Quad 303, possible improvement

[Reid Malenfant]

Hi Dave, all well & good However with some power avaialble in class A distortion measurements simply don't tell the whole story Once again, it's not about how much distortion is present, but the type of distortion become much more significant

In class A there can be no crossover distortion, so higher harmonics are out of the window & lower harmonics predominate which tend to be pretty innocuous.

[DSJR]

I accept that. Although I haven't measured the bias on my 303, the sound is like velvet with no aggro at all. Any grit or grain may be due to something else?????????

As long as no damage is caused though

[Reid Malenfant]

I was always very happy listening to my dads 303/33 combo, it blew away every music centre that anyone else owned back in the day

No idea about this one either tbh, but a little calculation reveals about 0.5W of class A or a tad more might be possible per channel with moderate heatsink temperatures. It might not sound like a lot, but if you don't listen to things loud it may well be worth the fettling.

I wouldn't expect distortion to increase too much once it enters class B either due to the output triple & it's huge current gain.

[Rare Bird]

Dave have you tried the '303' with say a 10K Passive pre?

[spendorman]

have you seen this? From the Revisionspot site...

"The next myth: high bias current

In the Quad service manual it is stated that the DC bias current of the power transistors must be between 10mA and 5mA. A popular “mod” is increasing the current to 35mA and in some cases even higher. Well, in real life the distortion is only 0.05% at a bias current of 0.5mA. At 5mA it is 0.01% and will not get lower at higher bias currents. Also the spectrum analyzer will not give a better (cleaner) picture at high levels of bias current. So Quad was (is) right! For practical reasons, stability of the circuit, 10mA is a good value for the bias current."

Very interesting, I still reckon that my amp with the increased bias sounds better. It could be that mine has some problems and that he increased bias overcomes. I doubt it though, it is in pretty good condition.

Tomorrow, my friend was supposed to be bringing me his 303 (which I set up as standard about a year ago) for the increased bias setting, but that is going to be delayed a while. He will have a listen to mine though.

I wonder why the the increased bias mod was popular.

There is no doubt in my mind that a 303 with bias that's low, can sound rough. It's mainly in the treble.

I purchased my first 303 in the early 70's. It was second hand and in good condition. I did not like the sound, gritty in the treble when compared to my Quad II's.

The 303 sat on the shelf for some years until I decided to set it up to spec. It was not massively out. But the improvement in sound quality was considerable.

This amp is one of the very early 303's (not the one I have done the mod on) and is still working fine, all original components. None of the electrolytics show any sign of leakage.

The other three 303's are much newer, one is a late model with the IEC power connector, strangely, all the newer 303's needed their big power supply and output capacitors replacing.

Well, perhaps it's not so strange that the newer amps need cap replacement, more like it's strange that the oldest one is still fine on it's original capacitors.

[spendorman]

Hi Dave, all well & good However with some power avaialble in class A distortion measurements simply don't tell the whole story Once again, it's not about how much distortion is present, but the type of distortion become much more significant

In class A there can be no crossover distortion, so higher harmonics are out of the window & lower harmonics predominate which tend to be pretty innocuous.

Spot on, in my view

[spendorman]

Dave have you tried the '303' with say a 10K Passive pre?

I have, worked fine.

[spendorman]

Hi Dave, all well & good However with some power avaialble in class A distortion measurements simply don't tell the whole story Once again, it's not about how much distortion is present, but the type of distortion become much more significant

In class A there can be no crossover distortion, so higher harmonics are out of the window & lower harmonics predominate which tend to be pretty innocuous.

Many years ago, I was given a Rogers Cadet III with a blown output transformer. I could not find a Rogers one, but G W Smith sold one that they said would be an equivalent. I cant remember exactly, but it was not straightforward to fit it.

Think I had to change the feedback and something else. However on a cheap test speaker it worked. I had a scope, various signal generators and did some basic tests, all seemed OK.

So, I connected the amp to the Hi Fi speakers, and was shocked that the repaired channel sounded so bad. The measurement (although basic) did not tell anyway near as much as my ears could.

Years later, that amp is still like that. Sowter can do a transformer, but it's costly.

[Rare Bird]

I have, worked fine.

I have aswell & it does work fine your right, for me it gives a more open sound it's as if the '303' has been set free to exhibit what a great performance it's capable of..don't get me wrong i totally adore the looks of the '33' but not it's sound regardless of what mods you perform on it.

[spendorman]

I have aswell & it does work fine your right, for me it gives a more open sound it's as if the '303' has been set free to exhibit what a great performance it's capable of..don't get me wrong i totally adore the looks of the '33' but not it's sound regardless of what mods you perform on it.

It was when I started running the 303 straight off my fairly good soundcard that I found how good it was.

Using the NAD 3130 as a pre for it does not seem to noticeably degrade the sound. The NAD as a pe amp seems to better than both my 33 and 44, my 44 is an early one though.