Quad 306 Power Amplifier

Hi All,

recently I came across to the service manual for my Quad 306 Power Amp. I use this Amp for more than 20 years without any problems. The produced sound is still exeeding nowdays. I am into the electronic business since 24 years. With all stages from repair up to an development engineer from A to D and back. But there is one thing that I didn't get.. How is the output stage from the Quad 306 working? Any hints from the old-shoolers?

Thanks, Felix

none schrieb:

Sorry for that. I hope this time with the name.

Felix

"Joerg" schreef in bericht news:aM94g.6223$ snipped-for-privacy@newssvr12.news.prodigy.com...

The basic amplifier is a fairly conventional complementary class-AB system, but with added extra current dumping, which used an additional class-A stage to fill in for any detected devation between the desired and actual output. IIRR the reviewers rather liked the results, with the exception of a few valve-mad audiphools who liked their amplifiers non-liear enough to add even order harmonics to the output.

I'd hardly call that output stage 'fairly conventional' by any means.

Graham

Hello Bill,

Sweet. That is indeed a maze of a schematic for an audio amp. Looks like they used feedback as well. Which makes me wonder why that wasn't enough.

Tube fans? I guess there is nothing that can convince them to use an amp where things don't glow inside.

Regards, Joerg

This is one of Quad's 'current dumping' designs. A curious arrangement.

Some info here...

Graham

"Pooh Bear"

The output devices T9 & T10 operate with zero quiescent bias, as can be deduced form the fact there is only one diode drop from the base of T9 to T8.

There are several major advantages inherent in the "current dumping" scheme, of which the absence of a bias trim pot and the need for bias thermal compensation is a relatively minor one. The big one relates to the output devices themselves - their performance has been rendered non crucial to obtaining low THD figures at all frequencies and levels.

The "current dumpers" could be and originally were cheap, rugged TO3 devices with no special properties in relation to matching, beta linearity, Ft or being made in complementary pairs.

With only two such NPN output devices per channel, a compact, rugged and reliable amplifier was built ( the 405) able to deliver 100 watts into 8 ohms and 160 watts into 4 ohms with very low THD, no thermal stability issues and as a result a small heatsink.

The 405's design was elegant, efficient and therefore low cost to build - and it dominated the hi-fi stereo power amp market in the UK for years.

Also, additional pairs of output devices could be very easily added in parallel to increase current output ability, and were, in later Quad models with higher power ratings.

BTW

Note the TLC 271 op-amp in the 306 acts as a DC servo to remove offset from the output and is not in the forward signal path though does come into play as a sub sonic filter. The need for matched ( diff pair) input devices has been thereby eliminated.

Another interesting feature is the lack of a PSU centre tap and deliberately asymmetric DC supply rails ( see T11 & T12).

Full of clever ideas.

....... Phil

Pooh Bear schrieb:

Thanks for the link.

The Quad engineers have created a clever design at those days. Less is more.

Felix

Hello Graham,

Feeding the summing node of a class C pair from the previous amp like shown in fig.1 is a pretty old trick. I have used it back in the 80's at the university. And no, I didn't invent it but read it in a really old book at the RF institute. IIRC the book was from the tube days.

Regards, Joerg