Hi all,
Just my comments on the 303: probably the amp I've spent the most in-depth experimentation with.
The 303 was a good amp in its day. The 33 preamp was mediocre. However, time changes and the 33 is now regarded as plain junk, and the 303 has been surpassed in quality by many newer designs of much lower cost and complexity.
I got a 303/33 at a garage sale in terrible condition, so I decided to restore it as a pet project. The process took all of about 100hours (with all the tinkering and changes took into account) over a period of about 8 months. There isn't pretty much anything I haven't changed, but most of my changes have been my own idea and not from NetAudio. Some of the changes made no difference, other subtle but by far the biggest change came from swopping the output transistors to MJ15003s - big improvement with much lower noise. This amp is now quiet enough for headphone use! The changes I made are as follows:
- output transistors to MJ15003
- PSU emitter-follower to MJ15003
- All small-signal transistors (except BC154) to MPSA06/56: they CAN be used for all of them and are easy to obtain.
- drivers to BD139/140 with 100pF Miller cap on B-C pins of BD140 for stability
- Added active current source instead of the original resistive one
- Changed PSU caps with 2x 4700uF
- Changed output caps to 3300uF
- Changed all other electrolytics
- Bypassed all electrolytics with 330nF polypropylene
- Changed the rectifiers to soft-recovery types with 22nF bypass caps
- EMI filter on the mains
- Changed all resistors to 1% metal film
- replaced all trimpots with multiturn types
- reset all settings
Furthermore I found that the 12V Zener diode and 1W 10k resistors in the (bad) regulated supply was grossly underrated - it seems like Quad likes to disregard thermal issues as they've inadequately sinked an LM7812 regulator in a tuner, causing it to fail every few years. I've replaced the Zener and the resistors with 5W types and they still get hot! PSU noise is lower now.
I have to agree with the comments on converting the output to fully complementary: I've tried it on one channel and couldn't hear any difference at all, so I changed it back. Rather just change the transistors to better ones (not faster, they may oscillate).
All in all the sound is very nice and warm and easy to listen to. Compared to a very entry-level DIY amp, Rod Elliott's P3A, it has much less clarity, detail and accuracy, but the P3A may be described as clinical in comparison. It does handle the bass quite OK (but the 15003s made a big difference there) and have little problem driving the ProAc 2.5 clones I use for lab experimentation (nice if you have University funding for audio research
) Perhaps the 303 struck some magic synergy with the ESL's, but for other speakers one can definitely do better, even on a modest budget.
These are my findings with all the mods I made. I would definitely not use the 303 as a tweeter amp, especially not for a 9900. Something like a Mini-Aleph or the Krell KSA50 clone (that's been revived on diyAudio) should fare much better. In a blind test compared with other amps I've asked some friends what they'd value the Device Under Test (the 303) and no-one went above $200.
Regarding using the 33 as preamp for the 303: It sounds worse even than a Rotel RA820AX integrated I use in my lab. I've began to change the electros but I get too much the idea of saving a 20year old dog thats been run over by a truck: perhaps possible but just not worth it.
I've found that many people stick to their 303s (and 33s argh) mainly because of sentimental value: many of them were still students or young people at the time it was on the market and they couldn't afford it.
Any other comments would be interesting.
Pierre
Just my comments on the 303: probably the amp I've spent the most in-depth experimentation with.
The 303 was a good amp in its day. The 33 preamp was mediocre. However, time changes and the 33 is now regarded as plain junk, and the 303 has been surpassed in quality by many newer designs of much lower cost and complexity.
I got a 303/33 at a garage sale in terrible condition, so I decided to restore it as a pet project. The process took all of about 100hours (with all the tinkering and changes took into account) over a period of about 8 months. There isn't pretty much anything I haven't changed, but most of my changes have been my own idea and not from NetAudio. Some of the changes made no difference, other subtle but by far the biggest change came from swopping the output transistors to MJ15003s - big improvement with much lower noise. This amp is now quiet enough for headphone use! The changes I made are as follows:
- output transistors to MJ15003
- PSU emitter-follower to MJ15003
- All small-signal transistors (except BC154) to MPSA06/56: they CAN be used for all of them and are easy to obtain.
