tube fanatic Posted March 11, 2015 Share Posted March 11, 2015 I posted a thread on the "Little Gem" amp concept a while ago, so I won't go into too much detail here. https://community.klipsch.com/index.php?/topic/153612-amazing-little-gem-flea-power-sep/ The amp proved to be very successful not only because it sounds so good, but also because its internally adjustable high frequency filter allows its sound to be tailored for particular listening tastes with a large array of speaker and room combinations. This version was built on request from someone who asked for the same ability at the bottom end. It was easy to implement since the design uses frequency selective negative feedback to compensate for the usual rolloff of the output transformers and speakers in the low bass range. The amount of feedback isn't overly critical, so it was an easy matter to replace a fixed resistor with a 25 turn potentiometer which allows the feedback filter cutoff frequency to be moved over a range of approx. 12 Hz while keeping the feedback amount sufficiently high. Although 12 Hz doesn't seem like a lot, it's amazing just how much of an effect on the sound is possible between one end and the other (in this case we're talking about a cutoff frequency of between around 51 Hz and 62 Hz- and remember that the cutoff isn't sharp, but rather a frequency dependent curve if plotted). This version also uses a different tube complement with interesting output tubes (35EH5s). These tubes have a huge power sensitivity and need less than 3 volts of peak drive to attain full output (they were originally designed for use in low cost phonographs employing crystal or ceramic cartridges which could put out a few volts). A low gain driver stage preceeds the output tubes to allow the amp to be used with sources having somewhat low output voltages. The bottom end response of the amp is among the best I've achieved as evidenced by the 320 Hz square wave shown below. Power output, like the original, is around 750 mw at 32 Hz (onset of visible clipping into 8 ohms resistive). Extensive auditioning with my RF-15s, as well as two different CW installations, has confirmed the desired goals and further reaffirms just how little power is needed to achieve clean, loud, listening levels with high efficiency speakers. I'm aware of two other versions of this amp currently under construction (one, with yet another tube complement, is by an esteemed forum member who, I'm sure, will provide an entertaining review; and the other by a fellow who found my original All American Five concept article on a friend's web site). Now it's time to put this to rest and move on to some things which are even more insane (tba in the future!) once I've cleared out a few more prototype amps.Maynard 4 Quote Link to comment Share on other sites More sharing options...
erik2A3 Posted March 11, 2015 Share Posted March 11, 2015 Very nice, Maynard - an absolute lesson on well-done P-to-P. Thanks for the description, as well. Input jacks up front and close to the input stage is excellent -- frequently a better practice IMO than immediately adjacent to mains inputs and other PSU components. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted March 11, 2015 Share Posted March 11, 2015 PM sent Maynard. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted March 11, 2015 Author Share Posted March 11, 2015 Carl, PM sent in reply to yours! Thanks Erik. Putting input jacks adjacent to output jacks, even with the use of shielded cable, runs against the grain for me, so I avoid it unless specifically requested to do it. So, what are you involved with at the present time? Any interesting builds or restorations? Maynard Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted March 11, 2015 Share Posted March 11, 2015 Carl, PM sent in reply to yours! Thanks! Quote Link to comment Share on other sites More sharing options...
erik2A3 Posted March 13, 2015 Share Posted March 13, 2015 Hi, Maynard A friend of mine recently gave me a vintage Quad 303 stereo power amp. He had initially sent it for power supply and ouput stage recapping, which I have already done, but I recommended new components, trim pots, etc. (the originals are dirty in the extreme, and had a reputation for drifting). He had since invested in some other components, and so very kindly just told me to keep the Quad to do with it as I liked. That's what's on my bench right now, with several other projects going on when I have time. My amazing wife also bought me the new 300B Transcendent Sound preamp/headphone amp kit, which is needed for room changes that necessitate long lengths of IC between preamp and amplifiers. I love building from scratch, and kits are fun to do too, especially since my time is very tight now during the school year and teaching. The TS kits are done very well by the way! Again, great upo non this amp. I have always been one to appreciate simple chassis that contain innovative and interesting circuits! Good to have someone like you around here. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.