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Folded/shunt cascode differential question
Folded/shunt cascode differential question
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Old 31st July 2018, 01:53 PM   #51
Sodacose is offline Sodacose  United States
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Join Date: Feb 2006
Location: mke, wi
Quote:
Originally Posted by Rod Coleman View Post
So long as the output resistor is well-rated for the power, nothing bad will happen, even if it is left running in this state for hours.
Thanks for this and the adjustment tips, Rod.

Of course the emitter is setting the anode voltage and the collector resistor is dropping voltage, effectively limiting current. It shouldn't matter if the tube is an open circuit. That makes perfect sense.

I let this project get a little dusty while I was distracted by other things. I came back and have to relearn it apparently. I'm also easily confused by PNP

Pics of the board:

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Old 8th November 2018, 04:20 PM   #52
Sodacose is offline Sodacose  United States
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This amp is still coming along:

Click the image to open in full size.

Boards are all assembled, just need to set currents, final wiring, and test. The EL34 outputs are in a garter belt bias configuration.
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Old 8th November 2018, 11:27 PM   #53
Hearinspace is online now Hearinspace  Canada
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Folded/shunt cascode differential question
Interesting to see your project SodaC. I was just telling Rod of my plans to use his shunt cascode circuit as the basis of an active line level XO and that one of the approaches I was looking at was in Small's subtractive crossover, which just happens to need a balanced amplifier stage.
Looking forward to hearing how your amp circuit works out. Thanks!
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Old 9th November 2018, 03:10 PM   #54
Sodacose is offline Sodacose  United States
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Me too

I'm planning to test the shunt cascode boards single-ended/unbalanced initially to get close on the trimmer settings and then wire up with the LTP CCS (simple LED and BJT cascode).

It's a bit fiddly with all the trimmers (voltage reference, anode CCS, tail CCS). I'm hoping things are fairly stable once dialed in. Provided this works out, I'm very interested in trying the shunt cascode single-ended for a phono pre as well.
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Old 10th November 2018, 12:08 PM   #55
tikiroo is online now tikiroo  New Zealand
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I just built and tested a folded cascode differential based on PRR's post #5 in this thread, intended for use as a phase splitter. I needed something with decent gain that could run off the low supply voltage available (only 155V, output stage is hybrid). Triodes had to be from an ECL85 as I'm using the pentode in the output stage.

See attached. Supply is from a simple Maida regulator, voltage source for the PNP bases is a resistor divider bypassed to B+ (helps with PSRR), anode current sources are just resistors. Gain is about 45 (~90 each side, halved due to grounding the second input). No issues with oscillation or instability, can swing >100V peak to peak, and perfect balance. On start-up the outputs sit at ~124V until the tubes warm up, then falls to the normal operating point with some undershoot.

All in all a very useful building block for the right circumstances.
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