Building a better amplifier with LM3886

LM3886 and other similar chips from National Semiconductor are the basis of amplifiers often known as "Gainclones". That's a nod and a wink to the iconic Gaincard amplifier from Sakura Systems, a very simple, very expensive and by all accounts very nice sounding japanese amp.

Many choose to construct a gainclone with point-to-point wiring, to reduce connection distances and number of parts. That's still a little too hard core for me. It's easy to fry stuff when you have a rats nest of solder and wires hanging off a chip just inside the front panel.
So I chose to go with one of the many PCB-options, a kit designed by Brian at Chipamp.com and sold by Mad About Sound.
What you get is four pcb:s and a bag of loose components. All are well withing acceptable quality, and the pcb:s are super nice, good layout and very, very sturdy. Everything is trough hole and makes for easy soldering.

The setup is: two separate power supply units for left and right channel and two amplifiers (yes, left and right channel. :) The PSU boards are really designed for two outputs each. I decided to solder everything on - there might be tweaks and modifications in the future, but it's possible to bridge rectifiers and save some room.
You have to get all the big stuff yourself: transformers, heatsink, chassi and connectors. I butchered my old, old Proton 520 amplifier and used the iron-core transformer from that unit. It's of dubious quality at best and has a truly astounding number of secondaries - everything from 70 to 25 volts out. It took some headscratching to get everything up and running. Still, no major fries. (If you're new to this: Mains power can kill you, so watch out.)
I re-used the heatsink from the same unit. It's larger than the LM3886 needs, at least with the 30 volts I'm running it on, but it was easier to leave it as it was. (And you never know... tweaks in the future maybe.)

I first ran the whole thing topless in the box with a pot lumped in for testing. It sounded tremendous: It's finally an amplifier that can fill my livingroom! Clarity was excellent, bass nice and neutral and the thing is completely silent - until you press play that is. :) It delivers a nominal 68W per channel into 4 ohms if I remember correctly, so it's quite possible to blow the speakers. Volume is painful at about 50% in 8 ohms. All done? NO.
Some preliminary testing showed that (as many have said before) the LM3886 really comes to life with a preamp. Also, I wanted to get all ratty connectors and controls in a separate box to ease future tweaking. I decided to build the Project 88 preamp from Rod Elliott, mainly because I already had the parts, after a bit of scrounging.

The power supply is the excellent TREAD board that Tangent sells, a nifty, pretty little thing that I warmly recommend. I lifted the voltage divider from Tangents article on virtual ground circuits. (Frankly there's no better site to go to if you want to start out in audio DIY. I bet he has a background in technical writing - everything is there, crystal clear.)
The whole thing was put on three separate boards to ease future tweaking.

The preamp box has six inputs, five in back and one in front, with a generic selector switch. The volume control is an Alps 50k with two resistors piggybacked. Not pretty, but I'm waiting for the energy to get a proper 100k stepped attenuator. Maybe santa will bring one, who knows. :)
The whole thing took a bit of trial and error. The main problem was grounding. The TREAD is powered by an external PSU, and the ancient mains wiring in my house has no grounded wallsockets, so there's still a tiny, tiny amount of mains hum in the speakers. It's damn annoying but not really noticable without ears to the speaker elements and volume off.
A preamp really lifted the amplifier sky high. The sound is much more dynamic. The reason is possibly that my DVD-player doesn't deliver quite enough Vrms out to drive the thing right.
Ok, so what now? Shall I put my soldering iron to rest and take up golf? No way.
Upgrades
The power amp: two separate toroidal transformers for L/R channel, possibly. Shielded internal wiring, better grounding plan. Maybe some experimentation with output / input capacitors, but probably not.
The preamp: I'm thinking about testing some of the preamps from Decibel Dungeon. I'm also very interested in trying the Aikido or similar simple tube preamps. I have a hunch that this might be a really good idea for this particular power amp. The PSU could use some work: probably an internal toroid. Shielded internal wiring. Maybe a fully balanced connection between pre- and power amplifiers. Everything on a single PCB to minimize wiring. A better volume control, and I think that a stepped attenuator is really the way to go here. Grounding needs improvement. Some sort of turn-off protection circuit would be nice. If the unwary turn off the preamp first there's a quite scary noise when the capacitors unload.
Also both amplifers need prettier on-switch knobs. :)
Construction time was fairly reasonable. The 5/95 rule showed up in big style - when you're almost done, 95% of the job remains. Putting everything in enclosures, wiring and fitting front panels was easily the biggest job. I had to improvise a fitting for the preamp's power connector from an old broomhandle to make the box fit on the shelf
What's next? Wait and see. DIY audio is about as addictive as a hobby can get. Still cheaper than golf, though.

Useful links:
Chipamp.com
Mad About Sound (Chipamp kits for LM3886 and other nice stuff)
Tangentsoft (TREAD psu)
DIY Audio (everything about everything)
Rod Elliot's site (Preamp and loads of other interesting projects)
Svalander (volume knobs)
My Flickr set with annotations
Decibel Dungeon (excellent gainclone discussions and many other projects)

Comments

[rabbit]

Vad har hänt? Inte nog med att du har blivit audiofil, du har även blivit hantverkare!

"That's why i stopped doing pot in the first place. Not because I didnt like it, but because I didnt want to build anything!

They say pot leads to other drugs, not it doesnt. It leads to fucking carpentry!"

[Gustav]

Jag är mållös! Du är kungen!