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    Dynaco st 70 with VTA Driver Board or Modified Original Board

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    gouldglen

    Posts : 4
    Join date : 2012-03-22

    Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by gouldglen on Sun May 13, 2012 9:45 pm

    Hi All, I am relatively new to this forum. I have learned a lot here already from reading. a friend of mine (self-taught technician) suggested to me that he could take the original dynaco driver board (7199 tubes) and get that circuit to out-perform a VTA or similar board. I am listening right now to a "new" st-70 with VTA board and original trannies. so far to my ears it sounds very good. I am very curious how folks feel about this challenge. I realize some here my have strong bias one way or the other. That is ok as long as you state your bias up front. I look forward to hearing from as many folks as possible as i find this subject quite intriguing.

    Cheers,
    Glen

    tubes4hifi
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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by tubes4hifi on Sun May 13, 2012 11:22 pm

    any one person can make a claim that something sounds better than something else, the only way to find out is to listen to things yourself. 25 years ago I listened to every stock and modified ST70 amplifier I could get my hands on.
    I wrote a report on the 6 most popular versions of the amp, posted here:
    http://www.tubes4hifi.com/st70mods.htm
    and a few dozen customer reviews of my own version shown on the bottom of this page:
    http://tubes4hifi.com/ST70.htm
    I haven't posted any more reviews here in the past 8 years or so, otherwise there would probably be in excess of 1000 positive reviews, so a few more than my own opinion.

    GP49

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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by GP49 on Mon May 14, 2012 11:10 am

    The story has faded into the annals of audio folklore by now except for those who still have a copy, but that "report on the 6 most popular versions" of the Dynaco Stereo 70 was originally an article in Glass Audio magazine. It generated some controversy as to methodology and the rigor with which the comparisons were done. Ultimately, Glass Audio owner/editor Ed Dell withdrew the magazine's endorsement of the review and sought out another reviewer to do a similar comparison. The reason given was a "conflict of interest" the nature of which was not revealed.

    baddog1946

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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by baddog1946 on Mon May 14, 2012 11:43 am

    All sound comparisons are subjective opinon. A technical comparison speaks for itself with measurements.
    If you understand all the mods done to improve this circuit and do not notice the acoustic results in a comparison with an original board I suggest you consult an audiologist. In many subtle and obvious ways the VTA is an long evolution of a proven topology.
    The VTA boards in all their forms IMHO are a vast improvement over the originals acoustically and technically. Many of the shortcoming of the original board and technology of the day's limitations have been improved without question. You can spend a lot of time and money to find that out.
    The VTA board has been evolved over 20+ years and seems to me to be near its technology development limit. A good idea taken a long way.
    You have your work set out for you if you think you can make the original board sound better than a VTA. Not withstanding your technical buddy's opinion.
    Baddog

    tubes4hifi
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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by tubes4hifi on Mon May 14, 2012 11:45 am

    the information is so old now (20+ years) that I had taken it down a few years ago, but have re-uploaded it now.
    The "conflict of interest" was due to one supplier not liking the fact that his own modification didn't sound as good as any of the others,
    and the fact that I got so many inquiries on the article about the "Vacuum Tube Audio" design that I started supplying it to other do-it-yourselfers who had already tried one or two of the others and had been as disappointed in them as much as I was.
    The second test by another independent reviewer came up with very similar results to my original test.

    GP49

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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by GP49 on Mon May 14, 2012 12:13 pm

    Thanks for putting some "meat on the bones" after all these decades, Roy.

    As I recall, the methodology brought into question was that one stock amplifier was listened to; then it was modified and listened to, then the modification stripped out and the next one built and listened to, and onward in sequence. I could probably be more specific if I didn't have so much else to do that I could dig out those magazines (there's a beautiful Garrard Type A record changer on the bench that needs to be overhauled and restored so I can get it out to UPS later this week)!

