6SN7 LTP and CSS Question

[LinuksGuru]

Hi,

Is it possible to use 10M45S as cathode current source in 6SN7 LTP shown on schematic? From simulation it appears that cathode bias voltage is only about 10V and I don't know for sure if this is enough for depleted mode mosfet.
Another option is a 10mA constant current (limiting current) diode like LM334Z (40V/400mW max, which seems enough). Currently have no data how LM334Z (which was designed for power supplies) will behave in audio circuits.

Thanks in advance for any suggestion(s).

 

[zigzagflux]

I would use the 10M45S CCS, but steal from the -61V bias supply.

 

[Koonw]

I have used 6sn7, ecc99 or 12bh7 with LM317 in the cathode, 4-12V, output balance can easily be adjusted by CCS resistor, cathode voltage is determined by ccs current value. So 3-12v in the cathode is quite (impedance may varied a bit) fine.

 

[grunf]

Hi,

Is it possible to use 10M45S as cathode current source in 6SN7 LTP shown on schematic? From simulation it appears that cathode bias voltage is only about 10V and I don't know for sure if this is enough for depleted mode mosfet.
Another option is a 10mA constant current (limiting current) diode like LM334Z (40V/400mW max, which seems enough). Currently have no data how LM334Z (which was designed for power supplies) will behave in audio circuits.

Thanks in advance for any suggestion(s).

Since the dissipation is only 100mW ( even 150-200mW will not be a problem), you can freely use small SMD depletion mosfets such as BSS139, BSS159 or BSP149, they all work well below 10V Vds, you can even make a cascade with them. In terms of noise, they are immeasurably better than LM317 ,334 and similar solutions.

 

[LinuksGuru]

Since the dissipation is only 100mW ( even 150-200mW will not be a problem), you can freely use small SMD depletion mosfets such as BSS139, BSS159 or BSP149, they all work well below 10V Vds, you can even make a cascade with them. In terms of noise, they are immeasurably better than LM317 ,334 and similar solutions.

Thanks for suggestion, I somehow missed this option. One problem that I see with depletion mode mosfets is an oscillation, which may happen and may not. How to suppress 10M45S oscillation is buried somewhere on this forum. LM334Z looks like based on Wilson CSS with bipolar transistors, and should not be prone to oscillation.

 

[Ketje]

I don't see any long tailed pair in that schematic ???
Probably an invisible concertina in front.
Mona

 

[grunf]

Thanks for suggestion, I somehow missed this option. One problem that I see with depletion mode mosfets is an oscillation, which may happen and may not. How to suppress 10M45S oscillation is buried somewhere on this forum. LM334Z looks like based on Wilson CSS with bipolar transistors, and should not be prone to oscillation.

I have been using depletion mosfets as CCS for many years. I haven't noticed any problems with oscillations and instabilities, at least for now.
In my audio system, I currently have 12 low voltage CCSs and 5 high voltage CCSs with currents from 2mA to 350mA and they all work without any problems.

 

[baudouin0]

MOSFET's have quite a spread of tolerance so you may need to adjust current. LM334Z will work fine. Bipolar current mirror is another option.