[Pars]

You won't be able to find the dual BJTs either (2SC3381 / 2SA1349 IIRC). Use matched pairs of singles such as 2SC2240 / 2SA970 or others; no big deal.

The dual JFETs (2SK389 / 2SJ109) weren't always matched that great (I think according to the datasheet they were only guaranteed to be within 10% or posibly higher matching). I know I have one that isn't matched particularly well. You should be able to match the singles much closer. Also, on diyaudio, there was a group buy for some "heatsinks" intended for singles such as these replacing the duals. These don't run hot, but the "heatsink" thermally bonds the pair together. Apparently they work quite well.

 

[amb]

The 2SK170/2SJ74 single-JFETs have identical specifications to the duals 2SK389/2SJ109. You could achieve better matching using singles than what's in the duals. The only downside with using singles is that the two devices are not on a single substrate, so there is no thermal coupling between them. The result is that there may be some DC drift with temperature. In reality this is not a problem on either the β22 or the dynahi. I've not seen a properly-built β22 drift more than a couple mVs of DC offset. The dynahi has a DC servo to counter drift, but its implementation has a very slow response and causes as much DC drift as it eliminates. But in either case the drift is negligible and of no concern.

Thus, there is no real disadvantage to using the single JFETs to replace the duals, as long as you match them well. When (and if) the LSJ109 dual P-channel device from Linear Systems becomes available, β22's PCB is already laid-out to accept them, but currently 2SK170/2SJ74 is the only solution. The dynahi PCB was laid out to support the old Toshiba 2SK389/2SJ109, and is compatible with 2SK170/2SJ74, but not the Linear Systems duals.

As Pars points out, the dual BJTs used for the dynahi's input cascodes are also obsolete. You will need to use single-devices as well.

 

[digger945]

Ultimately I ended up letting dgardner match up the 170/74's and output 3421/1358's for mine. This would have saved much time and money if I had done this first.

 

[NelsonVandal]

Quote:

Originally Posted by mcmurray /img/forum/go_quote.gif 2SJ74BL and 2SK170BL can be used as a replacement for the SK109BL and 2SK389BL.

Yes, and you can buy matched quads at amb's shop AMB audio shop

 

[FallenAngel]

Slightly off-topic, but what's the exact size of the Dynahi PCB?

 

[digger945]

Exactly 4" by 4.95".
I used Conrad MF75-30 heatsinks just because I didn't want to have to re-do them later, and they fit the sides of a Parmetal 12123 case like they were made for it.
There are a couple of changes I would make if I did this again, say with the DynaFET, or another Dynamite.

 

[digger945]

Parmetal 12x12x3, Conrad MF75-30 heatsinks, 1/8"x1" angles.
It's apart now, stuffing boards.
Grasshopper will hear soon....very soon

 

[Pars]

looks good

 

[Ferrari]

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif

Imagine the fully stuffed PCB’s with nicely matched parts, a balanced volume pot from DACT, feeding by two σ22 power supplies housing in a second (similar) enclosure. That will undoutedly be a great amp.
Very nice work so far, Scott!

 

[digger945]

Boards are stuffed, I'm mating them to the heatsinks now. Thanks for the compliments, I am getting very anxious to listen to this beast.