Sunday, December 11, 2005

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.

Can you revise / upgrade my 405 for me?

  • 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.



Anonymous Anonymous said...

Hello, A site about quad's upgrades just changes D1 to 18v.
Is it better than changing D1 and D2 to 16v?

3:52 AM  
Blogger Stefaan said...

This does not look like a good idea to me. The ideal voltage for an OPA604 (or other) opamp is +/- 15 Volt.

The "virtual mass" of the Opamp's output would also drift.

So better change both zeners to 16V.


9:33 PM  
Anonymous Anonymous said...

Thanks Stefaan for the voltage.

About MJ15031 and 2N3773, which capacitors do i have to replace?
For 2N3773, is it TR9 and TR10?
For MJ15031, is it TR2, TR3 and TR4?


2:24 AM  
Anonymous Anonymous said...

Maybe i can answer to my own question:
TR7 and TR8 are MJE15031
TR9 and TR10 are 2N3773
I didn't look at the tos package ;)
Am i correct for the TR?


2:33 AM  
Blogger Stefaan said...

Yes, that's correct.

Anyway they won't fit in any other way ;-)


4:59 PM  
Anonymous Anonymous said...

Now, i have R33 burning ;((( And it fried TR10,shortening b and c ;(
Which is the guilty component?


9:07 PM  
Blogger Stefaan said...

TR8 probably?

How about coming to Antwerpen one of the coming days with the 405, we'll take a look together?

BTW. R33 burning should not kill TR10 as it is not supposed to give a short-circuit (0 Ohm) when burning.


9:43 PM  
Anonymous Anonymous said...

I have a Quad 405 MK-1 and the right channel doesn't work because the 2 fuses of 4 Amp break when the amplifier is connected to the electric net. I would thank any orientation in order to locate the cause of this mishap. Regards, Luis

5:10 PM  
Blogger Stefaan said...

Hallo, Luis,

Sometimes a short-circuit in the output leads causes TR10 (BDY77) to crash. In this case the fuses blow.

Replacing TR10 with a 2N3773 would solve this matter (the BDY77 is no longer available and the 2N3773 gives better performance.

If this is not the reason all voltages should be checked in the circuit to find the reason.

If you want you can send me the circuit-board (not the whole amp !) and I'll check it out for you.


5:23 PM  
Anonymous Anonymous said...

I was about to contact you to send it to you when i found ;)))

I reversed D5 when i changed it to 1N4007 ;((((
Now all is running well with AD627 ;))))
Now, I will try the ztx753 with ludwig's input stage ;)))


2:11 AM  
Anonymous Anonymous said...

I also made another change:
I changed C8 to 82pf, L2 to 2uH, and replaced R38, R20, R21 with top level quality to stay at L2=(1/(1/R20+1/R21)xR38xC8)


2:15 AM  
Blogger Stefaan said...

Hallo, Seb,

Why did you replace C8/L2?


12:09 PM  
Blogger Stefaan said...

Hallo, Seb,

Some things are quite unclear to me though. I don't know ZBX753, and Google doesn't seem to know it either? Is this a new NPN?

Further improving the Class-A part of the amp might be a good idea, though.

Voltage regulators for the power-supply of IC1 are not really needed I think, the Burr Brown chips have excellent power-supply rejection. What's c20?

C2 has to be unpolarised, in fact. It is part of the DC-feedback (in order to get 0 Volt DC on the output) & removes the AC component of the DC feedback. This DC-component is unpolarized itself.

I don't understand how a wrong C2 could make R33 burn btw?

Kind regards, Stefaan

1:43 PM  
Anonymous Anonymous said...

Bonsoir Stefaan,

J'ai une petite question concernant les équivalences de composants concernant le 405.

Peux-tu me donner les équivalences actuelles concernant les composanst suivants :

- Transistor ZTX504

- Transistor ZTX304

- Transistor BC214C

- Transistor 40872 ( sur ton site MJE15031 mais j'ai vu aussi BD244C ? )

- Diode IS902

- diode LR120C

Autre question :

Connais-tu la raison de la présence de deux modules différents sur le 405 ?

En te remerciant par avance.


9:46 PM  
Blogger Stefaan said...

Bonsoir, Manu,

Cette question est tellement intéressante que je vais y considérer un nouveau article.


10:31 PM  
Anonymous Anonymous said...

Hello, would you please explain why you used 10 K for R4 and 47 mF bipolar for C2 for the general feedback.

I have same R6(100K) and C4(150mF) compare with your modification, but R4(22K) and C2(33mF) are in different value.

Without input signal I got 34 millivolts of DC offset at output.

8:37 AM  
Anonymous tegeje said...

When you experience more than a few (< 5 mv) DC offset at output, then that may be caused by the Signal Ground not being connected to the power Ground. On the board, both grounds are separated by the 10 Ohm Resistor R2.
See Keith Snook pages. He explained it in more details.

9:42 PM  
Anonymous Anonymous said...

Hi, is there some extra benefit to use discrete op amps (like Bursons, for example)in a Quad 405? Have you tried it?

4:21 PM  
Blogger Stefaan said...

I don't think Op Amps built from discrete components are better than the integrated circuits. Think of all the extra solder joints. Also in a intgrated circuit there are components that simply don't exist in the discrete world, like multi emittor transistors.

Joost Plugge

11:20 AM  
Anonymous José D'Amico said...

I tried Burson discrete OpAmps in my QUAD 405. They eat more power (more than 25 mA each), they whistle at power shut off and make some noise at high volume levels. Maybe PowerSupply?
I tried PS with 15V zener regulator 1N4744A (1 Watt) in series with 2.7 kohm 2W resistor, but think that the resistor value is incorrect for the current needed by discrete OpAmp. Do you have any suggestions? Thanks.

1:13 AM  

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