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tube fanatic

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  1. 3 hours ago, Jeffrey D. Medwin said:



    The Klispch speakers are generally hard to drive, with a SE power amp.  It's got high transfer efficiency losses, often a three way crossover to drive through, which is a killer for a sub 2 Watt amp to handle.  As I reported earlier above, you have to match the speaker to the amp. 



    An acceptable push-pull tube amp, could be built for Klipsch speakers, but no one has ever done that in audio to date. 


    Jeff Medwin

    Jeffrey,  in regard to your first point, it is totally untrue in my experience. As to your second point above, back in the day I built many wonderful push-pull tube amps for use with Klipsch speakers.  

    How do you come to these conclusions?



    • Like 2
  2. A detailed description of the listening room, listening level, proximity to the speakers when listening, types of music listened to, etc. would be helpful in terms of making recommendations.




  3. 14 hours ago, Backfire said:


    Another point is that what is commonly called the "damping factor" is a rather misleading, not to say almost meaningless, concept.  I sketched a couple of diagrams in a pdf, to show what I mean.  Its not a new point I'm making, and is perfectly well understood by those who analyse equivalent circuits, but it is one that does not seem to be widely appreciated.  The calculations I've included are a bit rough and ready, but they illustrate the key idea.


    The main point is that the speaker resistance (literally the resistive component of the speaker's impedance) should be being added to the output impedance of the amplifier, and it should appear in the denominator in the calculation of a meaningful damping factor.  (Not added into the numerator, where it is commonly put!)  Because the speaker resistance is generally a major contributor to the overall impedance of the speaker, it means the true damping factor will never really get much larger than about 1.

    damping-factor.pdf 411.16 kB · 11 downloads


    Well stated.  Yet, having an output impedance of 30+ ohms, as is often found with SEPs when no voltage fb is used, seems to make little or no difference when used with many speakers.  



  4. 16 hours ago, boom3 said:

    Thanks for the replies, however, let me re-phrase the inquiry:


    If one has two tube amps identical in every way, except for one output transformer having a lower secondary resistance than the other, which will be abetter match for Klipschorns?

    The DC  resistance of the secondary is of no consequence in general.  The output impedance is a different matter.  I have used high output impedance as well as low output impedance amps with Khorns and have gotten excellent results with both.  Again, you would need to experience different amps yourself and choose what you find most pleasing.  No one can tell you what will be best for your ears.




    • Like 1
  5. It depends on your definition of satisfactory.  Some users like very low DF SEPs, often with less than 1 wpc, while others prefer 60 or more wpc.  The only way for you to make a determination is to try various tube amps and draw your own conclusions.  



    • Like 1
  6. On 10/8/2020 at 10:32 PM, jajaja said:

    It was super cool! Decent sound, too. It was rebuilt by Don Sachs. I paid $675 and it lasted quite a while, but over the past few years I've had numerous minor issues. If I had technical skills I'd fix the problems myself, but around DC area techs charge an arm+leg to do even minor work. I just miss hearing my vinyl, so I'm looking for a lower maintenance amp.


    What problems are you having with the A300?  It is not a difficult amp to service.




  7. There are 2 ways to reduce distortion in SEP amps.  One is to use it at a fraction of its maximum output power, and the other is to use nfb.  I prefer the latter (as voltage fb) as it reduces the typically high output impedance as well as increasing the bandwidth.  If applied within reason, it does not ruin the resultant sound in my experience.  In fact, it can provide a very triode like experience in some systems.




  8. 47 minutes ago, Backfire said:



    All of which is a long-winded way of saying that I think it is almost certainly running in class AB, and not class A.  At low powers, up to a couple of watts, it will effectively be running in class A, but transitioning to AB at higher powers.  Nothing at all wrong with that, though!  It's just a little bit naughty, I think, when manufacturers make overstated claims about class A operation.  (Joule-Electra would not be the only one!)



    Class AB refers to a specific set of operating conditions which are usually somewhere between class A and class B.  So unless the amp has provision to change that, it is running in AB at idle through its maximum power output. It combines the advantages of both classes while minimizing their respective disadvantages.



  9. 11 hours ago, Jeffrey D. Medwin said:



    Are you talking about a bypass capacitor across the cathode resistor???   If so, most SET amps I see, maybe, just guessing  98%, regularly use electrolytic caps across the Rk, as a bypass.  IT is IN the circuit !!!!   Would these same people use a high quality electrolytic as a coupling cap. Why...NO !!   But they unconsciously use electrolytics across the Rk, of a 2A3, in its cathode return to ground.   That seems thoughtless to me !!! 


