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Old 24th August 2011, 03:14 PM   #1
radio is offline radio  Macedonia
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Exclamation Assistance to improve 100W amplifier!

Hi all!
I need help with amplifiers Improvement of seed (see Figure 1)!
Friends, what you need to change to the schematic to get better sound?
I want to transistor T3 (BC238B) was to replace the BD139 what to change for everyone to be OK?
Any help with this amplifier welcome.
thanks
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File Type: pdf Figure 1.pdf (85.1 KB, 170 views)
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Old 24th August 2011, 03:32 PM   #2
AndrewT is offline AndrewT  Scotland
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The front end needs a complete re-design.
Fit RF attenuator.
Remove P1 and insert a pair of accurately matched 100r as emitter degeneration.
Remove bc560b and insert a pair of matched bc560c.
Remove 5k6 and replace with a resistor that allows both halves of the LTP to pass exactly equal quiescent current (bias)
Remove the power grounds from the 4 signal ground connections and connect them as Signal Grounds separately to the Main Audio Ground (MAG)
Remove the base connection of T3 and fit the base between R11 and P2. Connect P2 wiper to P2 end terminal.
The ratio of C2:C1 is too low. Not 10:1, try >18:1, i.e. >=47uF

I don't know what a BSX47 is, but a BD is completely unsuitable for the VAS transistor.
R11 & R12 seem quite low in value. What is the base current passing into the Vbe multiplier (T3) and what is the current passing down the resistor ladder/string? What is left to pass as Ic of T3?

The OUTPUT stage:
What is T9?
Has someone changed the device from NPN to PNP?
If that change has been made, then I don't think it will work at all as shown.
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Last edited by AndrewT; 24th August 2011 at 03:46 PM.
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Old 24th August 2011, 03:39 PM   #3
radio is offline radio  Macedonia
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Hi Andrew!
Is there a chance to offer a redesigned schematic of this amplifier??
thanks
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Old 24th August 2011, 03:47 PM   #4
AndrewT is offline AndrewT  Scotland
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Start again.
This amplifier has not been designed. I'd call it cobbled together.
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Old 24th August 2011, 04:02 PM   #5
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Hi Andrew, if I posted a schematic for a simple Class A krell, would you be able to provide suggestsion for it too?

Have a look here:
http://www.amplimos.it/images/Krell%...EUR%201989.pdf
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Old 24th August 2011, 04:14 PM   #6
AndrewT is offline AndrewT  Scotland
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Nope, because that sounds more like setting a challenge rather than a request for assistance. If you have a Krell then keep it as such.
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Old 24th August 2011, 04:33 PM   #7
radio is offline radio  Macedonia
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Mr Andrew T, i understand better seed than English?!
So if you need to hand draw a schematic with modifications.
Thanks
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Old 24th August 2011, 04:36 PM   #8
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A request for assistance it is. Even if just conceptual.
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Old 24th August 2011, 04:42 PM   #9
jcx is offline jcx  United States
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the fig 1 amp schematic is not correct - bad bias, incorrect output connections - build it and it will not work

much better starting place would be Self's "Load Invariant" for 100W, CFP output and adds a few high loop gain tricks from Self's book

has the advantage of having been built, verified as working - can buy pcb, kits
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Old 25th August 2011, 05:49 AM   #10
radio is offline radio  Macedonia
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Question New schematics (Figure 2)!!

Dear friends!
I changed some things in the schematic (Figure 2 ),... are now asking why eating good again or should I change something?
All comments welcome!
thanks and cheers
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File Type: pdf Figure 2 (schematic) new.pdf (82.3 KB, 81 views)

Last edited by radio; 25th August 2011 at 05:52 AM.
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