That's what I do, some paste between the two heatsinks and also between the heatsinks and the front panel, so the 1cm thick front panel also dissipates the heat.
Instead of putting the mosfets in the middle of each heatsink I moved them 1cm closer to the frontpanel, that balances the heat better.
I finally disassembled my Alpha20 and I'm now transforming it in the Alpha Nirvana, almost there, should be done next week
@AKSA
Hugh,
Happy New Year!
Back to work!
Please refer to our discussion regarding the substitution of SJEP for the N-ch MOSFET (V141) on posts 2218-2220.
In order to obtain a Vgs of 1.5V, should I install a 5K pot for R122 and adjust for a voltage of ~ 1.5V? (With the stock value of 2k2 which is for the N-ch MOSFET FQA-40N25 the voltage at that node will be about ~ 5V which is adequate as the Vgs of these vertical MOSFETs is about ~ 4.5 volts).
How will this affect V113? Or will it not?
Additionally, do we alter the value of V122 (the zener) or V121 (the diode) so that the clipping function works appropriately?
Best,
Anand.
Hugh,
Happy New Year!
Back to work!
Please refer to our discussion regarding the substitution of SJEP for the N-ch MOSFET (V141) on posts 2218-2220.
In order to obtain a Vgs of 1.5V, should I install a 5K pot for R122 and adjust for a voltage of ~ 1.5V? (With the stock value of 2k2 which is for the N-ch MOSFET FQA-40N25 the voltage at that node will be about ~ 5V which is adequate as the Vgs of these vertical MOSFETs is about ~ 4.5 volts).
How will this affect V113? Or will it not?
Additionally, do we alter the value of V122 (the zener) or V121 (the diode) so that the clipping function works appropriately?
Best,
Anand.
Hi Anand,
Thank you for your post. This is very interesting to me, and I think it is doable.
With change in R122 the reduced Vgs will be accommodated in the circuit, notably from the LTP input stage. We expect there will be no gate current, like the mosfet FQA40N25. The only question would be the gate stopper, which could be different.
I would increase R122 from 2k2 to 2k7 to maintain 6.5mA through V113. I have no idea about the stopper, but Pass has used this device, and I would be guided by his value as it is unlikely to be the usual 220R for the IRFP240. No other changes, however.
Good luck, and please let us know the results!
Hugh
Thank you for your post. This is very interesting to me, and I think it is doable.
With change in R122 the reduced Vgs will be accommodated in the circuit, notably from the LTP input stage. We expect there will be no gate current, like the mosfet FQA40N25. The only question would be the gate stopper, which could be different.
I would increase R122 from 2k2 to 2k7 to maintain 6.5mA through V113. I have no idea about the stopper, but Pass has used this device, and I would be guided by his value as it is unlikely to be the usual 220R for the IRFP240. No other changes, however.
Good luck, and please let us know the results!
Hugh
@AKSA, @Nelson Pass
Thanks Hugh, I never thought about the gate stopper being different. In my rebuild of the Alpha Nirvana, my rails will be +/- 30VDC so hopefully the change of R122 to 2k7 will still maintain the optimum current through V113 (~ 6.5mA).
Reviewing the F6 document, Pass used 47.5 ohms as the gate stopper for his SemiSouth version (which was the original F6). So perhaps this value is a good one to try?!
I think the most interesting part is that the Semisouth JFET typically can have 10-20dB less distortion. The reason I was thinking about this was because I realized that vertical MOSFETS which are ubiquitous in solid state design were not originally designed for audio purposes (but running a lot of current through them, really, really helps as Pass has shown!). When Pass started getting excited about the SemiSouth JFETs, I paid attention. It’s unfortunate that they are not in current production nor is there a P-channel version in existence. Of course there are SITs, VFETS and Lateral MOSFETS but that would require some serious redesign.
Thanks for indulging this feeble mind.
Best,
Anand.
Yes, Anand, I would definitely use 47.5R as the gate stopper, and make it carbon composition, NOT MF.
I'm a little skeptical about lower distortion figures; generally this is a voltage driven device and transconductance is about the same as a FQA40N25, around 25S, but feedback reduces the distortion on the AN by about ten times anyways.
What appeals to me about the SemiSouth product (great shame the company is no longer with us) is the supernatural thermal robustness. In this role as a high dissipation TO247 it would last longer than just about anything I have seen, largely because of the die materials which have very high melting points.
Not convinced about 'indulging a feeble mind'. I'm sure in your field you are a titan!
Hugh
I'm a little skeptical about lower distortion figures; generally this is a voltage driven device and transconductance is about the same as a FQA40N25, around 25S, but feedback reduces the distortion on the AN by about ten times anyways.
What appeals to me about the SemiSouth product (great shame the company is no longer with us) is the supernatural thermal robustness. In this role as a high dissipation TO247 it would last longer than just about anything I have seen, largely because of the die materials which have very high melting points.
Not convinced about 'indulging a feeble mind'. I'm sure in your field you are a titan!
Hugh
Hi Hugh, Anand,
Great idea to try the SemiSouth SJEP, I also have a pair of E120R100 waiting for a good destination.
This Alpha Nirvana could be the ticket
I have already assembled the PS and the PCBs except for the mounting of the output mosfets.
So just in time for this SJEP variant
The LTspice sim looks good
Grts,
Danny
Great idea to try the SemiSouth SJEP, I also have a pair of E120R100 waiting for a good destination.
This Alpha Nirvana could be the ticket
I have already assembled the PS and the PCBs except for the mounting of the output mosfets.
So just in time for this SJEP variant
The LTspice sim looks good
Grts,
Danny
Attachments
How about using a IJW120R100T1FKSA1 as I do have a pair of these - https://www.mouser.com/ProductDetai...IJW120R100T1FKSA1?qs=3/hYFTG7CVevR3KjKjv20g==
Thanks
Thanks
Hi Hugh, Anand,
Great idea to try the SemiSouth SJEP, I also have a pair of E120R100 waiting for a good destination.
This Alpha Nirvana could be the ticket
I have already assembled the PS and the PCBs except for the mounting of the output mosfets.
So just in time for this SJEP variant
The LTspice sim looks good
Grts,
Danny
Thanks for making the changes to R122 as well. This gives me more confidence. Please try a 47 ohm grid stopper resistor as well…
Whether there will be a sonic improvement, we’ll have to wait and see. This isn’t new by any means, it’s been done to the Aleph J (called Aleph J SemiSouth) as well as several Zen Mod designs including Babelfish J2, Babelfish M25 R.2, etc…but from Hugh’s standpoint, he likes the idea due to SJEP being more thermally robust (for longevity) which if you are pushing 2A, 3A, etc…becomes more relevant.
Pass talks about 10-20dB less distortion levels here (whether you are using feedback or not). We’ll see if it pans out in the sonic trials.
Best,
Anand.
Yes, rated for only 1/2 that and that’s probably pushing it. But, maybe use in a cascode with a big IXYS or 3x IRFP240 in parallel like I did on LuFo and it only needs to dissipate only 10W.