    What's remarkable is that to my recollection, of all the modifications reviewed in that article (and it was DECADES ago) the only two that have stood the test of time and are still available now, in updated versions, are yours and the one that was least favorably reviewed! That's in addition to the original, stock Dynaco Stereo 70; although Dynaco is long gone, one can still build a totally new one.

    plexus

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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by plexus on Mon May 14, 2012 1:34 pm

    I am always getting into arguements with "audiophiles" who spew subjective evaluation results as if its The Truth. The problem is that subjective evaluation plays directly into our psyche and you know what a mess that is! I throw out any subjective evaluation conclusions by anyone other than myself and I do not readily share my own subjective evaluations unless I disclaim, more than once, that these are my own personal opinions based on what I heard and you are best to listen for yourself.

    Objective evaluation is a whole other animal. very hard to do with audio and cover off every single aspect that makes for a sound scientific experiment. you can usually only take it so far. but, its better than nothing. I consider a good objective evaluation much closer to The Truth than anything done subjectively.

    In other words, I don't really follow reviews. I will use them as an input to my ultimate decision making but in the end its my ears and brain (and heart) that will tell me what sounds good (to me) and what doesn't. I suggest you do the same. it will save you a lot of headache. but you have to do the same in reverse and that is never to impose your own subjective experiences on other people (eg. you can not say "X cables sound better than Y". you need to rephrase "X cable sound better than Y to me. you should listen for yourself."

    If you care the spend the money to join the AES ($100), there are a number of publications you can download on both subjective and objective evaluation in audio and also a number of research papers on subjective and objective evaluation of specific things like cables, sample rates, capacitors, etc.

    gouldglen

    Posts : 4
    Join date : 2012-03-22

    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by gouldglen on Mon May 14, 2012 2:55 pm

    plexus wrote:I am always getting into arguements with "audiophiles" who spew subjective evaluation results as if its The Truth. The problem is that subjective evaluation plays directly into our psyche and you know what a mess that is! I throw out any subjective evaluation conclusions by anyone other than myself and I do not readily share my own subjective evaluations unless I disclaim, more than once, that these are my own personal opinions based on what I heard and you are best to listen for yourself.

    Objective evaluation is a whole other animal. very hard to do with audio and cover off every single aspect that makes for a sound scientific experiment. you can usually only take it so far. but, its better than nothing. I consider a good objective evaluation much closer to The Truth than anything done subjectively.

    In other words, I don't really follow reviews. I will use them as an input to my ultimate decision making but in the end its my ears and brain (and heart) that will tell me what sounds good (to me) and what doesn't. I suggest you do the same. it will save you a lot of headache. but you have to do the same in reverse and that is never to impose your own subjective experiences on other people (eg. you can not say "X cables sound better than Y". you need to rephrase "X cable sound better than Y to me. you should listen for yourself."

    If you care the spend the money to join the AES ($100), there are a number of publications you can download on both subjective and objective evaluation in audio and also a number of research papers on subjective and objective evaluation of specific things like cables, sample rates, capacitors, etc.

    The intent of my original post was to spur some open-minded discussion on the possibility of taking the original dynaco driver board and possibly making it sound as good or better than the current replacement boards. I would also say that i value subjective opinions in audio more than objective ones. As far as the idea of "Truth" goes, I am involved most days with making music either in rehearsals or live on stage. I have also been involved with a number of recording sessions, both live and studio. My goal here is to get as close to the recorded event given the limits of hi-fi reproduction. I look forward to a productive discussion.

    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by plexus on Mon May 14, 2012 3:11 pm

    The original boards have issues that are going to make them a challenge to sound better than a replacement driver. First of all, the material the boards are made from is hydroscopic - it absorbs water and becomes a little conductive. also, typically, there is heat damage which causes the board material to decompose hence the areas of black. that black is carbon. carbon is conductive and resistive. so the boards themselves are an impediment to good sound (unless, subjectively, you prefer the sound of charred moist boards. Smile

    Next, the stock components, in a word, suck. maybe they were quality components 50 years ago but the stock components are low tolerance and low quality compared to even cheap electronic components of today.

