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jmyres's picture



jmyresjoined June 1, 2017

posted Wednesday, June 7, 2017 | 8:14 pm PDT

This thread has been created to discuss the design below. The person who started this thread can change this introductory post by clicking on the Edit button above.

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High-end Class-A Amplifier Model

I am rebuilding a vintage Class-A amplifier and wanted to model the circuit first to verify it works correctly.  I have a few quick questions...

- When modeling an amplifier, are voltage sources for each power supply rail (+-48Vdc in this case), plus a load resistance (8 ohm) all I need to add for the circuit to model correctly?

- Given the progressive deformation of the sine wave in each successive probe, is that mostly likely some sort of parasitic oscillation issue?

Final Note: Please zoom out all the way to see the entire cicuit.  Not sure how do I get the circuit to correctly fit within the preview window. 

Thanks for any advice!


The voltage across the ZD101

The voltage across the ZD101 zeners is about 0, not the expected 24V. Therefore the input long-tailed pairs are cut off and there is no gain. 

High-end Class-A Amplifier Model

Hi Jason,

This is pretty cool!

An simple 8 ohm load is a good start and plays an important role in modeling an amplifier.  Understanding your circuit with this simple load is the right thing to do at first.  When you get farther along, you might find it useful to model the nonlinear behavior of a loudspeaker.  See this Loudspeaker Example for more insight into this.  

Have you tried the "Enter Fullscreen Mode" button in the toolbar?  It's the best way to view larger designs.