Assistance to improve 100W amplifier!

radio · 24th August 2011, 03:14 PM

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

AndrewT · 03:46 PM

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.

radio · 24th August 2011, 03:39 PM

Hi Andrew!
Is there a chance to offer a redesigned schematic of this amplifier??

thanks

AndrewT · 24th August 2011, 03:47 PM

Start again.
This amplifier has not been designed. I'd call it cobbled together.

Dr.H · 24th August 2011, 04:02 PM

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

AndrewT · 24th August 2011, 04:14 PM

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.

radio · 24th August 2011, 04:33 PM

Mr Andrew T, i understand better seed than English?!
So if you need to hand draw a schematic with modifications.
Thanks

Dr.H · 24th August 2011, 04:36 PM

A request for assistance it is. Even if just conceptual.

jcx · 24th August 2011, 04:42 PM

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

radio · 05:52 AM

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

24th August 2011, 03:32 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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. Last edited by AndrewT; 24th August 2011 at 03:46 PM.

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24th August 2011, 03:39 PM	#3
radio diyAudio Member Join Date: Oct 2009 Location: Bitola MACEDONIA	Hi Andrew! Is there a chance to offer a redesigned schematic of this amplifier?? thanks

24th August 2011, 03:47 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Start again. This amplifier has not been designed. I'd call it cobbled together.

24th August 2011, 04:02 PM	#5
Dr.H diyAudio Member Join Date: Apr 2002	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

24th August 2011, 04:14 PM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

24th August 2011, 04:33 PM	#7
radio diyAudio Member Join Date: Oct 2009 Location: Bitola MACEDONIA	Mr Andrew T, i understand better seed than English?! So if you need to hand draw a schematic with modifications. Thanks

24th August 2011, 04:36 PM	#8
Dr.H diyAudio Member Join Date: Apr 2002	A request for assistance it is. Even if just conceptual.

24th August 2011, 04:42 PM	#9
jcx diyAudio Member Join Date: Feb 2003 Location: ..	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