Adding line pre-amplifier for my 300B Tubes

[jls001]

Sachin has complete kit. Parts are premium parts. I believe he also still have PS kit (except trafo).

 

[omishra]

Sachin has complete kit. Parts are premium parts. I believe he also still have PS kit (except trafo).

Also grounding is fully star ground eliminating any noise from circuit. No ground current passing through other component. Its not published separately but I am giving you my version.

Here you go -

 

[GeorgeO]

Omishra

I'm tempted to jump into your kitchen and sample the DCB that you have cooked up, after my Hiraga build-that is if my wife will allow me

 

[omishra]

Omishra

I'm tempted to jump into your kitchen and sample the DCB that you have cooked up, after my Hiraga build-that is if my wife will allow me

You are welcome.

 

[Hari Iyer]

Also grounding is fully star ground eliminating any noise from circuit. No ground current passing through other component. Its not published separately but I am giving you my version.

Here you go -

Thanks for the details of the circuit.

Was wondering if this is a unity gain buffer, can i use it after my output stage of my pre-amp to isolate completely? Please advise.

 

[omishra]

Thanks for the details of the circuit.

Was wondering if this is a unity gain buffer, can i use it after my output stage of my pre-amp to isolate completely? Please advise.

Yes, its unity gain buffer with matched pair of JEFTs. You can use it as output driver to drive power amp and interconnects of 50?. More near matching of JFETs gives 0mV offset. Input impedance is few mega ohms brought to ~1M?. Output impedance is ~50?. Good part is that does not have any feedback. So distortion is so small that it could be least measurable, up to 1V signal ~0.0001% (from pass B1 reference).

 

[Hari Iyer]

Also grounding is fully star ground eliminating any noise from circuit. No ground current passing through other component. Its not published separately but I am giving you my version.

Here you go -

Further looking at the ckt, the bottom JFET looks like a constant current source without the source resistance. This circuit can pass DC too if the input signal has DC and could potentially damage the tubes due to DC amplification. Why aren't any DC blocking capacitors used? My current class A am does have a DC offset of around 1mv to 5mv due to class A operation. This stabilizes only after around 15 min to near 0.5mv. Can this cause this DC to pass through. I have used my pre-amp with and without any DC blocking capacitors without any issues. But for precaution i am now currently using a DC blocking capacitor. What your say? Please advise.

 

[jls001]

As per Salas, the designer of the direct coupled B1, upto 5 mV DC offset is safe. On Om's PCB, most builders have reported offsets of less than or about +/- 0.5 mV. I've tested three different builds and mine at 0 mV and +0.56 mV was the highest and most asymmetrical I've seen so far.

So there is no need to worry. Too many people have built it and no one has had any problem due to high DC offset voltage.

 

[omishra]

It could pass DC, that is how it would work. But in sane setup dc should not cripple in. Still if you feel unsafe then dc protection is option.

 

[Hari Iyer]

Thanks for your suggestion. I may build them as i have a couple of JFET's lying with me and there are not much components to be soldered. Only after testing i could decide if the DC blocking capacitors add value.

 

[Hari Iyer]

Images for the DCB1, Class A pre-amp, power supply and VU meter full wave rectifier.