Gobo Stereo Audio Amplifier Kit
Gobo Stereo Audio Amplifier Kit (LM1875, 15W, Class-AB)
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.
Photograph 1: Gobo LM1875 Stereo Audio Amplifier Kit
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: Gobo Stereo Audio Amplifier Kit Packaging
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.
Photograph 3: Gobo LM1875 Stereo Audio Amplifier Kit Contents
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 nickle plated and have a good weight and feel to them. The power switch is illuminated and a nice anodized aluminium knob for volume control round out the parts of the Gobo amplifier kit.
Photograph 4: Antek AN-0512 Toroidal Transformer (12V dual secondaries) 50VA
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.
Figure 1: Gobo LM1875 Amplifier Schematic
Figure 2: Power Supply Schematic - Gobo LM1875 Amplifier Kit
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 secondaries. A full wave bridge rectifier is used to produce a symmetrical 16.8VDC supply. The power supply PCB has the option of accepting snubber capacitors (C3 and C4) across the main filter capacitors.
Gobo LM1875 Amplifier Kit Assembly
Photograph 5: Gobo-LM1875-Amp-Parts-PCB
Assembly of the Gobo amplifier kit is the fun part, but before you start building, I really suggest you read through the step-by-step instructions for the amplifier kit. It's clear that boxedkitamps.com has put a great deal of time and effort into developing the very detailed instructions for the amplifier kit and it is pretty much impossible to make a mistake by following the kit instructions. See the Gobo product page for a sample of the amplifier kit instructions. The Gobo LM1875 amplifier kit will take about 6 hours to complete. I finished the Gobo amplifier kit over a couple of evenings. With the thorough instructions the amplifier kit can be built by first time amp builders who possess only soldering skills.
Photograph 6: Finished Gobo LM1875 Amplifier Modules
Assembly of the circuit boards is rather straightforward when following the kit instructions. The completed amplifier modules are shown in Photograph 6. The printed circuit boards (PCB) have been cleaned with isopropyl alcohol and a toothbrush to remove solder residue and flux. Assembly of the power supply module is shown in Photograph 7. The set of "helping hands" greatly aids with assembly of the amplifier kit. Some of the resistors solder pads on the PCB were a little on the small side, but not a problem with a fine pointed soldering iron. boxedkitamp.com indicated via email that subsequent amplifier PCB would use larger soldering pads.
Photograph 7: Power Supply Assembly - Gobo Amplifier Kit
As noted in the instructions, for some of the wire hookups, the wire diameter exceeds the PCB hole diameter requiring some wire trimming. Don't twist the wire strands as it will make trimming more difficult. Once the power supply module was together, I was eager to test it out and verify the supply voltage. When testing out the power supply be sure to follow the instruction manual steps and the noted safety precautions. Photograph 8 shows testing out the symmetrical power supply for the first time. The power supply output is + and - 16.8VDC.
Photograph 8: Power Supply Testing - Gobo Amplifier Kit
The main fuse for the amplifier kit is located on the power supply module and covered with a removable plastic shield. As the protective paper coating is peeled away from the enclosure pieces you start to get a sense for the unique looking plastic enclosure of the amplifier kit. The amplifier kit uses a two prong plug and the finished amp is double insulated which makes it safe with the acrylic plastic enclosure. Regarding the lack of shielding offered by the plastic enclosure, there are no concerns as shielded signal wire is used throughout.
Photograph 9: Gobo LM1875 Amplifier Kit Assembly
The amplifier kit enclosure is made from a translucent blue acrylic plastic which has been laser cut. The rounded sides of the enclosure are very precisely cut accordion like hinges that allow you to bend and fold the plastic over to form the enclosure. The plastic, particularly the hinges are rather fragile during assembly, so take your time while peeling the protective covering and don't force the plastic. The PCB modules of the amplifier kit and toroidal transformer are mounted to the plastic enclosure and it is important not to over-tighten the connectors or you will crack the plastic. Follow the detailed instructions and the enclosure neatly fits together.
Photograph 10: Gobo LM1875 Amplifier Kit Assembly
Once the amplifier kit enclosure has been fully assembled it is surprisingly sturdy. I really like the unique look of the finished enclosure. Relative to a metal amplifier enclosure, the plastic enclosure design is very clever from a cost and weight perspective. With just a piece of acrylic plastic and laser cutting the resulting enclosure is well polished and looks real sharp with the laser etched boxedkitamp logo. Once powered up and illuminated, there is no doubt that the see-through Gobo amplifier will draw the attention of guests.
