Testing amps upto 300W into 8ohms

QA40x
1

Hi everyone,

I am designing a tapped dummy load for high-power amplifier testing with my QA403.

My immediate goal is to test a 200W into 8 ohms amplifier (40Vrms). Because this hits the absolute maximum input limit of the QA403, I want to build an attenuation network with enough headroom to eventually test amplifiers up to 300W into 8 ohms (48.99Vrms).

Based on the QA “Power Amp Basics” blog post, I plan to split an 8-ohm total load into a two-resistor voltage divider to target an attenuated voltage of 35Vrms at the analyzer input.

Proposed circuit values:

  • R1 (Top resistor): 2.28 Ohms (Dissipating ~86W at 300W total power)

  • R2 (Bottom/Sense resistor): 5.72 Ohms (Dissipating ~214W at 300W total power)

  • Connection: Differential tapping across R2 to avoid ground loops.

This should yield a total attenuation of -2.92 dB, dropping the 48.99Vrms down to a safe 35Vrms for the analyzer. I plan to use parallel arrays of non-inductive power resistors on a heavy heatsink to handle the thermal load.

Are my calculations correct for this target? Is there a more optimal resistor ratio or alternative approach you would recommend for this power level?

Thanks!

2

I would calculate more external attenuation like 18 or 24dB for safety.

I’ll send you later my schematic which works well at 2 and 4 ohm 1000W and 8 ohm 500W.

And there are some threads here about this theme.

3

Here it is:

image
image960×678 39.4 KB

4

Thanks! This is awesome!

5

I use huge load resistors

500W 8 ohms

in parallel

500W 8 ohms → 1000W 4 ohms

in parallel

500W 4 ohms → over 1000W 2 ohms

The oscilloscope output is only for controlling the signal curve, not for voltage measurements.

With this setup I get this result without signal:

image
image1920×1440 611 KB