RC Volume control for a lateral MOS AMP..

[jackinnj]

use a zener and 1n4148 in combination -- as the lateral mosfets are a bit pricey you might want to search the web for an overcurrent protection scheme as well.

we aren't trying to make this too complicated for you.

[Dxvideo]

Do you think should I add an overcurrent protection?
I supposed a 3 amp fast fuse series to spk out will enough for it!

[jackinnj]

It's an old "adage" that transistors are there to protect the fuses.

here's an elegant solution from Elliott:
http://sound.westhost.com/project53.htm

[Dxvideo]

But its a power limiter. I guess I wont need power limiting, because I am an avarage user and dont plan to exceed 50W RMS..
Does any circuit you know for short protection?

[AndrewT]

Hi,
the g/s Zeners will give some short protection.
For a single pair your may need somewhere between 5V and 8V between Gate and source.

This will allow a large current to flow at whatever voltage the supply can maintain.
The FETs will heat very quickly so then the rail fuses blow if the short continues and before the sink gets too hot.

Do not fit a fuse to the speaker output.

By the way, the Zeners pull a large current though the VAS when in protection mode. Those gate resistors or the VAS may blow.
To prevent this there should be a current limiter on the VAS as well. There is already a current limit inherent in a current sink. But you have a current mirror and both transistors may blow.

[Dxvideo]

Thanks...

[Nico Ras]

Originally posted by AndrewT
Hi,
the g/s Zeners will give some short protection.
For a single pair your may need somewhere between 5V and 8V between Gate and source.

This will allow a large current to flow at whatever voltage the supply can maintain.
The FETs will heat very quickly so then the rail fuses blow if the short continues and before the sink gets too hot.

Do not fit a fuse to the speaker output.

By the way, the Zeners pull a large current though the VAS when in protection mode. Those gate resistors or the VAS may blow.
To prevent this there should be a current limiter on the VAS as well. There is already a current limit inherent in a current sink. But you have a current mirror and both transistors may blow.

Hi Andrew,

I have not had any bad feedback of these potential hazards that you mention. The MOSFETs has built in zener protection. Furthermore, I know of many 100s of these amps built by both DIY and professionals alike since I first published it on my web site about nine years ago.

I know of five clubs that uses several of the higher power version at full blast every day for 10 or more hours, many of them have been working for several years without failure.

The original design is still in daily use standing in my office while playing the local radio station through several speakers throughout the factory and has been working for 24 hours per day for +9 years (>78,894 hours bar the occassional power failure). This is not too bad for such a risky design. During this period we have changed the radio tuner five times, but never the amp.

What is most surprising to me is that the capacitors must still be okay because the amp does not even hum, buzz crackle or anything and they are simple Rubicon 4700 uF/63V. The amp runs from a 50 - 0 - 50VAC/300VA transformer and is very warm to touch.

I am confused with the voltages and currents that you quote in your previous posting, these must be assumptions because they are not at all accurate.

But thanks for your comments anyway.

Kind regards

Nico

[Nico Ras]

Here is a picture of the original amp PCB published in May 1998

[Nico Ras]

And the original box knocked together from American oak shelving off-cuts that was lying around the garage.