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Old 17th December 2010, 08:29 PM   #41
cnpope is offline cnpope  United States
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It should also be added that Broskie's discussion is really only concerned with comparing the output stages themselves. I think that if one really wanted to build a Broskie (c)-type totem-pole amplifier, the technical difficulties of arranging a symmetrical way to drive it, especially with DC coupling, would be rather horrific!

Chris
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Old 17th December 2010, 09:32 PM   #42
banat is offline banat  Serbia
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This old article cood explains many OTL Tube Amp related things:6C33C-B OTL Amplifier - Background and OTL Circuits

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Old 17th December 2010, 10:18 PM   #43
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Quote:
Originally Posted by cnpope View Post
I think one has to read Broskie carefully, to see what he is claiming and what he isn't. He is NOT saying that a standard totem-pole configuration behaves the same as a circlotron. What he IS saying is that whether you make the "daisy chain" ordering tube/psu/tube/psu, as in the circlotron,or instead the ordering tube/tube/psu/psu as in the totem-pole, makes, of itself, no essential difference. What DOES matter is where you tie the thing to ground. In the circlotron, it is effectively the mid-point between the two speaker terminals that is tied to ground. In a standard totem-pole configuration, one of the speaker terminals is tied to ground (the one connected to the midpoint of the two power supplies). This introduces the asymmetry, and the very real difference as compared with the circlotron operation.

Schematic (e) is the standard totem-pole, and schematic (d) is the standard circlotron. He is NOT saying that they are equivalent in their operation.

What he IS saying is that schematic (c), a "minor variant" on the totem-pole in which the midpoint between the two speakers terminals is instead grounded, is completely equivalent to the standard circlotron output stage. He gives general arguments for why that is so, and he also reports the results of PSpice simulations for schematics (c) and (d) that confirm this.

I don't think he is saying anything controversial, because the the "minor variation" of the totem-pole configuration that he makes means that it is NOT the circuit that anybody making a totem-pole OTL ever normally uses.
But the interesting observation that he makes is that it is the decision of what point to tie to ground that makes all the difference in the performance of the circuits, rather then the decision of daisy-chaining tube/psu/tube/psu vs. tube/tube/psu/psu.

Let me just add that I personally have built three OTLs, two of which are totem-poles ("standard" Broskie (e)-type!), and one circlotron. Of the three, I think I like the circlotron the most.

Chris
Thanks Chris, for this clarification, which I am in agreement with. I have to say I was reading something a lot different in that article. His comments about 'magic' and stuff like that...
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Old 18th December 2010, 06:56 AM   #44
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Quote:
Originally Posted by cnpope View Post
It should also be added that Broskie's discussion is really only concerned with comparing the output stages themselves. I think that if one really wanted to build a Broskie (c)-type totem-pole amplifier, the technical difficulties of arranging a symmetrical way to drive it, especially with DC coupling, would be rather horrific!

Chris
My OTLs are DC coupled totempoles, but I cheat a little and use SS current mirrors to get the grid connection to the output tubes doen on the negative side. But hey, it's a single gain stage DC coupled tube OTL, and has been 100% stable and reliable for the ten years or so I've been building them. Admittedly, I only build guitar amps, and have not worried about the slight mismatch in the output swing since that only helps bring in a little 'character' to the guitar tone
I guess I should tweak the design to see what it can do in the HiFi department, but that takes time...who has any spare time these days?

I've got my first circlotron in the works, and looking forward to compare...whenever I get the time to finish it...
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