    The circuit topology (design) of the stock driver boards seems to be based more on the minimum to get ok sound quality out of them and also to be cost effective. the available tubes back then is different than now. also quality requirements back then are different than now - tubes then were common-place but now they are used mainly for people who want higher quality sound. i can't speak for the ST70 but certainly the MK3 driver design was more about inexpensive and "good enough" sound quality, for people back then. the 6AN8 is not a very good sounding tube for audio but it did save 1 tube over using dual triodes. probably good enough for people back then but not now. i think the same can be said for the tube selection and circuit design for the stock ST70.

    While you could mod the stock ST70 board to accept better sounding triodes like the 12_U7 family etc, you still have the poor quality stock components and deteriorated stock board to contend with. I think in the end its more cost effective to just get a replacement upgrade board like the VTA products.

    On my MK3's, my first attempt at an upgrade was to replace the stock board with new glass epoxy boards with the same circuit design. i also replaced all the stock parts with new quality matched parts. this did improve the sound but the biggest improvement was going with the VTA 6SN7 drivers and further improved with VCap CuTF's in the phase splitter decoupling.

    Bob Latino
    Admin

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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by Bob Latino on Mon May 14, 2012 3:40 pm

    The original Dynaco ST-70 driver circuit was designed around a circuit that used just two driver tubes - one (7199) driver for each channel. Dynaco did this to save one tube per amp. If a driver tube cost $1 at the time and 350,000 amps were sold - well - you do the math .. Dynaco saved many $$$$ by using this circuit.

    The original 7199 driver circuit is not linear at all drive levels and as the drive level goes UP the frequency response goes DOWN. 20 Hz to 20 KHz frequency response? - maybe at 1 watt output but certainly not a higher volume levels. Combine a poor driver circuit with a weak power transformer and power supply and you have a 25 WPC amp at best with a severely rolled off top and bottom end.

    Now the nice thing is that the ST-70 (in stock form) is a benign, mellow sounding amp that is not harsh or really bad sounding in any way. It just doesn't sound accurate when compared to better drive circuits.

    Almost any of today's ST-70 upgrade boards sound better than the original 7199 driver board. Below is the last page of Dynaco's 1975 product brochure. In November of 1974 they had to start rating their amps according to FTC guidelines. Check the photo below. Dynaco knew the amp would not make spec. Hey ... That "35 WPC" is now rated at "20 WPC". The 20 Hz to 20 KHz that Dynaco had on their spec sheet is now "50 to 10,000 Hz"

    The use of, or even the adjustment or alteration of that original driver circuit is (IMHO) like beating a dead horse. We should accept the fact that this circuit (at least by today's standards) is mediocre at best and should be put to rest.

    Bob




    Last edited by Bob Latino on Mon May 14, 2012 4:48 pm; edited 2 times in total

    plexus

    Posts : 94
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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by plexus on Mon May 14, 2012 3:59 pm

    wow. that says a lot (the dynaco reprint page you posted, bob). there it is.

    baddog1946

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    Location : Costa Rica

    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by baddog1946 on Mon May 14, 2012 7:19 pm


    I posted this a while ago but it speaks to the current issue so here it is again
    Baddog


    Hearing is not the same as listening
    Did you hear what I heard? You'll never know. Our experience of the music we listen to is shaped by extensive processing of the raw vibrations that enter our ears. That processing, by the auditive and cognitive centers of our brain, is itself strongly shaped by a long trail of experiences, expectations and culture. Plus another dozen or more factors.

    Most people are not so much interested in the mechanisms (both physically and mentally) behind listening. But listening is really where it all comes together. The enjoyment of music, the interpretation of performances, the judgment of audio reproduction quality is not done with our ears. It is done within our brain. The ears are the sensors that provide our brains with just a physical sample of the air vibrations that represent the sound. What those vibration mean, what they signify, and their quality is determined by the brain.

