Quad 405-2 Power Amplifier (with clone boards): Vintage Audio Review Episode

[@Radfordperson]

The 405 had a speaker protection board attached to the rear of the speaker sockets. The 405-2 had the speaker protection built in to the main boards. Therefore if fitting 405 boards into a 405-2 the amp will not have the speaker protection circuity.

[@vintageaudioreview]

Thanks for the info. The boards were different as far as components- I attributed that mainly a result of more modern components. Not having speaker protection is probably not a big factor with the unit the way my friend uses it.

[@Radfordperson]

@@vintageaudioreview In fact, the first batch of 405's were produced without the speaker protection board. It was very quickly added to further production. In 1976 I bought a new 405, it had the protection board. A few years later I upgraded it using the Quad -2 upgrade kit. This meant removing the protection board attached to the back of the speaker sockets, as the protection was on the -2 boards.

[@vintageaudioreview]

@@Radfordperson Interesting bit of info. The 405's seem popular according to the view count.

[@Radfordperson]

@@vintageaudioreview I still have my 405-2, it is in perfect condition, I also have a 33, FM3 ,44, FM4, 2, II's, ESL 57 and ESL 63.

[@vintageaudioreview]

@@Radfordperson nice that you have the collection Quads. One day maybe I will get a speaker to review...

[@christopher86043]

Wow, thanks for the review! They seem to be rather quirky amps, my 1976 405 has a voltage selector and 4 pin din input but doesn't seem much different from the 405-2, otherwise. As far as I can tell it still works fine. Along with the 33 pre-amp, style seems to count for a lot, with these models, and I'm still restraining myself from spending money needlessly on equipment that still works after 48 years! That heat sink is what got me.

[@vintageaudioreview]

Thank you for the comments! I believe the 405-2 also had the voltage selector and DIN input for earlier versions of it. The amp and preamp are a nice looking pair. If something is still working for you, then I would not be inclined to replace it unless you just want to have another piece of gear for your "collection".

[@christopher86043]

@@vintageaudioreview Yes, the earlier 405's were designed with DIN input to match the 33 (and 44) pre-amp's DIN outputs. The 44 later added RCA outputs to go with the later model 405-2. The 34 also started with DIN but switched by the end of its run. I ran into trouble when my 'homebrewed' RCA/DIN connector somehow burned out a wire, which killed one of the channels. I noticed this after opening the DIN end of the cable, and made a new one, but one of the reasons I had the 33 repaired was to simplify the situation and connect them as intended- DIN to DIN. I think this has improved their performance, though it's impossible to be sure!

[@vintageaudioreview]

@@christopher86043 I doubt the DIN cable improved the actual performance, but it certainly made your life easier connection-wise 😄

[@paulstubbs7678]

"Probe slipped", welcome to the club, I was working on a Toshiba SC-330 and accidentally took out a driver transistor, of course long out of production. The main DC filter caps take an eternity to discharge, I forgot and payed the price.

[@vintageaudioreview]

That looks like a nice amp- maybe one day one will come my way... I have not had too many issues with charged filter caps shorting out, but it is impressive how much of an arcing damage they can make when they do. Hopefully you were able to find transistors to make it work again...

[@Douglas_Blake_579]

Nice work. I've never played with the clone boards they now make for so many amplifiers, but it sure turned out nicely in your case.

[@vintageaudioreview]

Thanks for the comment, Douglas! This worked out to be a good case to test the clone boards, though had I not shorted out the one board, I probably would have attempted to repair it for a few more hours at least. The amp is running hotter that it was, but still working fine (the owner had me take it back as he thought it was smoking, but there was nothing wrong at all).

[@Douglas_Blake_579]

@@vintageaudioreview Me too ... when I get hooked into a repair I can be rather obsessive. I just hate it when I can't fix something... Of course it's running warmer... it's getting full power now. Smoking? Perhaps the owner needs to hide the cigars! (LOL)

[@vintageaudioreview]

@@Douglas_Blake_579 He does not smoke. I ran it for several hours into dummy loads- playing about a 30min. wave file that I put together- the QA40x can play a wav file at an amplitude of your choosing (not more than 0dbs I believe), which is a good way to play a "loud" levels w/out hooking up speakers...

[@Douglas_Blake_579]

@@vintageaudioreview Oops... I think you missed the joke .... You said he thought the amplifier was smoking...

[@vintageaudioreview]

@@Douglas_Blake_579 Yeah, I was a bit slow yesterday 😁

[@bobrose7900]

The original boards were compromised by budget constraints, but could be upgraded here in the UK to perform so much better. I'm not sure this mod is still available. It's a real shame the original boards were not rebuilt.

[@vintageaudioreview]

Thanks for the info and comment. I have the boards that pulled out, but felt it was more practical to purchase the clones, particularly for my friend's needs. If I ever get a dual +/ 50V dc power supply I might try to repair them at some point... for fun...

[@HipocratesAG]

love your channel, Thanks for a very nice content

[@vintageaudioreview]

Thanks for taking a few moments to let me know- I really appreciate it!

[@ragnarhairybreeks]

Good one. Enjoyed the mod. Am I just naive or is it truly surprising that clone boards are made for this amp? I mean, it’s not rare amp but it can’t have been produced in huge numbers. Cheers, Alistair Hold on… did a little searching, seems more of them made than I guessed!

[@vintageaudioreview]

Thanks for the comment/question. This amp is set up nicely for a clone board, as are several others I would imagine, but I have not had another case where I needed one.....

[@paulstubbs7678]

These clone boards look like they would be a good choice for making your own amplifier, I'm rather surprised at how well they work, as I've had a rather limited success with old HiFi gear.

[@vintageaudioreview]

You certainly have the beginnings of a nice 90-100w ch amp for a DIY project. Most HIFI gear that I have come across works decently- I enjoy the look of the gear as well.

[@ivanlukic2360]

Greetings from Serbia. There is an inverting mode opamp at the input. That is the reason for inverted phase at the output. You need preamp that inverts phase to get correct phase at the power amp output. In general the amp is not very well suited for 4 Ohms load. In that respect 405 was even worse than 405-2.

[@vintageaudioreview]

Thanks so much for the comment and info- glad I get folks from "across the pond" watching. As both channels invert the phase I guess it is not really important in the grand scheme of things. It appears that there were several renditions of the 405-2, so I really have no idea as to the actual design of the clone boards compared to a build of the 405-2....but overall the amp seems to be working well for the owner.