Does the Quad 520F hold up these says?

[Kevin Nolan]

I have a pair of Dynaudio BM6's which I rarely use but actually I think are a very fine speaker indeed, and when I use them I power them via a Quad 520f.

I'm thinking of using the BM6's more, and wonder if using the Quad 520f does them justice - whether it holds up these days and it not whether anyone can recommend an appropriate update / improvement amp to power the BM6's

For project studio use - but know abolutely nothing about where it's at with studio amps. Thinking that the amp should not go past the price, of, say, a pair of KH 120A's - or I'll just buy those! ( so not past £1000).

The Quad was respected in its day, so if someone expert in amps still thinks it holds up - then I'm happily use it: the BM6's wont be driven very loud, and they sound solid and balanced through the Quad as is.

Thanks!
Kevin.

[Hugh Robjohns]

I'm thinking of using the BM6's more, and wonder if using the Quad 520f does them justice - whether it holds up these days and it not whether anyone can recommend an appropriate update / improvement amp to power the BM6's

I know lots of people adore Quad amps and think they can do no wrong... And it's certainly true that the Quad 520f is well built and easy to service, and did a solid, reliable, workman-like job when it was introduced. I have one here in the studio driving PMC Wafer surround channel speakers.

An original model will probably need new reservoir capacitors by now, but other than that it's probably still going to operate to its original specs.

The 520f is a professionalised hybrid variant of the consumer Quad306 and Quad405-II. All models share similar 'current dumping' technology, and most of the bugs and design flaws from the original 405 were ironed out by the time they got around to the 520f.

Personally, I think you probably would hear a small improvement in control and detail with a more modern power amp like a Bryston or Chord, but you'd be digging around to find decent second-hand models within your budget.

So basically I'd say if you're happy with the Quad/BM6 combo -- and it's not a bad pairing by any means -- then I'd stick with it.

Oh... and despite the XLR inputs, most 520f models are actually unbalanced, grounding the cold side while passing just the hot signal to the amp's input via the input sensitivity control on the front panel.

Whether this matters or not depends on the nature of whatever you're feeding it from! Ground-loop noise and/or transient distortion will indicate a problem... Where true balanced inputs were required an optional transformer-balanced module was available for the Quad520f, but I've only ever seen one like that, and I didn't think it didn't sound as good as the standard version to my ears!

[Arpangel]

Don’t know if this is worth mentioning, but if you’re feeding the 520 from a balanced source, a mixer, like I am, I have to use an ART Clean Box, I’m not using a 520, I’m using a 405 2 instead, if I don’t use the Clean Box it’s very noisy with lots of hum, might be worth baring this in mind.

[James Perrett]

I'm still using an the slightly older Quad 405-2 for my nearfields and a home built Hypex based amp for the main speakers (which were previously powered by another 405). The Hypex may give very slightly more definition but the difference is extremely small and I wouldn't be unhappy to go back to a Quad if I had to.

I only switched away from the Quad on the main speakers when the mains transformer became too noisy on one amp and the other amp started to give odd crackles. I made one good 405 out of the two and put the Hypex modules in the other Quad case.

IMG Stageline make some amps that are very similar to my Hypex amp - the STA400D and the STA800D. I've not heard them but, if they work as well as mine, they're a bit of a bargain as you can pick them up for less than the cost of buying the modules from Hypex.

[Hugh Robjohns]

I owned a pre-loved quad 33/303 preamp/power amp combo back in the early 90s. I got them cheap because they had stopped working, and in the power amp I had to replace the output transistors, the reservoir caps and I vaguely remember an issue with the power regulator as well... And when that was working again I discovered that the volume control on the preamp was graunchy so I replaced that too.

The preamp was decidely on the noisy side (but by design rather than age), but the 303 amp was actually quite good, and both were easy to service.

I sold the 33/303 on around the end of the 90s and missed the elegance and EQ facilities so I picked up an original 405 a while back as it came with the 44 preamp which is what I actually wanted -- purely out of curiosity.

I was so disappointed with the power amp's noise floor and generally lack-lustre performance — compared to what I've been used to. And after doing some research I found lots of upgrades and recommended tweaks for the early 405s which I've applied (at least, all the sensible ones). They have improved it a lot, but I'm still far from blown away by it. Been spoiled with a clutch of Brystons since the early 90s!

The 44 preamp is a lovely classic Quad design, though, with wonderful modular construction, superb build quality, and the most fabulous EQ options.

But it also has the most ludicrous internal gain structure purely in order to use the then-fashionable cmos 4066 switches for source selection. I'm tempted to rebuild it with mechanical relays instead which would improve the signal path and gain structure dramatically.

Quad had some great ideas of the years, and I've always loved their industrial design, but they also did some really daft things in their circuit designs sometimes!

[Kevin Nolan]

Firstly apologies for the typo - should have been "...these days". Thanks for not making me look foolish !!

