I’ve always enjoyed electronic kits of all kind and like many of you will credit them for the foray into DIY audio. Over time as my DIY skills matured I found myself taking the DIY route for projects far more often than relying on kits. While I will always enjoy electronic kits, I generally won’t try one out unless there is something really unique about the kit. When I saw the Gobo Stereo Audio Amplifier kit from boxedkitamps.com, I was immediately intrigued by the unique looking enclosures available with the amplifier kits. Shown in Photograph 1 below is the completed Gobo Stereo Audio Amplifier kit with a translucent blue acrylic enclosure. The choice of enclosure finishes for the Gobo stereo amplifier kit include blue, dark grey and orange acrylic and bamboo.
The Gobo Stereo Audio Amplifier Kit is an class-AB stereo chip amp kit based around the LM1875 monolithic integrated circuit (IC). The Gobo amplifier kit is a complete kit in that it includes all the necessary parts and enclosure required to build a complete stereo amplifier. The LM1875 amplifier kit is easy to build armed with just soldering skills and the detailed step-by-step instructions. The amplifier kit comes with audio-grade capacitors and toroidal transformer. So based on what we see so far, if all goes well we should end up with a amplifier that looks and sounds like we spent a lot more money. Output power from the LM1875 chip amp will be about 12W per channel into 8 ohms and closer to 18W with 4 ohm speakers. From the product page the Gobo LM1875 amplifier kit has the following specifications:
- class-AB operation
- up to 18W per channel
- 0.015% harmonic distortion
- line-level input (such as an iPod, computer, CD player)
- gold-plated RCA jacks (one pair) Neutrik brand
- drives conventional speakers (4-8 ohms)
- audio-grade toroidal transformer
- 50VA power supply
- WIMA, Nichicon and Panasonic capacitors
- 1% metal film resistors
- Alpha brand audio pot
- fully shielded signal allows unshielded case
Photograph 2 shows the Gobo stereo amplifier kit packaging. The amplifier kit was packaged very well to survive delivery from Colorado, USA. Once inside the boxed kit, all the parts were bagged and neatly labelled. The contents of the Gobo LM1875 amplifier kit are shown in photograph 3. The odd shaped object that you see in the photo is the main portion of the acrylic plastic enclosure. Included with the kit are basic instructions, a wiring diagram and a parts list. The detailed amplifier kit instructions are available for download from the Gobo product page.
The Gobo Stereo Audio Amplifier Kit comes with some high quality audio components. The power supply features an audio-grade 50VA toroidal transformer (Photograph 4) made by Antek in NJ, USA. The capacitors are Nichicon Muse, Panasonic and WIMA. The potentiometer is a little light weight and is marked only A100k. boxedkitamps.com indicated via email that the potentiometer is an Alpha brand with a carbon element. The gold plated RCA jacks are made by Neutrik and appear to be good quality. The speaker binding posts are nickel plated and have a good weight and feel to them. The power switch is illuminated and a nice anodized aluminum knob for volume control round out the parts of the Gobo amplifier kit.
Gobo LM1875 Amplifier Circuit
The Gobo Stereo Audio Amplifier Kit is based around the LM1875 IC. The circuit generally follows the standard application note for the LM1875. For detailed information about the circuit and the LM1875 chip specifications and performance see the Texas Instruments LM1875 Audio Power Amplifier Datasheet – (PDF 3.18MB). Included within the Gobo amplifier step-by-step instructions is the LM1875 amplifier schematic shown below (Figure 1). The gain of the Gobo LM1875 amplifier circuit is set at about 23. C1 is the input DC blocking capacitor. R5 and C5 make up the Zobel circuit.
The power supply schematic for the Gobo LM1875 amplifier, also from the step-by-step instructions manual is shown in Figure 2. The power supply is fed from the toroidal transformer through dual 12V secondary’s. A full wave bridge rectifier is used to produce a symmetrical 16.8VDC supply. The power supply PCB has the option of accepting snubbed capacitors (C3 and C4) across the main filter capacitors.