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Quad 303

Radfordman

Radfordman

pfm Member
I have a good selection of amplifiers, but my favourites are Radford STA25 III and Quad II power amplifiers.

None of my transistor amps seem to give quite the same satisfaction. I am fond of the Quad 303, and have four of them. They are all set up pretty well to spec.

I thought that I might try an experiment, increase the output stage bias current to about double.

I have done this, 20 mV across the pair of resistors R124/125, instead of the usual 10 mV.

Heatsinks do not heat up excessively and I think that I am a little step nearer to the sound of the Radford STA25 / Quad II.
 
Hi Radfordman, I'm very interested in your mod- what does it achieve? Are you pushing the 303 into class A operation? Is it safe?
I was given an old Radford ZD50 transistor amp by a friend when it started to malfunction. He had had it for years and tried other amps but none managed to replace it. I tried it and indeed it is a fine sounding amp. I am using a 303 regularly but if it wasn't for the speaker threatening clicks and pops the Radford occasionally emits, and its ugly appearance (the 303 is IMO a beautiful thing ;-) ), I'd probably be listening to it now.
Mark
 
Yes that sounds very interesting. I have a very nice totally unmolested early 303 in fine working order that could do with a bit of a lift. I've always felt that the output coupling caps are the Achilles heel but I suppose I should respect that it is an old design. Anyone any thoughts on that?
 
Hi Mark

I believe that the treble especially at low levels is now better, less edgy. I suppose that increasing the bias does give slightly more class A power, though It won't be much. I suspect that there may be other reasons that an improvement is noticed.

I guess that it's a safe mod to do, other amps use similar output transistors (2N3055) and run at much higher bias current. This is what made me think of trying this. I have two NAD 3130's and they as standard run at higher bias current than the 303.

At your own risk though.

I agree that the 303 is beautiful.

Beware of the Radford ZD50, I have one, it is a fantastic sounding transistor amp, probably the best I have come across. However, about 5 years ago is was driving my B&W DM5's (not loud) when suddenly one channel stopped working.

The DM5 has a 2A fuse built in, this had blown and saved the speaker, I was lucky. Replacing the fuse, all was OK with the speaker.

The amp, despite it's protection circuits had put heavy DC though the DM5.

I have not managed to fix the amp as yet. It is not the easiest amp to work on.

The Radford STA25 III and ZD50 shown here are my ones:

http://www.adventuresinhifiaudio.com/03/07/2010/a-history-of-radford-electronics/
 
Chops54 said:
Yes that sounds very interesting. I have a very nice totally unmolested early 303 in fine working order that could do with a bit of a lift. I've always felt that the output coupling caps are the Achilles heel but I suppose I should respect that it is an old design. Anyone any thoughts on that?
Click to expand...

Have you looked at the condition of the caps? Well worth doing, tend to leak all over driver boards. Later 303's had them mounted the other way up, better.

If the caps are in good condition, I don't think they are really the Achilles heel.

If using with moving coil speakers I would increase the value of the output capacitors, say 4700 to 10000 uF. Should help the bass very slightly.
 
I didn't know the 303 used 2N 3055, those are out of the ark and seldom regarded as audio grade. They generally get used for high current switching.

As for a 303 being pretty, eh? It looks like a 1960s battery charger. Now the 33 is pretty, and the FM3, but the battery charger, no.
 
Funny you mention that .....I designed a battery charger for lead acid batteries as a school project 30 years ago using 2N3055 transistors!

Richard
 
stevec67 said:
I didn't know the 303 used 2N 3055, those are out of the ark and seldom regarded as audio grade. They generally get used for high current switching.

As for a 303 being pretty, eh? It looks like a 1960s battery charger. Now the 33 is pretty, and the FM3, but the battery charger, no.
Click to expand...


See here:

http://quadrevisionspot.blogspot.com/2007/03/quad-303-replacement-transistors.html

There are different grades of 2N3055, Quad often used the RCA equivalents.

Don't forget that the Quad 303 was born about 1968. 303's very rarely blow their output stage. In fact I think one can short the output at full power and it should survive.

