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Leach Amp pcb group buy interest

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#242
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#244
happy easter everyone!

Tad,

you can try the BC55x' for the longtail pair, they will never see more than 40 volts accross their collector emmiter juctions, you can then use the MPSA42/92 as upper cascodes if you plan to use 95 volt rails, that should work....or else use the MPS8099/8599 called for in the original article....

as for power supply, you can stack two torroids at least 1500va each.....i am not sure now, but ApexJr used to sell these hitachi 68,000ufd/100volts caps rated for 105*C....
 
#253
Tony said:
in the VAS position, each trannie's peak swing is the rail voltage, now if one is fully on, the other one is fully off, the off trannie sees the sum of the rail voltages, that is why limiting your rails to the Vceo rating is important....
Click to expand...

So you are saying that the off trannie has 2 X rail voltage?
Let me clarify this.
I have +-100V. My Vceo rating is 120V. When one VAS trannie goes off, it sees 200V therefore blowing the trannies 120V Vceo rating?
BTW, the VAS are T2 and T22 right?
 
#254
yes,
Each transistor will have a Vce applied when in the quiescent condition.
Use that initially to determine Pq of every device.

Then look at peak signal through the circuit. Re-calculate the Vce of every device. Then check -ve peak signal. Now select suitable devices for each location that satisfy the Vce, Pq and Ipk+Vce (soar)
 
#255
AndrewT said:
yes,
Each transistor will have a Vce applied when in the quiescent condition.
Use that initially to determine Pq of every device.

Then look at peak signal through the circuit. Re-calculate the Vce of every device. Then check -ve peak signal. Now select suitable devices for each location that satisfy the Vce, Pq and Ipk+Vce (soar)
Click to expand...

The first part quiescent state I get.
I think I get it now. The signal is superimposed on the dc so its 100V ac peak with 100V rail offset is that right?
 
#256
Luke,
Most of the mje340/350 specs are very good. Exactly what is the ft of these devices. I was not able to get a datasheet with ft on it.

My biggest concern is finding suitable devices for the signal TO-92 mpsa92 replacements. I get good hfe and ft but then the voltage rating is slack. Even the 0207 vishay resistors are hard to find in SMALL lots.😕
Tad
 
#257
tryonziess said:
Luke,
Most of the mje340/350 specs are very good. Exactly what is the ft of these devices. I was not able to get a datasheet with ft on it.

My biggest concern is finding suitable devices for the signal TO-92 mpsa92 replacements. I get good hfe and ft but then the voltage rating is slack. Even the 0207 vishay resistors are hard to find in SMALL lots.😕
Tad
Click to expand...

I think the mje340/350 are average general purpose, hfe min is low and I suspect ft would be around 4-6 Mhz. Theres not even a Cob figure?
The mje172 looks better, but max 100V.

For signal I think I have no choice but to use mpsa42/92. Why arent you looking at these, they are drop in replacements? I might try 2sa970 and 2sc2240, but getting the pinout from cbe to ecb is not easy.

In actual fact I will probably use lower rails, something like 80V.

Whats so special about 0207 resistors BTW?
 
#258
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
 
#259
I have +-100V. My Vceo rating is 120V. When one VAS trannie goes off, it sees 200V therefore blowing the trannies 120V Vceo rating?
Click to expand...

yes, the absolute value of Vdc is the sum of your rails, so +/-100 translates to 200 volts....so when choosing devices, specially in the VAS, i will get devices with Vceo rating of more than 200volts.

have healthy respect to Vceo ratings if you want your amps to last. high colector voltage and current leads to secondary breakdowns....a trait of bipolar devices....
 
#260
Each transistor will have a Vce applied when in the quiescent condition.
Click to expand...

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....
 
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