Quad 405 mod

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D

dcd

Member
Joined 2006
Tibi

Hi good to hear from you again, I see you are still paying homage to my schematic symbols :)

I should point ourt that the 2N2222 was simply a quick fix to model the circuit also Tr1 and Tr3 not ZTX753:

Tr2 is actually 2SC2547E single transistor appears to work OK and has excelent noise performance (in practice) with R12 in series with base. Tr1A/B and Tr3 are 2SA1085E.

I am not too sure the current source to replace R30/R31 is..... ? does it give any measurable or audible advantage?

If you are increasing the power of "the amp" you should rename it in accordance with the original QUAD policy, i.e. 405 is an amplifier that delivers 40*5W = 200W total into 8R, 606 = 60*6W = 180W total into 8R, I realise it went a bit off the rails with the 707 onward but them so did QUAD :-{


All

I am still trying to complete my "All variants of QUAD 405 .pdf" if you have any good scans of 405 schematics please email them.

Regards

Keith
 
4

405man

Member
Joined 2003
Many years ago my friend and I built some 405 clones copied from a board that Quad replaced under warranty for a college. Later having read an article on using the amplifier under test as the gain element in an oscilator we used the 405 a large air cored inductor and suitable capacitor to form an oscillator the argument being that the only distortion products would be from the 405 with a carefully adjusted bridge came out at 0.0018%. We also replaced the resisters with a current sink and this made no measurable difference so we decided that it was as well to dissipate the power in the 2 resistors. However it occurs to me that if you used the current sink you could then use a PNP transistor for bottom dumper and this might balance the output stage currents better. This might be worth modelling

Stuart
 
D

dcd

Member
Joined 2006
Stuart

even with a current sink a single PNP for Tr10 would either require a guaranteed hfe at low current or the sink would have to sink ? more than the 45mA that R30/R31 do and so the dissipatrion of both the sink and Tr7 would have to be re-considered.

the sink could be actively driven but that may creat more problems.

Complimentary Darlingtons or MOSFETs with low threshold may also work - if only i had more time and more nerve.

regards

Keith
 
D

dcd

Member
Joined 2006
Blue

I have seen this thread a few times before but just noticed that you have "borrowed" or "modified" my phrase "there are an infinite number of octaves (decades) between zero and 1Hz" next you’ll be saying "more is less" :)

Are you continuing to add to these discussions or have the "blokes" given you a hard time for your feminine views!! - You should try to catch "Little Britain" TV and radio programme from the BBC it often features "A lady"

Regards

Keith

I often look up but haven’t seen you there :-(
 
B

bulgin

Member
Joined 2006
blueangelaudio.com
Quad 405 Mods

Hi Guys

I recently posted elsewhere re my 405 Mk 1.

I started a week or two ago by fitting new bindingposts which were too short to pass through the small pcb behind them. These I modified by extending them and then my troubles started.

One of these new posts developed a dry joint which I took awhile to ID as faulty. In my efforts to discover the problem, one of my multimeters also started playing the fool and the two + speaker wires from the loom indicated continuity between them and anywhere on the chassis.

Anyway, all these problems were eventually sorted and all works: The posts are in, the din socket has been extended to two rca's and I fitted also a small toggle on/off in the second last blank hole on the right.

I have not yet refitted the pcb behind the posts. What are your wise opinions re this little board. Should I leave it out or put it back?

Next week, I'm going to replace the two electrolytics. I found a source for 10'000mfd 100vdc to replace the old items. Do you think this is OK or should I wait and get replacements from Quad UK?

There are other brown caps on the boards which also look a bit the worse for wear. What would you recommend here?

I also committed a bit of sacrilege - I repainted the top cover and side alu 'cheeks' with an auto paint finish (basecoat/clearcoat) and the colour I used was from a previous Renault - gris tungstene or 'gunmetal' metallic. The job turned out looking very nice.

Cheers

bulgin
 
C

Chris Daly

Member
Joined 2004
Has anyone noticed later Quad 405.2 boards incorporated a repositioning of C8 to collector of TR2 rather than above the R17 and C7, and that R6 and C4 swapped position with each other.

The DC Daylight mod is well worthwhile , I have done the input rearrangement to a Series 1 and 2 405 with excellent result. I am currently waiting on some OPA627 op amps which are SOIC types, DIPs are unfortunately prohibitively expensive.

There has been interesting discussion on the OPA627 inviting a current source namely a simple resistor from the negative supply rail to Pin 6 the output of the OPA627 the value of resistance is apparently 1.5k providing op amp supply is 15v neg. I am unsure of how DC conditions at base of TR2 in a 405 would cope with this or if such a mod is a good idea, it is thought the upper transistors in the OPA627 then conduct in Class A but I have yet to study the OPA 627 schematic. Has anyone had experience with this ? or is it just a bad idea?

Cheers / Chris
 
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