Studio 350 construction thread

[jamesfeline]

Thank you for your answer.
I have a couple of pcbs.

The problem is that i cannot find BF470, BF469 and 2SA1084, so i must design new pcbs with alternative parts. Most of them are smd now.
I will wait the protection circuit to add it on the new pcb.

I have those parts

 

[Billys8]

Thank you for your answer.
I have already order some of these parts,
but i am not sure, if i will keep the input stage as is.

 

[Ian Finch]

You should be able to substitute KSA1381, KSC3503 and KSA992 respectively in the same order as your question. The first 2 are were old glass TV tube drivers which are now scarce. The 3rd type is a small signal, low noise, high voltage type as is necessary for the input stage. Take care to check the pinouts though - I haven't checked all of them so you may have to twist some leads.

 

[Billys8]

Hi Ian.
I decided to use BF623 instead of BF470, and BF622 instead of BF469. The difference is only the case. They are SOT89.
The best alternative that i found for 2SA1084 is KSA992 as you already mention.

I am looking, also, for protection circuit suitable for this amplifier. I would like to add it on the same pcb.

Thank you!

 

[Ian Finch]

Watch the heat dissipation of the VAS transistors - They will likely run too hot. SOT89 is a lot smaller than TO126 and if you want to use SMDs, use SOT223 types such as BF721,2,3,4 which can also be attached by their larger collector tab to small sinks or to a few square cm of PCB copper.

 

[Billys8]

Ian you are right.
BF721 for BF470 and BF720 or 722 for BF469 are a better choice.
I will order them.
Thank you!

 

[Billys8]

Watch the heat dissipation of the VAS transistors - They will likely run too hot. SOT89 is a lot smaller than TO126 and if you want to use SMDs, use SOT223 types such as BF721,2,3,4 which can also be attached by their larger collector tab to small sinks or to a few square cm of PCB copper.

How can i calculate the heat dissipation ?

 

[Ian Finch]

The basic DC dissipation is simply the VAS stage rail-rail voltage multiplued by the bias current flowing through them. Calculating the current with an arbitrary signal is not easy at all so I suggest you simply fit the type used in the original kit which are a common, small TO220 size.

 

[Ian Finch]

Billy8, I neglected to mention that you will need a heat spreader, such as a piece of sheet copper about 0.5 mm thick and 1 x 2 cm to mount SMD parts to a heatsink. Solder the transistor tab to the copper, then bolt the copper to a heatsink with a thin mica insulator to ensure it is cooled adequately and the solder connection does not fail. That's why it makes more sense to use on-semi TO126 type semis KSA1381 and KSC3503.

 

[Billys8]

Thank you Ian for your feedback. I designing a new pcb, with changes and protection. Possibly i will use KSA1381E and KSC3503D, because it leaves me more space for the smd of the protection circuit.