Secondly, thanks a million for the very informative answers - and to Hugh for the expert guidance on wiring - if memory serves when I connected them up in the 90s I think this issue came up and was resolved - an expert over here in Ireland called Owen Drumm ( _seriously_ impressive character - as just one example - a DSP chip he designed (with a patent attached to it) resided in Calrex desks for years - one of dozens of similar stories I could tell you about him) - sold me the amp and helped me set it up - and he 'mopped' up a number of such issue for me in that project studio room.

So based on your views, I'm going to stick with the Quad - I was half expecting to be told to dump that piece of junk - but am plesently surprised to hear others using them still, and that the 520f will serve the BM6s well.

And on the BM6's - I know these have been a popular monitor over the years - but still - have them been underrated?? I've barely used mine over the years because almost as soon as I bought them I then bought a pair of Genelec 1030As - but powering on the BM6s recently - they're bloody well good! Very good - the sound is rock solid, and _Very_ neutral. I'm really impressed and feel I for one have hugely under estimated the value of these speakers!

Thanks again - really appreciate the insights - and the stories - your expertise is boundless, I'm afraid I just don't know the electronics behind audio equipment to this level, despite my day-job being a physicist!

[Arpangel]

I owned a pre-loved quad 33/303 preamp/power amp combo back in the early 90s. I got them cheap because they had stopped working, and in the power amp I had to replace the output transistors, the reservoir caps and I vaguely remember an issue with the power regulator as well... And when that was working again I discovered that the volume control on the preamp was graunchy so I replaced that too.

The preamp was decidely on the noisy side (but by design rather than age), but the 303 amp was actually quite good, and both were easy to service.

I sold the 33/303 around the end of the 90s and later missed the visual elegance and EQ facilities so I picked up an original 405 a while back as it came with the 44 preamp which is what I actually wanted -- purely out of curiosity.

I was so disappointed with the power amp's noise floor and generally lack-lustre performance — compared to what I've been used to as I've been spoiled with a clutch of Brystons since the early 90s! . And after doing some research I found lots of upgrades and recommended tweaks for the early 405s which I've applied (at least, all the sensible ones). They have improved it a lot, but I'm still far from being blown away by it. Struggling to understand all the hype and fervor associated with the design.

The 44 preamp is a lovely classic Quad design, though, with wonderful modular construction, superb build quality, and the most fabulously practical and effective EQ options.

But it also has the most ludicrous internal gain structure I've ever come across in a hi-fi product, purely in order to use the then-fashionable cmos 4066 electronic switches for source selection. I'm tempted to rebuild it with mechanical relays instead which would improve the signal path and gain structure dramatically.

Quad had some great ideas over the years, and I've always loved their industrial design, but they also did some really daft things in their circuit designs sometimes!

I had a 33/303 combo around 1975, loved it, but it got sold on for various reasons.
I now have a 405 2, and a 33 preamp, both were given to me, free, I wouldn’t have made an effort to buy them.
I have a great Trio amp from the 70’s, which is sonically superior to any of the Quad stuff, by far, that’s being used on our basement stereo, the Quads are on our main hi-fi, simply because my partner likes the look of those orange buttons, that’s the only reason.
The Quad is fine, it’s OK, but it’s not earth shattering, I can remember hearing my friends Trio LO7 power amps, with the matching preamp, they were earth shattering, a major turning point in my listening history, since then I’ve always been a Trio fan.

[James Perrett]

...since then I’ve always been a Trio fan.

It may be worth pointing out for the younger and non-British readers that Kenwood gear was branded as Trio in the UK during the 70's and early 80's. I presume that was an issue with the better known UK Kenwood company who made food mixers which has since been resolved as the Trio brand is no longer used.

[Arpangel]

Talking about amplifiers, you can’t without mentioning Ferrograph, the F307 amplifier was superb, preferable to Quad at the time, although I did like the Quad 33, nice laid back sound, very nice.
The F 307 was similar, but more so, rich, velvety sound

[Nazard]

The Quad 303 is a very nice amplifier, I have a very late one with a dark grey colour scheme and an IEC mains input.

The ESL-57 was introduced in 1957, at which time, the Quad power amps were the Quad II Monoblocks, first introduced in 1954, with KT66s as the output valves.

Going into the transistor age, Peter Walker needed a transistor amp to drive the ESL-57s, not an easy task, if you look at their impedance curve! So the 303 was born. Still, in my view, a very nice sounding amplifer, and, as Hugh said, very easy to fettle!

[Arpangel]

The Quad 303 is a very nice amplifier, I have a very late one with a dark grey colour scheme and an IEC mains input.

The ESL-57 was introduced in 1957, at which time, the Quad power amps were the Quad II Monoblocks, first introduced in 1954, with KT66s as the output valves.

Going into the transistor age, Peter Walker needed a transistor amp to drive the ESL-57s, not an easy task, if you look at their impedance curve! So the 303 was born. Still, in my view, a very nice sounding amplifer, and, as Hugh said, very easy to fettle!

