High Current Issue with Complementary 300W audio Amplifier

[rjj816]

I am working on a DIY 300 W audio power amplifier based on a complementary VAS architecture with a differential input stage, complementary VAS, bias/driver stage, and multiple parallel output transistors (NJW0281G/NJW0302G).

The amplifier works correctly in LTspice simulation. With the intended ±75 V supply, the simulated quiescent current is only around 10–11 mA.

However, during initial real-world testing at reduced supply voltage, I am getting a very high DC current draw:

I have used a transformer of 66 0 66 then i have a power supply

• 6v → approximately ±5 V rails, but about 2.2 A DC
• 11v → approximately ±7 V rails, but about 5 A DC
• Expected from simulation at these conditions: approximately ±8.5 V / 10 mA and ±17 V / 11 mA, respectively.

The output transistors and other devices heat up very quickly even at this low supply voltage. The power supply and transformer appear to be capable of supplying the current, but the amplifier itself is drawing far more current than expected.

I have checked the schematic against the simulation and am currently investigating the bias voltage, transistor pinouts/orientation, driver/output-stage wiring, emitter resistors, grounding, compensation, layout design rules.

One question I would particularly appreciate help with is: what are the most likely causes of several amps of quiescent current in a complementary-VAS amplifier when the simulation shows only ~10 mA? Also, what measurements would you recommend making first to isolate whether the problem is in the bias/driver stage or the output stage?

For more reference i can send the schematic as well which is working well in simulation in lt spice.

 

[gadget73]

doesn't sound like its being very nice at all. Maybe its more of an insulting amplifier.

Is it oscillating? Thats a common thing that won't neccesarily show up in modeling but may in real-world builds thanks to layout issues causing unwanted coupling or whatever. A scope across the output would probably give some clues here.

 

[rcs16]

It certainly would not hurt to post a pic of your schematic and/or the simulation file. Not that I think very many on AK use ltspice but I do.
Without that information kinda hard to expect us to give you any advise?
As said above scoping the outputs is good advise. One would think anyone bringing up a new design would be doing that as a first step.
Have you ever designed an amplifier before or worked on other similar topology designs? A 300W amp for a first design is taking a big bite.
Obvious look at the bias spreader voltage and range, it should be adjustable to compensate for component tolerances, as that determines the output stage current. What's done in sim is only an approximation of reality.