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    As for your design, I am glad to see someone using the 6GM8. (I wish there were more grid-frame tubes.) Below is the circuit I would try to build today, if I had the time. The advantages of DC coupling are retained and the higher rail voltage for the tube could easily be derived with the voltage tippler circuit I outlined in the that issue of the Tube CAD Journal you received. As for the headphone amplifier, I bet the FETs are much more quiet. About a year ago, I received a schematic and two pairs of matched FETs from a kind reader. The simplest HP amp possible:

           Single-ended hybrid amplifier with DC-servo loop

              Power supply for amplifier at left

Letter from Enro Borbely

 

     Dear Editor, I have received a copy of pages 20-27 of Vol. 3, Number 2 of your magazine from one of my customers. It is related to the Hybrid Tube/MOSFET amp design, that your contributor attributes to an Italian guy by the name of Generoso C. I am afraid he was criticizing the wrong guy for the design, because I designed the original Hybrid Tube/MOSFET circuit; he just copied my topology 100%. My circuit was published in issue 1/98 of Glass Audio, see copy of the original article on my homepage under ARTICLES (www.borbelyaudio.com) Nothing is wrong copying my circuits for private use, but it is illegal/unethical to produce/sell somebody else's design. I complained to Editor Ed Dell about this and he sent several notes to Mr. C, but received no answer.

     Just a couple of comments re your contributor's mod note. He is referring to my MOSFET design in Audio Amateur and calls the circuit the LINDER circuit. The guy's name was LENDER (Rudolf Lender, we were working at Motorola at the time, he was a Field Application Engineer in Germany and I was working at the Central Application Lab in Geneva, Switzerland). As for the modifications he suggested, I tried to make a very simple circuit, which I think worked very well with the current mirror. I think this was the simplest way of generating enough current to drive the input cap of the MOSFET. I left the generation of the second harmonics to the SE common source second stage, which also worked well, :-)) I guess I prefer a  controlled/acceptable amount of harmonics, even of the second harmonic variety. As for the suggested N-channel MOSFETs as the active ones instead of the P-channel, I am using Hitachi and Toshiba MOSFETs and have not seen any major difference between them in my designs. This might be the case with the IR MOSFETs, however they don't sound as good as the Hitachi or Toshiba in my opinion anyway. By the way, I have just designed a headphone amp using the same topology, however, I replaced the input tube with a dual JFET, see description on the homepage.

Best regards,

Erno

Germany

 

      It is an honor to meet you, even if it is over the net. I have read all your articles in Audio Amateur and I have enjoyed and learned from them greatly. Twenty years ago, I even built a pair of your 60 watt MOSFET amplifiers for a friend (very nice sounding; we used a 400 VA toroidal power transformer).

     A thousand apologies to Rudolf. I have built a complex I-to-V converter based on his design, which worked quite well.  

    The sincerest-form-of-complement version of your circuit is popular and I bet it must madden you to see someone else spoil your design and make money by doing so. But then, I am not the one ask about circuit design ownership or patenting, as I am not sure that patenting of topologies should be allowed anymore than I believe sentence structure should be copyrightable. Now, using your name would be an altogether different case and thus we can see the genius of Ray Dolby copyrighting the name, not the circuit. (I have been told that some of the remakes of famous amplifiers which were printed in this journal have been faithfully implemented — without any credit to me, of course.) In general, I believe designs and topologies are not as valued as much as is hype. I just read in a business book at the bookstore yesterday that many people make the mistake of putting 90% of their effort into creating a better product and only 10% into marketing, rather than inverting the ratio! This philosophy sickens me, but it is probably closer to success than I wish to acknowledge.