Quad 303, possible improvement

[spendorman]

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.

I believe Net Audio and others have figured it out, I know a guy who has the Net Audio mod. He has had some problems though.

Don't forget that there are complimentary power transistors in the 303, the drivers are, and they are quite big. However, the PNP ones are more fragile than the NPN ones.

Anyway, I am tempted to increase the output stage bias on my other three 303's and a friend has requested that I do his one.

[Reid Malenfant]

I don't see why you can't take it to 100mA bias if you wanted to, that would give some class A power at normal listening levels with only 6W of dissipation per channel Higher isn't out of the question either as 12W is naff all even on a heatsink the size that the 303 has

[spendorman]

I don't see why you can't take it to 100mA bias if you wanted to, that would give some class A power at normal listening levels with only 6W of dissipation Higher isn't out of the question either as 6W is naff all even on a heatsink the size that the 303 has

Absolutely, but I thought try a little first!

Perhaps someone else would experiment and report findings.

[spendorman]

I don't see why you can't take it to 100mA bias if you wanted to, that would give some class A power at normal listening levels with only 6W of dissipation Higher isn't out of the question either as 6W is naff all even on a heatsink the size that the 303 has

Bear in mind that the heatsink is also used for the series power regulator. On a standard 303 this can heat up the heatsink more that anything else (amp at idle or low volume).

This is another interesting point. Depending on which taps Quad used on the main transformer secondary (and they did vary this) more or less is voltage is dropped across the regulator.

I discovered this because at idle one 303 ran warmer than another, even though both had their output bias set to spec. This difference of temp was due to the different tappings used.

[Reid Malenfant]

If i find the time i might have a crack at it myself as i have a 303 & 33.

Just out of interest what is the standard quiescent current? A guess would be about 20 - 25mA

[spendorman]

If i find the time i might have a crack at it myself as i have a 303 & 33.

Just out of interest what is the standard quiescent current? A guess would be about 20 - 25mA

Have to look back, but I think emitter resistors are 0.3 Ohm, I measure across both to double the voltage and voltage is what I mentioned earlier in the thread. My memory is bad!

[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.

Here it is:


So it looks like about 16.7 mA as standard

33.4 mA as modded.

Just will check diag to see if I remembered the emitter resistor value correctly.

[Reid Malenfant]

Heck that's a little low, i guess they could get away with it though due to the output triple. 0.016A

[spendorman]

Heck that's a little low, i guess they could get away with it though due to the output triple. 0.016A

Just checked, emitter resistors (one is actually a collector resistor) are 0.3 Ohms.

[DSJR]

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."