F5-style pre

[Abrandt]

Hello guys!
Some pictures from my latest project.
I needed a preamp with some gain to drive my ZV9. The B1 I used had not the juice to drive the amp to full output with some programmaterial.
I wanted a solid state amp and I had already made the powersupplie so the +-15V was in place. I tryed some other preampideas from the internet but I was not fully satisfied. Then I got the idea to do something F5-like. And it worked out rather well I think. Please give me some feedback. More pictures if you want.
Merry christmas from Sweden, Abrandt

 

[Zen Mod]

YouTube

 

[EUVL]

Balanced version :
The F5X Preamp


Patrick

 

[indra1]

Hi Abrandt, could you kindly share your reason(s) for your choice to cascode the input jfets and your choice to reference the bases of cascode transistors to supply voltage instead of ground?

 

[xrk971]

Beautiful work! Love the bare wire 3D sculptures to the RCA’s.

 

[Abrandt]

Indra1
The cascod is mostly because I would try to do it and maybe to get a little less distortion. You could absoluteley make a version without them. Your question about the reference voltage, I don't understand. I have to get fixed voltage at about 5 Volt's at the bases. I could done that without the zeners maybe but ??
/Abrandt

 

[EUVL]

He was asking whether you connected the Zeners to Gnd and the resistors to the rails, as it should be.
The other way round was shown in your hand sketch schematics.


Patrick

 

[indra1]

Hi Abrandt, in case you have not done listening comparison, you may want to try 9.1V at the bases of the cascoding transistors by switching the zeners and the 4k7 resistors. I think the jfets will perform a bit better with ~8.5V at the drains.

 

[Abrandt]

OK I se now what you mean. Du you think that would make any differens? Are not the voltage fixed at the bases in both cases? And Indra1 do you say that the FET's do a better job with 8V at the drain? / Abrandt

 

[indra1]

Yes, I think so due to lower Crss and Ciss while IGSX is still quite low.

 

[Abrandt]

I just want to tell you that I now have changed the zener-location. No big differens either soundwice nor what I can messure (very simple messurements) though. But you are right, it ought to be this way . / Abrandt

 

[indra1]

Great, it means your power supply is good enough, otherwise you will notice lower noise. The slight difference if any should be noticeable by cleaner sibilants and whistles (not the clothing).

 

[Hikari1]

Abrandt:

Did you try mosfets on output as per your original schematic as well? Or did you just use bjt from the start?

 

[aspringv]

Awesome. True diy skills

 

[Abrandt]

No I haven't tryed MOS-fets. I read somewere that Nelson said -"fet's should have a rather high voltage to perform well". That's the reason I changed to bipolar. / Abrandt

 

[Hikari1]

I thought it was high current. It would be interesting to see what sounded better although a to-220 mosfet can take much more current than bc550.

It looks like Nelson is using small smd mosfets in preamps as the voltage stage these days...and bipolars just for cascode.