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Old 28th September 2004, 03:30 PM   #1
KT is offline KT  United States
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Default Gainclone single rail PS question

I'm just about done with my first GC! Before I plug it in, though, I wanted to clear up something about the power supply.

I looked at the schematic for Peter and Brian's GC kit. As you know this uses a dual secondary TX to get dual rails, a full bridge rectifier for each rail (4 diodes per rail, total of 8), and these two rails are referenced to ground between the two.

The PS I've built is different.

I'm using a dual secondary transformer, but only one bridge (4 diodes). I'm paralleling the dual secondary stages to increase current.

From there, the + pin of the rectifier supplies the + power for both the left and right channels. Similarly, the - pin of the rectifier supplies the - power for both the left and right channels.

With this type of setup, do I need to reference anything to ground? That is, do I need to tie the + and - pins of the rectifer together and bring this to ground? If so, would I place a small cap (4.7uF) between each pole and ground?

I'm confused because I've seen a schematic for a single rail supply that ties both rails to ground like this, but it used a center tapped transformer (center tap was also tied to ground).

Transformer I'm using is dual secondary, but not center tapped.

Thanks a lot,
KT
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Old 28th September 2004, 04:08 PM   #2
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Default Re: Gainclone single rail PS question

Quote:
Originally posted by KT
With this type of setup, do I need to reference anything to ground? That is, do I need to tie the + and - pins of the rectifer together and bring this to ground?

Stay quiet before you get hurt.
Glad you opened this thread on time!

-down, let's talk.
What voltage and current is your trafo?
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Old 28th September 2004, 04:30 PM   #3
KT is offline KT  United States
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Hi Carlos,

The transformer is a dual secondary Plitron at 22v per secondary and it's 120VA. I'm using it to power two LM1875 channels.

Thanks for your help. Advice is much appreciated.

KT
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Old 28th September 2004, 04:57 PM   #4
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Quote:
Originally posted by KT
The transformer is a dual secondary Plitron at 22v per secondary and it's 120VA. I'm using it to power two LM1875 channels.
That trafo is not good for the LM1875.
This chip can't handle more than +/-30v.
A 2*18 or 2*20v trafo would be the ticket here.
Go for something around 200VA.
Oh, and forget single voltage, use dual secondaries and dual bridges.
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Old 28th September 2004, 09:40 PM   #5
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Carlos, I read the datasheet which tells me max +- 42 volts => max 2 x 24-28 VAC.

2 x 22 V will work perfectly with only one rectifier bridge.
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Old 28th September 2004, 10:47 PM   #6
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Quote:
Originally posted by peranders
Carlos, I read the datasheet which tells me max +- 42 volts => max 2 x 24-28 VAC.

2 x 22 V will work perfectly with only one rectifier bridge.
P-A, pay attention, as you are giving an incorrect advice.
LM1875

At page 2 of the LM1875 datasheet:

"Supply Voltage 60V"

(or +/- 30V).
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Old 28th September 2004, 11:16 PM   #7
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KT,

although it might seem easy to establish a "single rail" topology like you were suggesting or planning, it is simply not possible. For a ground referenced amplifier you will need two supply voltages. You cannot parallel the secondaries, etc.

But you can use a single 4 diodes bridge only (two lines to the bridge and the oher two to ground). Only beware of the phases of the transformer's secondaries (must be in line).

Wasn't that not the Gaincards/Humpties orginal schematic to use only one bridge? And wasn't that you who opened the PowerHumpty first? Or was it somebody else named KT?

Klaus
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Old 28th September 2004, 11:21 PM   #8
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Sorry, I didn't read carefully enough.
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Old 29th September 2004, 01:05 AM   #9
KT is offline KT  United States
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Thank you for the advice, everyone.

Yes, I was thinking of using a single 4 diode rectifier bridge as seen in the 47 Labs gear. Obviously, I will need to do some more research before I finish the amp up.

I greatly appreciate the insight from Carlos regarding voltage limits. I was under the impression that 22v secondaries would be acceptable for the LM1875. I will look for a suitable transformer before I continue.

It was me who opened up the Power Humpty. I have it in my system now, so I haven't cracked it open for some time. I was planning on sharing some photos, so maybe when I successfully complete my first GC I will have a chance to pull the Humpty out of the system and open it up.

I can't promise how long that might be, however. Soon, I hope!

Is there any site that will give me a good tutorial on putting together a suitable 4 diode supply? I can pick things up fairly quickly, but I'm definitely not an engineer. Glad to have all of your help. Safety first, of course!

Best,
KT
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Old 29th September 2004, 04:20 AM   #10
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If you had two seperate transformers with single secondaries you can connect the secondaries to create a center tap.

And lohk says its possible to do this with a single transformer as well, and im sure hes right

Do some research on the windings though, if its two seperate transformers you just take the opposite wires and tie them together (hot to cold) to make the ground. Maybe one is more complicated.


Here is a power supply diagram: Full Wave (only the top left part )

You can make it simpler or as complicated as you want (take out the regulators).

Oh and it should have equal capacitance on each rail (I guess that pictures not a great one).
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