Quad Revision FAQ's: Part I: The Quad 405
Why would I think about upgrading or revising my 405?
There are several reasons:
- Most electrolytic capacitors tend to dry out after 15 years. Their value changes and in the worst case they give a short-circuit. This may cause a "rumble", a "hiss", more distortion and even a complete disfunctioning of the amp. Modern capacitors are smaller and perform a lot better. High-end capacitors have a life-time above 150.000 hours and less internal resistance.
- The Opamp technology in the 7-ties and the 8-ties was not at all as advanced as it is today. The LM301 that was used in most 405's can not match the performance of modern Opamps like the Burr Brown OPA604 (see the links on the left). CD's did not exist at the time the 405 was designed and CD's give a lot more dynamics in the sound dynamics vintage Opamps can't match.
- "Line-voltage" in the 7-ties was considered 500 mV. Today most CD-players or pre-amps produce +/- 1,5 Volt. Connecting a modern input-source to vintage Quads will cause distortion and an uncomfortable behavior of the Volume attenuator on the preamp. Reducing the input-sensitivity not only makes the 405 better adapted to modern input-sources but also reduces distortion and noise dramatically (<>
Instead of revising my 405, why don't I sell it on eBay and buy a new amp?
That's an option, indeed, if you have a budget of above 4.000 euros to match the same transparency and dynamics.
What useful modifications can be done to my 405?
- The capacitors have to be replaced by new ones. After more than 15 years capacitors dry out and change their specifications. Modern capacitors, by the way, are not only a lot smaller but also a lot better. For some caps it may be usefull to have "audiophile" caps inserted like Wima, Wonder Caps or MusiCaps if you have the perfect ear and a lot of money ;-) Good quality capacitors of a usual (and cheaper) brand will do as well. For 99% that is.
- The Opamp (usually a LM301 or a TL071) has to be replaced by a modern high-end audio Opamp. At the time the 405 was produced Opamp audio technology was new. Those veteran Opamps are absolutely no match for modern high-ends like the Burr Browns. CD's were not invented yet when the 405 was designed. CD's have much higher dynamics than LP's, dynamics those veteran Opamps can't match.
- Reducing the current-limiting may be an option. Quad designed the 405 for professional- and home-use at the same time. And for the ESL speakers. So they limited the current to 3 ... 4 Amps to stay on the (very) safe side.
- The output- and driver-transistors could be replaced by faster types available today like the Motorola MJE15031 for the drivers and the 2N3773 for the output-transistors.
- The internal wiring should be replaced. The cable-tree was good for reducing production-cost but for nothing else anyway. There is no use in buying expensive external cables when on the inside there is miserable bell-wire which is much too long.
For the rest the design of the current-dumping circuit is not critical about component tolerance. There is no use - for most components - to spend money on 1% tolerances when even 20% will do fine.
Will the 405 still have the typical Quad-sound after the upgrade?
Sure it will.
Even more than ever before.
The "behind the curtain" sound, due to the bad performance of worn caps will completely disappear and the transparency of the sound which is so typical for Quad will be outstanding. Modern Opamps will dramatically reduce distortion and improve dynamics.
But the current-dumping schematic remains the same of course. There is no amp below 4.000 Euro that can even match an upgraded 405 in a "blind test".
Many people who upgraded their 405 have thrown away half of their CD-collection because the OPA604's outperform the Opamps in most recording-studios.
You get used to the quality you're used to listening to music very quickly ;-) I don't ever go to pubs anymore. Because I can't stand what I hear there, especially when they put the music "loud".
What are the technical considerations when the Opamp is replaced?
- Most modern High-end Opamps like the OPA604 have a working voltage of + and - 15 Volt instead of + and - 12 Volt. The power-supply for the Opamp should be increased from 12 to 15 Volt by changing the Zener-diodes D1 and D2 with 16 Volt zeners. Otherwise there will be clipping at higher input-voltages and the undistorted power will be limited to about 70 Watt per channel.
- Modern Opamps eat more power. This may cause unwanted switch-off behavior ("Plops" in the speaker) when switching off the amp. See the other posts on my blog for solving this matter.
- In order to stabilize a "modern" Opamp one should add 2x 100 nF capacitors between pin 7/4 of the Opamp and the input-stage mass to avoid the input-stage oscillating and C3 has to be removed.
- For the local feedback I have the best experience with Bernd Ludwig's proposition (see the links on the left): replace R6 with 100K and C4 with 150 nF.
- For the general feedback I use 10 K for R4 and 47 mF bipolar for C2.
- This will get you an input-sensitivity of +/- 1 Volt, which is ideal for modern pre-amps and sufficient for the Quad 33 as well.
- Yes I can.No matter how old or how broken your 405 is, it can always be repaired and made sound better than a new 405-2. Just send an e-mail.
Next post about upgrading / revising the 33 soon.