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