCCC Caps
------------ +Rail

Where shoul be placed Input (Bridge) and output (amp)connections?

1- In at the left, out at the right
2- Both in the same place (center?)
3- Somewhere else?

Hi

Andypairo

skaara · 14th October 2002, 08:34 AM

It doesnt matter if you gi from left to right of reversed!
You just have to connect them (electrically and physically) like i told you and like shown on kristjans schematic

Andypairo · 14th October 2002, 09:24 AM

Ok but what about star grounding or similar layout techniques?
Caps are connected by soldering on their pins a 2mmq wire, so I souldn't have series resistance problems, anyway I wanted to minimize it.

BTW placing smaller caps (say 100 uf or so on) close to the board(s) can help or it's overkill?

Hi

Andypairo

13th October 2002, 07:58 AM	#1
Andypairo diyAudio Member Join Date: Sep 2002 Location: near Milano , Italy	Why two bridges are better than one? Somewhere else in this forum I read the suggestion to use two rectifier bridges instead of one, but i was wondering if there are other motivations apart from a (slightly) reduced output voltage and fewer thermal dissipation needed for each bridge.. I am building a Aleph 30 with a 500 VA 22V CT transformer (I know, 600> VA were better... but i had this one lying around), so I also need advice for lowering the rails voltage. I was thinking about a coil between bridge and filtering caps, I guess about 1mH should be enough.. suggestions? Thanks andypairo

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13th October 2002, 10:00 AM	#2
skaara diyAudio Member Join Date: Jun 2002 Location: Europe, Slovenia	You should do it like this: rectifier-caps-choke-caps

13th October 2002, 01:56 PM	#4
roddyama diyAudio Moderator Emeritus Join Date: Jan 2002 Location: Michigan	Hi Andypairo, I’m assuming you meant to say you have 44V CT. That would give you 22V to each bridge. A choke input PSU is going to give you too low a voltage for an Aleph 30. It calls for 25V rails and you’ll only have about 18V rails and less with higher loads. I would go with Skaara on this one and run a PI filter PSU (CLC). If you use your 1mH (assuming low DC resistance), and add input and output caps like in Kristijan’s schematic, your rails will run from 23V to 26V (light load to high load) according to Duncans PSU Designer. Hope this helps, Rodd Yamas***a

13th October 2002, 03:17 PM	#5
Petter diyAudio Member Join Date: Jan 2001 Location: Scandinavia	Even though kristijan-k has come up with an excellent solution, the original question remains unanswered: Why is 2 bridges better than one apart from the obvious (current capability, thermals etc.) First of all, as kristijan-k has written, there is no gain unless you have two distinct secondary transformer windings. If you do, you can treat the two rails as separate entities and then connect their zero's This allows you to take control of where large currents circulate, and not let that point be ground which is often by definition also "signal ground" and sensitive to noise induced from such currents. If you have a center tap transformer there is no benefit from such an approach. Herein lies the answer. Petter

13th October 2002, 08:20 PM	#6
Nelson Pass The one and only Join Date: Mar 2001	If you use one bridge with two secondaries and some net DC current flows through the ground, there will be a tendency toward saturation (and mechanical noise) in the transformer if the two windings are not precisely identical. Most of the time this is not a problem, but when it is, two bridges prevents it.

14th October 2002, 05:53 AM	#7
Andypairo diyAudio Member Join Date: Sep 2002 Location: near Milano , Italy	Thanks for the answers, I have 2 split secondaries and also a vibration problem (but only without inserting chokes,now I'm using a single bridge), you probably solved this problem... The trafo is a toroid, so the secondary windings should be very closely similar but I'll give this method a try. Another question: the service manual indicates 30000 uF per rail (and channel), kristijan-k's schematic more than twice (with the add of a choke). Is the latter common to both channels? I'll download also Duncan's PSU designer (Thanks roddyama) and will make some simulations by myself... Andypairo

14th October 2002, 08:34 AM	#9
skaara diyAudio Member Join Date: Jun 2002 Location: Europe, Slovenia	It doesnt matter if you gi from left to right of reversed! You just have to connect them (electrically and physically) like i told you and like shown on kristjans schematic

14th October 2002, 09:24 AM	#10
Andypairo diyAudio Member Join Date: Sep 2002 Location: near Milano , Italy	Ok but what about star grounding or similar layout techniques? Caps are connected by soldering on their pins a 2mmq wire, so I souldn't have series resistance problems, anyway I wanted to minimize it. BTW placing smaller caps (say 100 uf or so on) close to the board(s) can help or it's overkill? Hi Andypairo

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