    When I bypass the Rk of a DHT FInals tube, I use films, with as "S", because it takes about five high-quality film caps, of different types and uF values, in parallel, to get the Rk bypassing musically correct.  Also, it takes a lot of money, to do it right. 


    Here is the perspective of an EE from the Steve Hoffman forum.  I agree with him.  I believe that using multiple caps in parallel, regardless of type or price, craps up the sound.  My ENT doc friend showed me some frequency response plots of typical males in their mid to late 70s which have convinced me that some of what we believe we hear is wishful thinking.  Also, the nonlinearity of the human auditory system is so great, and so varied from one person to the next, that universal recommendations about audio components are worthless.






    John Dyson

    John DysonForum Resident

    Fishers, Indiana
    R. Totale said: 
    OK, but once it's doing its job of showing a short circuit to ground for audio frequencies, how linearly it grounds these things you will never hear or be able to measure with any tool pales in importance. If it can be done (which it can) with a 25 cent electrolytic the point of inserting a 10 dollar film cap eludes me.

    Again OK, but I try not to do any work or spend money based on feels.
    If I was making the decision for myself, I wouldn't change the cap unless I had a concrete reason to do so. Some people like to change things -- and that is fine also, perhaps part of the enjoyment.

    Put my EE hat on (I am an EE RF/Audio/Systems, SW Operating Systems/DSP -- and generally pretty good at all of them :)), so here is a quick (back of scrap pad of paper) analysis of the situation:

    Lets see -- cathode resistor/capacitor (I dont know the tube, so I'll guess a gm of about 2mmhos or 2mS.) That would imply an effective resistance of about 500ohms at the cathode (without bypass.) With an electrolytic with perhaps 0.1ohms of ESR, lets guess at 10% of that is nonlinearity. That would mean a delta of 0.01ohms with a 500ohm impedance at the node... The additional amount of distortion caused by the electrolytic changing by the 0.01ohms would be less (much less than) 0.002% additional distortion, and I am guessing VERY, VERY HIGH. In reality, the amount of distortion caused by the nonlinearity of the resistance would be pretty small. Now, we have an issue about the nonlinearity of the capacitance of the capacitor, and the best success there is to make sure that the capacitance has a reactance that is very small (relative to the lowest audio frequency) in relation to 500ohms. Also, unless the tube/valve is in a negative feedback circuit, the changing gm will totally swamp the nonlinearity of the capacitor (either the resistive or the capacitive reactance nonlinearity.)

    If I was making a design decision, I would simply try to choose a reasonably high quality capacitor with fairly low ESR -- the rest will take care of itself. The difference in distortion of a reasonably good electrolytic vs a perfect capacitor will be vanishingly small, and probably not even measurable.

    Of all of the things that might need to be improved (other than as a failed/degraded parts replacement) I don't think that changing the capacitor would be all that helpful. Audio source material and transducers (speakers, headphones, microphones, etc) are much more important.


  10. Here’s what Rozenblit had to say about OTL output capacitors on the Transcendent forum:




    Bruce Rozenblit03/29/04
    There seems to be a lot of confusion about what an OTL is on other message boards. OTL stands for output transformerless. The idea is to eliminate the output transformer from the signal path. That's what I do. It is a very difficult task because of the impedance mismatch between tubes and speakers.  

    There also seems to be a great deal of resistance to the use of an output capacitor. Many people for some unexplained reason, have gotten the idea that an output capacitor will greatly injure the signal. That is baloney. A transformer is about 1000 times more injurious to a signal than a capacitor. Most tube amps have output transformers but that is OK. Most speakers have internal coupling capacitors but that is OK. When an amp has an output capacitor, its the end of the world. I don't get it. 

    Many people have spent a couple of hundred dollars to replace the stock caps in the SE OTL with black gates and all have reported back that there was no improvement. All have wasted their time and money.  

    There is a big difference between conceptual idealistic notions and what works in the physical world. I concern my designs with what works in the physical world. The T16 is direct coupled and the SE OTL sounds even better but it has an output capacitor. Its the results that matter, not the concept. 
  11. There’s no question in my mind that the OPT is the weakest link in a conventional tube amp design.  Anyone who is willing to deal with a larger number of tubes should certainly consider OTL.  If I weren’t so cheap, and did not spend my time with similarly minded audio folks, I would do a few OTL designs myself.




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