- drivers to BD139/140 with 100pF Miller cap on B-C pins of BD140 for stability
- Added active current source instead of the original resistive one
- Changed PSU caps with 2x 4700uF
- Changed output caps to 3300uF
- Changed all other electrolytics
- Bypassed all electrolytics with 330nF polypropylene
- Changed the rectifiers to soft-recovery types with 22nF bypass caps
- EMI filter on the mains
- Changed all resistors to 1% metal film
- replaced all trimpots with multiturn types
- reset all settings
Furthermore I found that the 12V Zener diode and 1W 10k resistors in the (bad) regulated supply was grossly underrated - it seems like Quad likes to disregard thermal issues as they've inadequately sinked an LM7812 regulator in a tuner, causing it to fail every few years. I've replaced the Zener and the resistors with 5W types and they still get hot! PSU noise is lower now.
I have to agree with the comments on converting the output to fully complementary: I've tried it on one channel and couldn't hear any difference at all, so I changed it back. Rather just change the transistors to better ones (not faster, they may oscillate).
All in all the sound is very nice and warm and easy to listen to. Compared to a very entry-level DIY amp, Rod Elliott's P3A, it has much less clarity, detail and accuracy, but the P3A may be described as clinical in comparison. It does handle the bass quite OK (but the 15003s made a big difference there) and have little problem driving the ProAc 2.5 clones I use for lab experimentation (nice if you have University funding for audio research
) Perhaps the 303 struck some magic synergy with the ESL's, but for other speakers one can definitely do better, even on a modest budget. These are my findings with all the mods I made. I would definitely not use the 303 as a tweeter amp, especially not for a 9900. Something like a Mini-Aleph or the Krell KSA50 clone (that's been revived on diyAudio) should fare much better. In a blind test compared with other amps I've asked some friends what they'd value the Device Under Test (the 303) and no-one went above $200.
Regarding using the 33 as preamp for the 303: It sounds worse even than a Rotel RA820AX integrated I use in my lab. I've began to change the electros but I get too much the idea of saving a 20year old dog thats been run over by a truck: perhaps possible but just not worth it.
I've found that many people stick to their 303s (and 33s argh) mainly because of sentimental value: many of them were still students or young people at the time it was on the market and they couldn't afford it.
Any other comments would be interesting.
Pierre
Dear Blue and others interested in the Quad 303
The Quad 303 into ESL57's is without doubt a great
combination. I notice some contributors have experimented
with different transistor types which is interesting. I think
there comes a time where available replacements have to be seriously considered.
One of my 303's also uses BD139 and BD140 and MJ15003
as the PSU driver. It sounds somewhat identical to other 303's
which is very reassuring.
However Ft of the BD139 and for that matter BD140 is
very high. I have in the past used a BD139 as a VHF
Class C amp ! , so precautions of parasitics reentering
the earlier current source stages as I mentioned previously.
A note: The pinout of a BD139 and BD140 is quite different to
Quads original transistor.
Lets put this into perspective, its an amp designed in 1968
the Quad 303 will never compete with monster amps or for
that matter better layout topologys. Its singleton rather than differential input is outdated. Its use of a capacitor on its
output is outdated, but as I have suggested still quite
relevant to driving electrostatics.
Its sound into ESL57's is in my opinion bettter than most amps
Listening to its capabilities, its got extremely low distortion,
it reveals harmonic information that few amps will reproduce,
it has ample power to drive the ESL57, better still if used as
monoblocks. It has an open easy sound that lets the music
speak into ESL57's. Its available on the second hand market
at fairly low cost. Its robust and fairly easy to repair- if you
happen to buy an original unit with 1971 capacitors in it.
Blue I note your comments about the graph not being an
accurate interpretation and allowance for temperature sound pressure and some means of equally assessing the sound. to be made, I regret having recommended an inaccurate graph Unfortunately some means of equal measurement available
to us all, is unlikely to occur.
However properly adjusted with new capacitors the Quad 303 and given a room where the ESL is finely adjusted for its
best reproduction, the Quad 303 remains a valid means of driving and enjoying the enormous capabilities of the ESL57. No doubt due to Peter Walker taking his time to design this amp to properly drive the electrostatic speaker. Its a great combination. To my ears better than the 405 amp which does convey more detail,
but loses out in spaciousness and ease of listening.
Cheers Chris
The Quad 303 into ESL57's is without doubt a great
combination. I notice some contributors have experimented
with different transistor types which is interesting. I think
there comes a time where available replacements have to be seriously considered.
One of my 303's also uses BD139 and BD140 and MJ15003
as the PSU driver. It sounds somewhat identical to other 303's
which is very reassuring.
However Ft of the BD139 and for that matter BD140 is
very high. I have in the past used a BD139 as a VHF
Class C amp ! , so precautions of parasitics reentering
the earlier current source stages as I mentioned previously.
