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Leach Amp pcb group buy interest

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L

Luke

Member
Joined 2001
Tony said:
yes, the VAS tranies are biased at around 5mA quiscient, so that they are not really off all the time, the thing is even if you drive one to saturation, there is still this 5mA current present, and the supposedly off trannie is conducting this curent....
Click to expand...

Just one more question. There are 2 VAS transistors. Does each one not handle half the wave and then stop conducting when polarity changes?
 
L

Luke

Member
Joined 2001
tryonziess said:
Luke,
I just found this 2n5551 in TO=92 as a replacement for the mpsa92. I think is will work.

If you have never soldered components to one of Jens boards you will find that any deviation from the component spec's is liable to cause headaches. The 0207 deminsions are recommended as a start for component sizing. Look at the frontend real close and you will see a bunch of parts in a very small area. On my first Leach board I tried to use Dale resistors. Did not work. So I decided that using the Vishay 0207 where recommended might be a good idea. They come in .1% if you so choose and are aerospace quality. So that was my thinking. The main idea is to get a part to fit in the allotted space. They are also .6 watt rated in a small .250 watt case.
When you consider just how much this amp is going to cost to construct parts choices may be a good place to spend extra time. I would like to use MJL4281 outputs put 24 of them is too expensive. Especially if you are going to match components. So the mjl3281 from Avnet will be my first choice.
I also like the Mills output resistors, but at 2 dollars and change they get up there also. Probably will piggy back two Panasonic 3 watts like Nelson uses.
Money is short these days. And the price of 100+ volt electrolytics -- well.

Tad
Click to expand...

hi Tad,

Both Digikey and Mouser list this series as non stocked item, so min order is 5000. As for the output resistors, maybe we should do a little group buy to get the volume up cost down? I for one am keen if it gets you over the 50 mark, dont know what the item is but Ill look later and see what the price break is.
Had a look at Avnet and I cant search, see no point in rewarding idiotic companies by requesting a login as it only encourages them. Theres a company called peackock Welcome to Peacock they sell the toshibas and sankens etc. The think is they are around a dollar for the Toshiba 2sc5200 and 2sa1943, they charge a bank deposit fee and transport, but again if you get a couple together it may be worth a look.
Good luck with 0207 resistors.
 
TonyTecson

TonyTecson

Member
Joined 2003
Luke said:
Just one more question. There are 2 VAS transistors. Does each one not handle half the wave and then stop conducting when polarity changes?
Click to expand...

remember that the VAS is operating Class A, meaning for the full 360* cycle of the signal, not one of each transistor ever ceases to conduct current, the quiscient bias of about 5mA is maintained.

and yes, each transistor swings half the peak of the signal, we have two peaks, the positive, (0 to 90 to 180*) and the negative , (180 to 270 to 360*) peaks...if you take the sum of the peak to peaks, that will come close to your rail voltages, Vcc-(-Vcc), your plus and minus rails.....

imagine two rubber same sized bands tied together and knotted one end and then pegged at the other 2 ends....at quisceint state, i.e. no signal present....their lengths are equal, now imagine when signal is applied, one rubber band compresses and the other expands, their being pegged at both ends, symbolizing your rails........this in essence s what is happening to your VAS.
 
L

Luke

Member
Joined 2001
Tony said:
remember that the VAS is operating Class A, meaning for the full 360* cycle of the signal, not one of each transistor ever ceases to conduct current, the quiscient bias of about 5mA is maintained.

and yes, each transistor swings half the peak of the signal, we have two peaks, the positive, (0 to 90 to 180*) and the negative , (180 to 270 to 360*) peaks...if you take the sum of the peak to peaks, that will come close to your rail voltages, Vcc-(-Vcc), your plus and minus rails.....

imagine two rubber same sized bands tied together and knotted one end and then pegged at the other 2 ends....at quisceint state, i.e. no signal present....their lengths are equal, now imagine when signal is applied, one rubber band compresses and the other expands, their being pegged at both ends, symbolizing your rails........this in essence s what is happening to your VAS.
Click to expand...


Tony thanks for your persistence, Ive got it now:)
I think its going to be hard to find a transistor that can handle this sort of Vceo, I may stick with the mje340/350 as I have some in stock anyway.
Have you built the Leach yet?
 
T

tryonziess

Member
Joined 2007
Luke,
Ampslab has some nice Sanyo CRT transistors with 300 vceo and very high hfe and ft. The 2SA1381/2SC3503 and the 2SA1371/2SC3468.

