Phil Allison 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...-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.



** The current source transistors for the differential pairs are biased from a string of 4 diodes and a 47kohm resistor that connect from one rail to the other. This arrangement is very deliberate and not done as an economy.

If one or other rail fuse opens, all input stage bias is lost and the amp becomes dead - so no output current can be delivered. Done the conventional way, if one fuse blew it would send send the amp hard DC to the opposite rail.

A common way around the problem is to fuse only the output stage and leave the rest of the amplifier permanently powered. Phase Linear amps and many others are wired this way. The Quad 405 has a permanent connection for the negative supply for the input op-amp.

I know of one amp where the designer got it wrong, removing the plus side fuse was harmless enough but removing the minus side one was fatal. The output would swing to full opposite rail and deliver 90VDC to the loudspeaker. I wound up modifying about 30 examples for a hire business so this did not happen again.

Most amps have no rail fuses as an even simpler answer to the problem.



.... Phil