Cyrus 1 schematic question

[jaycee]

I'm still learning my way around amplifier schematics. While looking at the Cyrus 1 schematic, I see something wierd.

http://www.tcaas.btinternet.co.uk/cyrus1pwr.gif

The bit that's wierding me out is Q35/37, and it's feedback to Q15 via R105, C37, R67/69, is this as I suspect some protection circuit that removes the current source to the input pair should a fault occur on the output ? or is it some kind of feedback loop ?

[Richard C]

Yor're correct in that it is over-current protection and removes drive to the output stage by effectively switching off the input devices in the event of excessive output current.

[Tube_Dude]

Originally posted by jaycee
The bit that's wierding me out is Q35/37, and it's feedback to Q15 via R105, C37, R67/69, is this as I suspect some protection circuit that removes the current source to the input pair should a fault occur on the output ? or is it some kind of feedback loop ?

Hi jaycee


Yes,it is indeed a protection circuit ,that limit the current sourced by the CCS to the input pair , in the event of a short (or a punishing load) at the output.

Regards

[richie00boy]

It is a form of over-current protection for the output stage. When the current reaches a certain level Q15 conducts, shutting off the current for the LTP. Hence removing any drive to the output.

Whether it's better than the more common method of shunting the voltage applied to the output device base, I don't know.

[jaycee]

You've all just made me very happy that I am actually learning something

I may have a go at building the Cyrus 1 output circuit, but change a few bits, for example the output stage to a Sziklai using MJL4281/4302's. A friend of mine actually has a Cyrus 1 amplifier and he reckons it's brilliant, even though it's so old.