Australian Mosfet amplifier

[tommy1000]

I have found a Mosfet power amplifier article from Electronics Australia dated February 1994. Its only one page. Does anyone have the complete article or can point me to where to find it?
Thanks

[Meganinja]

Hi
Here it is







Hope it help, I built this years ago , sound is good but beaten by Leach amp and goldmund clone .

Regards

[AndrewT]

I had a scan through the spec and immediately noticed that the 4r0 performance shows a lack of current output.

I then see that the output stage is 2pr of Lateral mosFETs on +-75Vdc supply.
No wonder the 4r0 performance is starting to struggle.
It will drive 8ohm speakers well.

The IHF power figures show that the PSU is not upto the task of delivering high power into 4r0.
I wonder what happens if you load it with 2r0 or 1r3.
A 4ohm capable amplifier should easily manage to drive 2r0 and give reasonable performance into 1r3.

Double all those impedance/resistance values for 8ohm capable.

[blakkshepeaudio]

Spent a lot of time in front of these amps during the '90s.
Not good. Recommend you look elsewhere.
Took me 10 years to forgive Mosfets for this atrocity....

YMMV but I hated it.

[tommy1000]

Thank you for your comments, very helpfull.

[Meganinja]

Hi
Actually it sound not bad , not to say it is good if part with good pre , I used pre clone GM 29 + BW 802 series 3 with it and it become a GM , and best of all it is super cheap to build , but today there are many new designs such as GM Telos ...

Regards

[tommy1000]

Originally Posted by Meganinja

Hi
Actually it sound not bad , not to say it is good if part with good pre , I used pre clone GM 29 + BW 802 series 3 with it and it become a GM , and best of all it is super cheap to build , but today there are many new designs such as GM Telos ...

Regards

Thanks, do you have some info on the GM Telos? Have you build one?

[h_a]

A pity the schematic is so small, but from the data I can read certainly an ok budget amp.

Personally I would keep the toroids further away from the amp-boards - that should help with the S/N figure.

Hannes

[Ian Finch]

Originally Posted by AndrewT

....It will drive 8ohm speakers well.
...The IHF power figures show that the PSU is not upto the task of delivering high power into 4r0.I wonder what happens if you load it with 2r0 or 1r3.

You're quite right, Andrew, but then we would have zero chance of selling a full worst-case duty specification kit in the mass-marketplace where cursory specs, like 8R power and bragging rights were all that mattered.

Virtually all commercial product since the 1970s has been Japanese here, much of it similarly specified with sagging supplies that seemed to cheat on performance, but actually saved a lot of damage and grief as my experience with fewer failures showed. I can say that the guys who built kit amps in droves here were seldom audiophiles and scarcely were concerned with hi-end issues or conservative ratings. These were never intended for low impedance, difficult load duty or driving fencing wire either, as there is little merit in arbitrarily powering a domestic amplifier to work 24/7 into worst case loads. The need is for damage control, not to exacerbate it.

For comparison, many will know Cyrus product as up-market and fairly expensive small gear. What's under the hood? 2 x40W output and a teensy 85 or now 110VA trafo!!! I think that will make the design approach more obvious. It's well marketed, though - you need to buy an extra power supply for another staggering sum to get the "real deal" sound quality with this one.

[Nico Ras]

Hi Tommy,

This is a variant of the Hitachi App note, similar to Goldmund and many other amps uses this topology and it sounds excellent if components tweaked for optimum performance. The zeners + diodes on the gates are not required as the 2SK1058 and 2SJ162 have them built in.

I would change the gate resistors of the 2SK1058 to 100 Ohm and that of the 2SJ162 to 68 ohm.

Bandwidth of this amp and similar topologies are well above 300kHz and slew rate > 100V/uS is quite possible. Tweaking the components values can result in THD of 3 to 4 decimal places after the point.

Other than that this simple amp would run with the best offered in this forum.

Power output is a little lower than it could be due to the diode resistor volt drop supplying the pre-stages but this improves ripple rejection.

Changing the BC556 to 2SA970 and running the pre-driver driver off a higher voltage say, 90VDC and regulating it, actually gets this amp into the high-end stable of amplifiers and is a very transparent sounding amp.

Build it and tweak it, you will not be sorry. Optimum bias for these L-MOSFETS is around 180 mA for lowest distortion. A good power supply is a must if you want a top class amplifier.