Problem With Hight Power Solid State Amplifier

[vytas]

Please help me!
I Have an amp which is 500W per chanel. Power supply is 100V per rail or +-200V. It is Russian copy of Dynacord PAA990. When I run it till about 40W it woks fine, but when signal gets stronger it beats output transistors. What could it be?

[AKSA]

Vytas,

Normally this means it is oscillating.

A CRO is probably the best way to see this, but you can also confirm this with a variac; run it so the rails are at +/-85V or so. Lower rail voltage reduces instabilities in SS amps, so if it runs OK at this voltage then the problem is definitely instability.

How to fix it? Well, this goes to the core of the design, and is tricky to explain as well as implement. But generally more lag compensation (a cap across the base/collector of the voltage amplifier) will do it, though at sonic cost. Other methods might be degeneration on the input pair, or at the emitter(s) of the VAS. But in the last two cases some care is necessary to ensure DC operating conditions are maintained.

Cheers,

Hugh

[vytas]

Good morning AKSA

Your advices is very useful for me and it seems that you are right with oscillating appearance. I tried to decrease the DC and it was every think was ok, I am not shure for 100%, because I did not run it on hight power, but I think you are right.

I will see what can I do with caps, but it seems to me that there is one strange thing, it is double voltage amplifier I will try to copy schematic.
Any way this amp ran for 5 years, and during one show it get down (few russian output trans there are 14 per rail and one from voltage amplifier). After change it - problems begins.

[AKSA]

Vytas,

As transistor technology improves, they get faster, and when the output devices are fast, the global stability of the feedback network is threatened.

Unless you replace the output devices with the original, slower devices, this problem will continue. You either must reduce the speed of the outputs - which can be done by including a 10R resistor in series with EACH base (if there is not one already there!) - or you use the original parts.

Sometime you can remove the instability with a phase lead cap from the base of one of the drivers to the feedback node. Start with about 12pF, make it silver mica. This will ocassionally do the trick.

These problems come up again and again; a stable amplifier has its outputs replaced by faster, better transistors, and the fun begins...... It is perennial as the grass.......

Cheers,

Hugh

[vytas]

I did changes with original transisitors. I used the same mark. On the base every output transistor has 56 ohm resistor, on emiter has 1 ohm resistor.
There is one more idea that transistors are very close to their maximum power supply.
But as you mentioned I think that it is oscillating.
It is very interestin, maybe somebody has Dynacord schematic...
And I would like to thank you for your atention and advices. If you will find more ideas please let me know.

[MikeB]

hi vytas !

Maybe your replacement transistors are fakes ?
This would mean, if turning up the power these devices would
reach secondbreakdown, and boom...
What kind of transistors are these ?
Have you measured all resistors ? Maybe some are blown ?
As the amp has once blown, there is good chance that there are
still some blown parts. Maybe some of the 1ohm emitterresistors
are blown ? You could measure the bias at every single resistors
during idle operation. But be careful, 100v per rail is dangerous !

Mike

[vytas]

Thank you for attention,

I checked all parts transistors, resistors OAmp, caps. Every thing seems to me ok. And it works till about 40W, and then suddenly burns output transistors. It reaches some maximum power and burns. There are even protection, but it do not help. The worst thing is that I can't find a schematic of an amp.

[MikeB]

What transistortype are you using ?
If they are some toshiba 3281/1302 your replacements are fakes for sure.
There is also a possibility that the original transistors were selected
for high voltage.

Mike

[vytas]

Trasistors are KT8101A and KT8102A. Russian produce. I can't say anything good about them, but to change them to any better transistors is too expencive. I will try to copy schematic, but what will I get I don't know. One more thing I should notice - Russians call them „Forsage“. May be somebody nows this amp?