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Backfire

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  1. The "damping factor," as usually defined, is the speaker impedance divided by the output impedance of the amplifier. Since, probably, a manufacturer typically assumes an 8 ohm speak impedance when quoting a damping factor, this means that what is being stated is just a slightly convoluted way of saying what the output impedance of the amplifier is. That is, Output Impedance = 8 divided by Damping Factor. I think the concept of damping factor as a "figure of merit" is highly misleading. Figure 1 shows a simplified effective circuit for an amplifier (the left-hand box) connected to a loudspeaker (the right-hand box). The amplifier is modelled as an ideal (zero impedance) voltage source in series with a resistor R0 (the output impedance). The loudspeaker is modelled as an ideal (zero DC resistance) inductor in series with a resistor Rs (the DC resistance of the loudspeaker). The ideal inductor has an impedance Z, which is, of course, highly frequency dependent. The loudspeaker model here is very crude; better models look something like what is depicted in Figure 2. In each case, it is a resistor Rs in series with the rest of the model. If we stick with the very simplified model in Figure 1, and furthermore if we make the rough approximation that we can just add Rs and Z to get the net "speaker impedance" (i.e. we neglect the complications of the need to take into account the phase angle for the inductive reactance of the speaker coil), then the usual so-called damping factor DF is calculated as DF = (Rs + Z)/R0, which can get impressively (and deceptively!) large if the output impedance of the amplifier, R0, is very small. But it is clear from the circuit in Figure 1 that R0 and Rs are simply two resistors in series, and they both represent lossy components that tend to prevent the amplifier signal from controlling the speaker cone. A much more accurate measure of how effectively the amplifier controls the cone is to calculate Z divided by the total series resistance. Thus we can defined the "true damping factor" TDF as TDF = Z/(R0 + Rs). The crucial point is that the speaker resistance Rs should be in the denominator in this calculation, not in the numerator! It is completely misleading to do what is normally done when calculating the damping factor, and adding in the speaker DC resistance (which is typically of order 6 ohms) in the numerator. The speaker DC resistance totally overwhelms any resistance in the speaker connecting leads, and in a solid state amplifier it totally overwhelms the output impedance of the amplifier. It even overwhelms the output impedance of many tube amplifiers. As long as the output impedance of the amplifier is small compared to the 6 ohm or so DC resistance of the speaker, then it isn't going to matter too much what it is. The difference between 1 ohm output impedance and 0.001 ohm output impedance is almost negligible, as far as the control of the speaker cone is concerned. I made simplifying approximations to keep the discussion easy, but the basic point remains the same if one does things more precisely. The essential thing is that the DC resistance of the speaker is directly in series with the output impedance of the amplifier, and it is the sum of those two that represents the net effective "resistive loss" that characterises how well, or badly, the amplifier is getting its signal to the speaker cone.
  2. 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
  3. It probably depends what you mean by "for testing purposes." The pinout is the same as 12AX7 or 12AU7, but the 12DW7 is a funny hybrid, with two unequal triode sections; one has a lot more gain than the other. You won't get fireworks if you try 12AX7 or 12AU7, but it may only give a rough idea of how it's working.
  4. Yes, I was speaking a bit loosely. People sometimes speak of a class AB amplifier as "operating in class A up to X watts," where X is the maximum power it can put out before the tubes essentially stop conducting at the bottom half of the audio cycle. It can give a useful characterisation of how much power it takes before the tubes go into cutoff.
  5. I hunted around with Google, but I couldn't track down a schematic for the Joule-Electra. However, as far as I can make out, it is a circlotron. They can be very good indeed. I have one that I built a few years ago, using EL509 tubes in the output stage. I am a bit sceptical about the Joule-Electra claim that is is biased for full Class A at 100 watts into 8 ohms, though. That would mean 5 amps maximum current. With three tubes per side in the circlotron, that means a maximum of about 1.7 amps per tube. That is certainly not a problem for the 6C33C, as a maximum current. However, if it were truly running in class A up to 100 watts, that would mean the quiescent current in each tube would need to be about 0.83 amps. With a plate supply voltage of probably about 150 volts, that would be about 104 watts tube dissipation, which is well in excess of the rated 60 watts maximum for the 6C33C. It's not a problem to have the plate dissipation exceed 60W instantaneously, but it is a problem to have 104 watts steady dissipation. Usually in an OTL, 6C33C tubes are biased for about 200 mA quiescent current. 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!) I really like the circlotron design; a truly symmetrical type of output stage. I'm sure the Joule-Electras must sound superb!
  6. I completely agree with those saying that capacitors are not a big problem. And another point is that if one has any doubts as to whether a capacitor might have any deficiencies, one can easily check by using an oscilloscope to look at the signal across it. The ideal would be that a capacitor in the audio path (e.g. a cathode decoupling capacitor, or the final capacitor in a power supply) would have zero audio signal across it. Of course in practice, being of only finite capacitance, it will have some non-zero AC reactance, and so there will be a small audio signal across it (less and less as the audio frequency increases). The magnitude of the expected audio signal as a function of frequency is of course easily calculable. If the capacitor were causing any non-linear distortion of the audio signal, then this would show up in the signal one would see across the capacitor using the oscilloscope. If the distortion were enough to be audible, then it would easily be visible and measurable. By measuring the distortion, if any, in the signal across the capacitor one could easily estimate the distortion it would cause in the audio output from the amplifier. Almost certainly, unless one has made a really unsuitable choice of capacitor or it has some serious fault, the distortion it would cause would be completely negligible. But in any case, if the capacitor is causing any problems of this kind it is easily measurable and understandable using basic physical principles. I would be much more inclined to believe what the instruments were saying than the anecdotal accounts of a human who probably hears what he wants to hear.
