150w Greek Schematic

[sakis]

this is an amplifier constructed in greece almost 30 years ago one of them come to my shop for repair and there is a couple of things that trouble me

A) in the drivers emmiter IE bd 139-40 existing a network of 110R resistor and 0.1mf capacitor. (marked in red ) any body knows whats the reason ??

B) anybody things that this is a design worth upgreading/tweaking ????

C) there is no protection in the original design like base stoppers ...is there any chance that these resitors work as limiters ???? is this safe enough ???? and why other amplifiers do not use simular techniques ????

all opinion is very welcome

thank you very much

[Andy L. Francis]

Hi Sakis,

First. The pic is too big. Please resize the schematic!

Then.
It's the copy of the aussie ETI-480 KIT (Dick Smith electronics).

You'll find the answers to your questions here: http://home.alphalink.com.au/~cambie/ETI480/
Diyaudio thread: PCB pattern for 100W Amp

[teemuk]

The Sziklai pair receives degenerative feedback through the driver’s emitter. In the conventional setup the amount of this feedback is 100% but when the feedback path is divided by the shunt resistor (and a parallel capacitor) the amount of feedback is reduced. This allows extracting voltage gain from the output stage and consequently it can ease the aforementioned task of the VAS. The gain is approximately defined by the divider ratio.

The short circuit protection is also inherent because the emitter resistor limits the base current. I believe it’s not a very effective or bomb proof protection, though.

In my opinion, there’s more harm than benefits: Due to reduction in feedback, the devices now need closer matching and the overall linearity of the output stage is decreased. The circuit is prone to amplifying its thermal instability and it’s as unstable as the ordinary Sziklai. I firmly believe that employing this topology is at least one of the reasons why certain “Tiger” amplifiers have their notorious reputation.

edit:

see: http://www.swtpc.com/mholley/Popular..._1970_pg43.jpg

whole article at: http://www.swtpc.com/mholley/Popular...PE_Oct1970.htm

[Andy L. Francis]

ETI-480 schematic attached.

[ilimzn]

Practically every transistor in that schematic is working with an overvoltage (note taht Vce max for 2N3055/MJ2955 is 60V!), except the bias servo. The drawing has errors in the feedback loop (it is drawn as AC only, cap is in the wrong place), and it is also drawn 'in Greek' so it's not easily readable... It can also be noted the Sziklai pair is actually not conencted correctly so will not behave thermally stable without conencting the bias servo to the heatsink together with the output transistors (it is amazing how many times this comes up!) but it saves a few resistors in an implementation where the output stage is required to provide some gain, like here.

[Andy L. Francis]

C) there is no protection in the original design like base stoppers ...is there any chance that these resitors work as limiters ???? is this safe enough ???? and why other amplifiers do not use simular techniques ????

Base stopper as protection/limiter?
What do you mean?

[sakis]

for all these answers .....

all of these are usefull

special thanks to Andy:
i thought this cant be greek designed and especially 30years ago ....!!!!! it was stallen after all .... any way i think the guy who did is dead now but at the time he made some good money with these amps ....called them background 150 ( very crappy made though ) but working ....

to ilimzn :

sorry for the drawing and the size of the picture ....i had to download this program and learn how to run it the latest 2 hours
bc 549 is located on the heatsink dont bother about that .

thanks everybody for all the answers i will try to play with the amp a litle ...since is so simply made and with so simple devices.

Sziklai topology ....is the second amp i work with and dont know why but i like them so much ....

also as about protection and safety this particular greek amplifier was working for 30years inside a school (!!!!!!!!)
thats abuse any way you slice it !!!!!!!

[sakis]

of 2 transistors usually loacated in the output's base or drivers sometimes that monitors the current flow on the emmiter resistors and limits the drive of the output transistors

[sakis]

while repairing the amp seen these two resistors 110R that are placed from emmiter of the drivers to ground and even they are 110R 2W they seem kind of grilled meaning that they ve done some job ...limiting currents ....

will finish this tomorow and see how it goes mean while i will drop a note to Destroyer x since he is a real expert in the 70's amps to listen to his opinion also

[AndrewT]

Hi Sakis,
when do you propose to resize the schematic? post 2 request.