Will these work in this amp. I have concerns about stability issues with these very high frequency parts. The link Tony provided made reference to several crt use transistors.

Tony: Do you still need me to source those parts for you and ship to the Phillipines. Email me.
Has anyone received the international boards yet?

Tad
 
L

Luke

Member
Joined 2001
Tony thanks for the links. Fairchilds look good.
Tad there is an equivalent to the Sanyo part in a fairchild package, cheap too. Amps lab page wont load for medium power BJTs, maybe they dont carry them anymore?
So try the KSA1381 and KSC5303 in the E variant which has minimun hfe of 120 I think.
 
A

AndrewT

R.I.P.
Joined 2004
Tony said:
yes, the VAS tranies are biased at around 5mA quiscient, so that they are not really off all the time, the thing is even if you drive one to saturation, there is still this 5mA current present, and the supposedly off trannie is conducting this current....
Click to expand...
no, the VAS when biased to 5mA can swing +10mA and -4.99mA, i.e. a range from 0.01mA to 15mA and further if the VAS can survive it.
It is effectively a variable current source or sink whose current is decided by the drive voltage from the LTP resistor/active load.
 
T

tryonziess

Member
Joined 2007
I usually have no problem getting silver mica at Mouser or
Digikey. If either of you wish I can get them and mail to you. The U.S. postal service is many times cheaper than UPS or Fedex.
They can be shipped in a standard envelope for less than 4.00 dollars.
I still wish I could find the .6 watt resistors in 330r and 10r. The .6 watt item which most projects here rely on can be difficult to obtain in small lots.

What are your choices for all of the 0.1 uf film caps in the output stage. I was looking at the Epcos polypropylene from Mouser. They are about 35-40 cents apiece and come in 10 pcm.

Tad
 
L

Luke

Member
Joined 2001
tryonziess said:
I usually have no problem getting silver mica at Mouser or
Digikey. If either of you wish I can get them and mail to you. The U.S. postal service is many times cheaper than UPS or Fedex.
They can be shipped in a standard envelope for less than 4.00 dollars.
I still wish I could find the .6 watt resistors in 330r and 10r. The .6 watt item which most projects here rely on can be difficult to obtain in small lots.

What are your choices for all of the 0.1 uf film caps in the output stage. I was looking at the Epcos polypropylene from Mouser. They are about 35-40 cents apiece and come in 10 pcm.

Tad
Click to expand...

Hi Tad,

I had a look at mouser, and I saw lots of mica caps, but definitely no silver mica. Are they all silver mica perhaps, I dont think so as they have a few different types of mica caps to refine your search, but no silver.

How did you get the 0207 resistors, I can only fins them in 5000 minimum order numbers on mouser and digikey?

For the output stage caps Im using these http://www.diyaudio.com/forums/swap-meet/124065-fs-buf634-film-caps.html#post1881241

good guy, they are 0.15 but are mkp so its a good deal, I bought a 100 of the mks and 100 mkp. The 250V mkp are the right spacing to fit the board, just tested it. The MKS are smaller.

BTW I got my boards today, they are lovely:) I thought for some reason they would be scorched if you want to remove the 6 outputs.

Ive got to get a move on and finish my UGS power, its 60% there now.
Im looking at making some regs for the output of this amp and heavily biassing it for class A as much as the sinks will permit. Was inspired by another builder on another Leach thread.
Im thinking 70V rails should do it and then it would be bullet proof.
 
T

tryonziess

Member
Joined 2007
Luke,
A zizz wheel on a Dremel tool works very well to cut the boards. Using any kind of saw opens up the possibility of pulling the traces up. Nice neat and clean.
I wish I had gotten in on the UGS board buy. That is one sweet amp -- jfets and all. How far are you going to push the class A on the Leach 12? I have 1 amp per rail on my Leach 10 and it is a nice item.
I had assumed at the price the mica caps were fetching that they were silver. I will try to call Mouser or Digikey this week and confirm what is in them. The manufacturers cross reference number would probably be a better place to start.

Tad
 
A

AndrewT

R.I.P.
Joined 2004
AndrewT said:
no, the VAS when biased to 5mA can swing +10mA and -4.99mA, i.e. a range from 0.01mA to 15mA and further if the VAS can survive it.
It is effectively a variable current source or sink whose current is decided by the drive voltage from the LTP resistor/active load.
Click to expand...

Tony said:
if operated that way, then your vas is no longer class A.....
Click to expand...
why do you come to that conclusion?
 
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