In article , Ian Molton
wrote:
On Thu, 20 Nov 2003 18:46:40 -0000 "Mike Gilmour"
wrote:

Any audio tales of stoopid actions .i.e. like plugging in when mains
selector set to 110V...come on, we'll laugh with you not at you -
promise.

Ok, heres mine...

Just rebuilt Quad405 (40 odd quid in parts,mostly big transistors), and
realised that I needed a preamp.

thought 'I know, I'll use this tinypower amp with the gain set low'.

of course, it had a capacitor coupled output, so the first thingit did
was runthe Quad405 flat out into its (thankfully connected and working)
output protection board.

Anyone got a spare set out output transistors?

Not clear what you are describing above. Are you saying that the *Quad 405*
blew its output devices due to your injecting an input spike when your
'preamp' sic was switched on? Or are you saying the preamp output
devoices blew?

Anyhow, this is the event that made me decide to mod the amp. the
current limiting on the 405 was too severe, which is what allowed the
o/p trannies to blow (before the fuses could save them).

Puzzled. The point of the current limiting is to ensure the output devices
*never* blow up. The severe limiting on early 405's gets in the way of the
music sometimes, but should avoid failures. Hence I am puzzled by your last
sentence above.

Im modding the amp (rebuilding the PCBs) such that:

the gain is halved (1Vp-t-p inputs, less noise) use better quality parts
in the input,particularly one resistor that sets the performance of the
whole amp double the input cap values use a more modern op-amp and
adjust the supply to it to avoid hiss dc couple some transistors now
that higher voltage parts exist - like the 606 design. eliminates a
couple of passive components. apply the 405-mkII current limiter (better
performance, less blown outputs.

I'll make full parts lists and layouts available once Im ready.

Have you checked the stability margin? Reducing the overall gain and
changing various devices to different types may affect this.

Jim... did you ever look at the schematic I sent you ?

I am afraid I haven't. :-/ Too many other things have been distracting me
over the summer - many of them unfortunate. Sorry about this, but I had
completely forgotten. Can you remind me of the details by email?

Slainte,

Jim

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