I’ve been planning to build a Chip Amp for some time now. I’ve finally obtained some chassis parts and a transformer that should be perfect so I no longer had an excuse to put it off.
Here’s a shot from chipamp showing what the populated LM4780 PCBs look like.
I chose to use the LM4780 in parallel configuration. The LM4780 datasheet can be found here. This is essentially two LM3886 tied in parallel through 0.1 Ohm output ballast resistors; the advantage of this configuration being that the gain of each Op-amp should be more closely matched as they reside in the same IC. This configuration provides more output power than a 3875 or 3886 and allows compatibility with a wider range of loads without sacrificing audio performance. The circuit schematic is shown below.
The heart of the amplifier system is the power supply transformer(s). This is usually the single most expensive component in an amplifier. I was able to “recycle” one from an obsolete amplifier that was just about perfect. The 245 VA toroid fullfills the circuit power demands without being excessively large to induce mechanical hum. I’ve left some extra wires and connectors available to tweak with some added reservoir capacitors but I’m happy enough from my initial listening tests that I’m not sure I’ll bother. The power supply schematic can be found here.
I followed National Semiconductor’s recommended parts values fairly closely but chose audio-grade resistors and capacitors for every component in the signal path. Because of the small number of components in this circuit, the quality of components used becomes more important.
I used Panasonic FC and Black Gate capacitors and Caddock MK132 resistors throughout the design and Mills non-inductive wire-wound resistors for the output side. A common DIY modification to improve the audio quality of consumer audio equipment is to upgrade the bypass caps and audio path resistors. I figure I might as well build it properly the first time.
One of the most important components is the attenuator. All of your audio is passing through this device before it gets amplified. It just plain makes sense to use a decent one. You’d be surprised how many off the shelf amplifiers/receivers use crap here instead of doing it properly. If you’d like a quick and easy way to upgrade your amplifier, try replacing the stock attenuator with a proper one. This is an interesting read for anyone trying to decide which attenuator to use. I used the Alps “Blue Velvet” here. I would have loved DACT CT-2 but I like to eat too.
For me, chassis metalwork was the most labour intensive operation in this project. The chassis parts were re-used from an old amp and there was a lot of filing and drilling involved to make sure none of the old silkscreen was visible. Once this was complete, it was simply a matter of connecting the amplifiers to the power supply and cleaning up the wiring.
The LM4780 is a non-isolated chip meaning the case on the chip is at V- potential. This means that it must be electrically isolated from the heatsink. Enter thermal pads. These little beauties transfer the heat to the heatsink but are not conductive. They may be a little less efficient than thermal grease but in my case, the heatsinks are gigantic so heat is not really a concern. The amp boards are attached to the heatsinks using nylon screws (non-conductive). My first thought was that they would melt but apparently they’re good up to 400 degrees F. Of course, if needed, the heatsinks and attached amp boards can be removed for service.
The faceplate is a 5mm thick aluminum panel and is thicker than the threaded length of the Alps pot. For this reason, I couldn’t use a nut and had to glue the pot to the faceplate to ensure that there wouldn’t be any movement when turning the volume knob.
Here are some photos of the finished amplifier.
Conclusions and Listening Impressions
I was absolutely floored by the sound quality of this amplifier. I had read several gleaming reviews about gainclones and the great sound that is fairly easily achievable, but I usually take internet reviews with a grain of salt.
After initial listening, my doubts have been eliminated. Imaging is clear and distinct. The amplifier handled bass effortlessly and drove the Forests with little to be desired. I was quite surprised by the output of these little chips. I will definitely be trying out more of these.
It will be interesting to compare it to the discrete mosfet amp on its way. Needless to say, I’m looking forward to doing some more critical listening with this new addition in the near future.