Can anyone explain this circuit..

[pflodin]

Hello again

As some of you might have seen, I have had a broken amp that I now have repaired sucsessfully much tanks to the friendly people here on this forum. Now when that amp is back in buisness again, I can move on to the next project.

I recently bought a couple of pro power amplifiers on a flea market (!), Two complete 2x500W (they are labeled "JBN PA1000 Power Amplifier", JBN was a swedish brand that disappeared some time ago) amplifiers and a LOT of parts.
The aplifiers are ofcourse not working, but it seems like it's just the output transistors that are blown. I have taked with a guy that was a little familliar with these amps and he said "these amps blows output transistors all the time, ans sometime even the driver trannies..".
Maybe that's the reason I got a box with 50 new power transistors with them....

The question is, Why does it blow out the transistors all the time ?

Is it something espesially strange with the power circuit of this amp ?

I have attached a verry quick drawing of the power section of this amp, Its werry rough an a lot of the components are left out, but I wonder, why all these resistors ? in other "NPN only" circuits I'w seen there has been only one per transistor..

What output power is realistic to expect from this circuit ?

/Peter

[djk]

This amp has the same kind of ±V as the Hill, but with fewer outputs. If it has enough VA in the transformer it should be able to swing 500W/ch/4R.

The 'extra' resistors in the negative half outputs are probably there for a current limiting circuit.

I would consider using the MJ15024 in this amp as well as it will handle over twice the power that the 2SD types will. If you pull the spec sheet on the 2SD you will find that the maximum junction temperature is only 150*C vs 200*C on the MJ part.

[Upupa Epops]

It's old qasicomplementary s..t, get it to the ashcan .

[OliverD]

Re-use the case, heatsinks and power supply to build something nice

[pflodin]

yes, there is a lot of nice components to be used in future projects, like 1kVA 2x55V toriodal transformers, large heatsinks, 10000uf 100V caps , 19" cases etc.

But still, I have at least 8 more or less complete amplifier modules
(heatsink with 6 transistors and PCB mounted direct on it), most of them will only need two or tree new transistors. The modules are intended for +-80V rail, but I was thinkig that I might try to run them on +-60V rail (have transformers for that) and use them to convert some old monitor speaker boxes to active types. I dont want to mount a 1kva transformer in the speaker box , but a nice little 250VA torioid should fit perfect, and I don't need 500W @4R in a monitor box.

/Peter

[tschrama]

It's old qasicomplementary s..t, get it to the ashcan .

Upupa Epops, I only see NPN devices....you see something I don't see?

[Upupa Epops]

We both see the same, but we have different opinions about sense of this work - I mean, that it is lost time with not guaranted results.

[tschrama]

Hi Upupa Epops,

I didn't mean I disagree with you, I just didn't see the quasi-complimentary topology... Maybe it was a quasi-complimentary design, but there are other options, don't you agree?

I would love to see a bit more of the complete schematic

greetz,

Thijs


PS
I think it would be a far more atractive chakkence to repair the amp, find out why it blows up so much and improve it...I get the impression that these a BIG amps...

[phase_accurate]

O.K. quasi-complementary is not the last word in terms of audio quality admittedly.

But as far as I understand he wants to use them for sound-reinforcement. So this might be the smaller problem compared to the reliability issue.

So if he can prevent them from going up in smoke, he might be more than just content with them.

Regards

Charles

[Circlotron]

Originally posted by Upupa Epops
It's old qasicomplementary s..t, get it to the ashcan .

Originally posted by phase_accurate
O.K. quasi-complementary is not the last word in terms of audio quality admittedly.

The GainClone crowd seems to like it well enough.