A note: The pinout of a BD139 and BD140 is quite different to
Quads original transistor.
Lets put this into perspective, its an amp designed in 1968
the Quad 303 will never compete with monster amps or for
that matter better layout topologys. Its singleton rather than differential input is outdated. Its use of a capacitor on its
output is outdated, but as I have suggested still quite
relevant to driving electrostatics.
Its sound into ESL57's is in my opinion bettter than most amps
Listening to its capabilities, its got extremely low distortion,
it reveals harmonic information that few amps will reproduce,
it has ample power to drive the ESL57, better still if used as
monoblocks. It has an open easy sound that lets the music
speak into ESL57's. Its available on the second hand market
at fairly low cost. Its robust and fairly easy to repair- if you
happen to buy an original unit with 1971 capacitors in it.
Blue I note your comments about the graph not being an
accurate interpretation and allowance for temperature sound pressure and some means of equally assessing the sound. to be made, I regret having recommended an inaccurate graph Unfortunately some means of equal measurement available
to us all, is unlikely to occur.
However properly adjusted with new capacitors the Quad 303 and given a room where the ESL is finely adjusted for its
best reproduction, the Quad 303 remains a valid means of driving and enjoying the enormous capabilities of the ESL57. No doubt due to Peter Walker taking his time to design this amp to properly drive the electrostatic speaker. Its a great combination. To my ears better than the 405 amp which does convey more detail,
but loses out in spaciousness and ease of listening.
Cheers Chris
with all this discussion about 303's and ESL57
I should have also said the pre amp I use is an
Audio Synthesis Passion
Regarding the Quad 33 preamp it has a very good tape
output section in it and is ideal if you own an FM3
tuner for buffering that. The FM3 really excels when
driven by the 33. The 33's tape output is very good
for driving sources like FM4 or FM3's which seem to
lack any ability to properly work on their own.
33's may also be good with tape decks when
driven by the tape output. Re- use and recycle
being the motivation and wording.
Cheers / Chris
I should have also said the pre amp I use is an
Audio Synthesis Passion
Regarding the Quad 33 preamp it has a very good tape
output section in it and is ideal if you own an FM3
tuner for buffering that. The FM3 really excels when
driven by the 33. The 33's tape output is very good
for driving sources like FM4 or FM3's which seem to
lack any ability to properly work on their own.
33's may also be good with tape decks when
driven by the tape output. Re- use and recycle
being the motivation and wording.
Cheers / Chris
i recall when there was no other amplifier in the same pricerange as the 303 able to drive the ESL63's without getting into trouble.
And even much higher priced models that could not handle the Quad's combination of low impedance and very high phase shift..
Quite a remarkable achievement of Mr Walker and his mates.
Great sounding amplifier ? No Way !
Can it compete with modern complementary designs, no !
Has the 303 been built by diy folks ? A great many !
Why bother, leave it original, and find a nice set of Quad II's.
Anyone ever see the remarkable tonal control of the 33 again ?
And even much higher priced models that could not handle the Quad's combination of low impedance and very high phase shift..
Quite a remarkable achievement of Mr Walker and his mates.
Great sounding amplifier ? No Way !
Can it compete with modern complementary designs, no !
Has the 303 been built by diy folks ? A great many !
Why bother, leave it original, and find a nice set of Quad II's.
Anyone ever see the remarkable tonal control of the 33 again ?
It seems the 303 has/had quite a religious following with ESL57/63s! Not having heard them I cannot make any comments in that regard. I have noted its excellent ability to handle bass and difficult loads (the ProAc 2.5 is fairly insensitive and needs some juice to open up).
As a note to Chris: I first used the BD139/BD140 drivers with the miller cap on the BD140, and then pondered if it perhaps may survive without it..... NO! It oscillated severely and almost blew the output stage. In fact, no other device I tried with the necessary ratings was slow enough to work without compensation. The only way would have been to use something way overkill like 2N3055/2955 which is inpractical. Moral of the story? If you're going to change the drivers, add the miller cap! Some people like Krell like to add it on both the NPN and PNP but in most cases (like the 303) the PNP alone is sufficient. All the other semiconductor changes are plug-in upgrades, provided they are of the specified models. It would be tempting to use MJ21194/21193, but they are much faster and could go unstable without compensation. The 15003/4 is a superb transistor for audio with very good SOA so no need to change it anyway. Its improvement over the original output cans also allows higher biasing. The heatsink can take a bit more idle heat so its not a problem, but I've added extrusions to mine and increased the bias 5 times. The only question is whether it sounds better, but I think so I can recall Douglas Self had some sound reasons why a Class AB amp shouldn't be biased too high into Class A...
