Depletion MosFET Buffer

[6L6]

SOT-223 are BIG for a surface mount part. Nothing to worry about.

 

[EUVL]

You need to thermally couple both devices through a heatsink to reduce DC drift.
Can of course be done, need to plan upfront.
Or add a DC Servo as in the Tubecad link :
https://www.tubecad.com/2004/blog0023.htm

Thanks Patrick. The tube cad stuff is always good. The ones I built had a servo exactly like he shows.

BSP129 is not holly. You can also try DN2540, etc.
But I prefer LU1014 cascoded a la Zen V9, biased at 200~300mA.
Just need to simulate first to find the optimum bias.


Patrick

 

[jean-paul]

Please explain what the use of this circuit is. Sources generally have low output impedances and 2V rms outputs. Amplifiers usually have 10 kOhm or higher input impedance. My wires are good buffers with better PSRR or do I miss something?

 

[Mark Johnson]

Allowing vacuum tube sources to drive active equalizer (active crossover) networks with low-Z inputs.

 

[jean-paul]

Ah OK. Thanks.

 

[EUVL]

100mA bias is enough for most headphones.


Patrick

 

[jean-paul]

Wouldn't it be with +/- 24V rails and DC coupled that there will be very loud power on/off effects and/or considerable DC combined with a smell of heating voice coils?! And a gain of 1x? No muting?

 

[EUVL]

Slow start, DC protection, Muting, .... are all add-on's, just like power amps.
www.diyaudio.com/community/threads/xen-shpp-simple-headphone-protection.292681/



Patrick

 

[jean-paul]

That is not an answer to my question. Sorry. For headphones it seems itself a quite inadequate addon.

Well let’s leave it at that. It becomes hot so all is good.

 

[Vix]

Please explain what the use of this circuit is. Sources generally have low output impedances and 2V rms outputs. Amplifiers usually have 10 kOhm or higher input impedance. My wires are good buffers with better PSRR or do I miss something?

For example: Imgine you really wanted to build some Zen amp variant, but hey, you have a hefty mosfet at hand, with high input capacitance. To remedy this, you apply feedback with low resistances. Ups. You have an amp with 600 ohms input impedance.
So, why not add a buffer like this one...
Or, you want to build an M2-like amp...but hey, you want to put this buffer in front of a transformer...

 

[jean-paul]

Todays sources laugh at 600 Ohm.

It is good, I am gone shopping with my drag racer.

 

[Abx74]

The most important question is: "will it drive F4 ?" 😆

 

[grunf]

Here is a little buffer using depletion mode BSP129 SMT MOSFETs.
It will drive low impedance loads at .000% thd range. Frequency response is very extended to video range.
It needs some heatsinking or lots of copper because the current is close to 100 mA.
The circuit is the old White video buffer with silicon.

Great idea. I have a couple of amplifier schematics and I/V stages with depletion mosfets but for now only in LTspice. The picture shows the FFT of a simple CFA A class amplifier with two depletions, two bipolars and two laterals at a power of 1W into 8ohms.

 

[Aleph5]

I've had this on deck for a while, one channel balanced. I haven't built this version yet, but have tested several without feedback or the servo where I used DN2535. The Toshiba JFETs here simulated better, as I recall, but the Supertex parts are amazing for distortion and BW.

I have a few BSP129s and have always been a fan of the SOT-223 package. Just right size for SMD. Not had cause to use any yet.

 

[Mark Johnson]

Would someone please check my power dissipation arithmetic?

_

 

[Zen Mod]

I could, but don't have BSP129 on hand

that's why I gently asked before, and Wayne confirmed

anyhow, if in doubt, just toss it in bucket of oil

 

[schultzsch]

I am using this heat conductive adhesive to connect the small heatsinks to smd chips.
Then some gpu ram heatsink like these attached to the sot223 capsule together with a bit of copper surface on the pcb can fit the bill for this application.

 

[jackinnj]

Advanced Linear Devices wrote a white paper on DMOS circuits, must be 15 years ago:

 

[wayne]

I did notice that SOA graph after I toasted a finger.
But the data sheet says 240 Volts , .35 Amps. 🤞

 

[jean-paul]

If stuff should be as rudimentary as can be and it must also be high current then SMD maybe is not the way to go. TO220 likely is.

Or use a wire and have a light load.