let's tweak this amp!!!!!

[Flavio88]

hi everybody!!
i want to obtain more power from this power amp by using +\-60v
supply and paralleling 2 pairs of output darlingtons... i can directly do this or i have to change some transistors?(input pair, etc...)thanks.....amplifier schematic

[MikeB]

bc639 (t4) needs to be replaced by something more powerful (higher voltage, higher wattage), that's the at least change to keep it from blowing. The output protection also might need to be readjusted.

The input can't be correct, there is missing a resistor, this way c1 presents a naked capacitance to the signal source...

Mike

[Leolabs]

The VAS boostrap section can be replace by an active current source.

[destroyer X]

Questo circuito e molto buonno fratello.

A good idea to increase the power capacity of that BD639..... install something bigger there...and that can have better gain...better than 180 of DC gain.

I think the good transistor for this position may be 2SC2238 or 2SC2328...have to check those ones data sheets, to discover wich one is for audio driver applications.

Alike the Lamborghini Murcielago, this amplifier has a very nice performance.

Increasing the voltage and re-adjusting bias in the provided trimpot, will produce a very good increase in the power....but you will need one more output pair....more than 200 watts RMS over 4 ohms and undistorted power will not be difficult to reach.

Good idea is to remove all that output protection...aaaaagh!...it sounds very bad! and killed dinamics to protect output devices....grumpf!..not a good thing...no one wants a Ferrari with a limiting speed device!

Those enormous capacitors used in the output...from Colector to Base.... hummm...they avoid oscilations...but avoid also good sound....go reducing them and monitoring R22 with the Finger-O-meter, as this one turning hot will means that hi frequencies, from oscilations, are going to ground passing inside this resistor....searching this point of oscilation you will know the mininum possible capacitance to work there...increase one more step to be safe (normal capacitor values standard...those steps).

For instance, as an example...if you reduced using the capacitor standard steps and the amplifier started to oscilate using 100 picofarads..... this may represent that will not oscilate using 120 picofarads...one step more...use 150 picofarads.

Those Darlingtons use to produce some harshing sound...as they start to oscilate when driving high audio frequencies...... this is the reason people install big capacitors over them....this will avoid harshing when driving those High level and High frequency sounds.....but will produce a permanent distortion, even when not driving those high frequencies...so...it is some medicine that will kill the one is sick..... overdosis.

Good luck.

arrivederci

Carlo

[Flavio88]

thanks everybody for the suggestions!!!
Carlo,
can i use 2sc2240 or bd139 in place of bc639?
and the input pair? the bc556 can work with rails of +/-60v?
i would use this amp as subwoofer amp, then suond quality is not my primary target, i would only obtain more power with double output pair and higher supply rails...
and... can you explain how to paralleling output darlinghton? ex: i need to doubling also power resistors(1ohm)and collector-base capacitors?
Grazie fratello!!!

[destroyer X]

Selezione C capiche leggionari?

Use the selection type C, understood Roman Soldier....but it will work near the upper limit...a little bit dangerous...maybe you can find something more adequated...having more voltage capabilities..to be more safe to you...2N5551 and 2N5401 are units hardly used and can fit your needs too.

The output darlington may have a small resistor in series with the base....both 4 bases will need those 10 ohms resistors.... all emitters must go to througth emitter resistors.... each one twice the schematic value or something near that.

The capacitance from colector to base must be reduced, and it is more safe to use one to each transistor.

As you will use to bass...to subwoofer, the annoying treble will not bother you.

Yes, the BD139 C (atention to selection C) will fit your needs as voltage amplification...it is very good in that position.

Good luck,

Ciao

Carlo dei Danesi

[MikeB]

Originally posted by destroyer X

Yes, the BD139 C (atention to selection C) will fit your needs as voltage amplification...it is very good in that position.

It's a bad choice, as the amp will get +/60v, it has to withstand 120v, way too much for BD139, it will blow... (Max rating of 80v)
I guess some nice Hitachi or Sanyo video amplifier will be good here.
Or simply a MJE340, often used here.

Mike

[destroyer X]

Peak to peak....in the practical work...real work it will not go...but will blow because this one is for 80 volts swing.

Yeah...it is better to try another one.

Those higher voltage transistors are found internally to PC supplies...they will fit your needs without problem.

regards,

Carlos

[ash_dac]

Originally posted by Flavio88
hi everybody!!
i want to obtain more power from this power amp by using +\-60v
supply and paralleling 2 pairs of output darlingtons... i can directly do this or i have to change some transistors?(input pair, etc...)thanks.....amplifier schematic

I need comments about "100W power amp. elektor april 1982"

[Flavio88]

ok..ok...lets see if i understand...
-for the bc639 i will use a mje340
-for the input pair i will use 2n5401, but can i use 2sc2240(i just have 2 or 3 of them)?
-for paralleling the bdx66/67 they need to have theirs emiter resistors(1ohm 9w, for each), a base-collector capacitor each, and all the bases must have a 10ohm resistor in series.

please explain me pratically and simply...
thanks for the great help...