Yes, I think the 33 has a preferable sound to my ears, than my 405.
The 33 is a nice slow "plodding" amp, which I like, very smooth too, it rounds off things, whereas my 405 is a bit more aggressive, more modern sounding.

[Hugh Robjohns]

For the avoidance of confusion, the Quad 33 is/was a preamp. The matching power amp was the 303.

The Quad 44, and later the 34 were the preamps designed to accompany the 405 and 405-2 power amps.

[Drew Stephenson]

I remember plugging a Cambridge Audio C500 pre-amp (modern, budget-ish) into a 303 as a repair test and being very impressed by the result. Something about the combination was definitely more than the sum of its parts.

[Arpangel]

For the avoidance of confusion, the Quad 33 is/was a preamp. The matching power amp was the 303.

The Quad 44, and later the 34 were the preamps designed to accompany the 405 and 405-2 power amps.

I ended up with my 405/33 combo purely by chance, being given them at different times, I’d be interested in trying a different preamp with the 405.
Although I’m thinking of replacing them both with a Croft Integrated, a friend went mental about these, he thought it was so good there was something wrong with his amp.

[BMountain]

Yes, it most definitely does. I use ome to power my Yamaha NS 1000 M´s and can´t find anything better that is not a whole lot moreexpensive.

[ajay_m]

Interesting discussion on quad amps and replacement transistors for them here https://www.vintage-radio.net/forum/sho ... p?t=172062

I can say that some amps homebuilt by myself from the original quad current dumping design were prone to catastrophic output failure putting a fairly large amount of DC across the speakers. This is not something I'd want to happen to expensive speakers and I think quad made a number of design changes over time to improve reliability. Other than that the performance of these amps is pretty good and as always, the distortion of any speaker system is orders of magnitude greater then amps like these.

[Arpangel]

Interesting discussion on quad amps and replacement transistors for them here https://www.vintage-radio.net/forum/sho ... p?t=172062

I can say that some amps homebuilt by myself from the original quad current dumping design were prone to catastrophic output failure putting a fairly large amount of DC across the speakers. This is not something I'd want to happen to expensive speakers and I think quad made a number of design changes over time to improve reliability. Other than that the performance of these amps is pretty good and as always, the distortion of any speaker system is orders of magnitude greater then amps like these.

I've definitely got to make a move replacing these Quad's on our living room stereo, I just don’t feel they have any vibe, or make you want to listen to them.
I do like the looks though, I'll keep them and use them somewhere else.
But what to replace them with? that's the question, I am a big fan of vintage Japanese amps, when I use my Trio it's like the music has jumped into life compared to my Quad's, but its getting very unreliable, and I’m reluctant to buy anything vintage now.

[ajay_m]

The main failure point on older class AB amps would be primarily the very large filter capacitors on the linear power supply. These could be up to 100,000uF and are probably best preemptively replaced, along with the much smaller X or Y rated caps and/or MOV devices on the mains primary side. Though luckily a failure of these primary side components generally results in a bit of smoke and a blown fuse.
Thermal compound on the main output transistors can degrade over time (an issue also with PC CPUs too), causing devices to overheat and fail. Finally if the output stage quiescent current is set by a trimpot, this can become a weak link over time and if contact between wiper and track fails, some designs can cause catastrophic output stage failure. And the output devices may well be unobtainium these days as well.

[Arpangel]

The main failure point on older class AB amps would be primarily the very large filter capacitors on the linear power supply. These could be up to 100,000uF and are probably best preemptively replaced, along with the much smaller X or Y rated caps and/or MOV devices on the mains primary side. Though luckily a failure of these primary side components generally results in a bit of smoke and a blown fuse.
Thermal compound on the main output transistors can degrade over time (an issue also with PC CPUs too), causing devices to overheat and fail. Finally if the output stage quiescent current is set by a trimpot, this can become a weak link over time and if contact between wiper and track fails, some designs can cause catastrophic output stage failure. And the output devices may well be unobtainium these days as well.

My Trio amp is faulty, I can use the aux in for CD, but the phono input has gone down, massive distortion on it.
Not really worth repairing.

[snipedog]

I have a pair of original input balancing circuit boards for the Quad 520f power amp. Both in full working order, and quite difficult to find as I believe they were a custom order from Quad. Simple install with no soldering required and an effective solution to earth loops. I was about to put them up on eBay. PM me if anyone's interested ?

[MarkOne]

I've definitely got to make a move replacing these Quad's on our living room stereo, I just don’t feel they have any vibe, or make you want to listen to them.

When I was in the HiFi business, we spent all of our time, developing and testing products that did nothing but make the signals bigger with the smallest distortion possible and the lowest noise floor possible.

In fact the very opposite of 'vibe'

Because... "High Fidelity"

[Hugh Robjohns]

I was about to put them up on eBay. PM me if anyone's interested ?

I'm interested and have sent a PM... or you can mail me: hugh@soundonsound.com