Vivid Audio Koala
The Koala project began in September of 2006 with a request from a customer for a dedicated amp for the legendary AKG K1000 headphones. Following right on the heels of the Chameleon, we proposed an amp based loosely on this design- a round 2 channel integrated with IT coupling- but with one significant difference. The Koala would be Single ended OTL (Output Transformer-Less) with an output impedance of about 30 ohms. The output tube would drive the K1000s through a bank of output coupling caps to block DC.
Other details- Voltage Gain tube: W417a, Output Tube: 6080 or 6AS7G
Veneer: Bubinga, Waterfall grain pattern
Base: Solid Walnut with Purpleheart and Sassafrass accents
Top: Hand-finished Copper laminated to aluminum.
After completing the amp, we shipped it to our customer who listened to it for about 6 weeks for so. Realizing the raw potential of the Koala, and wishing to push the design to the max, we began considering some part upgrades. The Koala was shipped back to Atlanta for a major refit.
Take a look in the Koala photo gallery at image number two. This is the final Koala circuit, now with significant upgrades. Note the large VH Audio output coupling caps and smaller VH OIMP cathode bypass caps for the W417a. Also notice that the 24k gold Vivid input cables and the custom output cables are hardwired to the Koala circuit, eliminating input RCA jacks and speaker binding posts altogether. Lastly, notice the Goldpoint stepped attenuator (a thing of beauty, btw, both sonically and otherwise). This replaces the choke loaded "law fake" dual 100k pot as the chokes were picking up hum from within the amp. After we traced the hum to these law fake chokes we were able to remove the high value electrolytic caps in the power supply and reinstall the original Solen film caps. This brought back an enormous sense of life to the music with only a teeny tiny bit of hum. Those electrolytics really suck!
The large white resistors are the cathode bias resistors for the output tubes, which are drawing lots of current. The 6080/6AS7G tubes are not terribly consistent in this regard, ranging from around 70mA to a scorching 140mA (in this circuit).
In case you are wondering, the large iron on the top plate (from L to R) is PS choke for the L channel, power transformer, and PS choke for the right channel- there are no output transformers on this one. Inside are two more PS chokes and the Interstage Transformers.
I would be remiss if I did not offer a word of thanks to our customer in Cincinnatti, who never lost his faith in us or his enthusiasm for this project, which took the better part of 7 months, and with the upgraded components, roughly doubled the original cost. Without his dedication we would have never succeeded in extracting the absolute maximum sonic performance from this project. Thanks Steve!
Lastly, I would like to thank Eric Belt for his contribution to the hum fixing, correcting the power supply voicing, and his general dedication to problem solving in the completing of this project. Thanks Eric!