some op amps were quite noisy and had good headroom so to speak so maybe
this is part of the reason.
Brian

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"Don Pearce" wrote in message
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On Mon, 16 May 2016 13:21:52 GMT, (Don Pearce) wrote:

On Mon, 16 May 2016 13:20:49 GMT, (Don Pearce) wrote:

On Mon, 16 May 2016 14:12:56 +0100, "Dave Plowman (News)"
wrote:

Got a Quad 405-2 in for repair. Late model with IEC in and out, and the
on-off switch.
Heavily modified. PS PCB added with two lots of rectifiers and caps and
the twin secondaries of the original transformer feeding each one. Dunno
just what difference this makes in practice.

Amps are modded too. Most noticeable are some caps on the reverse side
of
the PCB and Burr-Brown OPamps in sockets - Quad has soldered in the
TL071
by then. A quick glance looks like they've upped the volts to the OPamp
too.

But the odd thing is the gain of the amps is a lot down over the
original.
Quick check shows something like 10dB. Why would this have been done -
or
have I got to look for additional faults?

Both channels the same? Sounds like a deliberate change rather than a
fault. I mean, if they dropped the gain of the 405, they had to
increase the gain of the preamp accordingly. The only schematic I have
shows an OPA 604 which is a FET input op amp from Texas instruments.

d

Ignore that. My schematic is for the original 405, not the -2

d


This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB. Does that sound about right?

d

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Dave Plowman (News) wrote:


But the odd thing is the gain of the amps is a lot down over the original..
Quick check shows something like 10dB. Why would this have been done -



** The only practical reason is to reduce residual hum and noise when the amp is operating in a typical installation - which these days may include supply grounded items like TV receivers and computers.

Unbalanced hi-fi gear is prone to ground loop hum which can be very tedious to fully eliminate and the 405 having a fixed gain of 56 times makes that task so much harder.

The Quad II valve amp had a gain of only 7.1 or 10 times - depending if it were set to 8 or 16 ohms. As a result, the input level was 1.5V with noise and hum around 95dB below 15W and noise alone nearly 110dB below - remarkable for anything using valves.

With the 33/303 combo, Quad increased amplifier gain to 40 by making the input level 500mV. 405s followed suit with the same input level producing 100W output - so it would mate perfectly with the 303 pre amp. Connecting a supply grounded accessory ( like a tape deck or tuner) normally resulted in audible hum at higher gain settings.

Reducing the gain of a 405 to 17 times is an excellent idea, makes any residual noise 10dB less and really helps with hum issues.

Here's that link again:.

http://www.desmith.net/NMdS/Electron..._upgrades.html


..... Phil