Quad 303 : Upgrade output stage to be fully complementary - diyAudio
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Old 19th June 2004, 01:06 AM   #1
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Question Quad 303 : Upgrade output stage to be fully complementary

Hi!

I've upgraded the caps and transistors in my 303.

Currently I'm using my 303 for the tweeters (Scan Speak D2905/9900) in my 2-way active system. Actually I think that the 303 has a softer more relaxed sound than my Quad 405,2. Above all I insist on soft sounding tweeters, but I must admit that the 303 is not as detailed as the 405,2.

I'm not and expert, but I know that the quad 303 uses old output technology. At:
http://www.net-audio.co.uk/quad303upgrade.html

I read:
Quote
Additional Modifications for the experienced.
The 303 can also be upgraded further by modifying the output stage to be fully complementary instead of the original quasi complementary design. Quasi complementary designs use two NPN output devices with one NPN and one PNP driver to produce a push pull output stage...
UnQuote

303 standard (left) and 303 Modified (right):
Click the image to open in full size. Click the image to open in full size.

Has anybody outthere tried this?

Or do you have comments?

Regards, Ask
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Old 19th June 2004, 02:38 AM   #2
anatech is offline anatech  Canada
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Default Mods

Hi Ask,
I normally wouldn't do this unless I had a channel blown, but why not? You may hear more of an improvement with the current source mod. You don't need to use a constant current diode, it's just easier. You can construct your own constant current source as well.
I have not tried either of these mods but they look sound.
-Chris
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Old 19th June 2004, 09:38 AM   #3
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The 303 is one of the worst SS amps i've ever heard. Is it the regged PS starving it of current? Are the output caps too horrible?
No idea. Just the concept of improving something that bad seems strange.
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Old 19th June 2004, 03:37 PM   #4
anatech is offline anatech  Canada
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Hi analog_sa,
I agree with you that the 303 isn't the best amp I've heard. It would seem an excellent candidate for improvement. The power supply is required for stability and noise reasons. Other brands have done this as well to good effect.
I would say that it's just an old design that could use updating. I think that Ask is on the right track and the 303 will sound much better at the end. Ask will gain some experience, and that's always worthwhile.
-Chris
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Old 20th June 2004, 09:32 PM   #5
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Default Quad 303/405,2 -> D2905/9900

hmmmmm....

I think that my upgraded Quad 303/405,2 can drive my D2905/9900 fine!

* I agree that a Quad 44 with TL71 opamps has bad treble
* I agree that an upgraded Quad 33 has even worse treble
* I agree that a NON-upgraded 303/405 has very bad treble performance

My source is an Arcam Delta 270 with LClock 2 and AD8620 opamps - I tried to connect a Marantz 6000 KI Signature which has softer treble on my system. Since I can tell the detailed difference in CD-Players, this test leads to the conclusion that my 303 is not an overshadowing bottleneck for the tweeter performance. Besides this I've heard many other systems...

Maybe analog_sa never heard an fully upgraded trimed Quad 303+44?

Regards, Ask
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Old 20th June 2004, 09:42 PM   #6
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Quote:
Maybe analog_sa never heard an fully upgraded trimed Quad 303+44?
Thank goodness, no
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Old 14th September 2004, 01:38 AM   #7
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I think it was Stan Curtis of Cambridge fame who
said complementarys are as similar as a husband
and wife having the same surname.

Your suggestions here of changing the output
to a complementary output. The triple Quad designed is
a valid design. even Doug Self hasnt ventured comment
on triples. Id take care changing any polaritys here
as you may then be just providing drive for the
final transistor and discounting Quads triple design.

Re sound of the 303 , I much prefer the 303 into
electrostatics than a 405. the output capacitor
much maligned is of huge benefit not only in
protecting against DC but allowing current to precede
voltage which helps the electrostatic be driven.

Cheers / Chris
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Old 14th September 2004, 10:32 AM   #8
TonyF is offline TonyF  New Zealand
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Hello Chris ( again) I agree the 303 does well into QUADs. esp my old ones. But I also use it on my Lowther horns.

Don't know why they sound so bad in SA. But then, everyone to their own taste.

Lowthers with no crossovers may go well with the Maynard amp !
Cheers Tony
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Old 14th September 2004, 11:37 AM   #9
tubenut is offline tubenut  South Africa
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Default Maybe it is the dry air @ altitude :))

..
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Old 5th March 2005, 01:42 PM   #10
Blue is offline Blue  Cayman Islands
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Default Complimentary or just consecutive numbers

I have to agree with Chris Daly (Hi Chris)

The fact that Doug (I designed the noisiest pre-amp with most electrolytics) Self has made no comment would suggest that it is a good design, but then I know it is... it just doesn't travel well but can be rejuvenated without expensive capacitors or changing too many active components.

The output triples give a very high gain "complimentary" pair with the bits that matter, the output devices being truely equal in performance, and the bits that cause the quiescent current to change, the first in each pair, run low current and kept away from the output devices (take note Doug).

Quad went to great lengths to ensure that the linearity of each half was the same although this is not reflected in the fact that the resistor values are the same each side. They also went to great lengths (100R emitter resistors) to prevent high frequency instability which may occur with higher hfe and Ft devices.

the real test would be to set the quiescent current (Iq) at the normal 20mA and apply a 1kHz triangle wave with the feedback removed, you should see no visible crossover distortion with full output swing in resistive load.

If the crossover is minimum at a lower Iq then the amp may be more sensitive to temperature changes, If crossover is minimum at a much higher Iq then you will be generating your own temperature changes.


Chris you know I can't let you get away with a comment like...

"huge benefit not only in protecting against DC but allowing current to precede voltage"

If the current was to lead (or lag) the voltage at the output then the frequency response (into a resistive load) could never be flat. With a 303 connected to an ESL57 you may well find that it isn't but for other reasons like the load is not resistive.

As any good chippy would tell you "measure twice and cut once"

Best regards

BLUE
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