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Old 8th December 2006, 11:02 AM   #1
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Question heretical unity gain OUTPUT stage...

This is very much a thought in progress, so forgive the lack of component ID

I've been toying with this idea for a couple of days now having read SY's excellent posts about the line stage. I wonder, what happens if you go bigger?

simple question would this work? I cant see a real problem from an SE viewpoint. (apart fom general drive difficulties). Park the output at 0v DC and theres no need for the parafeed cap.

OK, so double it, and feed each half in anti-phase and bingo.

or not, this seems either really obvious or really dumb.

any views?

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Old 8th December 2006, 11:31 AM   #2
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One problem- your addition of the transistors to boost the voltage range of the servo takes the inverse feedback and turns it positive. You can fix that by changing the servo integrator from an inverting to a noninverting integrator and let the transistor change the overall phase to inverting.

One other point: in the preamp circuit, it's important to keep the output at zero volts DC. With the transformer, it's more important that the two cathodes have equal current draw than that their voltage is zero. In other words, the differential voltage between the cathodes should be zero, but voltage to ground can be anything. It's pretty easy to rearrange the servo to do this, then use a single current source to ground from the primary center tap. Saves a bunch of parts and would probably work well as long as the stage is kept in Class A.

"...apart from general drive difficulties" Yes, indeed, that is the Achilles' Heel of CF output stages.
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Old 8th December 2006, 01:29 PM   #3
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thanks SY - instant guru feedback which I'll admit I'm gonna need to think about...

The transistors were only there because I couldnt think of a OP-amp that would stand the necessary grid voltage. about -50v- 60v for the 6AS7G I had in mind, originally.

BUT your second paragraph sums up nicely what seemed objectionable from the schematic. all I need is both ends of the transformer at the same potential (I thnk) As it is, it looks like I'd have two OP-amps 'fighting' each other? (one other point indeed!)

Although I need to redraw the schematic. I think you're saying just one half of the schematic should be servo controlled. which seems more reasonable. In fact a current mirror would be simpler?

Just to explain exactly what I had in mind, the idea here is to do whatever's necessary to make the o/p stage happy regardless of the PS or driving requirements. Then work back from there.

Oh Hang on, Cathode followers were selected because the o/p is near 0v, but since that doesnt matter anymore...

My head hurts... I need to go think

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Old 9th December 2006, 03:09 AM   #4
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ok ok ok now I see,

The push pull nature of the original idea renders a lot of it useless

To continue the discussion though, attached is the simplified version, which IS pretty trivial.

Following the thought to its logical conclusion you'd get a CCS loaded output stage. The servo isnt really needed at all (and is still wrong) Balancing the sinks on both sides of the transormer means that any mid voltage will do...

I think.
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Old 9th December 2006, 03:36 AM   #5
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Default Meanwhile in another part of town

This is the single-ended version.

Again, maybe a bit obvious. But, this time the O/P does need to be at OV.

Assuming that there are no howlers. I'm struggling to find any information on the best transformer impedance (for parafeed generally) Lower would be better I think? output impedance of a CF is low anyway (47 ohms for a parallel 6AS7G)

As always any advice is appreciated.

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