DIY LM3875 Chip Amplifier (Gainclone) Kit
DIY LM3875 Chip Amplifier (Gainclone) Kit
The Beast is a LM3875 IC (chip) amplifier, built from a popular chipamp kit. I'm hard to please and always looking for new things to try in audio. If it involves a DIY project, all the better. Mark and Gio had been after me to try a chipamp for some time. I really didn't need any more amps and the spousal concern factor has been growing! My collection of gear exceeds size of my listening room and stray pieces are being stashed throughout the house. If you have looked at the other projects on the site you will see that a modified K-12 is my primary serious listening amp. It does what I ask and more. But nonetheless, I took the leap and forged into new territory. When I started, I really didn't have a clue where it was all going. But I figured if it was worth doing it ought to be done well. My original plan was to build a chipamp with a solid state or tube buffer. I actually bought the parts. But being a bit of a conservative guy (translation, I like to do things cheaply if I can) I couldn't find a suitable enclosure for the project. I had plenty of small enclosures but nothing big enough. Scratch that idea. So I went ahead and built a chip amp without the buffer. I used the LM3875 Amplifier Kit (non-inverting) available from AudioSector.com. Nice printed circuit boards (PCB), good quality parts at a good price (thanks Peter Daniels). I had a suitable transformer that didn't make it into another project, so off we go.
More information about the LM3875 chipamp kits is available from the Users Guide for the Non-Inverted LM3875 Kit - (PDF 1MB). The schematic for the non-inverting gainclone / chipamp is shown in Figure 1 below.
Figure 1: LM3875 IC Chipamp (Gainclone) Schematic
Photograph 1: Beauty and The Beast - 12AX7 Tube preamplifier and LM3875 Chipamp
Test Build - DIY LM3875 Chip Amplifier (Gainclone) Kit
The first version (fortunately no photo as it would be just too embarrassing) was a collection of parts sprawled on a shelf with the rest of my gear. With all the hype on chipamps, I wanted to evaluate it for myself before I got serious about making it look pretty. It passed the smoke test. That is always a concern when something is powered by a 200 watt transformer. No smoke or fire, so I popped in a CD. Pretty nice sound, not top of the heap, but not trash either. I sat back to listen. For the first few hours it was kind of bright (really it was too bright for my tastes). So I let it burn in for a while. It cooked for several days as I forgot to turn it off. You see, I don't put line switches or power indicators in anything unless it is absolutely necessary. My system has two kinds of equipment, those that stay on all the time and those that get turned on with a master switch. I'm kind of a minimalist in some ways. Lights, switches and wiring can be sources of noise and hum and I hate hum. Given some limitations, I believe less is better. Now back to the amp. Over a period of days either my ears went south or it got mellower. I figure the amp changed and it was good enough to pretty up and do a few modifications. The first mod was to put snubbers on the amp boards next to the filter capacitors. I used 470 nf polyester caps to clean up the power a bit. I believe it worked although I can't point to any data that supports it. It just sounded cleaner and was easier to listen to. I next tried bigger filters, but took them out in favor of the 1500 uF Panasonics that came with the kit. Like I said earlier, I didn't believe everyone when they said 1500 uF would be enough. I figured 5000 or 10000 uF would be better. I was wrong, it didn't seem to make any difference I could hear.
So how did it sound? Very low noise, very quiet background, the term I would use is precise. Well controlled and balanced. This is well and good as the 4 ohm Dayton MTMs I use might easily be a difficult load for some amps. I didn't include a Zobel network (less is better again) and didn't have any problems. The amp is DC coupled straight through.
My original design had the heat sink on the bottom of the amp chassis and during hard use it kept shutting down. Thank you National Semiconductor for saving me from myself. It looked cool sitting on tall feet, unfortunately it didn't run cool. So change of plan, flip the amp over (and see its ugly belly). A modest heat sink stuck on a piece of tin, how tacky.
Enclosure and Construction - LM3875 Chip Amplifier (Gainclone) Kit
To make both parts look alike I used two plastic project boxes from Radio Shack. The power supply found a home in an old computer printer switch enclosure. Everything fit like I planned.
Photograph 2: Power Supply for LM3875 Chip Amplifier
Photograph 3: Power Supply for LM3875 Chipamp
Still the basic set up was rather utilitarian, I wanted a distinctive look. It came from an odd place. I discovered face plates designed for home theater wiring systems at a local hardware store. It turns out that they are available from many sources (Parts Express, Radio Shack, MCM Electronics, to name a few). They come with snap in jacks of all types. The jacks are gold plated and pretty decent. The plates run about $1US and the jacks about twice that.
I did run into a problem during assembly though. The jacks had a tendency to loosen up. So a small dab of glue and the problem was solved. Hint; use the glue after you have soldered the connections. Why you might ask? Well in short, glue smokes like crazy when you try to solder through it.
Photograph 4: The Beast's Guts
The next step was to get everything into the boxes. The chipamp was easy as there are almost no parts. The only thing that might trip someone up is that it is a good idea to use heat sink grease between the chips and whatever you use to absorb the heat. Even with the heat sink on top they get pretty warm if you work them hard.
Photograph 5: LM3875 Chip Amplifier Enclosure
Measurements - LM3875 Chip Amplifier Kit
Next it was time to take the amp to my work bench and test it. I always listen first then measure and scope my projects. That way I have no preconceived ideas about how something should sound based on the measurements. Measurements are helpful to refine, but I allow my ears to be the final judge on any changes. So how did it measure? In short excellent, as the scope traces (see photos) demonstrate. The amp is one of those rare pieces of audio gear that acted much like a wire that amplified. It had flat response from DC to 100 kHz. 100 kHz square waves were pretty square with only mild rounding of the leading edge. Noise was below the resolution of the scope. My ears said something in the order of -90 dB.
Photograph 6: 100 kHz Square Wave Response - 0.2 volts/division into 8 ohms (resistive)
Photograph 7: Finished LM3875 Chip Amplifier (Gainclone) Kit
Impressions - LM3875 Chip Amplifier Kit
When driven by my solid state Velleman preamp the amp was detailed and accurate. It had a good sound stage, no noise. The Velleman behaves much like a buffer in this set up. There was a sort of marriage of parts. It was a good blend. However, when I attached Beauty (12AX7 Tube Preamp kit) to the Beast, it didn't sound as good as I thought it should. The tube preamp just didn't sound right. There seemed to be something going on in the upper ranges that I just couldn't define. To get the details of the fix, see the Beauty (12AX7 Tube Preamp kit) article as the description is a bit lengthy. Needless to say, I solved the problem and everything sounded like I thought it should.
Final Thoughts. After listening to the tube-chip combination for a while, I really would have loved to have this quality of sound reproduction at any time in the past. It easily surpasses everything I've owned except for my modified K-12 Tube Amp kit. The only area the K-12 beats the Beauty and Beast is in midrange detail and sound stage. For sheer power and presence the tube-chip combo is excellent. Very clean presentation and lots of impact. Horns and percussion are great. Signal to noise is extreme. With either preamp there just isn't any background to detract from the music.
If anyone needs a quality music system, this is the way to go. As a plus the price is right. The cost of building the Beast from all new parts is under $100US. With the gingerbread and stuff to make it pretty add another $50US. Good listening everyone.