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transistor amplifier design finalization
transistor amplifier design finalization
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Old 14th June 2019, 04:11 PM   #11
brian92fs is offline brian92fs  United States
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transistor amplifier design finalization
Other thoughts: You don't have actual VAS or Output devices in your model. It's hard to come to any real conclusions without a these. Many on this forum use the models Bob Cordell has developed. You may want to give these a try.
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Old 14th June 2019, 05:58 PM   #12
brian92fs is offline brian92fs  United States
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transistor amplifier design finalization
You'll also want to do some additional analysis in simulation:
  1. Noise Analysis
  2. AC Frequency response
  3. Square Wave testing
  4. PSSR testing - add some ripple and noise to the supplies
  5. Slew Rate
  6. THD analysis at different frequencies and power levels
  7. Bias stability - vary supply voltage to determine if bias levels are stable with fluctuating voltage (from mains fluctuations or transformer regulation)
  8. Review dissipation in devices to ensure operation within SOA
  9. Open Loop Gain analysis to assess Gain Margin and Phase Margin. Including loop gain review to check for excessive NFB that may lead to instability.
  10. Probably many others I can't think of at the moment

Getting this "right" in simulation is the easy part. Then comes layout / PCB design and component choices which is likely to be more challenging.

Once a prototype is available, you'll likely need a scope to check for stability issues. And you'll probably want to repeat some of the simulation analysis in "real-life" on the prototype - simulations don't always hold up in reality. Equipment to test distortion would also be a nice to have.

Not trying to discourage you - just pointing out the road ahead. Taking a know stable design and tweaking it or designing your own PCB for it to accommodate different device choices would be simpler path.
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Old 14th June 2019, 06:08 PM   #13
Fossilshark is offline Fossilshark
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These are all excellent points, and I do remember considering Bob Cordells output triple (I.E. an output device) so I think i'll add that in. I started working on the PCB before your comments so I'll just keep redesigning until its right. thanks!
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Old 14th June 2019, 06:23 PM   #14
brian92fs is offline brian92fs  United States
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transistor amplifier design finalization
You may want to stick with the EF2 output stage or consider CFP. I understand that EF3 can produce superior results, but can be harder to stabilize. Also, I don't think there's much advantage to a EF3s OPS for a low output single pair setup. For a low output design, you may want to consider a CFP output stage. Many people prefer this when only a single output pair is used.
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Old 14th June 2019, 07:33 PM   #15
Mooly is offline Mooly  United Kingdom
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transistor amplifier design finalization
This is a really great attempt actually. I through some different models into the sim and gave it whirl... its... different. Very good in parts

The driver dissipation is relatively high at idle, more than the output devices but so what. It works well.

The only real concerns are the stability, and here it seemed easy to provoke oscillation. The usual Zobel and output inductor did little to help (surprisingly) and a tweak of the compensation was needed. Running the amp at such a low gain is one of the contributory factors... but it can be worked around.

Distortion is shown at 1 watt and 15 watts into 4 ohm. Pretty good.

Squarewave testing was where things came undone a little initially as can be seen by the small added capacitive loading.
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Old 14th June 2019, 07:37 PM   #16
Ylli is offline Ylli  United States
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And I hope you are not planning on actually using the 2N3904/3906 pair in the output. As posted Q12 is dissipating 5.8 watts. Well above the 2N3904 rating.
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Old 14th June 2019, 07:41 PM   #17
brian92fs is offline brian92fs  United States
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transistor amplifier design finalization
I would add when I tinkered with it, reducing the VAS current seem to help with stability issues.

Excellent point on the overall gain. Forgot to mention that. The high NFB factor can cause problems. Perhaps its better to reduce this and then reduce the supply rails to arrive at the same output goal?
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Old 19th June 2019, 10:10 PM   #18
Fossilshark is offline Fossilshark
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I finally got to testing and I managed to get the square waves a little better by reducing the current in the VAS. I also raised the supply voltage to 20v as well. I think I'm happy with this little design for now at least until I hear how it sounds.
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Old 19th June 2019, 11:11 PM   #19
Mark Tillotson is offline Mark Tillotson
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Real drivers and output devices have less bandwidth than small signal transistors and more capacitance and stored charge, this affects compensation and stability and distortion, you need to check what effect this has on the simulations.
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Old 20th June 2019, 07:00 AM   #20
Mooly is offline Mooly  United Kingdom
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transistor amplifier design finalization
I'm not so sure that version will work to well tbh.

Q15 and R19 are messing with the DC offset. They had no effect in the earlier version but here, with your altered values... just take them out altogether for starters.
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