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Old 21st February 2008, 09:03 PM   #1
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Default LM1875 power supply question (easy)

I have the chipamp power supply which has four outputs: V+, V-, PG+ and PG-. The Quick Kit QK50 has only three inputs for V+, V-, and G. Do I simply tie the two power supply grounds PG+ and PG- together and connect this single G output to the amp G input?

I am using an Avel 18V trafo with dual secondaries. Power supply works so far with 27 V on each side. That seems a little high for the LM1875.

Peace,
Tom E
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Old 22nd February 2008, 06:33 AM   #2
ratza is offline ratza  Romania
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Default Re: LM1875 power supply question (easy)

Quote:
Originally posted by madisonears
Do I simply tie the two power supply grounds PG+ and PG- together and connect this single G output to the amp G input?

I am using an Avel 18V trafo with dual secondaries. Power supply works so far with 27 V on each side. That seems a little high for the LM1875.

Peace,
Tom E
Yes, just connect PG+ and PG- together to obtain the common ground. LM1875 will run very hot at +/-27V, but on 18Vac you should have just below 25V.
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Old 22nd February 2008, 06:45 AM   #3
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Hey, that's exactly how I run mine. Yes, it needs a big heatsink, but it sounds fantastic.

I didn't isolate the chip from the heatsink--I isolated the heatsink from the chassis (totally automatic because my amp chassis is wood, not metal). And there's a air vent under the heatsink. So, that works nicely at keeping the chips cool.

There's an "LM1875 Thoughts" thread and a lot of thoughts indeed. Some are better than others, but all provide interesting differences to help meet personal tastes.

The most noticable things about the LM1875 are its powerful dynamics, an extra dose of authentic clarity (wideband, not peak), and its fierce "driver command" that can run most woofers at a higher speed resolution. Okay, LM1875 is just wonderful.
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Old 22nd February 2008, 11:29 AM   #4
AndrewT is online now AndrewT  Scotland
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Hi Mad,
the PSU DC voltage depends not only on the load current but also on the transformer regulation and smoothing capacitor value and mains supply voltage. It will vary from hour to hour and day to day.
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Old 22nd February 2008, 11:32 AM   #5
Paswa is offline Paswa  India
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Quote:
Hi Mad,
so, we need a regulator?? not we??
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Old 22nd February 2008, 12:16 PM   #6
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Quote:
Originally posted by Paswa


so, we need a regulator?? not we??

need is such a strong word.

It depends from person to person, or usage to usage. For normal music listening or a movie sound track , I don't think it is needed. But it will probably sound slightly better. depending on a non regulated power supply you used to compares quality( transformer VA size, filter cap size , and a few other things)


But if you want to experiment , or want to be sure that your maximum output voltage and current is ALLWAYS exactly the same then you need one.
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Old 22nd February 2008, 03:00 PM   #7
Paswa is offline Paswa  India
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There is a regulator IC called LM138 or LM338, I find it here http://www.national.com/mpf/LM/LM138.html
Unlike the LM78xx serise the LM338 baby can handle upto 5 Amp . I never used it but hope to use it later. You could also join me. I think a single LM1875 will never draw more than 5 AMp. Hope you will reply
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Old 23rd February 2008, 07:59 AM   #8
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That amplifier chip doesn't need a voltage regulator chip whenever the power supply is providing power within tolerences for the chip. If the power raises or drops a bit, there's no noticable effect.

EDIT: However, there are some amplifiers that could benefit from a voltage regulator chip. For instance, Class D amplifiers and Tripath can be kept within their narrow "sweet spot vs. power vs. safety" if that power is regulated. So, thank you for the news of this easy-to-apply regulator. I will try it on my Tripath. But, not on my LM1875's.
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Old 23rd February 2008, 11:03 AM   #9
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by Paswa
the LM338 baby can handle upto 5 Amp ........... I think a single LM1875 will never draw more than 5 AMp.
but the speakers might ask for more than 5Apk on fast starting transients (& fast stopping transients).
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Old 23rd February 2008, 04:29 PM   #10
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The peak transients of the regulator 12Apk and the amp are a near perfect match, but is there any point to using the regulator?
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