Simple cirklotron with Lateral MOSFETs

[juma]

Being a good buddy as he is, Jacco shared some of his 2sk135s with me, and not having the counter-part (2sj50) I decided to use them for a nice circlotron amp.
Different topologies were tried and here is what gave the best sonic result - Source Follower ie. power buffer driven with opa1632. I really love this chip (it's not an OpAmp but a full differential amp) - it sounds most natural and most appealing and I didn't found better sounding circuit as long as you don't ask for more than 12-13Vpeak per phase (per output) which is all I needed to drive this circlotron to full power.
I used two pairs of MOSFETs per channel 'cause I want them to drive 4 Ohm loads easily without worries about Pd.
The amp is very stable (thermally too) - DC offset drifts 2mV from 20 to 60 degrees Celsius on heatsink. Offset to GND is +/- 3mV in the same temp. range and the total bias drift is 20mA for 1.2A bias (600mA per MOSFET). BTW, Jacco sent me pretty well matched MOSFETs (8 pieces are in 50mV range Vgs difference).
With not so good matching Rsource should be 0R47. Off course, 2sk1058 can be used instead of 2sk135.
R29 and R30 - 100k multi-turn pots are used to set the MOSFET's Vgs => Id => DC offset in pretty much the same way as in F5.
The sound is very rewarding, and since this is my first circlotron now I understand why Broskie, EUVL, Circlomanen and some other guys often use this topology.

[flg]

Originally Posted by juma

Being a good buddy as he is, Jacco shared some of his 2sk135s with me, ...The sound is very rewarding, and since this is my first circlotron now I understand why Broskie, EUVL, Circlomanen and some other guys often use this topology.

touché Jacco & Juma. It's one of my old favorites too! Now we just need to convince N.P. to work his magic on one of those variants too!

[revintage]

(it's not an OpAmp but a full differential amp)

Hey Juma,

Even TI says OP-amp about the 78dB open loop gain, "Black Widow" in the middle of your PCB. The topology of the OP is like a typical poweramp with folded-cascode, except for its separate output-buffers with some kind of error amp(?). But it might sound OK anyway.

Still the 2SK135(I have bunch too) output stage looks simple and straightforward.

[juma]

Originally Posted by revintage

.... But it might sound OK anyway.
...

Hi revintage,
that's the thing in audio, quite often you won't know unless you build it
How is the progress with your gain stage (JFET/Gyrator) ?

[RLa]

Hello!

Getting a circlotron running (and correctly biasing the output stage) is not a very easy task. I like the use of capacitors and the special opamp that also has an inverted output. However, isn't it a problem when the output swings to positive (opamp output 4 is positive), the bias of the negative (lower) side is turned off since the output 5 is negative and the sources of both output halves are referenced to the ground through R5, R4 and R17, R8? Won't this cause some distortion that does not happen in a conventional push-pull output stage? This might not be a problem because circlotrons usually sound very good I have built about 4-5 such amps but with a bit different schematics.

[revintage]

Originally Posted by juma

How is the progress with your gain stage (JFET/Gyrator) ?

Going slow as always. Spent more time with my woman, wine and whiskey than expected during the holidays .

But got the sand(2SK223, 2SK170 and IRF610) I ordered anyway. So if I could stay away from the above mentioned it could soon be done.

[juma]

Originally Posted by RLa

... the special opamp....

Come on, i put in the post #1 - it's not an opamp, it's a full differential amplifier, a great SuSy chip

Originally Posted by RLa

... isn't it a problem ...

No, it's not - read the OPA1632/THS4131 datasheet

[juma]

Originally Posted by revintage

...woman, wine and whiskey ...

A DIYer's bane

[Zen Mod]

Fugly !

and overcomplicated .....

[juma]

Originally Posted by Zen Mod

...overcomplicated .....

Sure, we'll wait for Nelson to find a perfect active device, then it won't be hard to make it less overcomplicated