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Little red rooster - my LM3886 based amp

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doors666

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I have been working on a few designs for a while now, finally managed to put one in a semi-proper chassis. This one is an LM33886 based amplifier. This one is going to be used as soon as I wake up, so it will be called Little Red Rooster:) and of course the blues theme. Some pictures are also attached, I will post more detailed information soon. I got some extra boards for my projects.

I am really happy with the way this pcb layout and overall design has turned out. I have tested it with ipod, mobile phone (mp3s) and odac with flacs as sources, with or without my B1 preammp. Its absolutely quiet with no hum, even with my ears an inch from the speakers. There is no turn on/turn off thump. The dc offset is about 2mv for one channel and less than 1mv for the other one.

I have tested it with shitty test speakers, continuums (BBC LS3/5a or harbeth P3ESR clones), overnight sensations (DIY), B&W602 S2. Yet to test with alpairs.

Sound is very neutral, to me, it sounds much better than my topping tp20, which its going to replace with my desktop speakers (alpairs and continuums).

I tried various genres, blues, jazz, hard rock, metal, rock n roll, hindi crap quality mp3s. Sounds good and very musical with them all.

I still have some work to do in the chassis, mainly replace the heat sink with a better single HS for both channels and the face plate. I will reroute the wiring while redoing the sinks as the speaker wires are quite close to the sinks right now,, and get better speaker binding posts, the good ones I have unfortunately turned out to be non-insulatable from the chassis. I was desperate to make and hear it, so connected it however I could. Will also put a loop breaker also when I replace the heatsink, though there is no hum, its still better for safety also.

Since there is a slot for a fan, will put one fan with probably 9v supply to keep it quiet. It will have a switch also, and I plan to use the fan only with the floorstanders (diy) which are 87db 4ohm MTM monsters (ER18MTM) and keep it switched off when not needed.

Transformer is 22-0-22 250va. I am using a thermistor (10R, 2A) in the neutral line for softstart. Supply uses 4700uf 63v panny caps (they measure at about 5500uf though:), with ESR of around 23mR). On board large caps are 2200uf SMG and 150uf panny fc. There are some SMD parts (all are 1206 for ease of soldering) to keep the feedback loops, bypass caps and input stage as small and as close as possible. There is space for a rail to rail SMD cap which I havent installed or experimented with yet. Coupling cap is arctronix 2.2uf 250v, waiting for wimas though. All SMD caps are NP0/C0G, I also want to try SMD film caps for these as well as thin film resistors, yet to order these.

There is an RF filter at the input, the entire input stage is shielded with a ground pour shield on both side of the pcb. zobel and thiele networks are there at the output. Schematic and pcb layout are also attached in case anyone wants to take a look. I will also upload some scope pics soon.

I have also made an LM1875 amp build which sounded really good, but havent put in a proper chassis yet. So hopefully I will put a write up for that too soon.
 

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doors666

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Some amplifier PCB pictures...

Circuit schematic and pcb layout....
 

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doors666

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Hi
What is your opinion about continuum? Are you using any sub ?
Regards

Those are really really good speakers, specially at that price point. The designer jeff bagby has stated somewhere that his are better than the harbeths. I havent done a comparison myself.

I have a sub but I usually dont turn it on. These have a decent amount of bass. This is my music only setup in a smallish room.
 

somakmaitra

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They are really excellent speakers except the bass is a bit lacking .The voicing is just awesome.

Regards
Somak
 

doors666

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That's nice project.How much power it pumps per channel?

This is straight from the datasheet for LM3886 -

68W Cont. Avg. Output Power into 4? at V CC = 28V
38W Cont. Avg. Output Power into 8? at V CC = 28V
50W Cont. Avg. Output Power into 8? at V CC = 35V
135W Instantaneous Peak Output Power Capability

They are really excellent speakers except the bass is a bit lacking .The voicing is just awesome.
Regards
Somak

Thats a nice looking build.

yeah the voicing is superb, and they are lacking a bit in bass, but not too bad considering its still a small sealed bookshelf. There is a floorstanding version of it somewhere on the tech talk forums. it also got the approval of jeff. IIRC its identical design with just a taller cabinet. It does a bit better in the bass department without compromising the voicing.
 
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doors666

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Atlast I received my good caps order. I changed the feedback cap from panasonic fc 470uf/25v and a 0.1uf poylester to elna silmic 220uf/6.3v and a 10nf wima FKP2. This made a huge difference in sound quality. Improvements were across the board, bass, treble, clarity, tonality, soundstage, everything became much better. Then I changed the arctronics R46 2.2uf to a wima 2.2uf, which is a physically smaller cap also. Things imrpoved, but not as much as when changing the feedback cap. I guess that means that the arcotronics cap is pretty decent. I heard a lot of difference moving from generic chinese polypropylenes to arcotronics.

Finally I found some time and managed to changed the website with a new layout and improved the build guides also

some pics -





Finally closed the chassis, for the time being:)

 
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