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My latest DIY effort - LM1875 power amp

Andy_H

Andy_H

Behind the Wall of Sleep
Based on my success with Bauhausler's LM1875-based Squelette project, I decided to build another amplifier utilizing his circuit design, with a few modifications. This one will be a straight power amp so I can play around with different preamps. And while I found the original Squelette chassis design clever and distinctive, my metalworking skills were not quite quite up to par and I wasn't entirely happy with how that aspect of my build came out. Thus I decided to use a pre-made enclosure for this one - an old Technics CDP with issues that I realized I was never going to get around to fixing, so I gutted it to reuse the chassis.

I got the power supply components wired up yesterday - tests out fine. Today I will be working on getting the boards filled with the other components.

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There's a lot of room in there for such a compact little amplifier circuit. I haven't settled on the positioning of the components yet, but I will be reusing the power inlet and RCA jack locations from the Technics for simplicity's sake. I was also wondering if I should cut a piece of aluminum to make a shield between the transformer and the board. Finally, I need to figure out what to do for the faceplate - probably fabricate something plain and simple from wood or a sheet of aluminum. The only thing that needs to go on the front is the lighted power switch.

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Your xfmr already has electrostatic shielding, and aluminum won't provide magnetic shielding. Anyway, power amps don't generally require magnetic shielding unless clearances are minimal and you're unlucky. It does happen, but very rarely in low-power amps like this. If you encounter buzz/hum when it's built, try holding a steel shield in place manually before drawing conclusions.
 
this reminds me I've got a pair of LM1875's boards built just sitting around, I need to get a suitable transformer
 
The board is now complete and ready for signal testing, but that won't happen tonight. I'm ready to call it a day.

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It lives!

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The front panel of the enclosure still needs done, but it's been playing fine for about 1 1/2 hours. DC offset is good around 1.5 mv. (That's AC mv on the meter as pictured.) I suppose I'm about ready to take it off the cheap Sonys I have it hooked up to in case of meltdown. It seems to be running fine but it's still a little scary to connect something I built myself to my prized Polks or JBLs. :para:
 

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BinaryMike said:
Kudos! Always a pleasure to see successful first-flight reports.
Click to expand...

Thanks! I think the fact that I built this same circuit once before was key to this build going smoothly. I made all the mistakes first time around and learned what not to do on this one :). It also went much more quickly this time. I'm learning, slowly but surely.

Right after my previous post I moved the new amp into my main system and listened to several albums. This design makes for a really impressive little amp. At least in my space it has no trouble putting out enough volume - hard to believe it's only 11 watts. I have a couple of much larger power amps, one or the other of which usually resides in my main system (Adcom GFA-545II and Nikko Alpha III) and while the LM1875 wouldn't dethrone either of them it puts up a respectable performance, especially considering that it costs ~$50 and can be assembled from scratch by a novice builder. Bauhausler made a real winner with this design.

I love this world of DIY audio! I have a whole bunch of other projects I want to make: a CMoy headphone amp, a Pass B1 buffer (board and JFETs on order for that one), a Hagerman Bugle phono stage, maybe an LM3886 chip amp, and, someday when I have a little more experience, a Pass F5 …
 
National Semi invented a real winner in the LM1875. I believe the same chip was also sold with slightly different specs as an industrial power op-amp: LM675. I've used them successfully as DC motor drive amplifiers, among other things. The LM1875 eventually led to large family of audio power amp chips with very impressive performance per dollar.
 
All done!

OK, here it is, all finished up. The thing that took a while was finishing the wooden faceplate. I had a scrap of a poplar 1x3 that conveniently was just the right size. I took my time sanding it nice and smooth and applied a Minwax Ebony stain and Polycrylic finish, and got it all buttoned up tonight. Hooked into the main system and I'm listening right now - it's a really a pretty impressive little amp. I need to experiment with some different preamps next.

A few more nude shots:

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And all wrapped up:

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Not a perfect fit and finish, but not too bad for something built mostly from scraps, on a shoestring budget. :)
 

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