Quad 303 better for bass or for treble?

Quad 303 power amp is great but not ideal for bi-amping.

Dear AllenB,

Their is some thing lovely about the Quad 303. The price and styling are without comparison on UK ebay for power amplifiers. They sound so much better than the Quad 405. I would always recommend replacing all capacitors in a Quad 303 as they are very old (I would recommend doing this in all audio kit over 20 years old and the 303's are older than that).

I have 3 Quad 303's and refurbishing them makes them sound a million times better than they sound without new Capacitors. Sadly they are not good at three things:
  • Bass response is poor when comparing to a Quad 405 and Quad 306 (capacitor coupling).
  • Handling of low impedance loads is not great.
  • Bi-amping/active cross overs.
Quad 303's wont break with hard to drive loads of low impedance as they are intended to drive Quad ESL 57's which have an 2 Ohms impedance but this is in the high frequencies so the powers are low and apart from the JLH class A amp the beat all the limited number of amps that I have heard driving Quad ESL's.

Another problem with the Quad 303 for bi-amping or connecting directly to a computer is that they regulate the Earth rather than the positive rail which means you need to avoid shorting the 303's Earth with your other amps earth (I killed the power regulators two or three times before I realized, I should have looked at the circuit), I would suggest using a single ended to balanced driver in front of each Quad 303 and if using just one and a computer definitely use an optical connected DAC if your using a desktop computer. (laptops don't define the earth so it wont matter so much but is still a good idea)

When I have finished a few more projects, (specifically my Active cross over and rebuilding my Quad 306's) I want to compare my Quad 303 power amps with my Quad 306 (after capacitor replacement to give them a chance) because my tests currently favor my Quad 303 for sound (excepting bass compromises) but Quad 306's are being used due to the annoying earthing issue I mentioned earlier. The Quad 306 is a lot better than the Quad 405 even before replacing the old capacitors in my 306.

In terms of treble sound, I think Quad 303's are very nice, though a JLH 15 watt class A amp that I built (but never had big enough transformer so it started to hum) did sound better and moving from the 1969 to 1996 circuits showed to me that the capacitor coupling in the Quad 303 output is responsible for the poor bass (even with triple size compared to the circuit from Quad) as this was equal to the 1969 amplifier topology (using a coupling capacitor) and when I moved to the 1996 topology (no output coupling capacitor) the Bass in the JLH outperformed the Quad 303.

I hope this does not lead you to strip out the electrics of the Quad 303 and use it as a gain clone as a Quad 303 is a classic amplifier well worth restoring, but a well built JLH class A amplifier will sound nicer but is a lot more work.
 
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Another problem with the Quad 303 for bi-amping or connecting directly to a computer is that they regulate the Earth rather than the positive rail
This doesn't actually mean anything. What is regulated is the voltage difference between the positive rail and earth. I'm sure you had a problem of some kind but it is really caused by the black speaker connection of the 303 being floating.
 
I am going to restore a 303 and use it in a bi-amping situation. Should I replace the output caps with PP types and use it for the treble, or put some big electros in it and use it for bass?

That might work-out. If you choose the right size it can be used as part of the XO.

In today’s world they do not hold up well if stock.

dave
 
  • Bass response is poor when comparing to a Quad 405 and Quad 306 (capacitor coupling).
As long as somebody is reading this 2010 thread: and I suspect Allen B the OP is not:
The poor bass response of the 303 is not because of the output cap. I have a similar dynaco ST120 and the bass is fine with a 3300 uf output cap on each channel. The difference is that the rail voltage regulator in the 303 has no capacitance buffer afterwards. Collector of the pass transistor TR3 appears to be directly coupled to the rail of the power amp. This limits supply of the negative rail to 2 or 3 amps max. The ST120 has a 3300 uf cap between the rails after the regulator transistor. Ability to power a whole cycle of a bass waveform while the line voltage passes through zero once or twice is an important ability in reproducing bass.
There may also be a deficiency in the 303 transformer. The dynaco transformer can source 6.75 amps @ 70 v without stress, which I've tested with resistors. I don't know how capable the 303 transformer is.
As far as installing a polypro output cap, at 2000 uf or higher, it is not physically possible IMHO. 3300 uf 100 v polypro cap would be the size of a kiddie pencil box. You need two of them.
If I was going to hot-rod a 303 I would put a 3300 uf cap between the collector of TR3 and the positive rail. I would make sure driver transistors have at least 30 mhz Ft, and the output transistors have 3 mhz ft. 38494 output transistor may be a 400 khz transistor, if the originals survived this long. RCA made 50 mhz TO5 transistors in 1970, I have two survivor 2n5320/22? in my ST120. The highs are great with 3 mhz output transistors (the originals were burnt up when I bought the ST120) .
 
