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 Post subject: LM3875 first go
PostPosted: 23 Feb 2016, 08:37 
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Joined: 26 Sep 2015, 13:59
Posts: 58
Hi,

I'm attempting build a LM3875 chip amp (I have only built valve amps up to now). I normally like to use a regulated power supply if possible to keep all things nice, stable and smooth. However, I notice that none of the schematics on this site use regulated supplies. Is this becasue the current draw is too high for a LM317/337 combination? If so why not use some pass transistors? Also the amount of smoothing capacitance looks way lower than I would have expected, I usually use at least 20,000 uF in a 6.3v heater supply to get things quiet! Perhaps the LM3875 has some special tricks up its sleeve??

Please go easy on me, I'm new to the modern world of silicon!

Mark


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 Post subject: Re: LM3875 first go
PostPosted: 23 Feb 2016, 09:13 
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Joined: 18 Aug 2015, 07:38
Posts: 164
This is one thing I also noticed when building this amp. It works as is and a regulated PS is not needed. I understand others have increased the PS capacitance but others say it compromises the mid-range sound. Class A amplifiers have much more capacitance in the PS which is needed for Class A amplifiers but for this amplifier it is not needed. More technical members will be more able to explain why.


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 Post subject: Re: LM3875 first go
PostPosted: 23 Feb 2016, 09:35 
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Joined: 04 Jun 2008, 20:59
Posts: 3746
Location: Arizona, USA
Hi, I somehow just lost a reply to this so I'll try again. You are essentially correct on the current load and small capacitors. The IC seems quite happy with rather cruddy power as long as there is enough of it. It is undoubtedly a push-pull design with some internal regulation for the driver stages. For a regulator if you want one I would use a LT108X series ones. They are low drop out (about 1-1.3 volts min) and go up to about 7 amps. Do both positive and negative rails. I did one of these amps a while back from a partial kit. It is posted on the site as Beauty and the Beast. It had small filter caps too. It didn't seem to matter. Output was clean, quiet and very linear. To my way of thinking a bit sterile sounding. But this is typical of most solid state amps. I have gone to an all tube system and really like that sort of sound so while good, the IC amp is on the shelf. Naturally YMMV. One thing that seems a problem with the amps is they are rather sensitive to ground loops. I see the issue quite regularly on various forums. Careful attention to grounding and shielding is in order. I suspect that most builders mess up on connecting the signal ground (and internal power ground) to the chassis directly. Some form of separation should be used, either a pair of rectifier diodes or a resistor / X2 capacitor (indeed both things can be used at the same time). Another likely source is connecting the Left and Right signal grounds together in more than one place. Just don't. Often this is when using shielded wire inside a chassis and the shields are joined at both ends. These are things you probably already know, but trip up a lot of new builders.

Good listening
Bruce

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 Post subject: Re: LM3875 first go
PostPosted: 01 Mar 2016, 13:08 
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Joined: 26 Sep 2015, 13:59
Posts: 58
Great help and advice, as ever, thanks.

I now have my LM3875 amp up and running. When I first turned it on I thought it was dead becasue it was so quiet! But all was well. I followed Bruce's rules for grounding that I have used in my valve amps.

I decided to build the most basic version with the minimum of components in PSU and amp circuit and it works pretty well. I will try a few tweeks over the next few weeks to see what might improve things - including a regulated power supply and more capacitance close to the chip.

I am using my home built forewatt valve preamp as input, fed by a highly modified Music Fidelity V-DAC. My first impressions are of a superb and detailed midrange and sound stage. Bass is extended and controlled but a little dry. High frequencies are, to my ears, lacking. It souds like there's a cotton sheet between me and the music. Perhaps it will loosen up with time. On the whole an amazing sound for such a simple device. However, it woun't make me throw away my valves just yet!

Mark


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 Post subject: Re: LM3875 first go
PostPosted: 08 Mar 2016, 08:05 
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Joined: 18 Aug 2015, 07:38
Posts: 164
RevM wrote:
Great help and advice, as ever, thanks.

I now have my LM3875 amp up and running. When I first turned it on I thought it was dead becasue it was so quiet! But all was However, it woun't make me throw away my valves just yet!

Mark


Tubes rule! For the money the 3875 amp does sound better than most commercial offerings with a much higher price range. A great build for anyone and you better get some chips while you can. Mouser still had a few the last time I looked. Fakes and counterfeit ones are going to appear next. A good tube or SS Class A is another step up in sound. I have built 4 Class A SS amps but they still will not replace my best SET.


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