Try Ale Moglia's JFET-DMOS cascode gyrator for low output impedance.
Ale has some PCBs.
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Hi Euro21
it is nearly the same wi use Michael and me...but instead of the upper dn2540 ..wi will use Ixys
Michael have it under test ,and he said it sound better ..the idear came from Kevin K&kaudio
Best Bjarne
it is nearly the same wi use Michael and me...but instead of the upper dn2540 ..wi will use Ixys
Michael have it under test ,and he said it sound better ..the idear came from Kevin K&kaudio
Best Bjarne
Hi Bjarne, the difference is the lower FET. Big sonic difference with the JFET and the DN2540 performs poorly here.
@Bela: the BF862 is superb but you need an additional 15V Zener from drain to source otherwise will die during power on, it's very sensitive with the lower VDSS max.
@Merlin: I'm back home next week and will be shipping the boards as soon as I can
@Bela: the BF862 is superb but you need an additional 15V Zener from drain to source otherwise will die during power on, it's very sensitive with the lower VDSS max.
@Merlin: I'm back home next week and will be shipping the boards as soon as I can
Bela,
Looking at your diagram again, I just realised that the BF862 hasn't got the stones for this the 4P1L at 35mA. The IDSS is 20mA and you are pushing the limits of a very sensitive JFET if you want to run it at 30-35mA and you will need a negative VGS I think above -1V. Check the data sheet.
Best JFET for this at 20mA or higher is the J310 or SMD version (MMBFJ310).
Ale
Looking at your diagram again, I just realised that the BF862 hasn't got the stones for this the 4P1L at 35mA. The IDSS is 20mA and you are pushing the limits of a very sensitive JFET if you want to run it at 30-35mA and you will need a negative VGS I think above -1V. Check the data sheet.
Best JFET for this at 20mA or higher is the J310 or SMD version (MMBFJ310).
Ale
Hi Ale
i do not understand why the lower fet in gyrator plays bad with the upper (i hope you will explain why,,,always like to learn new things about the 4P1L)
wi use dn2540 as the lower,,,and ixys as the upper---i now michael use it now with wery good result
Best bjarne
i do not understand why the lower fet in gyrator plays bad with the upper (i hope you will explain why,,,always like to learn new things about the 4P1L)
wi use dn2540 as the lower,,,and ixys as the upper---i now michael use it now with wery good result
Best bjarne
I also regularly use IXTP01N100D, even at lower (200-300V) voltage (I lost a handful DN2540 due to the transients), but don't have usable Ixys DMOS spice model.
I regularly use J310, but the problem is the same as Ixys DMOS: no precise Spice model over 25-27ma, so I use instead of J310 model (I have four different models) another FET model.
Hi Ale, don't you think that a Mosfet like BSP129 is better suited for current above 20mA due to higher Gm?
Hi Bjarne, the gyrator version you're using will work well but is not optimal. The lower FET in a cascode circuit like this is running under challenging conditions for most of the MOSFETs. The VDS is very low due as is set by the VGS requirements of the top FET. This means that VDS is less than a couple of volts where the reverse capacitances go to the roof (check the data sheets). The JFET perform much better here and the HF response of the gyrator is far better. I measured above 100kHZ on my 01a Preamp. Have a read at my 01a preamp blog entry http://www.bartola.co.uk/valves/dht-pre-amplifier/01a-preamp-gen2/.
For valves with low current requirements (<10mA) you can use the 2SK170 and BF862. They do sound beautiful. For higher currents, like what you need for the 4P1L, you will need to use different FETs given the current needs. The J310 works really well here.
You will need the protection zener between gate and source and you will need a zener between drain and source for a sensitive device like the BF862. Otherwise it will not withstand the power on. Been there than that.
Hope this helps
Ale
Hi Indra,
I've used the BSP129 before and works really well. However, the 2SK170/BF862/J310 jFETs are superior in the treble response.
Have a look at the data sheet and you will see that the BSP129 reverse capacitance goes to the roof (100pF) at VDS lower than 4V. If you also check the transconductance plot you will see that it goes down below 100mS.
It's true that it has a x10 transconductance compared to the MMBFJ310 (10mS) however the latter has a capacitance between 1pF to 5pF at the same operating conditions. The BF862 has a Crss of just 2pF. I've tested the gyrator circuit with the BF862 recently and was amazed with its performance.
Cheers
Ale
Thanks Ale, I confirm the good top end response of 2SK170 and 2SK68A for current < 6mA, but no experience with gyrator for currents above 10mA yet. I'm planning a valve stage to drive source follower operating at 15-40mA. Still examining options to try.
Hi Indra. Haven't tried the 2SK68, thanks for the feedback.
The BF862 is great and can do 20-25mA max. In addition to J310, I used the 2SK369 with great results and can do 30mA, however it's now obsolete. I use it on my RIAA stage and I love it.
I generally use the 2SK170BL (12mA) which I still have some, but given I now have the PCB which can accommodate the BF862 SMD device, I use this instead.
The BF862 is great and can do 20-25mA max. In addition to J310, I used the 2SK369 with great results and can do 30mA, however it's now obsolete. I use it on my RIAA stage and I love it.
I generally use the 2SK170BL (12mA) which I still have some, but given I now have the PCB which can accommodate the BF862 SMD device, I use this instead.
Will the MOGLIAA Gyrator have a lower output impedance than a transformer coupling? Assuming the LUNDAHL transformer ...
With BF862 as "lower" FET the Rout about 220R (in my 4P1L preamp simulation).
