Hollowstate Devices - Duet The making of a 300B amplifier!

!

My name is Matthew Santa Maria (Theaterboy ID on eBay) and I have the pleasure introducing the new Hollowstate Devices - Duet Amplifier. !!Before I start the story on the Duet, I will describe how I first became aware of this amazing amplifiers designer. During a visit to my local Hi-Fi store a discussion started about testing tubes. After I mentioned I owned a TV-7 tube tester, the guys at the store said they knew an expert technician - Daniel Nelson - who was considered the guru of TV-7’s and he could calibrate my tester. They had always steered me in the right direction before so a couple of days later I dropped it off and said have at it. !!As technicians capable of doing this kind of work are becoming fewer in number than the old bluesman that we have all grown so fond of listening to on our audio systems - I was not really sure what to expect from this service work. I was just happy there was someone even willing to do the work at all!!I received my tester back and was so pleased with the work I would have gladly payed twice what I was charged. My tester had been throughly gone through and things I never expected were done. Worn tube sockets replaced, gasket seal replaced, and the missing testing accessories were included - along with the calibration. It had a tag on it with the date of service and was signed Dan Nelson. !!

A couple of years passed and one day I was testing a batch of old tubes and I was getting a short indication on every EL-84 tube. I decide to call Dan and ask if the TV7 had issues with this tube. He confirmed it was just a glitch in the tester design and not to worry if the tubes showed shorts. !!At this point in our conversation Dan mentioned he had built a 300B amp he would like me to hear. Over the years I have worked on the retail and manufacturing side of high end audio. As Dan and I spoke more about his amp, I was recalling the dozens of times someone had told be about a home brew audio product they had built. Some of them did sound good, but they all ended up being something I wouldn’t be happy owning let alone recommending to anyone else. Then I recalled how impressed I was with Dan’s service work on my TV7 and said “Let’s Do It”.!!

I gathered my best single ended amplifiers - AudioNote Ongaku, Cary 805 mono’s & CAD300SEI - and readied myself for Dan’s visit... !!Nothing could have prepared me for the experience I was about to have. I am happy to say the Duet is one of the few amplifiers I will always have on hand when I want to listen to music. It is simply superb!!!I will not bore you with the details of the whole listening session here, but I will say I came away from it wondering how this was possible. I was looking at an amp hand built by Dan, an individual designer on his own. No team of folks helping with all the details that go into producing a piece of electronics; taking it from a concept to a finished product. The Duet handily stacked up sonically against some of the very best single ended amp designs and had unique qualities and features the other amps on hand did not even begin to tackle. !!While the Cary 300SEI is available with remote volume control none of the other amps in the session had this benefit. Input switching was manual on the other integrated amps (Ongaku & CAD300SEI). The Duet possesses both remote input switching and volume control. I could switch from my Record Player, CD player and then my music server and adjust volume with a remote - I was in heaven!!

How does the Duet sound? This is where I would normally describe the relatively small differences the best amplifiers have. Terms like resolution, imaging, soundstage, tonal balance and any of the other audiophile performance language terms we have all used to describe our listening experiences.!!While I have fallen into that camp most of my life. Now that I have reached middle age and listened to hundreds of audio products; I find these things to be more of a trap. If they are used as a single parameter to judge the sonic performance of a component in a given moment in time, one misses the big picture. These days I strive to listen to a component’s whole presentation all at once and get a read on how a piece moves me emotionally. While the Duet can hold its own with the best in all the sonic performance areas mentioned, nothing I have heard close to its price can best it in any of these parameters. The most magical thing the Duet does supremely well is get the rhythm & pace of the music right. It is impossible to not tap your foot or have the spontaneous urge to get up and dance when listening to music through the Duet. I can pay no higher complement to any component. !!

One of the other things that's always disappointing to me about modern high-end gear is the lack of visual appeal. Most minimalist gear eliminates all the very cool meters and scopes of the most iconic vintage audio components. These features added valuable input to the owners and had the added benefit of looking very cool as well. When today’s audio designers do add some visual elements they either overpower the units visual look or end up seeming very toy like. Dan’s approach was elegant, adding visual delights that also provides useful information.!!On either side of the volume control are Nixie tube meters that move from the center left and center right with the music signal. The meter is calibrated so 1 watt is about where the power switch and input indicator selector are placed and as you get towards the end of the meter you are at 8 watts. There is also an LED just above the volume knob that will light when the amp approaches max power. !!

Dan wasn’t satisfied with the tactile feel or cosmetics of the available off-the-shelf knobs for the volume control - so he turned his own brass knobs in his shop from solid bar brass stock. !!

