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Old 25th May 2004, 07:06 PM   #11
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Thanks, that's good to know. Do you know if the basic layout has changed? I just found this on a separate discussion, and thought the design looked odd. At least, different from stuff I'd seen before. And, I couldn't understand how it worked.
I've been trying to map the circuit board, but meeting with some frustration because the component identifiers stenciled on the board differ from the component identifiers on the schematic and components that can be clearly traced differ in value from the schematic. I believe the layout to be the same, hence my confusion as to how the design values can vary so dramatically from the components on the board. I don't have my notes with me or I'd give you some specific examples.
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Old 25th May 2004, 07:47 PM   #12
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>>GG has low gain<<
That's not quite true. GG topology has a LOW I/P impedance and a HIGH O/P impedance. As the load impedance approaches infinity, the gain approaches mu. Really HIGH load impedances are not practical with resistive loading. As a result, a small voltage gain is observed. Combine a GG with a Gary Pimm style CCS and the picture changes considerably. A GP CCS presents a load in the GOhm range to the tube, while its mu follower O/P is excellent at driving something downstream. A GP CCS loaded GG yields the "full" mu of the tube.
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Old 25th May 2004, 08:04 PM   #13
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But the two CF stages are still common cathode
A cathode follower is the opposite of common cathode. No voltage gain, therefore no Miller effect. And since the cathode follows the grid, Cgk is suppressed.
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Old 25th May 2004, 08:09 PM   #14
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Originally posted by Eli Duttman

Combine a GG with a Gary Pimm style CCS and the picture changes considerably. A GP CCS presents a load in the GOhm range to the tube, while its mu follower O/P is excellent at driving something downstream. A GP CCS loaded GG yields the "full" mu of the tube.
That is not quite true either GPís MF circuit has only high impedance in the low frequency range but not at higher frequencies. If you select proper fets it is possible to get a few megs at 20 kHz, but thatís it. But honestly I didnít try the fet-penthode mu-follower combo myself.

Anyway the load resistance/capacitance is reflected back to the anode of the amplifying triode. Something GP does not mention, however this is only important driving low impedance. I personally see not much advantage of such very high load impedances, but anyway the mu-follower sounds very good to me.

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Old 25th May 2004, 08:19 PM   #15
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A cathode follower is the opposite of common cathode.
Oops! You're right, of course, I stand corrected.
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Old 25th May 2004, 09:34 PM   #16
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I've been brainstorming a non-inverting gain block for use after the RIAA equalized circuitry in a HIGH gain phono stage. The object of the exercise is something approaching the proverbial straight wire with 20 dB. of gain.

I'm thinking in terms of a grounded grid 6922 loaded by a Gary Pimm CCS. The 6922's grid would be connected to ground by a 100 Ohm resistor. NFB would be applied to the grid by a 1 KOhm resistor connected to the mu follower O/P. I/P buffering of the block would be by a small signal enhancement mode MOSFET voltage follower DC coupled to the 6922's cathode. A 1 KOhm resistor serves to connect both the FET's source and the 6922's grid to ground. The FET would be forward biased by a 9 V. Lithium battery in the gate circuit. A 10 MOhm gate resistor allows a SMALL PTFE cap. to be the DC block battery bias requires. The 6922 idles at 2.5 mA., while the FET idles at 3.5 mA. That places the 6922's cathode 6 V. above ground.

Opinions please!
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Old 25th May 2004, 09:37 PM   #17
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Opinions please!
Aw, you already know what I'm going to say.

Too complicated.
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Old 25th May 2004, 09:45 PM   #18
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I agree, there is complexity. A Porsche is complicated too and it gets the job DONE. Will my conception get the job DONE?
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Old 25th May 2004, 09:47 PM   #19
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Rube Goldberg would be proud!
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Old 25th May 2004, 09:51 PM   #20
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Hi,

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A Porsche is complicated too and it gets the job DONE.
No, they're about as simple as they come....probably why they're so reliable too...

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Rube Goldberg would be proud!
Me too...

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