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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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In Reply to: Re: Ideal OTL posted by Ralph on March 29, 2002 at 08:02:12:
Dear Ralph,THANK YOU SO MUCH!!!!!!!
Like I had said before, without fail, you blow the smoke out of the room and simply clear things up.
Your answers were EXTREMELY helpfull.
The fact that the 12SX7 is not being produced, has tripled in value and difficult to get, makes it abundantly clear that its a non reality for a production amplifier. I also got the impression from your email and post, that its not that much better, or that at least whether or not it is better is debatable. IMHO, your choice not to use it is the only one sensible to me.
The facts you mentioned about the 6C33C-B vs. the 6AS7G output tube were very educational. I am glad you posted it since I am not sure that all the folks who are familiar with the AOG are aware of those issues.
[Ralph also explained to me over the phone, that negative feedback has a fundamental flaw. In theory it should work just fine, according to the math of Fourier and Shannon. However, in reality there is a phenomenon called "propagation delay", which is a measureable time delay as the feedback is looping through. This is not accounted for in the theoretical math equation, and in fact, Ralph stated that when this point is raised the mathmaticians don't have an answer!
On top of this, the propagation delay results in the negative feedback reducing even order distortion, but increasing odd order distortion. It also affects the low frequency dynamics by restricting them, while at the same time sending the high frequencies into ringing! Therefore, when the other amps are using this approach, there is a tendency for their sound to take on a somewhat "shrill" characteristic, in contrast to the relaxed characteristic I found when auditioning the Atmasphere amps.]Lastly, the damping factor issue was also very helpfull. From what you have shared, it seems to me that the Rp/2+mu=Zout calculation, has to be accounted for both halves of the output tubes. You pointed out something that I haven't found anyone else to mention: the 6AS7G is a DUAL TRIODE! Therefore it has essentially two tubes per tube to be accounted for. Furthermore, they are in parallel. All of these factors affect the manner in which you calculate the Rp value.
Again, I think this is very educational for the AOG website. I am sure most of those guys already know all this stuff, but maybe some of them are like me, and didn't know how you really calculated your spec. Your method, also makes intuitive sense, and explains why I kept finding so many discrepancies with other reported figures using the same formula.Therefore, using Ralph's application of the Zout formula when applied to the Atmasphere amps, he states that there are two ways to approach the Rp value:
Method 1.) Double the output tube number per amp, in order to account for the Dual triode nature of the 6AS7G output tube. In this case the Rp is determined by dividing the 6AS7G plate resistance of 280 by 28 tubes (14 dual triodes X 2) for the MA1 monobloc, with a mu of 2.
280/28=10, Rp=10.
Rp/(2+mu)=Zout; 10/(2+2)=2.5ohms!2.5ohms is about 7%off of the measured internal impedance that Atmasphere specifies for the MA1. Ralph says that in parallel and differentially balanced, he believes that the plate impedance is lowered in the real world, which is why they measure 2.3ohms, rather than 2.5ohms as is predicted by the formula.
Method 2.) This time don't double the number of the output tubes, but approach the problem from the parallel nature of the circuit. This according to Ralph, has the affect of reducing the plate impedance of the 6AS7G by half; from 280 to 140. Now determine the Rp value as 140 divided by 14 tubes, and again you end up with an Rp value of 10! Which as was just shown above will result in a Zout of 2.5ohms.
Thanks again Ralph.
Gratefully,
David
PS:
Ralph also noted another aspect of the amp's impedance behavior. Siting Speltz discovery, he mentioned that Atmasphere had found similar behavior. More specifically, the above impedance value is really calculating the impedance at full output power. When Atmasphere measures the internal impedance empirically, they measure it at full power. This is because, according to Ralph, to use any other power level is arbitrary, and will not result in a definitive answer. This is how they obtain 2.3ohms, at the 140 max power of the MA1 (same method for MA2, and M60).In this case, as the power demand increases, the paralleled tubes dump more current, and the impedance drops significantly. However, when the power demand is low, the impedance rises. This helps to explain why the transients on the Atmasphere amps are so exceptional. As Speltz notes, when the load becomes harder, the amp holds onto it even tighter.
I hope this isn't boring too many of you out there. And maybe it was a little helpfull.
Take care everyone, and have a great weekend!
David
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- Re: Ideal OTL - david 12:23:35 03/29/02 (0)
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