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Old 21st May 2008, 04:17 AM   #41
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Thank you Peter for all the information and help!.I Have build three of your LM3875 kits.and I am 100% happy with the result.
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Old 21st May 2008, 02:25 PM   #42
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No problem, I'm always willing to help

Here's wiring done for inputs and outputs. By proper internal layout we can minimize cables length. This is not critical, but won't hurt either

Next, versin with potentiometer.
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Old 21st May 2008, 03:14 PM   #43
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I've been using Noble pots for years; while they are not perfect, for the price they offer great value. Available from percyaudio and partsconnexion.

Most suitable value is either 25k or 50k (log taper, stereo). Picture below shows the connections, pin 1 & 5 are not being used in normal application (those are for loudness option)
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Old 21st May 2008, 03:19 PM   #44
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And here how it's implemented in the amp.

I use SG (signal ground) on amp board as star ground for input signal. The ground wire from RCA and from ground pin on the pot (pin4) connect to SG. Signal wire from RCA goes to pin 2 on the pot, from pin 3 it goes to IN on amp board.

Those are the connections for one of the channels, the other channel is done exactly same way. Please note that so far the grounds are kept separately.
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Old 21st May 2008, 03:44 PM   #45
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Quote:
Originally posted by Peter Daniel
The amp basic schematic shows only 4 resistors per channel.
R1 is optional and its value can be anything between 200R and 1k (or so). I usually don't install that resistor at all, placing piece of wire in its place. Alternatively, if you need coupling capacitor to protect the amp from DC that may be produced by a source component, a small electrolytic cap can be installed here: 4.7uF or bigger.

Hi Peter,

This is a great thread!

I have built myself a gainclone based on your schematic with R1 (220 ohm) installed. Can you give some comments on the sonic difference when we put a piece of wire in place of R1, for this particular design?

Thanks,

Joshua
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Old 21st May 2008, 06:37 PM   #46
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Without R1 the sound will be more neutral and transparent, especially with Premium kit.

I supply that resistor "just in case", to provide some isolation for amp's input.

I had a case when DAC was connected to the amp (without series resistor) and some RF interference was occuring when pot was completely turned down. With a resistor in place, the problem was gone.
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Old 21st May 2008, 07:00 PM   #47
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Talking about interferences, there are certain instances, I guess mostly due to location and local house wiring, where the amp is sensitive to switching appliances and house lights, and may also pickup quite heavily radio frequencies. In my experince, this is some 5% of users that need to deal with that problem and the cure is quite simple, by installing small polystyrene capacitor (300pF or so) between inverting and non inverting amp inputs (pin 7 and 8 in attached diagram).

That issue was also discussed here:
LM3886 very prone to mains noise
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Old 21st May 2008, 07:38 PM   #48
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Quote:
Originally posted by Peter Daniel
Without R1 the sound will be more neutral and transparent, especially with Premium kit.

I supply that resistor "just in case", to provide some isolation for amp's input.

I had a case when DAC was connected to the amp (without series resistor) and some RF interference was occuring when pot was completely turned down. With a resistor in place, the problem was gone.

Very useful information, thanks Peter!

Joshua
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Old 21st May 2008, 07:50 PM   #49
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Peter,
Very nicely done!
Kudos.


7/10
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Old 21st May 2008, 08:53 PM   #50
sneih is offline sneih  Indonesia
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hi peter,
ur thread give me great tutorial...
thx u so much


nb:
i stiill waiting ur power supply section

gracias
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