    In his latest book "I am a strange loop", Douglas Hofstadter says it as follows:
    "The passage leading from vast numbers of received signals to a handful of triggered symbols is a kind of funneling process in which initial input signals are manipulated or "massaged", the results of which selectively trigger further (i.e., more "internal") signals, and so forth. This baton-passing by squads of signals traces out an ever-narrowing pathway in the brain, which winds up triggering a small set of symbols whose identities are of course a subtle function of the original input signals.[...] In the interest of clarity, I have painted too simple a picture of the process of perception, for in reality, there is a great deal of two-way flow.
    Signals don't propagate solely from the outside inwards, towards symbols; expectations from past experiences simultaneously give rise to signals propagating outwards from certain symbols. There takes place a kind of negotiation between inward-bound and outward-bound signals, and the result is the locking-in of a pathway connecting raw input to symbolic interpretation."
    The brain does NOT take the ears at face value

    And in case you are wondering, the brain does NOT take the ears input at face value. It is as if the brain says: OK, ears, I get that, but let me look at a couple of other inputs as well. What mood am I in really? How does that equipment actually look? Color, size, cost? Did I build it, or did I lay out hard-earned money to buy it? Or is it from a manufacturer I heard bad things about? Etcetera, etcetera.

    How DO you become aware of what you hear? I mean, it's not as if you 'feel' those vibrating hairs in your inner ear and think, aha, that's a piano! No, somehow, the brain makes you 'aware' that you hear a piano. Pretty mysterious, that whole process. But scientific research has come a long way to unravel at least some of it...
    A landscape of sound.

    Imagine that the brain has many areas that react to a certain input. There is an area that mainly acts on sound, one that mainly acts on colors, one that mainly acts on how you feel (what Antonio Damasio calls 'the body state')

    The important word here is 'mainly'. Because, there is extensive interconnection between those area's, there is an exchange of information to built your perception. Call it a landscape of neuronal activity, all over your brain, that couples and integrates all those inputs (external and the internal body state) and which leads to a particular state of 'perceiving' the world around you, sound, sights, emotional state, etc. A nice example, related to the total integration of sensory inputs to the perception of audio, is described by Floyd O'Toole, VP of Acoustical Engineering at Harman, in an article called Audio science in the service of Art. The article is quite long to download, but this excerpt makes this particular point.

    So, we are unknowingly building, revising, redoing that perception landscape while we go about our business. But the actual vibrations of those hairs in your inner ears are only ONE input of many others. That particular vibration information gets integrated in the total picture. But unavoidably, in that process, much of it is thrown away. The brain uses this, discards that.

    Now, if you are trying to listen attentively to a particular sound, the 'gain' of the sound area is turned up and that of the other areas is turned down. You can be so immersed in listening that you hardly know what's going on around you. You can tune the attention of your brain. In fact, that gain can be turned up so high that you think your hear a sound that isn't there. You can create a certain landscape in your brain that is totally imaginary.



    Just making it up
    Lets taken an example from vision because vision experiences are much easier to discuss. Focus your attention on your mother's face. Got that? OK. Now, I assume your mother is not right there with you (if so, try your father's face). Do you realize what is going on? You 'see' something that isn't there, that is constructed by your brain from memories. A figment of your imagination if there ever was one.
    Now, very likely you also have a particular emotional feeling, connected to your mother. You may remember a nice meal she cooked, or you remember her serious illness last year, or you feel guilty because you haven't called her lately. All constructs from your brain, created without physical perception inputs.

    It's even worse. If indeed you remember that nice meal, you probably 'see' how she smiled at that occasion when you complimented her. Or, if you remember the sickness, you probably 'see' her as a sick person. If it was a serious sickness, chances are that you actually 'feel' bad. Not only does the brain create a perception all by itself, it also selectively 'shapes' that creation depending on which memory you happen to access and it influences your 'body state' to match the perception. When you imagine a bad situation, your body also starts to 'feel bad'.



    GP49

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    Re: Dynaco st 70 with VTA Driver Board or Modified Original Board

    Post by GP49 on Mon May 14, 2012 7:33 pm

    plexus wrote:wow. that says a lot (the dynaco reprint page you posted, bob). there it is.

    To put it into the perspective of real-life music reproduction, how often is full power required above 10KHz?
    If this really ever does happen, see if the tweeters still work afterwards!

    AND the 50Hz limitation on full power at the low end is power supply-related, beyond the capability of ANY
    redesign of the audio circuits in a Stereo 70.

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