MOSFet Class A SE with inductive loading.

[mhouston]

I have tried a few searches on the site but can find no threads on single MOSFet Class A amps with inductive loading. Has anyone successfully done this and can tell me about it and what they did? Any schematics? Should this continue as a new thread?

I have built my own single MOSfet amp with resistive loading but understand inductive in the source leg can be achieved.

[Brian Donaldson]

Good Idea. Choke load doubles your efficiency.

Some one made one using a modified microwave oven transformer, but I don't know if it was mosfet or bipolar. I think it was circlotron. Search microwave, choke, SE amp.

[lineup]

yes

I have seen schematics of a circuit here at diyaudio.com
with use of inductors in outputstage, to increase power
I do not remember if it was MOSFET and/or single end, but is probable

topic name was 'My Class A' or something like that

maybe someone else can search and find it

[lineup]

I found it!

This thing is nominally a 50 watt per channel stereo setup with a source follower output stage and an inductor in the source lead.
It runs a 28 v rail with 3.5 amps current so that's about 100w dissipation.
The advantage of using an inductor is that you only need half the supply voltage because the inductor will swing the missing half rail for you.
Therefore the efficiency is at best a horrible 50% rather than a woeful 25%.
The downside is that suitable inductors are heavy and expensive.
But I already had two of them so that was lucky.

Topic by: Circlotron

Thread: My first ever Class A amp.

MOSFET Class A with inductor loaded output stage.
Schematic: MOSFET Inductor Output


Enjoy!

[lineup]

Originally posted by mhouston
I have built my own single MOSFET amp with resistive loading
but understand inductive in the source leg can be achieved.

As far as I can undestand
you should be able to quite easy convert that amplifier, you have there,
to use a large inductor for load, instead of power resistor.

Gudd Lock!

[mhouston]

Originally posted by lineup


As far as I can undestand
you should be able to quite easy convert that amplifier, you have there,
to use a large inductor for load, instead of power resistor.

Gudd Lock!

So my 15ohm load resistor in the drain can be swapped for an inductor. Sounds too simple. Care to make a quantitive call for the inductance value? And are we talking 60mh or 10mh.

[lineup]

Originally posted by mhouston
So my 15ohm load resistor in the drain can be swapped for an inductor.
Sounds too simple.
Care to make a quantitive call for the inductance value?
And are we talking 60mh or 10mh.

I have no idea. I am not too good with inductors myself.

But you can get a hint from Circlotron schematic.

He has single supply 28V and 3.5A bias in MOSFET.
He uses 85mH in parallell with 8 Ohm speaker.

If I were you, I should read all his thread (see my link).
Maybe you find out some formula or way how to decide inductor value/size.

Circlotron is active, maybe he will join in to assist.
Circlotron, Last seen online: 22.02.06 10:41 AM (Today)

[mhouston]

Hugh Dean drew a MOSFet output stage with a choke in the source leg for me. His calcs. produced a 63mh unit at about 2A. I’m just trying to find out who else has been down this path. A Hammond 10mh 5A choke cost me $AU53.

I can get chokes wound but at this size (and I need two) the cost may out-weigh the benefit. I would like to try with no voltage stage before so a single fet (I’m using the 2sk1058) and inductive loaded. I realise the power will be well down but I have not found this a problem in the past.

[lineup]

Originally posted by mhouston
Hugh Dean drew a MOSFet output stage with a choke in the source leg for me. His calcs. produced a 63mh unit at about 2A. I’m just trying to find out who else has been down this path. A Hammond 10mh 5A choke cost me $AU53.

I can get chokes wound but at this size (and I need two) the cost may out-weigh the benefit. I would like to try with no voltage stage before so a single fet (I’m using the 2sk1058) and inductive loaded. I realise the power will be well down but I have not found this a problem in the past.

Hammond ... hmm, yeah
When looking for good Chokes for SolidState power amplifiers
we could turn to Tube Valve stuff. They know these things.

But is far from impossible to build your own good COILS
for Inductive loaded Class A amp output stages.

For example Nelson Pass is good with inductor construction.
His ZEN Lite uses a lamp light BULB as load for Class A.
Light bulbs are somewhat inductive ...

Zen Variations - Part 1: Zen-lightenment by Nelson Pass
(c) 2001, Nelson Pass
http://www.passdiy.com/pdf/zenlite.pdf

Attachment shows a more advanced variant:
Balanced Zen Lite with four 300 Watt Light Bulbs

[Tyimo]

Hi Lineup!

His ZEN Lite uses a lamp light BULB as load for Class A.
Light bulbs are somewhat inductive ...

Sorry, but is it not a good example, because He used the bulbs as resistive loads.
There is the ZV7 with choke loading.

Tyimo