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hahfran 19th December 2007 09:39 PM

Quad 303 triple cascade
Dear all
in long gone days when I was a student I owned for a short time a Quad 303. I had to sell it when I went overseas to another university. Now that I am retired and still have the schematics
I want to do something entirely different from what I did during my active occupation, computer theory , mathematics and that like, so why not build an amp like the old Quad?
I do still remember it had a wonderful musical sound.
But apparently the special triple cascade of the Quad output
was a particular charcteristic of only this amp and was not utilized in other amps. Although it should provide excellent quiescent current stability 'online' so to speak as the power transistors junction temperature is the input variable of the feedback.
Does anyone know major disadvantages of the Quad concept?

Dieter F.

jmateus 19th December 2007 10:11 PM

Interesting and coincidental.

I had a Quad 303 long time ago and enjoyed it very much as the sound
was very smooth and pleasant. For some reason, I remember, it had
a problem at the time ( I'm talking about 1980's) and I decide do scrap it, I didn't see no reason of keeping it around because of the output
cap technology.
I don't know too much about the 303 topology, what I know is the beautiful
sound it produced. After all these years after founding one of the
boards I scraped in one of my boxes I kind of thought "why don't I revive this amplifier that gave so many hours of pleasure?", this was
my motivation of doing what I did: I made another board from the existent one, copying the layout and constructing it from scratch also following the layout and schematic.
Fortunately I still had kept the original transistors (38495 and 38496) so it wasn't too difficult to reproduce the original. The rest
of the transistors was a matter of some criterious substitution.

Coincidentally, because I just finished the whole amplifier a couple of days ago, I improved the power supply with beefer capacitors
and made the output caps much bigger.

The result? I think I've got to make an even better 303 that it was before,
I can't tell you how much pleasure it gave me after I turned it on and started playing my favorit CD's.

I've never seen too many posts about the 303, I know there are
a lot of mods that can be done on this amp, but for now I'm going to enjoy the sound until I I think it's time to tinker with it.

That's why I thought there was a coincidence of finding a post about this amplifier.

john_ellis 20th December 2007 11:26 AM

I have built Quad-303-alikes with (epi-based ST) 2N3055, BD139/BD140's and although I was not able to compare an original the newer epi-base transistors allow higher frequency response (at least 2x from 30 kHz (I think?) to > 70 kHz) than the original, and with a some mods. it seems possible to reduce the susceptibility to transient distortion (slew rate if you prefer).

Maplin (still) sell TO-39 case transistors 2N3019, 2N4033 which were ST and MOtorola similar to RCA's 40361/40362 which others seem to have used at the time. Don't know how these compare (yet) with the BD139/BD140 but they may replace old Quad transistors. Rated at 80V.


AndrewT 20th December 2007 12:16 PM

was the 303 a quasi complementary output stage?

burbeck 20th December 2007 12:52 PM

Quad 303/33
I like these amps i have owned a couple of them, and repaired a few. mainly capacitors drying up.
features that are good or "odd" about them, depends on your veiwpoint. still not bad for 40 years old.

0.5V input level @100kR (nice with a 10k vol control "passive" pre)

Stabilised power supply

single supply (output capacitor) fit a good one

Inverting (i think ?)

Use MJ15003 for output

Use 2n3773 for regulator

links to info etc


hope this is of interest

hahfran 21st December 2007 05:13 PM

There are a couple of variables to consider why the old 303 produced such a musical sound.
The foremost important imo is the spectrum of distortions.
If this is like the spectrum of harmonics which a musical instrument
produces with acoustical output power as a variable then the amp's distortion were basically inaudible as distortion but where subjectively interpreted as a property of the musical instrument, either pleasant, or unpleasant. That would require crossover distortion must be a minimum.
Insofar the property of the Quad triple cascade , to keep the quiescent current constant , could play an important role and may even be improved when the driver stage transistors are kept at a constant junction temperature by means of a sort of thermostatic device.

I will post some design ideas soon

Dieter F.

john_ellis 21st December 2007 09:05 PM

Hi Andrew T

The Quad 303 was a quasi-complementary with triplets.
The first I made used ZTX304/40362/2N3055 for the upper and ZTX504/40361/2N3055 for the lower. At that time a 2N3055 was a real 2N3055 (1 MHz).

Another version used MPSa06/MPSa56 small signal first stages. I've also made some with BD139/BD140 in first and second stages recently, to check the performance with the newer 2N3055's.

The newer 2N3055 is an epi base and not dissimilar to MJ15003. (2.5 MHz min). Not only does this make an audible improvement it is clearly measurable as having double the bandwidth (when the capacitors are reduced as well).


bigpanda 25th December 2007 01:50 AM

Hi John,

Happy holidays.

You said you made the 303 with BD139/140, what changes do you have to make to the circuit ? I intend to try out with MJ15003/4 and BD139/140 but wondering if any modification have to be made. Would you mind sharing with us?


john_ellis 29th December 2007 10:35 PM

Hi Bigpanda

I don't think that any changes are needed to use the BD139/Bd140 in old 303 circuits, except that the pin-leads are different from the old TO5/TO39 cans, and therefore some track hacking is needed, if you use these in an original board.

Probably 2N5551/2N5401 can be used for the smaller signal stages, but I haven't tried these.

ST still sell (through Maplin) the 2N3019 and 2N4033 which might replace the old devices directly (80V rating, should replace 2N2102/40361 and 2N4036/40362), if anyone needs TO-39 replacements.

I'm working on the updated circuit diagram and hope to post later.


bigpanda 30th December 2007 01:11 AM

Tks John,

I think this is the first time I have more complete info on the 303 alternatives since I visited this forum. PCB is no problem. I had it made for to126 device but I just don't know what to use. The only thing that is missing is the BC109. But like you said, I can try using the 5551/5401. There seems to be no excuse for my 'waiting' now.

Tks again

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