High power Quad 405 experiment 390W into 1 Ohm

[405man]

Over the holiday period I was upgrading a Quad 405-2 as here

Quad 405-2 upgrades

Some details of the operation of the 405 can be found here

http://www.keith-snook.info/wireles...d-1975/Current Dumping Audio Amplifer DCD.pdf

The schematic of the 405-2 I have used was one of the last made and can be found here

http://www.keith-snook.info/amplifier-hifi-schematics/QUAD 405 Service Data.pdf

But I thought I would do some investigation first as I recovered some BUX22s from some scrap equipment at work. The 405-2 is standard apart from having the current limiting hybrids removed and the current sensing resistors shorted and the BUX22s fitted.
I have a QuantAsylum QA400 and a resistive network to scale the output of the amp to suit but the level is not yet optimised so I have not exploited the full dynamic range of the QA400 which is why the distortion is slightly higher than I would have expected. Listed below are the power and distortion levels into various load resistances

8R 125W 0.012%
6R 170W 0.015%
4R 200W 0.024%
2R 300W 0.022%
1R 390W 0.021%
R.5 250W 0.031%

As the amp is running without any protection I am reluctant to reduce the load resistance much further though I did misread the DVM and briefly reduce the load to 0.2R. I shall now upgrade one channel with new opamp etc and see what results I get

 

[scopeboy]

Yikes. Those transistors are monsters. 🙂 Just be careful that you don't burn out the drivers.

 

[JonSnell Electronic]

What sort of power transformer are you using to produce 390Watts?

 

[405man]

I am using the standard 405 mains transformer, but I have never measured its VA rating, the time I tried the load resistor was a bit under rated and the smoke threatened to set off the fire alarm so I had to abandon the test, at a guess its about 350-400VA. As there is only about 20V RMS across the 1R load I should measure the voltage the transformer sags to, a job for tomorrow

Stuart

 

[Mooly]

If the 1 ohm was smoking it may well be way higher than 1 ohm under those conditions.

 

[405man]

Its a few years since I tried to measure the VA using just a transformer and a load, The 1R load gets hot but the reduced duty cycle of the QA helps and I have not tried to drive the 1R for long periods as the 405 heatsink is a bit small

Stuart