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Old 11th July 2009, 09:41 AM   #781
AndrewT is offline AndrewT  Scotland
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make sure the GC is not a GC.
Add all the components to convert it to a normal AC coupled chipamp, with correct non-inverting gain, a Proper PSU with proper Decoupling, a Zobel or better a Thiele Network on the output and low pass+ high pass filters on the input.

When that is all attended to design/build your buffer/pre-amp.
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Old 11th July 2009, 10:41 AM   #782
PJPro is offline PJPro  England
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Default Re: passive pre

Quote:
Originally posted by kele1975
If you have interesting links where I can buy quality case for preamp,I will be grateful.
How about Modu?

Clicky
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Old 11th July 2009, 10:52 AM   #783
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Location: bogu iza nogu...
Default Re: Re: passive pre

[QUOTE]Originally posted by PJPro
[B]
How about Modu?

I was just looking at their site.Prices are good,but the shipping is a bit expensive.And design is too plain.But thanks anyway.
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Old 14th July 2009, 09:15 AM   #784
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Thanks for all the input cap advice. I have it in a more permanent home that doesn't involve an ipod , so I think I'm going to opt to leave the input cap out for now.

Peter I think you did a fine job getting the point across that the amp is DC coupled. I don't see any reason you should be in the hot seat. I did heed your advice and check the bias before connecting speakers, and the amp was functioning fine with the ipod for several days before Murphy decided to pay a visit.
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Old 17th July 2009, 06:25 PM   #785
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Default use of LM3875 kit from Audiosector with a 4 ohm loudspeaker

Hi all

I’m very interested in build a LM chip-based amplifier for using it in my secondary system, connected to an old Meridian 206 CD player and a pair of DIY loudspeakers. My doubt is what chip to use. The LM3875 kit from Audiosector is very interesting, because there is a Premium version available (chips selected for very low DC-offset, high quality components), its rectifier PCB is very small (the same size as the amplifier PCB) and Peter Daniel gives very good technical support for builders here. However, I’ve read that this chip is less appropriate for low impedance loudspeakers that the LM3886, so I hope you can give me some advice.

The loudspeaker I’m going to drive with the chipamp is a small DIY loudspeaker from the Intertechnik catalogue, the model called “Kohärenz”, based on the Seas MP14S/H719 4 inch 2-way coaxial driver. My 15x15x30cm enclosure (internal volume= 4.3 litres) is a little taller than the described in the kit.

The loudspeakers are used in a small room (11 sq. metres), placed directly on a desk and against the rear wall. I like its coherent, clear and controlled sound. I listen to them from two different positions (one at 60 cm and other at 2 m distance), mainly classical music at low volume, and I’m not a bass-freak. According to the Intertechnik catalogue, the Kohärenz bass driver is 4 ohms nominal, and the impedance vs frequency curve is as shown in the attached image. It is rather low, with a minimum of 3.5 ohm at 300 Hz.

Do you think the LM3875 is suitable for this specific loudspeaker and my listening habits, or would the LM3886 be a better choice? If so, is it possible to use the Audiosector kit with the LM3886 chip or I need to buy the Chipamp.com kit?

Thanks in advance

Jose
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Old 17th July 2009, 09:30 PM   #786
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"Due to high demand," National is charging a "small shipping fee" for samples. Small like $72.00 for five chips!
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Old 18th July 2009, 10:30 PM   #787
jh0n is offline jh0n  United States
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just finished building my first gainclone - i'm measuring 23vac out of the transformer, and 30vdc across the binding posts. volume pot seems to have no effect.

any idea where i should start looking - i don't believe this is normal. no shorts/cold joints/etc. that i can find.

thx in advance.
-j.
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Old 20th July 2009, 01:50 PM   #788
AndrewT is offline AndrewT  Scotland
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Default Re: use of LM3875 kit from Audiosector with a 4 ohm loudspeaker

Quote:
Originally posted by spendorspain
the Kohärenz bass driver is 4 ohms nominal, and the impedance vs frequency curve is as shown in the attached image. It is rather low, with a minimum of 3.5 ohm at 300 Hz.

Do you think the LM3875 is suitable for this specific loudspeaker and my listening habits, or would the LM3886 be a better choice
There are many that use and recommend 4ohm speaker driven by chip amp from a lower voltage PSU.

I say stick with 8ohm or greater.
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Old 21st July 2009, 06:46 AM   #789
40 watt is offline 40 watt  Canada
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Default Re: Re: use of LM3875 kit from Audiosector with a 4 ohm loudspeaker

Quote:
Originally posted by AndrewT
There are many that use and recommend 4ohm speaker driven by chip amp from a lower voltage PSU.

I say stick with 8ohm or greater.
My baffle is cut for an 8 Ohm woofer [16cm] and a 4 Ohm tweeter. I don't know what came over me to do that. To maintain the 8 Ohm load I've decided to add a second tweeter in series, but mounted offset in the rear of the speaker. I had some Infinity Reference Standard 2.5's that did that, though they were four Ohm, and were probably parallel, I can't recall. I have in hand 8 of the little neo tweeters so if I use an extra pair for impedance peace of mind I won't sweat it. The price was right...

Any thoughts on how the rear-firing tweeter would sound? I don't hear much of that these days but Linkwitz modified his Orions for an additional rear -firing tweeter and waxed eloquently about it. I can say that no ears died from my Infinitys either.

I can't quite get my brain around a series hook-up that allows the front tweeter to be L-Padded down to match the woofer but the rear one would be more effective if allowed its full efficiency, about 4 decibals higher, and end up at the desired 8-Ohm nominal.

I guess if I HAD to the rear tweeter could be after the L-Pad for simplicity, but I know it will nag me if I don't at least try some configuration...

Anybody have a quick and dirty schematic?
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Old 21st July 2009, 02:57 PM   #790
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Default Power Supply

Newbie here, thinking of building one of these, dual mono....
Q. Is there any point in building two discrete power supplies when I can buy two SMPS of 250VA from China (my local provider) for less than the price of a single decent torroidal transformer?
Thanks in advance
/Jon
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