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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Here’s my latest effort in a sonic upgrade strategy for FM tuners that I’ve been pursuing for years and have finally given a name: the Tu(be)ner. It grew from my dissatisfaction with the sound of most transistorized tuners versus their older tubed counterparts, although the advent of solid-state technology has delivered vastly improved RF capability.My theory for the degradation in sound quality in transistorized tuners parallels what many have found when trying to improve the sound of CD players - in most cases, the “back end†of the system leaves a lot to be desired. The stereo audio signal emerging from the Digital to Analog Converter in the case of CD and, I have found, the Multiplex chip in an FM tuner is clearer, has more dynamics, separation and frequency extension than the signal that appears on the output jacks. CD “tweakers†found that improving the “back end†audio amplifying components and/or circuitry markedly improved the sound. I’ve done the same with some tuners but regressed the process by doing the amplification with vacuum tubes.
The application turns out to be relatively simple. You need to build up a stage with a gain of 20 to 30 dB and, relatively high input and low output impedance. My first efforts were an amp consisting of two cascaded 12AU7 tubes, although based on some listening suggestions provided by Don Scott, I did change the output cathode follower to a 12BH7. The circuit is tapped directly across the audio output from the multiplex chip.
The older tuners I originally modified years back had plenty of space inside to mount the extra components; on the inside back panel of a Sherwood Micro CPU tuner and a minibox added to the top chassis of a Marantz Model 20. Those tuners also had room to accommodate the additional power transformer and associated rectifier/filter components. Since then, other tuners I acquired and considered modifying were of “slim-line†construction, only 3 or 4 inches tall with little room to add the amplification components even if I out-boarded the power supply. I finally hit upon miniaturizing the amplifying circuitry by use of “pencil†or subminiature tubes, the last “gasp†of the tube industry. A dual triode is packaged in a glass tube 3/8†diameter by 1†tall! I changed the circuit to a “totem pole†or mu follower and mounted the tubes and the associated components on a Radio Shack 2†by 3†proto-board. I installed this first in a Nikko Gamma I by drilling two holes in the top shroud of the tuner through which the tubes protrude and support the board via rubber grommets. This arrangement only requires a 2†X 3†by 1†deep under the mounting holes and can probably fit in most tuners.
I’ve uploaded two pictures of the Nikko modification to my folder in the Inmate Photo Gallery. The output of the tube amp is connected to the “variable output†of the Nikko so you can quickly compare that audio with the “stock†signal available from the “fixed outputâ€. Not shown is the outboard power supply that supplies the filament and B+ voltage.
As I indicated before, most sonic attributes versus the “stock†audio output are noticeably improved. Next on the operating table is what I am already referring to as my Sumo “Charlie the Tu(be)nerâ€. Also have a client with another Marantz 20. I’ll keep you posted.
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Topic - Tu(be)ners - stellavox@excite.com 18:32:39 08/25/04 (0)
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