Quad quandary?

[meBob]

Hello All,

I have a Quad 405 mk2 and I am now considering changing the sensitivity to 1.5v from (0.5v) and also upgrading the caps to 15000uf while I'm in there. I would be doing the upgrade myself.

Would the benefits of this be clearly audible? How close to the 606 would the upgrade get?

I could buy a 606, but I would probably be also replacing the caps and other bits 'n' bobs in it and so there would also be extra expense in buying one of those. The 405 mk2 has ( 4 months ago) been back to Quad for a full service and been given a clean bill of health (and sounds good).

One of the main drivers of this upgrade is that my Kenwood pre with a sensitivity of 1.5v is not getting on too well with the 405, all the action is condensed before the 9 o'clock position. I'm also doing a full upgrade on the Kenwood and so would like to have a usable amp/pre combo.

Thanks,
Rob

[Light Dependant Resistor]

Hi Rob
Sensitivity of the 405 is I think most neatly adjusted downward by lowering the TL071 opamp feedback resistor R6 value
The relationship of feedback R6 to input resistance R3 is creates a gain of x 15 of the input signal then seen at the
op amp output . If we say we need the gain three times less 0.5 to 1.5v this suggests we need a gain of x5 instead of
15 , accordingly R6 needs to be 110,000 ohms.

Increasing capacitance to 15,000uf vs 10,000uf creates added stress on the standard bridge rectifier Quad install
However you could use a higher amperage rectifier such as a 35A Block type. Rewiring for a diode bridge per channel
has some benefit too. This site has many pages of such modifications: http://www.keith-snook.info/quad-405-mods.html

The replacement capacitor size although higher in uf is likely to be similar or even smaller, as capacitor physical size has
changed dramatically over the years since the 405 was made. The replacement capacitor voltage needs to be carefully
considered as well, I would suggest 80v vs the 63v Quad provided.

Barry or Alan, and other forum members may also have opinions on these mods.

Cheers / Chris

[graham67]

hi Rob,

I own late Quad 405-2, a Quad serviced 405-2 and a full monty Net Audio 405-3.

One of the nicest combos is using the late 405-2 12565 Issue 7 boards in conjunction with the Dual Mono PSU which uses two 10,000uf caps per channel, instead of one.

If your boards are in good shape, I definitely agree the PSU is where you will get maximum benefit if implemented correctly.

Regards,

Graham.

[nutteronthebus]

As always have a look at R7 & R8 3k3 resistors on the board as they are rated at .25W and draw .37W ( replace with 1w type )

[meBob]

Light Dependent Resistor:
I was under the assumption that the 405-2 could handle 2 15000uf with no other updates needed, I think I read that on the Dada forum, I'm not sure though if that was the official opinion of Dada.

I've noticed when jumping up in volts the capacitors tend to become taller, I'll check the availale height in the chassis.

Having said that, before I do anything I'll have done my reasearch fully and be ready.

Thanks for your reply.


Graham67:
I hadn't considered the Dual/Mono psu, but it's just become a possibility.

I have a 405-2, SERIAL:67224 and BOARDS: M12565-6

If I understand correctly you have a standard 405-2 (no mods), a standard late 405-2 with a Dual/Mono psu and a fully modified net 405-3 (standard transformer/psu?).

What kind of a sonic difference is there between your standard 405-2 (no mods) and the late 405-2 with a Dual/Mono psu?

Thanks for your reply.


Nutteronthebus:
Thanks for that, while I've taken it apart that's a definite upgrade I'll do, I wouldn't be happy at all if the 405 failed at a later date with something that could have easily and cheaply been avoided in the first place.

[graham67]

Graham67:
I hadn't considered the Dual/Mono psu, but it's just become a possibility.

I have a 405-2, SERIAL:67224 and BOARDS: M12565-6

If I understand correctly you have a standard 405-2 (no mods), a standard late 405-2 with a Dual/Mono psu and a fully modified net 405-3 (standard transformer/psu?).