After our listening session, Dan removed the bottom cover and started describing some of the choices he had made and some of the design

Over the next several weeks as I got to know Dan better, I realized that none of this was an accident. He had spent numerous years as the Engineering Manager for a company that manufactured Wet Process Machinery for the printed circuit board industry. Dan and his group designed this equipment for companies like Western Electric, GTE, GM, HP, & IBM which could run as high as $500,000.00 in the 1980s through the 1990s. !!In his role, Dan was charged with pulling together all the different engineering teams and delivering prints to the manufacturing side were he would guide the products to completion. !!Today besides building amplifiers, Dan repairs & restores high precision test gear used by companies that have to have accurate results. This testing equipment is highly complex and run as much as $70,000.00 new. He takes part in a myriad of other projects as well. When I was at his shop last, he was working on a project for a national optometry foundation. It was an optical device to help people with lazy eyes, train the eye in question, and help it return to normal. He explained he took on the project because the majority of devices on the market were out of the reach of many folks budgets and he had come up with a more cost effective version.!!!!

Then Dan turned his attention to the first batch of the production Duet’s in the works. He invited me inside his workshop to check things out. This is when I finally understood the level of skill and meticulous attention to detail Dan brings to his projects. !!In front of me were five Duet chassis in different stages of production. Dan also took the time to show me the four binders he had produced with Bill of Materials, Schematics, Purchase History, and Prototype information.!!!!

At this point I was still fascinated with the level of refinement the Duet possessed so I asked Dan to take me through his journey in more detail. !!Dan explained that after years of listening to highly regarded Solid State amplification, he decided to build his first tube amp to see what his valve amp enthusiasts friends were all abuzz about. He took some iron from an old Fisher product and built up a Push Pull tube amp on a chassis he already had on hand. Much to his surprise this relatively low power tube amp sounded much better then his well reviewed, 100 watt, solid state amp. Now you tube lovers are grinning, as you already knew the outcome. And you folks with power hungry speakers are thinking, all these low powered tube guys are crazy. The result of this first tube amp build was a fire had been lit in Dan’s psyche that fueled a long journey to arrive at the Duet  

First Push Pull Amp Build

!The Duet’s circuit had been in the works since 2007 with three prototypes created before this final design was arrived at. His first single ended 300B effort was pivotal as during this time he came up with the drive circuit that eventually became the front end of the Duet. This time he used purpose built mains & SE output iron. Now Dan had really upset his preconceived ideas on amplification. This 8 watt per channel amp had him mesmerized with its sonic virtues. Now, you low power, single ended, triode guys are grinning again as you had already discovered one of the best keep secrets in audio. These super simple amp designs, which are mostly modern incarnations of the vintage Western Electric 91 amps theater amps from the 1930s have a special rightness to their sonic presentations. Once you have heard it, you can never go back! Oh and you guys with power hungry speakers are thinking what the hell are they talking about - only 8 watts!!!!!

First 300B Prototype Amp Build

At this point Dan turned his attention to achieving and surpassing the results he obtained with the first 300B project and designed a set of Monoblock amplifiers around the Russian GM70 tube. The Monoblock configuration was certainly the way to go as it opened up a huge soundstage sonically. Ultimately Dan’s desire to accomplish this sonic result while retaining the real world benefits of a single chassis pointed him back to what would become the current Duet!!

First MonoBlock Amp Build

Through this 4-year prototype process, Dan made some refinements to the input circuit and tried many different parts in the signal path and power supply to hear what effects they had. For example, most designers of tube amplification will agree to avoid electrolytic capacitors in the signal path. He discovered they had negative effects on the power supply as well. Even using the best versions of available electrolytic’s in the power supply, the sonic performance drop was significant. A huge gain in sonic performance was achieved by eliminating them altogether.!!

The third prototype and first version of the Duet was created much as you see it now except that Dan originally used an acrylic front panel, wood side panels, and the Dual mains transformers were US voltage only.!!This was the amp I first got to listen to and I was blown away with what I was hearing. Dan and I talked about the final production amp a bit and it was decided the front panel would end up being powder coated aluminum with slots cut to allow the nixie tube power meters to be more visible. The side panels were also changed to power coated aluminum and the mains transformers were sourced with both 120V & 240V operation capabilities. !!He built the Duet because of the previously mentioned fire that was lit after he built his first humble but superb sounding tube amp and then it was impossible for him to stop. He wanted to push what was possible in tube amp design to the sonic performance limit - which for a select group of tube amp lovers can only mean a Single Ended, Triode, Class A design.!!This all adds up to about 1250 man hours spent getting to the final Duet. !!

Dan and I invite you to join our Triode Tube Tribe and know it will be a fun journey. !!If you are prepared to carve out a space in your place that is dedicated to making music happen and willing to match up the proper components, cables and speakers. We can promise you an experience that will put a smile on your face every time. !!Not everyone in your circle of family or friends will get it, but that’s half the fun. You can just tell them that the Duet was born from the original designs from the beginnings of audio. And that 300B tube based amplifiers were used to power the loudspeakers when the first talking theaters were transitioning from silent films. They may still have a dumbfounded look on their face but it will reaffirm you as the audio gunslinger you know you are down deep. No sense hiding from your true nature anymore!!Regards!Hollowstate Devices - Team!!