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    "Traditional" driver circuit designs versus "modern" all-triode designs

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    JunkyJan

    Posts: 100
    Join date: 2008-12-09
    Location: BC, Canada

    "Traditional" driver circuit designs versus "modern" all-triode designs

    Post by JunkyJan on Mon Aug 30, 2010 11:59 pm

    Hi Gentlemen

    I came across this posting on the web: http://santafemusic.blogspot.com/2008/09/tube-be-or-not-tube-be.html and as you can see, the author pooh-poohs modern driver circuit designs and claims that the original Hafler designs are superior. This is not the first time I have come across this line of thought.

    You opinions?
    -- 'Jan

    Bob Latino
    Admin

    Posts: 1903
    Join date: 2008-11-27
    Location: Massachusetts

    Re: "Traditional" driver circuit designs versus "modern" all-triode designs

    Post by Bob Latino on Tue Aug 31, 2010 11:45 am

    JunkyJan wrote:Hi Gentlemen

    I came across this posting on the web: http://santafemusic.blogspot.com/2008/09/tube-be-or-not-tube-be.html and as you can see, the author pooh-poohs modern driver circuit designs and claims that the original Hafler designs are superior. This is not the first time I have come across this line of thought.

    You opinions?
    -- 'Jan

    Hi Jan,

    I agree with what the author said except he only compared the original Dynaco circuit with the Curcio circuit. I have owned and heard a set of Dynaco Mark III's with the Curcio driver circuit and IMHO a Mark III with a the Curcio circuit does sound a bit "cold" in comparison to a Mark III with the original 6AN8 driver circuit. That fact does not, however, make the original circuit "correct" or "right". The original circuit sounds warm, smooth and very "tubey". The stock bottom end is a little soft and does not control speakers as well as modern circuits do. The stock top end is rolled off just a little.

    Understand also that when Hafler designed that circuit (and power supply) there was no digital music. Vinyl records were the signal source of the day and although theoretically they are capable of a 50 to 60 dB dynamic range most records rarely exceed 30 - 40 dB. CD music is capable of 90 dB. Dolby Digital up to 120 dB and DVD audio up to 144 dB. To deal with those kind of dynamic ranges you need a power transformer and B+ DC power storage supply that can give large instantaneous doses of "juice" when needed on musical peaks. Most STOCK Dynaco power supplies just can't do it. Changing out the quad cap to the 80, 40, 30, 20 which almost doubles the B+ power storage on an original Dynaco amp will allow them to better deal with digital music signal sources.

    It is hard to find a "bad" sounding original Dynaco tube amp. Almost anything you play through them will be smooth, mellow etc. - but that fact - doesn't make them sound accurate and tonally correct to the original recording.

    Disclaimer > Everyone knows that I use the VTA driver boards in my amp kits so take that into consideration when reading what is below.

    The VTA driver board for the ST-70, Mark III etc. walks a fine line giving (IMHO) both tube smoothness and tonally correct sound reproduction. The bass is tight and the top end is smooth and has good extension. Roy Mottram developed that circuit in the 1980's and has refined the circuit over the years to it's present state. You can also up the "tubiness" of this driver board if you want by running the amp in triode mode. You can also alter the feedback line on this board. Raising the feedback resistor value from its present 7500 ohms (R7 left channel - R8 right channel) will give less feedback, increase gain and even more "tubiness" if you want. Dropping the value of R7 and R8 gives more feedback and less gain and less tubiness. Some have even removed the feedback line altogether. If you do this then gain and tubiness will go up but it is possible that the amp may not be stable with all speaker loads. I have done just that here (removed the feedback line to see what would happen) on all the speakers I have here and the amp played fine with no issues other than increased gain. I did notice that the amp without any feedback lost a little bottom end control.

    Bob


    Last edited by Bob Latino on Tue Aug 31, 2010 2:33 pm; edited 1 time in total

    JunkyJan

    Posts: 100
    Join date: 2008-12-09
    Location: BC, Canada

    Ah - the NFB 'curse'!

    Post by JunkyJan on Tue Aug 31, 2010 2:18 pm

    Bob, I was wondering the same thing - the common opinion over at DIYAudio and Audiokarma is "Before going to the trouble of replacing your VTA / Poseidon / whatever board with the original Hafler board, you want to go listen to an original ST-70 - sure it sounds good, but in a 'Fuzzy blanket' sort of way - it sounds DEAD".

    Sorry, but I choose Accuracy and Detail over colouration and fuzziness - even if it means that it exposes weak components in the rest of the system.

    Regarding NFB: I always wanted to ask about Negative Feedback (but were too scared in case it leads to a Religious War)! Bob, seems to me there was a line of thought in the Audiophile world in the 1990s/early 2000s that NFB is a BAD THING and should have been banned by the Pope, the UN Council on Human Rights as well as the SPCA. I therefore find it VERY interesting to know that the VTA board will still stay (relatively) stable when the NFB line (the 16 Ohm output on the OPT IIRC) is disconnected.

    Supposedly, running without NFB should have the same effect as running in Triode mode (versus Pentode/Ultralinear mode), and I have been meaning to try it. I don't think I will bother now....

    Thanks!
    -- 'Jan

    Bob Latino
    Admin

    Posts: 1903
    Join date: 2008-11-27
    Location: Massachusetts

    Re: "Traditional" driver circuit designs versus "modern" all-triode designs

    Post by Bob Latino on Tue Aug 31, 2010 2:55 pm

    Bob, I was wondering the same thing - the common opinion over at DIYAudio and Audiokarma is "Before going to the trouble of replacing your VTA / Poseidon / whatever board with the original Hafler board, you want to go listen to an original ST-70 - sure it sounds good, but in a 'Fuzzy blanket' sort of way - it sounds DEAD".

    Jan,

    Younger listeners who grew up with solid state throw a stock ST-70 tube amp into their system. Their music system had a solid state amp in there and all of a sudden their "ears don't bleed" anymore from the hardness of their SS amp. They think they have found the holy grail of amps - an original Dynaco ST-70! What they have done is gone to the other end of the spectrum because an original ST-70 IS very listenable and smooth. The stock ST-70 doesn't give them the listening fatigue after 1/2 hour like their SS amp did. They also have a larger and deeper soundstage now that they didn't get with their SS amp. They may think they have found audio nirvana but there are are better tube audio amp designs out there that can give a more accurate music listening experience than a stock Dynaco amp.

    Bob

    viridian

    Posts: 14
    Join date: 2010-02-19

    Re: "Traditional" driver circuit designs versus "modern" all-triode designs

    Post by viridian on Wed Sep 01, 2010 4:26 pm

    I kind of disagree with the premise that modern amps have triode front ends and classic amps use pentode stages in the front end. There are several current marques that use pentodes in the front end, Shindo and Yamamoto come immediately to mind. Likewise many "traditional" amps had all triode front ends. It's more a design decision than it is a matter of tradition or currency.

    As mentioned, the traditional Dyna circuits can sound quite fine, but there are better circuits available now, and different sounds to suit ones taste, from the more analytical sound of the Curcio boards to the more fluid sound of the VTA board. It doesn't seem that the blogger is really poo pooing (love that turn of phrase) modern circuits, but the Curcio circuit is clearly not to his taste and does not suit his system. Not surprising to someone who has heard Thiels.

      Current date/time is Sat Aug 23, 2014 4:26 am