Radfordman
pfm Member
I did not know that the 303 was partially rediscovered by Hi-Fi Answers. Anyone got a scan of the article? It would be interesting.
Quad current dumping amplifiers
- Thread starter johnfromnorwich
- Start date
I did not know that the 303 was partially rediscovered by Hi-Fi Answers. Anyone got a scan of the article? It would be interesting.
I have tried a 32.5 into both 405 and 405-2. The connection requires a special cable as the 4pin DIN naim is a different arrangement to the 4pin Quad.
The standard 405 is too noisy and has too high a gain for a 32.5, but my altered 405-2 with reduced input gain works OK. these comments are about the system connected to quite high sensitivity speakers.
Tony L
Administrator
I think it was Answers, certainly one of the 'flat-earth type' of mag, it would have been early 80s. I don't think it ever got a specific article, I just remember recommendations to Quad owners in the help pages to try a better pre such as a 101 or Crimson before dumping the whole Quad setup as the 33 had been identified as the main weakness in the combo. I don't think the 303 was exactly loved by the golden eared types, just regarded as better than expected and worth keeping for a while if sufficient budget was not available to change out the Quad combo for a newer pre-power.
Tony.
Radfordman
pfm Member
I have four 303's all set up pretty accurately. At idle they all run at different temperatures even the output stage bias currents are correct. One runs virtually stone cold and the warmest one is quite warm to the touch.
Initially, until I had set them up right, I guessed it would be bias current out doing this. I then realised that it must be something else and am pretty sure that its the big series power supply regulator transistor. The more it has to drop to give the 67V, the hotter the heatsinks will be.
Sure enough checking out a few voltage, this seemed to be the case. The secondary of the mains transformer has several tappings. It depends on which of these are used.
Higher tapping means more voltage dropped by the big series transistor, means more heat. Strange, not all the 303s use the same tappings.
Can anyone shed any more light on this?
Initially, until I had set them up right, I guessed it would be bias current out doing this. I then realised that it must be something else and am pretty sure that its the big series power supply regulator transistor. The more it has to drop to give the 67V, the hotter the heatsinks will be.
Sure enough checking out a few voltage, this seemed to be the case. The secondary of the mains transformer has several tappings. It depends on which of these are used.
Higher tapping means more voltage dropped by the big series transistor, means more heat. Strange, not all the 303s use the same tappings.
Can anyone shed any more light on this?
Tony L
Administrator
Mine, an early MkI just back from a Quad service, runs very cool. In use into the Heresys at typical TV levels the cooling fins remain cold to the touch, but not quite as cold as the other end of the amp. No perceived 'warmth' as such.
I remember back in the early 80s using my first 303 as a monitor amp when recording the band I was in at the time. It was sitting on the floor driving a friend's pair of HB2s straight from the mixer output. When it came time to pack up I nearly dropped the thing it was so f***ing hot - I'd stupidly placed it on the carpet which must have blocked the vents on the bottom panel. Luckily no long term damage done, but a lesson learnt regarding those bottom vents!
Tony.
Radfordman
pfm Member
Yep, my early one runs the coldest (or coolest!) and the uses the lower voltage tap on the secondary of the transformer.
Tony L
Administrator
Do you have any preference for the Mk I or II? I've no idea if there is a sonic difference, though the amp boards are apparently different. Mine has a four digit serial number, 9693, so is well prior to the change which occurred at s/n 11500. It appears to have been bought by it's previous owner in 1970 at a cost of £43.00 (I have much of the paperwork for that system including the Monitor Golds).
Tony.
Radfordman
pfm Member
Early 303's had a slightly different biassing arrangement for the output stage, I forget the details, they can be seen in the circuit diagrams available.
By Mk2, do you mean later 303's with the IEC mains socket? I have just one of these.
By Mk2, do you mean later 303's with the IEC mains socket? I have just one of these.
Tony L
Administrator
No, there are two 303 schematics for the amp boards, one for prior to 11500, one for after. These tend to be referred to as Mk I and Mk II, though it's not an official Quad designation. I'm just curious as to whether there is any sonic preference one way or the other.
Tony.
Radfordman
pfm Member
Yes, the difference as far as I can remember, between the two circuit diags is the biassing arrangement.
I don't think I can associate any difference in sound quality to this,that is provided all is in good condition and set up properly.
Very early 303's used four separate rectifiers for the main power supply, later used an encapsulated block rectifier (AEI I seem to remember).
Other small differences original PS and output caps were 2000 uF (nominal) later were 2200 uF (nominal). Not that that would make much difference. I like larger value output caps if using with some moving coil speakers.
Early 303's had the above caps mounted with connection towards the driver boards. When they leak, it can make a mess of them.
Late 303's have those caps mounted the other way up and longer wires to accommodate this. This reduces the chance of nasty leaking electrolyte getting on to the boards.
Tony L
Administrator
My first 303 did just that back in the early 80s, it spewed one of it's caps all over the circuit board and a channel died. I think that was an old amp, almost certainly a Mk I. I bought it second hand in 1978. The one I have now also has the caps in with connections downwards, though it has never leaked. When I had it serviced Quad replaced all four with ones that are a lot smaller physically, but they didn't changed the orientation. Looks like they have used 2200uf caps, at least I can read that on one of them through the vents, I've not popped the lid off for a proper look. I like this 303, it's in immaculate unmarked condition, in fact far better than the one I bought 32 years ago which had a slight ding on the top!
Tony.
andy831
pfm Member
I will bring my IEC 303 down, stick it in your system for a month or so and see what difference there is yourself Tony
Radfordman
pfm Member
There is (or was) a Quad conversion kit older 303's for mounting those caps the other way round. I seen to remember that there were extension wires for the caps and a tagstrip.
Radfordman
pfm Member
I have put the Quad details for inverting the capacitors here, you will have to scroll to find it:
http://docs.google.com/fileview?id=...EyMDEtMGE0MDY5ZTEzZmI0&hl=en&authkey=CNP-z5UJ
http://docs.google.com/fileview?id=...EyMDEtMGE0MDY5ZTEzZmI0&hl=en&authkey=CNP-z5UJ
Tony L
Administrator
Thanks Andy, perhaps we should have a 'Quad appreciation afternoon' at some point and compare your pre against the passive too.
Yes, I was aware of that, and a little surprised that Quad didn't do it to mine when recapping it, though I guess modern caps may suffer less. It seems most amps have caps mounted directly to the circuit boards these days, so thats connections downwards.
Tony.
Radfordman
pfm Member
I suspect they would have done that if you had asked. They probably tried to keep the price down a bit. Anyway, the new caps will probably last many years.
I know that the Quad service Engineer that does the older equipment is very good. He's a friend of a friend. I have to admit that when replacing caps on some of my 303's, I have not inverted them.
andy831
pfm Member
That sounds like a plan, I need to pop down and collect my records, I will be in touch to agree a time / date
Martyn Miles
pfm Member
I use a Quad 405 in my PA system. Much nicer than some of the PA amps. ( Stageline, for instance) When I play music through it people comment on the quality. I've never actually tried it on my own speakers, LS3/5as. I'm told it is a good match. Martyn .