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Designing my headphone amp
Designing my headphone amp
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Old 17th September 2021, 08:13 PM   #191
NickKUK is online now NickKUK  United Kingdom
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Join Date: Dec 2019
Quote:
Originally Posted by hpasternack View Post
The formula for the instantaneous rate of change with respect to time of a voltage
v(t) = A * sin(wt)
is
v'(t) = A * w * cos(wt)
This is all you need to know. There is no guessing, no mystery.
The dV/dt figures on the capacitor data sheet are not relevant to the slew rate of the amplifier you might design using the capacitors.
Ok. That makes sense. Reading around the subject from this morning, I'm aware the slew rate in terms of an amp becomes the lowest component slew rate, be it due to opamp used, cap or particular filter with deliberate control over slew rate for stability. Coupling caps would be on the signal path hence are one part of the chain.
__________________
V1 - low gain tube HPA OTL, fully balanced cascode 12BH7A, 12BH7A CCS, 12BH7A drivers and ECC99-BJT hybrid circlotron. (build)
V2 - pure digital tube HPA, RF-tube DSD512 modulation with mosfet output stage. (initial design)
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Old 17th September 2021, 08:31 PM   #192
hpasternack is offline hpasternack
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Location: New England
Quote:
Originally Posted by NickKUK View Post
Ok. That makes sense. Reading around the subject from this morning, I'm aware the slew rate in terms of an amp becomes the lowest component slew rate, be it due to opamp used, cap or particular filter with deliberate control over slew rate for stability. Coupling caps would be on the signal path hence are one part of the chain.
You go to engineering school, they teach you (hopefully) how to think systematically about this stuff. You can jump to the wrong conclusions and just confuse yourself if you go at it willy-nilly. Just saying.
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Old 18th September 2021, 06:15 AM   #193
NickKUK is online now NickKUK  United Kingdom
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Join Date: Dec 2019
Quote:
Originally Posted by hpasternack View Post
You go to engineering school, they teach you (hopefully) how to think systematically about this stuff. You can jump to the wrong conclusions and just confuse yourself if you go at it willy-nilly. Just saying.
Hehe, my day job is digital and IT, with some oddities like quantum cryptography hence a passing familiarity/critique with complex number wave functions and sine waves. So the world of amp design and tubes is a welcome change and a chance to have fun, learn and make mistakes (especially when not following process or procedure). I don't see much difference between systems/software design and amp design. There's quite a few step-by-step guides and books on that I've found in reading.

The requirement for low Zout was always part of the requirements, only shifting requirements at the end of design in an attempt to drive bigger speakers in addition to headphones is why there's some gnashing of teeth.
__________________
V1 - low gain tube HPA OTL, fully balanced cascode 12BH7A, 12BH7A CCS, 12BH7A drivers and ECC99-BJT hybrid circlotron. (build)
V2 - pure digital tube HPA, RF-tube DSD512 modulation with mosfet output stage. (initial design)
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Old 19th September 2021, 02:56 PM   #194
NickKUK is online now NickKUK  United Kingdom
diyAudio Member
 
Join Date: Dec 2019
Screenshot 2021-09-19 at 15.48.56.png

I've simplified the backend to remove unneeded components:
* the mirror BJT in the tube line isn't required - they are all connected so a cap bank provides all the required current that's required.
* removed cap banks - leaving one for the combined tubes and one for the BJTs base isolation. The multiple banks were, in fact, in parallel.
* stripped out resistors used for quazi-biasing - these, in a 60 second sim end up causing the system to fight. Biasing has to be via a negative to the grid (just like the original circlotron).

I've tuned up the tube side by disconnecting the BJTs (I may make this a switch) - this is the output at full 4V peak input and a full 160mA peak-to-peak across 55ohm:
Screenshot 2021-09-19 at 15.45.14.png

That's enough not to worry about BJTs.. I'm just waiting for a sim to finish with the BJTs connected.. it balances nicely - the BJTs provide power and the tubes backoff their peak-to-peak voltage swings.

Screenshot 2021-09-19 at 16.00.04.png

Bottom combined through load current (green)
Middle combined tube peak-to-peak (blue)
Top a single BJT current combined bias and signal (pink)
__________________
V1 - low gain tube HPA OTL, fully balanced cascode 12BH7A, 12BH7A CCS, 12BH7A drivers and ECC99-BJT hybrid circlotron. (build)
V2 - pure digital tube HPA, RF-tube DSD512 modulation with mosfet output stage. (initial design)

Last edited by NickKUK; 19th September 2021 at 03:02 PM.
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Old Yesterday, 07:18 AM   #195
NickKUK is online now NickKUK  United Kingdom
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Join Date: Dec 2019
Just leaving this simulating for today - pure tube direct connected version (no isolating caps). This is essentially an M60 in class A only operation.

Screenshot 2021-09-21 at 08.28.09.png

It's very well behaved on startup - load current with 1.5V peak input with the front end tuning the same as the capacitor version.

Screenshot 2021-09-21 at 08.17.40.png

In terms of safety sims I need to try:
* Tube open
* Tube short
* PS short
* PS open
* startup with the above
* shutdown with the above
Logically, through inspection the amp rebalances but I want to check.

EDIT: updated 240Vrms.
__________________
V1 - low gain tube HPA OTL, fully balanced cascode 12BH7A, 12BH7A CCS, 12BH7A drivers and ECC99-BJT hybrid circlotron. (build)
V2 - pure digital tube HPA, RF-tube DSD512 modulation with mosfet output stage. (initial design)

Last edited by NickKUK; Yesterday at 07:29 AM.
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