Rebuilt Quad405 Problems

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Thomo

Thomo

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Joined 2006
#1
#2
I'm also Quad405 happy owner🙂

With voltmeter test dc supply, to be shure that you have + and - rail. Maybe you don't have one rail so thats why you have same volt on spk out for both amps.
If thats ok then check the board connections. If you made your new boards maybe mistake is on them, some off connections, or some shot connections....
Allo check the supply for IC, check the zener diodes, maybe one is soldered backward. Can happen, once i did that 🙂

You must put some DC protection, for speakers sake! 😉
 
#3
Hi. Thanks for your help.

I checked the supplies to IC and there was a break in the positive supply track - on both channels!

I don't really know what the person who owned this previously was doing - but £45 for a Quad 405 in any condition, I consider that a bargain!

Anyway I am now reading only a few millivolts of dc offset now, I've installed velleman speaker protection and am about to connect some old speakers.

What mods have you performed on your 405? or is it just stock?

Kind regards, Lee.
 
#4
Ah, your welcome😉


Well, i don't know which is my version, because friend gave me schematic so i made it by that, some 10 years ago. My input IC is TL071, also there are 3 small coils which are not present in original schematic.
My quad405 is somewhere between original and last modified version 🙂
I can say that i am very satisfied with quad405. Testing with some regular hi-fi comercial amplifiers it's like heaven and earth-can not be compared 😀

How much millivolts is dc offset? Small dc is not dangerous but some 100mv is much, by my standards 🙂
Check the volts after zener diodes, that is Ic-s pin 4 for "-" and pin 7 for "+", they must be equal. If one reds 15,3 and other 16,2 (or something like that) i would change zener diodes and check resistors before them, use 1% 2w resistor there. I'm not shure if zeners are 15 volts on every schematic, on my they are.
 
#5
Hi. Just letting you know that all is well. The amp is making very sweet music, and with a few hours burn in it should improve further (if you believe in such things).

The dc offset is very low only around 6 millivolt.

Just connected to my Tannoy LGM's and I am very impressed. Much better than the 405 I modded for a friend, and also much better than my Gainclone. I will go dual mono with the power supply as soon as I can afford the caps.

Thanks for your help, kind regards, Lee.
 
#7
Quad 405 Reborn

Hi , I start two weaks ago to build again the new version . 🙂 Schematic
An externally hosted image should be here but it was not working when we last tested it.
PCB
An externally hosted image should be here but it was not working when we last tested it.
and final picture
An externally hosted image should be here but it was not working when we last tested it.
other picture
An externally hosted image should be here but it was not working when we last tested it.
😉 all the best Alex mm 😎
 
#9
Hi Alex,
Looks very nice, but I have a concern that the PCB layout doesn't match the schematic.
The problem I noticed is then the schematic shows the boot strap capacitor is a 47uF in parallel
with a 3.3uF. But the PCB connects one of them to the negative rail. Is the PCB layout the
version built?

If this is the version built in the picture, it appears that the 47uF is connected backwards across
the lower 560R, so it won't live very long.


- Symon
 
#10
QUAD 405 REBORN PCB corection

Hi , Symon u right I have been corected PCB error , and I hope now its ok
4p481o0.jpg

All the best Alex mm 🙂
 
#12
Hi,

Question for anyone knowing the history of the Quad 405. When first introduced it boasted innovative circuit design. I believe they called it current dumping.

The schematic posted here, however, is an old fashioned quasi complementary design. What happened to all the innovation? When did Quad abandon it?

Mark
 
#13
Quad used NPN output devices for robustness (and cost I guess). The 'Current Dumping' scheme means that the lack of linearity of the upper and lower totem-pole transistors isn't a problem, so you can use a quasi-comp output without any performance issues.
 
#14
The Quad 405 is an error canceling amplifier and uses a reactive bridge to combine the output of a class A stage with an unbiased output stage. If you look at the last page here

http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/Schematics/QUAD-405-Evolution.pdf

It is easier to see the configuration than in the original Quad schematic

This is a link to an original article from Quad

http://quad405.com/currentdumping.pdf

and this one to further discussion about this inovative design

http://quad405.com/jaes.pdf

Stuart
 
#15
Alex mm

You appear to have copied one of the features of the 405 PCB accurately - you have an electrolytic capacitor along side the R30/R31

L2 should not be 3.3uH - If you are aiming for accuracy and as you now have the bridge implemented correctly you should make it the calculated value.

The original design articles show how the bridge is derived using resistors and everyone now believes that the use of an inductor L2 is simply to reduce power losses when in fact it could be a 0.3-0.5R resistor with little loss but for the fact that it is fitted to prevent switching spikes from getting to the output and as such it is best that L2 (and all the inductors) are a bit lossy to prevent ringing.

Keith

DCD
 
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