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Old 9th June 2012, 07:18 PM   #1
MarkL1 is offline MarkL1  United Kingdom
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Join Date: Jan 2012
Default LM3875 problems/upgrades

Hi Iv just finished my first diy amp. Its the audiosector premium lm3875. So far its had around 25hours burn in time and the detail in the highs and treble are very good, But it seems the mids and the mid lows are missing and sound very muddled.
Would swapping the 1500uf capacitors onto the power board and buying some 100uf elna cerafines capacitors at the amp boards. Described here
http://www.diyaudio.com/forums/chip-amps/172926-adding-larger-capacitors-audiosector-lm3875-kit-2.html
Is there any other options to try?? Are elna cerafines at 5 each any good??

Another problem, its very loud even on 1/8th of a turn and i think it sounds better when i turn the volume on my computer down and the amp up. See similar problem here
http://www.diyaudio.com/forums/audio-sector/178960-audiosector-lm3875-volume-control-preamp-options.html
would the pot influence the sound quality, is it worth buying a 10k, better quality pot?

Also is it worth using better quality than my cheap silver plated copper hookup wire??

Thanks for your help

My amp uses
50k alps blue beauty
300va alirlink transformer 2x22v
Cheap 1 per meter silver plated solid copper 0.5mm hookup wire
Cheapish no brand gold plated brass rca and binding posts

My speakers are commercial monitor audio bx2s 2 way
Freq response 42hz-30khz
Sensitivity 90db
8 ohms

My inputs are from a laptop or bluray player fed into a dac, arcam rdac
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Old 9th June 2012, 07:25 PM   #2
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Location: Wirral UK
Hi Mark.

Do you have the PG pads connected anywhere? The pads look empty on the photo.

Nice case.

John
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Old 9th June 2012, 07:44 PM   #3
MarkL1 is offline MarkL1  United Kingdom
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Following the build instructions i didnt think i needed to

http://www.audiosector.com/nigc_kit-users_guide.pdf
see page 29
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Old 9th June 2012, 08:05 PM   #4
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I'd say missing lows are most likely the speakers beeing incapable of producing frequencies below 60-70Hz, and thus sound muddled as the woofers hit xmax which for speaker this small is about 1mm travel in either direction.

It can also be caused by the capacitors in the feedback path, the datasheet specs 22F for this cap, i use 100F and get nice deep bass.

I think the datasheet circuit assumes a small bookshelf system or the use of a subwoofer.
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Old 9th June 2012, 08:24 PM   #5
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Hi,

Is your second picture for real ? That is the way your actually using it ?

If so it will sound utterly abysmal in the frequency areas you describe.

You need 20" tall or so good speaker stands. Keep the speakers away
from the side walls and use the distance away from the back wall to tune
bass, but basically the further way from the back wall the better. Try to
make the distances of the speakers from the floor, side and back not
similar, this will smooth the mid bass and help utilise room bass gain.

rgds, sreten.

A 10K pot will make no difference whatsoever.
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When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 9th June 2012 at 08:46 PM.
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Old 9th June 2012, 08:49 PM   #6
hurtz is offline hurtz  Germany
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Join Date: Jun 2010
There appears to be no PSU which can result in a voltage drop at low frequencys considering the high(er) current demands resulting in insufficent excursion (muffle). This is very easy to check, wire up a voltmeter (preferably analogue) to the positive/gnd/negative rail and watch if the voltage drops.

1500uF per rail is very low, even with a 300VA toroid. It is much easier to achieve the desired stiffness using a smaller toroid with a larger PSU.

Different brand capacitors/cables will only help if you believe in them ;-)
If you must experiment with this, grab a friend to do blind-tests.

What kind of soundcard do you use? If its an onboard model it is more likely to introduce noise/distortion at maximum level.

You might want to measure the output of your amp to see if the "muffle" is introduced by the amp or is a result of amp+speaker.

cheers,
hurtz
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Old 9th June 2012, 09:02 PM   #7
MarkL1 is offline MarkL1  United Kingdom
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Join Date: Jan 2012
The set up is pretty poor at the moment, but im comparing the sound quality to my commercial marantz pm6003 amp. im Moving home soon so i decent set up will follow with speaker stands.

the laptop is fed into a usb dac. so no soundcard involved.

Im looking for a cheap solution so i might try using 100uf caps
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Old 9th June 2012, 09:20 PM   #8
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USB DAC = usb soundcard, external soundcard.
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Old 9th June 2012, 09:34 PM   #9
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Quote:
Originally Posted by MarkL1 View Post
The set up is pretty poor at the moment, but im comparing
the sound quality to my commercial marantz pm6003 amp.
Hi,

A poor set up is no way to compare amplifier sound quality*.

But you've opened a can of worms so I'll go for it. No way
will a chip-amp compare with a well designed hifi amplifier.
(Unless you take some very misguided opinions seriously.)

My budget favourite is the Pioneer A300-X, about 70 used.
A very nice amplifier you couldn't hope to build for the price,
and basically IMO something a chip-amp cannot better.

The Marantz is a highly rated 300 amplifier, I don't know
much about it but you can't expect a chip-amp to compete.

rgds, sreten.

*I'm lost for words as to how awful the set up in picture 2
would sound, and how it is impossible to make any sensible
judgements regarding amplifier quality. Quality doesn't come
into it, only differences that are in the end probably irrelevant.
__________________
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When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 9th June 2012 at 09:43 PM.
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Old 9th June 2012, 10:13 PM   #10
hurtz is offline hurtz  Germany
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So you have basically no idea what you are doing or what those caps really do in the signal path but want to switch them regardless? And after reading that for lower frequency = greater cone excursion = higher current demand 1500uF might not be enough, you want to switch them for much much smaller ones??

If you want to have a quick ready to go solution you shouldn't DIY.

Also for a comparison you don't need the perfect listening spot, it should just be the same. If the difference in sound is so obvious that you will hear the difference after rewiring the speakers there is something wrong. Is the muffling also occurring at low levels? If not you are probably driving the amp into clipping or into Spike(over-current protection).

@sreten
SInce a chipamp is basically a classic AB-amplifier in a smaller package any well designed chip-amp will perform as well (or bad to its specific respects) as the by you quoted hifi-amp (by wich I assume you are talking of solid state).
Also you might want to back up your claim with actual data rather than calling people misguided.

cheers,
hurtz
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