On Tue, 22 Dec 2015 01:25:37 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:

Trevor Wilson wrote:


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.


** The amp is fully symmetrical / complementary from input to output. I note it still has relay input muting and relies on rail fuses for self protection - but has enough outputs, 10 x 150W devices per channel, for that to work.

http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png

Such a topology allows for a very wide operating voltage range, down to a +/- a few volts and this stops the thumps.

But I am a bit worried about the bias arrangements - having six transistors in the bias loop makes for thermal instability.



I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?


** What about the 330pF cap - C608 ?

Along with the 33kohms, it forms a pole at 160kHz.

There is no load isolating inductor at output either, so it might not like capacitor values around 10nF to 100nF plonked right across the terminals.

.... Phil

I see that but in that position it is a kind of luke-warm pole. I
expect it to be placed so it reinforces the Miller capacitance between
collector and base of Q613 and Q615. That has the advantage of not
creating two poles associated with the voltage amp - itself a
potential source of instability.

d

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