LM1875 Power Supply+Transformer Considerations - diyAudio
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Old 16th November 2014, 08:29 AM   #1
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Default LM1875 Power Supply+Transformer Considerations

I will be building a LM1875 amp, based on to IC's for stereo sound.

1. Why do almost all DIY amp projects use Torroidal transformers (donut shaped) vs the normal ones found in wall warts? Will there be any performance regression if I use the other type of transformer?

2. Can the LM1875 be used with a single rail(non split) power supply? How? What are the benefits of the split PS?

3. I could also use an old laptop adapter 19V 3A, so If I split the rails, I'll get around +/-9V, which is low.

4. If I use 2 chips, can I use the same power supply rails for them, with a common ground? Or would it be better to have 2 separate transformers + rectifiers?
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Old 16th November 2014, 09:02 AM   #2
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1. Usage depends on situation. You can use a wall wart for a DAC or a RasPi or maybe a headphone amp but not a high powered amp. For this project you can use torroidal or a EI type transformer.
2. For LM1875 the recommendation is split power supply. However you could use a 0-48V non-split also see 20W Audio Amplifier using LM1875
See also
LM1875/lm1876 single supply?
3. True 19V is low.
4. I dont see the need for 2 transformers in this case but that would be ok too.
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Old 16th November 2014, 09:05 AM   #3
cspirou is offline cspirou  France
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You can use any transformers you want as long as they have a sufficiently high wattage. Toroidal transformers do have an advantage because of better magnetic coupling which is more efficient and less likely to have an external magntic field interfering with your project.

Using the same power supply for two channels is quite common and will work fine for your project. Using separate power is called a mono configuration and is done because a common power supply can have crosschannel distortion and this is greatly minimized with separate supplies. But for me the little improvement hasn't justified the cost and extra work involved. I would do dual mono if I am integrating my amp with my speaker however.

If you go to chipamp.com they sell lm1875 kits in stereo and mono configurations
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Old 16th November 2014, 10:39 AM   #4
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So, is the non-split circuit okay? Then why do people use a split config?
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Old 16th November 2014, 12:03 PM   #5
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split supply has an advantage, the thing won't require a DC blocking cap on the output.

Single supply has an advanatge, as if anything fails, DC can not get to the speakers.


Split supply is the one to go for.
Better sonics.
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Old 16th November 2014, 12:15 PM   #6
AndrewT is offline AndrewT  Scotland
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The single polarity supply version uses a big smoothing capacitor in the Power Supply Unit (PSU) and it also needs a big capacitor in the output feed to the speaker.
The dual polarity supply uses two big smoothing capacitors in the PSU and does not need a big capacitor to feed the speaker.
Both are electrically the same and both require the same number of big capacitors.

The biggest difference is that the Negative FeedBack loop (NFB) of the dual polarity version isolates the capacitor effect from the speaker.
This can give a slightly better performance, particularly if the wrong size of capacitor/s is/are used. The single polarity version can be configured, but it is more complex, to include the speaker capacitor inside the NFB loop. This would negate any differences in performance between the types.

Both versions require DC blocking capacitors in the input and in the NFB loop.

Summarising:
both types can with care perform equally well.
__________________
regards Andrew T.
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Old 16th November 2014, 01:59 PM   #7
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So.. if I'm using a split power supply, I'll connect the 2 outputs of the transformer to the rectifier, which then has the 2 capacitors connected between -V & +V, with the middle point as Vground. I might not be able to source A center tapped transformer.

Since I need around +/-28VDC, I'll need 18VAC at the rectifier, so a 36V output at the transformer. How many Amps should the tranny be rated for?

Click the image to open in full size.

Do I need fuses?
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Old 16th November 2014, 02:14 PM   #8
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Quote:
Originally Posted by carlmambo View Post
Since I need around +/-28VDC, I'll need 18VAC at the rectifier, so a 36V output at the transformer. How many Amps should the tranny be rated for?

Do I need fuses?
+/- 28 VDC is too high for the LM1875 - I wouldn't recommend going over +/- 24 VDC. I've used +/- 19..20 VDC in several projects, obtainable with a 15-0-15, 3A trafo.

Since you've already got fuses in the layout above, you might as well use them. I use fuses only on the primary side of the trafo.
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Old 16th November 2014, 02:44 PM   #9
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Okay, if I want +/-24VDC, I'll need around +/-VAC at the rectifier.

1.Is the above layout good, expect for the fuses, which I won't be using

2.Do I really need a center tapped transformer? Can I use a normal one with 2 outputs?
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Old 16th November 2014, 02:48 PM   #10
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Click the image to open in full size.

Sorry for the worst diagram ever, but can I get away without using a center tap transformer? (the red lines show how a center tapped transformer would be connected).

In my case, I'll try and use just the remaining part of the circuit, and use the dual capacitor middle pt . to get a Virtual Ground. The issue is that I doubt the shop nearby will have a cantre tapped transformer.
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