# Testing amps upto 300W into 8ohms **URL:** \ **Category:** QA40x\ **Created:** [June 17, 2026, 10:28pm UTC](https://forum.quantasylum.com/t/testing-amps-upto-300w-into-8ohms/2313 "2026-06-17T22:28:33Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![nick-fl](https://avatars.discourse-cdn.com/v4/letter/n/dbc845/32.png) [@nick-fl](https://forum.quantasylum.com/u/nick-fl)\ **Post date:** [June 17, 2026, 10:28pm UTC](https://forum.quantasylum.com/t/testing-amps-upto-300w-into-8ohms/2313/1 "2026-06-17T22:28:33Z")
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! ---
**Author:** ![cfortner](https://avatars.discourse-cdn.com/v4/letter/c/cc9497/32.png) [@cfortner](https://forum.quantasylum.com/u/cfortner)\ **Post date:** [June 18, 2026, 6:10am UTC](https://forum.quantasylum.com/t/testing-amps-upto-300w-into-8ohms/2313/2 "2026-06-18T06:10:43Z")
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. ---
**Author:** ![cfortner](https://avatars.discourse-cdn.com/v4/letter/c/cc9497/32.png) [@cfortner](https://forum.quantasylum.com/u/cfortner)\ **Post date:** [June 18, 2026, 4:55pm UTC](https://forum.quantasylum.com/t/testing-amps-upto-300w-into-8ohms/2313/3 "2026-06-18T16:55:57Z")
Here it is: ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/0/0f25ce877b16243be1dc804b9928afe08c6dce14.jpeg) ---
**Author:** ![nick-fl](https://avatars.discourse-cdn.com/v4/letter/n/dbc845/32.png) [@nick-fl](https://forum.quantasylum.com/u/nick-fl)\ **Post date:** [June 18, 2026, 5:42pm UTC](https://forum.quantasylum.com/t/testing-amps-upto-300w-into-8ohms/2313/4 "2026-06-18T17:42:04Z")
Thanks! This is awesome! ---
**Author:** ![cfortner](https://avatars.discourse-cdn.com/v4/letter/c/cc9497/32.png) [@cfortner](https://forum.quantasylum.com/u/cfortner)\ **Post date:** [June 18, 2026, 6:19pm UTC](https://forum.quantasylum.com/t/testing-amps-upto-300w-into-8ohms/2313/5 "2026-06-18T18:19:13Z")
I use huge load resistors 500W 8 ohms → 500W 8 ohms in parallel 500W 8 ohms → 1000W 4 ohms in parallel 500W 4 ohms → 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 and DUT: ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/9/91287147e9dfadcdc7628c1df1ce155195349bfa.jpeg) Edit: > **[MOUMOUTEN 100W 1(Ohm) Gold Aluminium Shell Resistor, Heatsink Housing Repair...](https://www.amazon.de/-/en/MOUMOUTEN-Aluminium-Resistor-Frequency-Industrial/dp/B0DRGC56JR)** > > MOUMOUTEN 100W 1(Ohm) Gold Aluminium Shell Resistor, Heatsink Housing Repair Test Metal Resistance for Inverter LED Lights Frequency Divider Industrial Control : Amazon.de: DIY & Tools