Quad 405

I managed to get two of those Quad 405's, asked the seller to bridge them also. This is my first ever Quad amp, hope I'll like them.

Any 405 users on here?





Item number: 330324480624 (ebay)

 

Used it in the past and now use the baby brother 306.

Nice amps but generally only with easy 6 ohm+ loads, unless you aren't planning to push it very hard.

405-2 is a little better with speakers where the 405 hasn't enough current capability.

 

My speakers are 8 ohms, so should be no problem.

 

Yeah, It'll be fine with TDLs.

 

What speakers are you using on the 306?

 

Quad ESL57s and AuioSmile Kensai.

 

Not quite. If you are running 'bridged' each amp effectively will 'sees' a 4ohm load*. That's in the area where the 405 will not be happy. It'll need a small mod for best performance used like this - remove the current limiters (easy). Otherwise you have a power amp capable of huge voltage output, but still limited to 5A current.

The effect of this is not subtle - because as standard, a 405 is capable of c.100W into 8ohms, an easy 150W continuous into 6ohms, but about 50W into 4ohms and just 17w into 2ohms! A bridged 405 will perform about the same - because of the current limiting.

Fix that, and you should probably see about 250W per channel into your 8R speakers.


*In a bridged amp, twice the current demanded at the output because because there's twice the voltage swing pk-pk, so

 

The two Quad 405's have been upgraded and modified, with all the latest components. Before buying the Quads, I have told the nice gentleman what speakers I will be using (Via Phone). I also told him the specifications of my loudspeakers, if it will be fine being used with bridged 405's. Anyway this Man I bought these amps from is very genuine, he also collects Quad Pre's and amps.

Also the Quads have new protection built in.

He is more of a home made Quad man in respect


Here have a look at his work, He is not bad. Item number: 330324480624 (ebay)

 

Martin, there is also a kit to 'parallel bridge' the 405 IIRC. That should give more current while keeping voltage swing the same.

I'd worry personally about disabling protection on the current dumper range of amps as the heatsinks aren't at all suited to continual use at high powers. Big amp in a small box.

 

You might be surprised Rob. It's mean continuous dissipation that causes problems, and that's actually pretty small for most music (whereas the 405 dissipates c.32W at idle, which is by far the greatest component of the heat load). Bridge the pair of amps in one case and you haven't actually added anything to the total net Pdiss or thermal load in that case for a given delvered loudness.

But the paralleling of amps is not a happy compromise at all* - much better to properly 'bridge' and solve the problems it brings in tow IME. The existing o/put devices can cope with 8A quite safely - and much more if you try other, more modern equivalents. The fact is, this amp was designed when 5A was plenty, and conservative, and speaker designer didn't take the P with low impedance in the hope of sounding more 'efficient' than the design really is

(I really much get my old 405/2 out of storage and try it again..!)



*I'd go further - it's the kind of dumbass buck-turner that audio is all too full of IMO. Fannying at the edges, rather than doing some very simple engineering for a much better result.

 

Very true, though with the heat issue I was considering the total dissipation of the unit, ignoring the stereo/bridged options.
My old 405 (mk1) used to get frighteningly hot after a few hours of hard running.

Then again these things are littered around recording studios to this day so perhaps the fact they get hot enough to fry eggs isn't that much of an issue. My Grandfather bought us a 405 back in the early 80s after I regularly used to hear one in a studio driving Tannoy Little Reds when doing my saturday job as a yoof.
It seemed to cope remarkably well with live feeds played at very realistic levels.

 

Dead right.

Don't they call the properly engineered 405 the 606 Mk2 ?

 

Maybe

Either way, you *can* hammer the 405 and 405/2 hard, because of course there is *no* critical bias setting, nothing to drift with heat nor age - still a unique acheivement IIRC. In fact even with the internal feedforward 'bridge' disconnected they still turn in better than 0.1%thd at full power!

 

Pardon me for my silly question, but why do some recording studios like to use the Quad 405?

Also does Tannoy speakers go well with the 405?

 

The 405 is built like a brick shit-house and, as Martin mentions above, the circuit topology means it remains stable and within specification for many years without service.
The protection circuits mean they are practically unbreakable, though they can get bloomin hot.

They go very well with older Tannoys which present a nice easy load and are pretty sensitive.

 

OK Thanks

 

The little 306 is also surprisingly capable and can actually deliver as much current as a 404/2, though naturally it swings less volts.
Lovely with 57s. I've serviced and restored one - not that there is much to 'service' apart from replace a few caps.

 

I'm a bit curious to know what speakers go ideal with the Quad 405?

 

Most of 'em.

 

That's grate, then I can put a 4-6ohm speaker on the Quad then