Many designs used the 2N3055, for example, NAD in the 3020, 3130, A&R in the A60, fine sounding amps. Leak also in the Stereo 70.
 
To get a step closer to the Quad and Radford, try some 0.5-1 Ohm resistors in the speaker line.
That raises output impedance to match the valve units.

The difference heard will depend on the loudspeaker loading.

Of course PJW couldn't find any difference into a benign load :)
 
Thanks for that, that's interesting. I've often wondered why I've never seen a 2N3055 in an amp, it seems they were out there all the time, albeit as equivalents. They were the de facto tranny of choice in the 60s as you say, so it's hardly surprising they went into the quad, and when I was into hobby electronics (1980s) they were just the item of choice in loads of circuits needing robust power trannies. I'm not surprised the things seldom blow in an amp, bloody things are bulletproof.

A lead acid charger with power trannies - now that's posh. Most of them don't even bother with full wave rectification!
 
Robert said:
To get a step closer to the Quad and Radford, try some 0.5-1 Ohm resistors in the speaker line.
That raises output impedance to match the valve units.

The difference heard will depend on the loudspeaker loading.

Of course PJW couldn't find any difference into a benign load :)
Click to expand...

It's a thought, my speaker cables are not that thick, so may have appreciable resistance, but that would be there on both the solid state and valve amps.
 
steve said:
I've often wondered why I've never seen a 2N3055 in an amp, it seems they were out there all the time, albeit as equivalents.
Click to expand...
The reason is the absolutely horribly-low current-gain it 'offers'. On paper the 3055 will pull the skin off a rice pudding; but in practice the necessary driver stage can mostly do that even without the 3055. It's why Quad had to invent a stable 'Triple' for the 303.

Fortunately those days are long behind. Which isn't to say a good triple isn't still without merit - Bryston have made a business of it!
 
stevec67 said:
As for a 303 being pretty, eh? It looks like a 1960s battery charger. Now the 33 is pretty, and the FM3, but the battery charger, no.
Click to expand...

I like the 303, the case fits the function, it has a lovely heatsink and panel moulding and is simplicity itself to access parts for service. It's shape makes more sense once you realise it's the same as a single Quad II, i.e. it's designed to slot into owner's existing cabinets etc. It may not as beautiful and timeless aesthetic design as the later 405, but it's still a classic, one of the most easily recognisable amps ever made.

Tony.

PS if you want to see a battery charger look no further than the similar period Sugden A51.
 
Tony L said:
I like the 303, the case fits the function, it has a lovely heatsink and panel moulding and is simplicity itself to access parts for service. It's shape makes more sense once you realise it's the same as a single Quad II, i.e. it's designed to slot into owner's existing cabinets etc. It may not as beautiful and timeless aesthetic design as the later 405, but it's still a classic, one of the most easily recognisable amps ever made.

Tony.

PS if you want to see a battery charger look no further than the similar period Sugden A51.
Click to expand...

That's more like a power station for a village!

Always wanted to try one of those!
 
Just an update:

A few days on and the amp with its increased output stage bias is running fine, and I still believe that it sounds better.
 
An interesting article on the 303 here. There seems to be a whole range of views from leaving them exactly as Mr Walker intended right through to the major rebuilds like the Net Audio monos.

Tony.
 
I have a 33 303 which has recently gone down on one channel. Going to send it off to Quad to sort it out as love the sound. However got hold of an Armstrong 621 and it has that same lovely warm sound with some grunt when needed. A gem indeed. The Quad though does feel a lot better built! Just an aside, sorry to stray off course!
 
I still love my quad 303s, I use them every day. one is used to biamp a speaker and it works very well indeed. (this is a 5.1 system, but don't tell Tony)
 
It's interesting to see the QUAD 303 and 405 gradually rebuilding their reputations these days. In the 1970s, they were considered greatly inferior to Naim's early (and much more expensive) 160 and 250 models.

Much is made, then and still today, of Peter Walker's apparent 'cheeseparing' use of inferior components in what could have been much better products. With modern use of better replacement components, his amps are said to sound much better.

If Peter Walker had been prepared to condone a little more financial outlay, would his amps have been more successful?
 
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