Quad 303, possible improvement

[spendorman]

I have a fairly 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.

[Rare Bird]

Try using MJ15003/MJ15004 in fully complementary output as Net do..

[spendorman]

Try using MJ15003/MJ15004 in fully complementary output as Net do..

Thanks, thought about that, but decided may try that at some other time. Also thought about just trying MJ15003 and keeping the circuit as original.

[Rare Bird]

O Mr Spendorman your PM box is full

[spendorman]

O Mr Spendorman your PM box is full

Fixed now, thanks

[DSJR]

Andr'e is right and the option once output devices are changed to remove or significantly relax the current limiting (4A to at least 8A) would help with more modern low impedance speakers. Possibly the fully-complimentary conversion which can be done with current transistors.

Please check out the Dada website as well

Of course, if Quad thought that the original tweak was a good thing, don't you think they'd have done ot at the time?

Others' work on the equally antiquated Naim amp design shows that there is an optimum bias current level and increasing it makes no beneficial difference - see the Acoustica site for links regarding this..

[spendorman]

Andr'e is right and the option once output devices are changed to remove or significantly relax the current limiting (4A to at least 8A) would help with more modern low impedance speakers. Possibly the fully-complimentary conversion which can be done with current transistors.

Please check out the Dada website as well

Of course, if Quad thought that the original tweak was a good thing, don't you think they'd have done ot at the time?

Others' work on the equally antiquated Naim amp design shows that there is an optimum bias current level and increasing it makes no beneficial difference - see the Acoustica site for links regarding this..

303 as I guess you know uses NPN devices for the final output. One cannot make this fully complimentary using the "current" transistors, one needs an NPN and PNP pair.

I am a great one for leaving things standard, but this bias mod has definitely improved the amp.

It might just be a one off with this amp that is improved by increasing the bias, I have three more 303's to try the same mod on.

[DSJR]

That's fine, but i'd like to know why this was never done by quad, that's all. Maybe there's a perfectly logical and genuine reason why not But don't ever think that they'd not have done it because they hadn't thought of it at the time

[Reid Malenfant]

There wasn't any complementary devices about when the 303 was developed & going over to a fully complemetary output stage would involve modifying circuit boards. Why fix what aint broke

Obviously a competant modder could easily figure it out, but you'd need an oscilloscope & all sorts of loads to test it on for stability issues once modified.

Looks like i was answering the wrong question

[spendorman]

That's fine, but i'd like to know why this was never done by quad, that's all. Maybe there's a perfectly logical and genuine reason why not But don't ever think that they'd not have done it because they hadn't thought of it at the time

Well, I don't know, but in my opinion Quad 303's (in standard form) are not that good in retaining their output stage bias setting. Over the years I have serviced quite a few.

Increasing the bias a bit over standard, I suppose makes it less likely that over a period of time, the amp will likely have at least the specified bias current.