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Old 19th May 2014, 09:44 PM   #1
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Default I need help with voltage regulators

Hello !

I'm planning to build a 2.1 amplifier this summer, but fact is that I'm using a board like this :

Assembled 2.1 Channels LM1875 NE5532 Audio Power Amplifier Board 25Wx2+50W(Sub) 11504-in Other Electronic Components from Electronic Components & Supplies on Aliexpress.com

This amplifier board is designed to be fed by a 12V-0V-12V transformer, but I'm going to connect it to a 18V-0V-18V one in order to reach the 2*25W + 50W allowed by the LM1875 chips. I'm also going to replace some capacitors with other higher voltage electrolytic capacitors and with some MKP capacitors on the audio signal path.

However, I'm wondering about the design of the voltage regulator section ! It's here to ensure the two NE5532 operational amplifiers are supplied with a constant 12V. But I noticed that the actual board schematics do not respect the manufacturer's schematics. The voltage regulators are KA7812 and KA7912 from Fairchild Semiconductors. To see the differences, I joined the schematics of the KA7812 ( they are the same for the KA7912 ) on the actual board, and from the Fairchild's recommendations. On my drawing, the 6800 F capacitor is one of the two filtering capacitors feeding the LM1875 and the regulators.

Here are the complete datasheets for the two voltage regulators :

KA7812 : http://www.alldatasheet.com/datasheet-pdf/pdf/99450/FAIRCHILD/KA7812.html

KA7912 : http://www.alldatasheet.com/datasheet-pdf/pdf/97973/FAIRCHILD/KA7912.html

Is the current design okay or do I have to modify it ? Is there a way to improve it ?

Thanks
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Old 19th May 2014, 09:59 PM   #2
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You didn't read the fine print! In the application notes it says C1 is needed if
the regulator is not near the power supply cap but in your case it is fairly near the
power supply cap,
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Old 19th May 2014, 10:08 PM   #3
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Thanks for your answer I forgot to write it, but noticed it. In fact I'm wondering why a 100F capacitor is used instead of a 0.1F capacitor ( C0 ), and why a resistor is used before the regulator
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Old 19th May 2014, 10:20 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Your looking at 4 ohm main speakers and an 8 ohm
sub to make it work, take note of the 4 amp max.

In actual fact 12V-0-12 VAC makes sense after
rectification for the whole thing to work very well,
with a decent but relatively modest heatsink.

YMMV but at the cheap cost of the board stick
with the recommended supply. 18-0-18 VAC
is far too much for 4 ohm and bridged into
8 ohm, in any real sense and a waste.

rgds, sreten.

You can't optimise L+R for 8 ohm
without also needing a 16 ohm sub.
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Last edited by sreten; 19th May 2014 at 10:24 PM.
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Old 19th May 2014, 10:24 PM   #5
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Well it could be one or more reasons. Using a larger cap for C0 will lower the voltage reg. noise. Now the 47 ohm resistor may be there to sort of protect the voltage regulator. When you power the circuite off I believe the voltage in C0 will try to flow back through the regulator to the power supply usualy this is of no concern but with a large value for C0 it can harm the regulator. Usually one would use a diode in parallel with the voltage regulator. But in this case that 47 ohm resistor should provide protection and it was probably about 1 cent cheaper than using a diode.
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Old 19th May 2014, 11:06 PM   #6
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Quote:
Originally Posted by sreten View Post
Hi,

Your looking at 4 ohm main speakers and an 8 ohm
sub to make it work, take note of the 4 amp max.

In actual fact 12V-0-12 VAC makes sense after
rectification for the whole thing to work very well,
with a decent but relatively modest heatsink.

YMMV but at the cheap cost of the board stick
with the recommended supply. 18-0-18 VAC
is far too much for 4 ohm and bridged into
8 ohm, in any real sense and a waste.

rgds, sreten.

You can't optimise L+R for 8 ohm
without also needing a 16 ohm sub.
All my speakers are 8 ohms speakers. I thought 18-0-18 VAC wouldn't harm anything with a proper heatsink and a 12cm fan thermally activated. Maybe I'm going to reduce costs with a 14-0-14 VAC transformer to still gain some extra watts. The heatsink I'm going to use is a 8*3*2 ( width*height*depth ) inches aluminium heatsink.

I'm actually running my home made speakers with a Logitech Z313 amplifier ( with a STA540, 4 channels chip ) and it works very well. But the amplifier struggles to really play loud with only 5 watts per satellite and 10-15W for the sub

Quote:
Originally Posted by woody View Post
Well it could be one or more reasons. Using a larger cap for C0 will lower the voltage reg. noise. Now the 47 ohm resistor may be there to sort of protect the voltage regulator. When you power the circuite off I believe the voltage in C0 will try to flow back through the regulator to the power supply usualy this is of no concern but with a large value for C0 it can harm the regulator. Usually one would use a diode in parallel with the voltage regulator. But in this case that 47 ohm resistor should provide protection and it was probably about 1 cent cheaper than using a diode.
Thanks for your detailed answer I read on the web that some people put a ceramic capacitor in parallel with the C0 capacitor to prevent HF perturbations. So, is putting a 0.1F ceramic capacitor useful in my case ?

I read on the KA7812 datasheet that the minimal input voltage is 14.5V, and I fear the resistor lowers the voltage too close to 14.5V. Do you think replacing the resistor with a diode as I did in the joined file could improve performance and/or lifetime of the regulator ? If yes, I believe that the diode should be placed in the other direction for the other regulator.

I'm sorry if my questions sound dull, I'm just a beginner who spent lots of time reading forums but without much practical experience and I'm not really self-confident
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Last edited by Subway2400; 19th May 2014 at 11:08 PM.
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Old 20th May 2014, 09:20 PM   #7
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Okay, I think I won't modify anything for this first project

Let's begin with some easy projects before getting in complicated ones

Anyways, thanks for all the informations
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Old 21st May 2014, 12:27 AM   #8
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Note that LM1875 has 60V supply max rating (ie. +/-30). Your initial thought of 18-0-18 will get very close to that with a +10% tolerance supply. Your supply could easily exceed the max rating, depending on the tolerance of the mains input voltage, and the actual output voltage on the 18-0-18 rated windings.
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Old 21st May 2014, 11:33 AM   #9
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I wanted to put a 18-0-18 supply because of the datasheet stating that for " Vcc = +25V and -Vee = -25V ", a LM1875 would produce 25W per channel with a 8 ohms speaker. 12-0-12 would produce +17V and -17V, with only 12W per channel instead of 25W. But because many members on the forum told me that a 18-0-18 supply is too much, I finally decided to avoid using it

So, what about a 14-0-14 supply ? The LM1875 would be fed with +20V and -20V, producing 17W per channel. The power dissipation would be 12W for one LM1875.

Does it seem correct for you ?
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Old 21st May 2014, 01:37 PM   #10
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Unless you regulate the DC voltage feeding the amp, then the design issue turns to how you can stay under the maximum DCV given all the variables in your unregulated DCV.

You may be lucky and always have a pretty constant ac mains voltage. Some aren't so lucky, and the introduction of solar inverters has certainly increased the max level some experience in my area of the woods.
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