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AllenB

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2008-10-18 11:31 am
and I suspect Allen B the OP is not
I'm here, but with not much to say. I'd heard about these amps when I got my hands on one by chance and I wanted to find a use for it. It's hard to identify and be confident of its strengths when an amp needs work and may not be performing as well as it should. In any case I got it working and usable but over time I found mine had a characteristic sound that I wasn't liking, and it wasn't really what I was looking for in a subwoofer amp so it got pushed aside.
 
As there is life in an old thread I'd add that upgrading a 303 is quite possible to do and use modern 3MHz 2N3055's to replace the 38494 which were rumours suggest the 2N3055, ft 800MHz. But they struggled to work well over 10kHz (fhfe=10kHz) unless the base drive was improved.
I haven't actually upgraded a 303 but built a 303 circuit and halved the main frequency compensation capacitors, eliminated a large one in the input stage (10nF IIRC) and that went a long way to giving a more respectable 70kHz B/W with better sound (to my ears). Some "sprog stopper" capacitors were added in the drivers as without those the upgrade would oscillate. At the time I built two versions both using BD139/BD140 drivers, one with the same devices as pre-drivers and the other with ZTX304/504 sandwiched between a ZTX108 for thermal stability. Probably 2N5551/5401 could be used instead of 304/504 which appear to have gone obsolete.
Low frequency performance will be impaired by the 2000uF output cap. That needs to be 22mF which you may be able to find modern devices to fit in the space of the old types. A PSU rail cap may be helpful too, not so much for bass but treble as the regulator transistor might not keep up with the faster o/p trannys.
 
The 303 PSU regulates the negative line, but that is grounded.

No it isn't. The ground is grounded. The negative rail floats up and down according to what the regulator is doing.

the main PSU capacitors negative voltage will swing up and down with the load rather than the positive rail which stays fixed.

Correct. That's what the regulator is for.

The difference is that the rail voltage regulator in the 303 has no capacitance buffer afterwards.

Yes it does. There is 2u2F for stability of the regulator. Nothing more is required.

Ability to power a whole cycle of a bass waveform while the line voltage passes through zero once or twice is an important ability in reproducing bass.

The 303 (or any) regulator will never see that, due to all the upstream capacitance. If you think it's a problem, increase the PSU capacitance, taking care not to decrease the transformer conduction angle too much. Or shunt the 2u2F with something larger, but I doubt you'll get any benefit.

The bass response of the 303 is -3dB at 15Hz. This is due to the input RC and the DC servo. You can easily lower the servo's cut-in frequency by changing the 12uF capacitor to 22uF or 47uF, or more if you like. Also change the input C to at least 1uF.
 
"No it isn't. The ground is grounded. The negative rail floats up and down according to what the regulator is doing."

Depends on whether you are talking about the negative rail from the rectifier or the negative rail from the regulator.
The negative rail from the regulator stays more or less at ground. That was what I referred to. Although I would agree it may move slightly ("bob" rather than float?) due to small currents through the 2.2 ohm resistor links to the earlier stages, which connect to the actual ground. So yes it can "float" but nowhere near as much as the negative rail from the rectifier is the one which mostly varies in response to the demand.

Well, you are right about the 2.2uF being on the output of the regulator. That was not my comment, but I doubt it is adequate for an upgraded 303 with a much improved bandwidth. That is what I was suggesting a larger capacitor for. The regulator transistor was a 40411 which was essentially a 2N3772 as far as I know which had an even poorer response (200kHz min) than the old 2N3055's. It was adequate in those days on the original design. Possibly. It was as you say included to stabilise the PSU rather than keep the impedance low at high frequencies. But we are discussing intermediates here, perhaps between 10kHz and 100kHz. Not sure about the ESR of the old 2.2uF capacitors either.

The low frequency response of the 303 is limited by a combination of all the LF time constants. Including the output capacitor. The time constants are 10.6, 6 and 10Hz. So not just the input and "servo" nor just the output capacitor. If the output capacitor is increased, then yes, the other two need to be increased as well. And probably the 300uF PNP decoupling capacitor, which also has a 10Hz response but that is inside the feedback loop. A larger capacitor here will keep LF distortion low.

So if you want a 303 to behave as a 303 did when it was introduced, no changes are needed but as the 2N3055 hometaxial devices are obsolete then you may want to take advantage of the higher frequency transistors available now.
 
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