What kind of a sonic difference is there between your standard 405-2 (no mods) and the late 405-2 with a Dual/Mono psu?

Thanks for your reply.
[/QUOTE]

hi bob,

I have two quads std 405-2 and full 405-3 however I did swap the boards around to see how they sounded :-)


the 405-3 is a slightly cleaner sound, not as warm as the 405-2. it is still unquestionably Quad just not so vintage. However the dual mono PSU with the stad boards retains the warmth but adds better bass and a more open sound. IMO, the DMPSU offered the biggest bang for the smallest outlay.

I based my opinions playing into Tannoy HPDs and Rogers LS6 speakers.

[Barry]

A rule of thumb when it comes to power amplifier resevoir capacitors is to use 1,000uF for every 10W of output power. The Quad 405 (all versions) is ostensibly a 100W amplifier into 8Ohm, so uses a 10,000uF capacitor on each 50V supply rail. Assuming the effective resistive load is 8Ohm, then the time constant CR is 80ms, which means the supply capacitor can easily support a signal of greater than 25Hz. Increasing the time constant by increasig the value of the resevoir capacitor will allow the power supply to support a lower frequency signal; hence the desire to change the capacitor value from 10,000uF to 15,000uF.

The bridge rectifier used is a 12.5A device, the PM4A2Q:

https://www.datasheets360.com/pdf/-8222690884954299516 ,

which has a maximum surge current rating of 240A. The reactance of a 10,000uF capacitor is 0.16Ohm at 100Hz, so the instantaneous current drawn from the 50V supply rail is 314A. This is, I assume, within the surge rating of the bridge rectifier, since the ESR of the capacitor also needs to be taken into account. However replacing the capacitor with a larger 15,000uF value (and hence an instantaneous current draw of 470A; subject to the caveat above) could exceed the rating of the rectifier. Therefore I would concur with Chris, that ideally the rectifier ought to be change for one with a higher current rating.

I have a pair of 405s which have been 'monoblocked', that is only have one amplifier per case with separate power supplies. The resovoir capacitors have been changed to F&F 22,000uF devices and a 35A rated bridge rectifier is used for each power rail. The amplifier boards have been upgraded to Keith Snook level 3. The input has been modified to be truely balanced with a sensitivity reduced to 1.0V for full output; achieved by reducing R6 and increasing C4 pro rata.

Also agree with the advice given by Dave: R6 and R7 are under-rated. Upgrade them to 1W devices, and distance them from the PCB (which you will have to do on the M12565-6 board). I had an early 405 where one of these under-rated resistors did indeed fail, sending the 405 into a paroxysm of oscillation!

[meBob]

graham67:

Thanks for 405-3 info on Wam, the 405 I've bought was because of its warm/fatigue free reputation, so I probably wouldn't get on with the Net Audio kit. I'm enjoying the original sound of the 405-2, but any improvements to the bass or soundstage would be welcome.

However, the PSU mod is looking like a worthwhile benefit and I'll be doing some of the other board mods.

[meBob]

LDR & Barry:

I am grateful there are wiser folks than myself on this site to give me the odd nudge in the right direction every once in a while.

I'll be ordering some bits from mouser shortly and the component below will be in the order:

35A
https://www.mouser.co.uk/ProductDeta...%2fgxEV9jMc%3d

Barry, thank you for your concise and clear explanation.

"paroxysm of oscillation!" - That sounds like an absolutely wonderful condition to be in!

Should I stick with 15000uf or what would be the limit a stock 405-2 would handle with the above 35A bridge installed?

[Barry]

You ought to be able to increase the size of the reservoir capacitors to 22,000uF if you use the 35A rated bridge rectifier shown. However whether it will improve the SQ over that obtained with a 15,000uF is debatable.

Choose capacitors with a 63V rating and an operating temperature as high as possible.