  7. Yes, I think you are right; most OTL amps these days do not have a capacitor between the output and the speaker. I have four different home-built OTL amps myself, and none of them has an output capacitor as such. Same is true, I believe, of commercial designs like Atmasphere or Bruce Rozenblit's Transcendant Audio amps. There are still capacitors in the output path, because the audio signal passes through the power supplies. The same is also true, of course, for an SET or push-pull amplifier. It is not true that OTLs require paralleled output tubes. Two of my OTLs, for example, each use just a pair of 6C33C tubes in the output stage (totem pole), so no paralleling of tubes at all. They both give 25W into 8 ohms. In any case the alleged "choir effect" of paralleled tubes is a non-existent phenomenon, in my opinion, that is not recognised by any reliable authority.
  8. Yes, the 6C33C OTL is about 25W into 8 ohms. I'm just using the single power supply for the two channels, if that's what you mean. The big toroid on top is 117-0-117 secondary, for the main positive and negative HT supplies for the output tubes, and also, with voltage doubling, for the input and stages driver. There are a couple of smaller toroids under the chassis, for the heaters. The main HT transformer is a bit of an overkill in terms of power handling, I think. I don't have any problems with it. The design of the OTL is based on one by Tim Mellow; it appeared in Audio Express, February 2010. The power transformer for that other OTL is a bit bigger, and sits in my front garden...
  9. The first is my everyday amplifier, a conventional type of OTL amplifier using 6C33C output tubes. I quite like the second one too; again OTL, using 6082 output tubes. The novelty with this one is that there are no transformers at all; it runs directly from the mains supply. It's based on one of the earliest OTL designs, by Dickie and Macovski in 1954. Regretfully, my constructions tend to be rather utilitarian in comparison to some of the beautiful ones built by others!
  10. Taking this, and some of your other assertions, at face value, you must surely admit that by many people's standards they are extraordinary claims. It is therefore reasonable to ask for proof of extraordinary solidity. Knowing, as we do, that the human ear and mind, as with other senses, can easily be deceived, the surest way of being certain that the claimed effect is genuine is to carry out double blind listening tests. You spoke, for example, of hearing a "disgusting skew" if L1 and L2 in an L1/C1/L2/C2 power supply were unmatched. That sounds like something that it would therefore be very easy for you to demonstrate as audible (to you), if you were to take part in properly controlled double-blind listening tests. You are not talking about a tiny, subtle effect; you are talking about something that "sounds disgusting" to you if there is a mismatch. It should be incredibly easy for you to demonstrate that you really are hearing that, and that it is not a result of some expectation bias on your part. When you think of all the years you have probably spent arguing with the many people who don't believe your claims, and the fact that it seems to be very important to you for people to believe the claims you make, it would be relatively easy for you to produce quite compelling supporting evidence by taking part in a rigorously-controlled double-blind listening test. Maybe consider doing something like that?
  11. It seems to me that a characteristic feature of your posts is that you shy away from anyone's attempt to engage you in technical discussions that challenge your assertions. For example, a while back in your Dyanaco preamp thread some questions were raised about your use of the term "transfer efficiency" in the context of different choices of power cord. I presented some estimates showing that any difference in power transferred from the wall socket to the amplifier when using a 12 AWG Mil. Spec. wire as opposed to cheap zip cord was completely dwarfed by day-to-day or hour-to-hour variations in the voltage supplied by the local electricity company. Instead of showing a willingness to engage in a technical discussion to back up your assertions, you simply responded by asserting "I am not going to enter into any technical discussions with you. There is a lot that technical people THINK they know, but do not know at all. It is hilarious to me, how much educated people, who can not think out of the box, get goofed-up in audio, and I find they also often have crummy sounding home audio systems. Have a great day." This kind of reply is pretty much your stock response to more or less anyone who has some technical knowledge and who tries to challenge you on one of your "technical" assertions. I can well understand why Maynard would be sceptical that anything would go differently in any further attempts he might have to discuss technical matters with you.
  12. Yes, 40uF would seem to be pushing the limits of what is recommended for a 6X4, so certainly switching the 40uF and the 20uF around so that the 20uF is the one coming directly after the rectifier would give a bit of extra margin.
  13. It looks like it just has standard, rather basic, bass and treble controls; it doesn't look as if it has RIAA compensation (which is good, because you don't need or want that, when driving from a CD player). So the best thing would be just to try it as is, after following Maynard's suggestions about recapping and adding a 3-wire mains cable. One amp takes its input from the left-channel output of the CD player, and the other takes the right-channel output. Tone controls are like a red rag to a bull in some "audiophile" circles, but they have their uses and plenty of people like to have them. These amps are never going to be super hi fi, so the best thing is just to try them out, after the basic safety replacements a la Maynard, and see what you think of them.
  14. I think perhaps you are under a misapprehension about the time-scale of the signal travel time through the amplifier, in comparison to the time-scale of the musical rhythm. The time delay is totally insignificant. There may be other objections that could be levelled at feedback in some circumstances, but a time delay that interferes with the beat of the music is not one of them.
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