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Old 23rd October 2008, 02:56 PM   #1
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Default Help with first Power Supply for LM1875

Hi, the first thing I should say is I am new to the world of DIY audio, so goes easy on me.

I have been doing a lot of reading up and started to build a LM1875 amp. I have already got two mono LM1875 kits (and soldered them), a very nice Modu enclosure, and all the back panel connectors.

I am now moving on to building the power supply. After reading many articals in these forums and a few guides this is the circuit I have come up with:

Click the image to open in full size.

As you can see I have opted for a 160VA, single 230V primary, dual 18V secondary toroid transformer. Along with two bridge rectifiers and 4,700uF capacitors for smoothing.

So basically, do I have the wiring all correct? Specifically do I have the grounding of the signal and mains correct?
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Old 23rd October 2008, 03:23 PM   #2
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Looks good to me! The only thing you may want to experiment with (and there are a LOT of opinions on this) is the amount of capacitance you use. Also, you may or may not want to try a snubber power supply, which adds a series resistor and capacitor in parallel with the main capacitor.
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Old 23rd October 2008, 03:41 PM   #3
AndrewT is offline AndrewT  Scotland
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2channels fed from +-4700uF is getting close to Peter Daniel's implementation.
Except he uses 96dB speakers and locates the smoothing (1500 to 2200uF) on each amp PCB.
Either go PD's route or go Carlos FM's version. Completely different ways to approach the PSU.

Only power up your project through a mains light bulb tester, until you KNOW it is wired correctly.
Use the light bulb tester each time you add on the next stage or modify any stage.
I suggest you power up the mains transformer alone, then add the rectifiers and check again. Then add the smoothing and check again, etc.
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Old 23rd October 2008, 03:47 PM   #4
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Page 7 in the datasheet might be interesting for you, since you want to use 25 V rail voltage. Those considerations were based on 8 Ohm speakers. The situation is worse for 4 Ohm speakers.

Your schematic looks correct. How about using fuses also on the secondary side?
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Old 23rd October 2008, 05:10 PM   #5
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Thanks for the quick reply. I was thinking of perhaps having four 2,200uF caps instead of two 4,700uF as some people on the forums said it gave them better results.

My next question is, do i "need" the connection from the ground coming out of the power suppy to the mains earth screwed to the enclosure?
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Old 23rd October 2008, 05:16 PM   #6
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YES...U need to "Earth" as drawn !!
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Old 23rd October 2008, 05:38 PM   #7
AndrewT is offline AndrewT  Scotland
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The Safety Earth must be permanently connected.
All exposed conductive parts must be connected to Safety Earth.
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Old 23rd October 2008, 05:39 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by simonlarusso
I was thinking of perhaps having four 2,200uF caps instead of two 4,700uF as some people on the forums said it gave them better results.
if you decide to go this route then read Peter Daniel's construction guide and his advice on building your chipamp.
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Old 23rd October 2008, 06:54 PM   #9
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Quote:
Originally posted by simonlarusso
My next question is, do i "need" the connection from the ground coming out of the power suppy to the mains earth screwed to the enclosure?
You need to connect the earth to anything conductive that can be touched without the need of any tool. That is the enclosure, can be the heatsinks, if they are mounted outside of the enclosure and so on.

The connection from earth to the internal ground should be made, if no other component in your audio chain has one. It serves to improve ground stability and noise immunity. It can be omitted, if another component in your chain already has such a connection, because then you may get ground loops that lead to hum.
Professional equipment often comes with a ground lift switch that leaves a choice for both cases.

The important point is that the earth connection to the exposed parts must never be interrupted as long as the amplifier is plugged into mains.
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Old 23rd October 2008, 08:55 PM   #10
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Quote:
Originally posted by AndrewT
Only power up your project through a mains light bulb tester, until you KNOW it is wired correctly.
Use the light bulb tester each time you add on the next stage or modify any stage.
I suggest you power up the mains transformer alone, then add the rectifiers and check again. Then add the smoothing and check again, etc.
Yes I am going to make a bulb tester, this is simply a 100W bulb added to the "Live" wire of the power cable and all in an insulated box of course?

Quote:
Originally posted by pacificblue
Your schematic looks correct. How about using fuses also on the secondary side?
The kits I bought have a fuse on both the +25V and -25V as part of the PCB design.

Quote:
Originally posted by AndrewT
if you decide to go this route then read Peter Daniel's construction guide and his advice on building your chipamp.
Ok thanks, I will do a bit more research and decide on what values I will use for smoothing caps.

Quote:
Originally posted by pacificblue

You need to connect the earth to anything conductive that can be touched without the need of any tool. That is the enclosure, can be the heatsinks, if they are mounted outside of the enclosure and so on.

The connection from earth to the internal ground should be made, if no other component in your audio chain has one. It serves to improve ground stability and noise immunity. It can be omitted, if another component in your chain already has such a connection, because then you may get ground loops that lead to hum.
Professional equipment often comes with a ground lift switch that leaves a choice for both cases.

The important point is that the earth connection to the exposed parts must never be interrupted as long as the amplifier is plugged into mains.
Perfect, thanks for the explaination. Being new to this I have a lot of silly answers that keep popping up.

Ok, thanks for all your replies. I have slighty modified the circuit and think I might just use one bridge rectifier as this will be simpler to build and keep cost down. This is my new schematic:


Click the image to open in full size.

As I said above I have not commited to what cap values to use yet. So is it all correct?

Thanks in advance
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