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Old 15th March 2006, 02:17 AM   #1
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Default Input capacitor on BrianGT amplifier?

I'm currently ordering the parts for a BrianGT amplifier someone generously gave me the PCBs for.
However, I noticed the lack of an input capacitor. Why is this? Won't this cause problems with DC offset?

(Also, on an unrelated issue...has anyone noticed an increase in sound quality with the CarlosFM style unregulated PSU vs. the Chipamp.com style PSU?)
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Old 15th March 2006, 03:13 AM   #2
PierreG is offline PierreG  Canada
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Look here

Just added caps and snubber bit from CarlosFM design PSU

or use the search button
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Old 15th March 2006, 03:29 AM   #3
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Thanks for the link, but the LM3886 kit already has an (admittedly simplified) PSU.
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Old 15th March 2006, 04:34 AM   #4
Tweeker is offline Tweeker  United States
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The capacitor in question is (or is not, actually) at the input to the amplifier, and not PSU related.

An input capacitor is not required if DC is not present. Neither the BrianGT amp nor the datasheet app note circuit use one. A transformer or output capacitor may be needed at the preamp output though.
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Old 15th March 2006, 04:57 AM   #5
Nordic is offline Nordic  South Africa
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It is however a most convenient way of setting up lower frequency rolloff.

Just know that if you use it this is the most important cap in your system, and should be of the best quality you can possibly afford.
Not the most expensive thats out there, I think if you look at the most expensive lines of caps at say RS components you get a good feel for "real world" performance otherwise they would stock all those funny caps too.

I changed from 0.5 x 1.5cm input caps yesterday to proper audio caps measuring some 4 x 2 cm, it is not a slight change, its like my amp grew up, the very room feels larger when I close my eyes.
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Old 15th March 2006, 05:06 AM   #6
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There are some rather decent looking 4.7uf polypropylene caps at ApexJr for 2/$1. I guess I'll try a pair of those.
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Old 15th March 2006, 06:20 AM   #7
Nordic is offline Nordic  South Africa
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Real audio quality caps with 5% tollerance can be had for $10... come on you probably spend more on sweets in a month... don't be a 2 dollah ho, be a 10 dollah ho
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Old 15th March 2006, 10:55 PM   #8
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Quote:
Originally posted by Nordic
Real audio quality caps with 5% tollerance can be had for $10... come on you probably spend more on sweets in a month... don't be a 2 dollah ho, be a 10 dollah ho
Eh, they look okay.
Can you reccomend a brand?

Also, ApexJr has quite a few other inexpensive (surplus) capacitors. Because this is going to be external anyway, can you reccomend one?
http://www.apexjr.com/capacitorsR.html
Also, can you reccomend an inexpensive pot?

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Old 15th March 2006, 11:27 PM   #9
Tweeker is offline Tweeker  United States
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Go with a film and foil polypropylene capacitor versus a metallized capacitor. Theres nothing wrong with the apexjr cap, and its a steal, but this is not a cap to scrimp on. Certainly beats mylar or electrolytic caps.

Metallized capacitors have some advantages over film and foil capacitors in that they can be self healing, but that is utterly irrelevent to this application.
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Old 16th March 2006, 02:51 AM   #10
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Value for input cap?
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