Photograph 11: Finished Gobo Stereo Audio Amplifier Kit
Before you use the amplifier for the first time you need to check that there is no DC offset at the output terminals of the amplifier. To check, use a multimeter to check across the amplifier outptuts for DC voltage. No source is required. There is no need to worry about DC offset less than aboout 25 mV. There was zero DC offset with this Gobo amplifier kit build.
During operation the Gobo amplifier kit is dead quiet and you will not hear any hiss or hum even with your ear near sensitive speakers. There is a very slight "turn on pop" when powering up the Gobo amplifier. Also, when you turn off the power there is a short "thump" which occurs as the symmetrical power supply capacitors drain unevenly. The intensity is not large so there should be no concern regarding potential speaker damage.
Review and Sound - Gobo Stereo Audio Amplifier Kit
Now on to the important stuff, how does the little amplifier kit sound? To evaluate the sound of the Gobo Stereo Amplifier kit I tried it out with my very simple rec. room Hi-Fi system consisitng of just a CD player, amplifier and speakers. The source is a NAD C516BEE CD player and the speakers are a pair of Dallas backloaded horn speaker enclosures loaded with Fostex FE206E fullrange speaker drivers. The high sensitivity horn loaded speakers (97dB+/1W/1m) match well with the Gobo LM1875 amplifier kit which has an output of about 12W into 8 ohm speakers. Like most well implemented chip amplifiers the Gobo LM1875 amplifier sounds far better than it should considering the affordable kit price. It's been a number of years since I've enjoyed an IC based amplifier (see DIY LM3886 Gainclone) and the Gobo amplifier shares the common sound traits of these chip amplifiers. To my ears I find the overall sound of the chip amplifier to be very accurate, neutral and detailed. Despite the small size of the amplifier, the bass response is strong and accurate. The midrange and top end response sounds natural and detailed with no colorations. Coupled with a sensitive single driver speaker system the combination is very revealing of recordings.
Photograph 12: Gobo LM1875 Amplifier and NAD C516BEE CD Player
Overall, the amplifier kit went together easily, looks and sounds excellent. The Gobo amplifier will fit in well with small Hi-Fi setups and also as a desktop amplifier with a computer or mobile source. With the detailed instructions, the Gobo amplifier kit can be put together by first time amplifier builders with just soldering skills.
Photograph 13: Gobo LM1875 Amp with Fostex FE206E BLH Speakers
I was really intrigued by the plastic enclosure that came with the Gobo amplifier kit so I asked for some additional details regarding the enclosure design and fabrication process and here is the response from boxedkitamps.com.
Our unique enclosure was designed by us and manufactured by Pololu in Las Vegas. It's made from 3mm Trucast acrylic which is laser cut flat. The round sides are called "living hinges". Although fragile looking, it is only fragile before assembly, and quite sturdy after assembled. The enclosure will support considerable weight and really has only ever broken from a solid, sharp blow, in our experience. The enclosure is also highly heat resistant. Part of our testing included shorting out the transformer until it tripped the breaker--smoking hot. There were no burnt marks or signs of melting. It had a tiny warp on the bottom where it was in direct contact. I really like the results of the laser cutting process. Without any further finishing, the sharp corners are eased and the edges look polished.
We can get away with an acrylic enclosure because everything inside is shielded and double insulated (like power tools). We really wanted the kit to be complete and a metal enclosure would double the cost. It is meant to be an entry level kit after all. The amp produces great sound as is, but with many paths to upgrade--including subbing a metal enclosure--for which we offer a "caseless" kit. - Ken Knowles, boxedkitamps.com
The Gobo Stereo Audio Amplifier kit is easy to build, looks and sounds great. With a little bit of DIY effort you will end up with an amplifier that sounds far better than the cost of the kit. With the well documented step-by-step instructions the amplifier kit can be built by first time builders who have soldering skills. The Gobo amplifier kit is available for purchase from boxedkitamps.com. The Gobo Stereo Audio Amplifier kits are currently on sale for $100 US (regular $195 US) which IMO sounds like a fantastic deal. For additional information about this build, see the Gobo LM1875 Amplifier Kit thread on the forum.
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