8 Channel Amplifier

This amplifier is a long time whish of mine fulfilled. When designing crossovers, I listen to the partially finished speakers through the crossover emulator of LspCad. The first time I used this software, I employed 2 stereo hifi amplifiers, which were carefully set to the same volume. This is far from optimal, and thus the need for a multichannel amplifier was born. My soundcard has 8 analog outputs, and I wanted to be able to use all these outputs. I had a few requirements for the amplifier. It would need to have low gain (my soundcard has fairly high level outputs and there is no mixer application to turn the volume down with). It could also be low power per channel (I wasn't planning on listening to very high levels). The amplifier boards needed to be physically small. And off course, sound quality needed to be good and equal for all 8 channels.
Off course, I could have watched ebay for a few weeks and then buy a second hand rotel 8 channel amplifier, but that's no fun. I asked on a maillist for suggestions on how to proceed. I was thinking of building an amplifier based on chip-amps, but wondered if it was also possible to go the "discrete route". I had no experience in building discrete amplifiers and knew nothing more about them other than they were supposed to be better than chip-amps.
I got a reply from Lukas Louw that he was playing around with an opamp based amplifier at that time and that it seemed a good match for my requirements.

The first amplifier was this one, The input stage is an opamp, followed by a complementary voltage amplification stage and then followed by a complementary feedback pair output stage. Because of the fact that I planned to use 8 channels, the circuit was converted to a differential input amplifier to reduce the risk of ground loops.

first prototypeThis is the first prototype board I made. I made this using the Eagle Software. This board is small enough to be made with the freeware version. And it's clearly visible that this is one of my first PCB projects with more than 10 components. Sadly, I couldn't get this amplifier to behave. It was very prone to oscillation, and after some further testing we decided to give up on this circuit and switch to a fully discrete design

The discrete version was also balanced, to prevent ground loops. The opamp is now replaced by a complementary differential amplifier, and R34/C20 are added. These are to reduce open loop gain and help improve stability. I first built a proto-prototype, with the transistors soldered to a small piece of proto-board and the in- and outputs plugged into the opamp-socket. I ran some tests on this amplifier and it worked! That was a big relief, after days of tinkering with the previous prototype, I was starting to doubt myself. But this “christmastree amplifier”, with all these parts dangling out in the air actually performed quite well. It had a wide bandwidth, and square waves also posed no problem.
Because of these good results, I started on a new PCB design. I set out to decrease PCB size to 50mmx80mm, so that 4 boards could be made out of one standard eurocard sized board. And with help and a lot of time spent on this board, I succeeded. The result can be seen below, in the picture on the right. There is a 4r7 resistor mounted, for testing purposes. It was somewhat too light, so during testing, it sometimes got so hot that the solder came loose..

Christmas tree amplifierPrototype discrete amplifier.

Stereo amplifierThis new PCB worked very well, and there was still some room left on it. That room was just enough for a DC offset adjustment pot, input attenuation resistors and seperate connections for the feedback. After this, the PCB was about completely full. This time, I built a stereo prototype, so I could not only measure the performance, but also actually listen to it. All testresults were very good, including the listening tests. You can read about these tests on the next page. This latest design is the one I used for building the 8 channel amplifier.
These are the files for this amplifier:
Schematic
Partslist
Top layer (600dpi gif)
bottom layer (600dpi gif)
Silkscreen
Eagle-board file