Pass bjt cascode amplifier from the 70s or 80s..

[kees52]

Kees,

I haven't looked at your circuit in the utmost detail but the theory looks sound enough and it should work ok. I would consider dropping the direct coupling in the feedback path and running a feedback shunt cap. This will let you set the DC offset with a simple voltage reference at the gate of the input fet, such as a string of LEDs. This worked for fetzilla at least. The use of a fet input and vas, and lateral fet output gave fetzilla a very stable bias and offset with no servo needed. However this was without cascodes, so I'm not sure if your experience will be different.

If you need parts, you could try digikey. They deliver nearly anywhere and stock most parts at good prices and in small quantities.

I have tryed digikey, but shipping is also here expensive except if I order more then 65 euro, otherwise shipping is 18 euros.

So I do collect, that is better and I need it also to test, only the DN2540-N5 she don,t have.

I don,t like caps, I have a servo in the amp I use now, (playing in home) that is a hybrid who is dc coupled to the mosfets, and sound very very good.

a servo has no invloence on audio if it timebase made very low, The feedback as implemented is current feedback, as it is in source of Jfet what is
low impedance, and so high bandwith, I will try your idea if it is not working properly.

 

[kees52]

I will try this way, I want to now how it sounds with this dc coupled feedback, playing with these two resistors I can go low in offset, a little pot of 500 ohms or so for finetune, and maybe not needed. simulation gives a rockstable offset set with feedback resistors.

I now already it will be fun.

 

[kees52]

It is sunday and I had not much to do, so I simulated some things, I tought maybe this time a compound stage and a double feedback loop. yes stil it is symetrical but simulation says second harmonic is present witch make 0.18 % distortion, the others are 0.000 somewhere, see pictures, now we can talk about the netagives or positives of this kind of amp stage, I myself have never try it, but it can sound really nice and warm if I look at distortion and it is fast.

 

[kees52]

here it is complete with idle current adjustment. feedback can from output or driver, from driver then more second harmonics and more stable, it oscillates in multisim, cause are the jfets, bad models from them also..

some time ago I had test a hybrid with compound stage, as far as I remember it did sound very sweet, it was in the startdays of my hybrid making.

 

[GregH2]

It's a nice looking circuit and I hope you build it. I also hope you build the asymmetric version and see which you prefer the sound of more. There's nothing wrong with a little H2. Did you manage to find those parts?

 

[kees52]

some error R28 there has also by a coil parallel, otherwise 2.2 ohm in series with speaker, boooom.

I find parts only I need to order minimal 70 euro, not a problem because I am quick there, amps cost mony, but you may also build it, it is DIY freedom.

I do first the hybrides, the one now give problems, it blows the gate protection diodes if I forgot the power mosfet supply, then the diodes are in series with the speaker, and the 6C19 120 mA tube with his mosfet mu stage easely blow the diode, maybe a 1.3 watt version is better, or keep voltage on the powermosfets then the swing trough the speaker follow the driver sinus, and the diodes stays alive.

If I see it the right way I just explane, amps are sometimes strange things.

 

[kees52]

my last post, the hybride, oscillations are the cause, because of earth problems, soooo what sensitive is a amp for that but now, a beautifull square wave 20 khz.

oke offtopic because here we have allfets.

 

[GregH2]

Looks like a lot of fun! I'm yet to build a valve amp, but I hope to eventually. Glad you sorted the problems out.

 

[kees52]

Looks like a lot of fun! I'm yet to build a valve amp, but I hope to eventually. Glad you sorted the problems out.

thanks and yes it is fun, and tricky with so much voltages who are high.

but a tasergun is more painfull.