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Old 5th November 2013, 10:23 PM   #1691
tommy_o is offline tommy_o  United States
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Quote:
Originally Posted by stijn001 View Post
Thanks AndrewT, in future I will.

Does anybody have the schematics for the LM3875 AMP module?
Are you asking for the datasheet?
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Old 7th November 2013, 06:14 PM   #1692
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Hi Tommy_o, I was after the the schematics to go with Peter's lm3875 board. I've worked it out by now thanks.

I've build the premium kit amp. In many ways it sounds amazing, very detailed and refined.

However I still have a few issues:

- Once in four of five power-ups I get a oscilating sound, "whu" sound- 1 sec silence- "whu" sound etc.., coming from both channels. I have separeted PSU boards and it contineus even when I turn my pre-amp off. It doesn't sound like a hum either, very strange. It goes away after 1 min max. Does anybody have a clue what might be going on?

- Secondly. I'm picking up very high frequency noise coming from the pre-amp. I don't think my pre-amp/dac is particularly bad, it's just that the GC is brutal. I'd like to trade in some gain against raising the noise floor by increasing the shunt resistor at the LM3875 input. Would that work? Also what would be a good high frequency filter, if anybody has got any suggestions.

Lastly I think a EMI filter is needed too in front of the transformer. Things like my doorbell are clearly audible through the amp.

Any other options suggestion to my issues above would be highly appreciated.
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Old 8th November 2013, 06:47 AM   #1693
AndrewT is offline AndrewT  Scotland
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I want to respond, but Peter takes exception to my comments.
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Old 8th November 2013, 07:35 AM   #1694
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Should I start a new thread in the D-amp section?
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Old 8th November 2013, 07:55 AM   #1695
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Quote:
Originally Posted by AndrewT View Post
Beginners should never omit the "optional" stability and filtering components.
I'm starting to suspect this is a pretty valid comment.

Which options are recommended as the boards allow for quite a few?

Which is a great feature by the way!
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Old 8th November 2013, 11:44 AM   #1696
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Right, I’m taking the following approach.
I’m going to change out R1 to a 1k resistor and am going to put a 470pf HF filter cap across the RCA’s, to start with

Also:
I’m still wondering if I should use a C-in, Peter suggests an optional 4.7uf or higher. I read elsewhere that C-in should be atleast between 5-10uf. As this cap will be inline I suspect it will have quite an influence the amps sound. What value/ size voltage wise do people recommend here? Also as there doesn’t seem to be a position other then in the place of R1, would I need to place these caps outside of the board?

The possible benefit of a 10uf at C1/2 is not clear to me, can someone explain what I stand to gain with adding these optional caps?

Thanks,

Last edited by stijn001; 8th November 2013 at 12:03 PM.
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Old 12th November 2013, 04:02 PM   #1697
AndrewT is offline AndrewT  Scotland
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The input cap is also referred to as the input DC blocking cap.
It does two quite separate jobs.
It blocks accidental DC entering the Power Amplifier.
It applies a high pass filter to the signal sent into the Power Amplifier Input.

The value of the input capacitor does NOT affect it's ability to block DC. However some types of capacitor pass more/less leakage than other types. For least leakage (=good) do not use an electrolytic, nor paper. Some of these can leak significantly.
Use a film type.

Thge value of the capacitor does affect the roll off frequency (or passband) of the filter passing the signal.
There is an enormous variation in user preferences and some technical requirements.
The F-3dB (= 1 / 2 Pi R C) is usually set lower than 20Hz. Some will set it lower than ½Hz.
I like 1Hz to 2Hz for that F-3dB, to ensure that 4Hz is within 1dB of flat. -1dB @ 4Hz is probably not audible, but this error adds to all the other errors both inside the Power Amplifier and before the Signal reaches the Power amplifier. You should listen to your own system and decide how low you need the filter frequency to be such that it is inaudible.

RF filtering.
The series resistor and the following shunting resistor to Signal Ground form a low pass filter that attenuates the high frequencies. Again there is enormous variation in user preference. Listen and decide what suits you.
Start with F-3dB @~50kHz and see if that is audible. You may have to go as high as 300kHz to make this RF filter inaudible for you and your system.
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Old 15th November 2013, 02:03 PM   #1698
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I've ordered a pair of AURICAP XO Series, 4.7uF Metalized Polypropylene Cap, to use as input caps, which I've read good things about. I've already tested with 2.2uf and that seemed to solve the Vdc problem.

Is there an online calculator or formula in which I can plug in the two resistor values to determen the RF cut-off frequency?
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Old 15th November 2013, 03:21 PM   #1699
udailey is offline udailey  United States
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Yes. If you read AndrewT's informative post you'll know what you need to Google.
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purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
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Old 15th November 2013, 09:29 PM   #1700
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Thanks udaily, if you are so in the know, you could have just answered my question. But instead you chose to try and patronize me. That gives me the impression you are not a very pleasant person, prove me wrong. Please be assured that, I did "read" his comment very well, I Googled "rf filter calculator" and several other combinations and did not find what I was looking for, hence I asked the question. Don't believe me if that's your disposition, but don’t blame me for it.
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