Regarding the 33 preamp, I've tried the tape loops but it still lacks in the top end plus the gain is too low. A simple two-stage opamp based preamp gives much better performance with the 303 on CD source.
Jacco: I totally agree with your comments, except as to the "leave it original" part. The very least that should be done is to replace the caps and reset the
- 67V PSU output
- bias voltage
- bias current
to get it in the same state as it was new. Otherwise performance may be degraded and damage to the amp/speakers inflicted. Most of the other upgrades are not strictly necessary, but the output transistors made a huge improvement (without changing the character as Chris observed). Its not very expensive and is truly worth it.
In fact, I've recently replaced two older amps' (Lindsey-hood 20W class-A and an oldish Musical Fidelity) 2N3055 output stages with MJ15003s and the improvement was astronomical. Quite a few Lindsey-Hood builders have remarked the same.
Pierre
As a note to Chris: I first used the BD139/BD140 drivers with the miller cap on the BD140, and then pondered if it perhaps may survive without it..... NO! It oscillated severely and almost blew the output stage. In fact, no other device I tried with the necessary ratings was slow enough to work without compensation. The only way would have been to use something way overkill like 2N3055/2955 which is inpractical. Moral of the story? If you're going to change the drivers, add the miller cap! Some people like Krell like to add it on both the NPN and PNP but in most cases (like the 303) the PNP alone is sufficient. All the other semiconductor changes are plug-in upgrades, provided they are of the specified models. It would be tempting to use MJ21194/21193, but they are much faster and could go unstable without compensation. The 15003/4 is a superb transistor for audio with very good SOA so no need to change it anyway. Its improvement over the original output cans also allows higher biasing. The heatsink can take a bit more idle heat so its not a problem, but I've added extrusions to mine and increased the bias 5 times. The only question is whether it sounds better, but I think so
I can recall Douglas Self had some sound reasons why a Class AB amp shouldn't be biased too high into Class A... Regarding the 33 preamp, I've tried the tape loops but it still lacks in the top end plus the gain is too low. A simple two-stage opamp based preamp gives much better performance with the 303 on CD source.
Jacco: I totally agree with your comments, except as to the "leave it original" part. The very least that should be done is to replace the caps and reset the
- 67V PSU output
- bias voltage
- bias current
to get it in the same state as it was new. Otherwise performance may be degraded and damage to the amp/speakers inflicted. Most of the other upgrades are not strictly necessary, but the output transistors made a huge improvement (without changing the character as Chris observed). Its not very expensive and is truly worth it.
In fact, I've recently replaced two older amps' (Lindsey-hood 20W class-A and an oldish Musical Fidelity) 2N3055 output stages with MJ15003s and the improvement was astronomical. Quite a few Lindsey-Hood builders have remarked the same.
Pierre
jacco vermeulen said:
With different voltage, different bias, different output devices, oversized powersupply, added foil caps, dual mono.
The 303 was very stable, imo, due to the kind of components used up to the output stage.
As i am doing with the Leach, and will do with the Super Leach, but not fondle around too much with the original circuit.
PWatts said:
I can vouch for that ... i've owned many 303s, and much as i love them, can't listen to them for long. The last set i converted to monobloks by removing a channel, and then sold them off to a fellow who got them totally revamped (still haven't talked the guy into telling the orld what he did)... he considers them now one of the best amps he has ever had (and he has has some decent ones)
An externally hosted image should be here but it was not working when we last tested it.
http://www.t-linespeakers.org/FALL/Q303.html
dave
PWatts said:
Pierre,
With the more powerfull output transistors you should look at the current protection. That is in the stock situation a very conservative 4 amps peak, and will give a nasty sound when driving low impedance speakers at loud levels.
The current protection is not output-voltage dependent (they got that smart only later with the 405), so if you know what you are doing here can greatly upgrade the drive capability of the modded 303.
Comparing the protection circuitry to the 405 would make it clear: the 303 only samples the voltage across the output emitter resistors, while the 405 also samples the supply level (which will in turn give the Vce as the protection circuit is connected to the output of course).
Jan Didden.
303 monoblocs
Sorry to revive such a long dead thread but don't suppose you remember what you did do you? Was it just a matter of removing one output board & it's caps?
Graham
Sorry to revive such a long dead thread but don't suppose you remember what you did do you? Was it just a matter of removing one output board & it's caps?
Graham
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