Noise with a modified Quad 405

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vzs

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Joined 2005
#1
Hi,

I modified a Quad 405 M1 to Keith Snook's Quad 405 evolution, Quad 405 - DCD Mod-3, but leaving C7 and R17. Although the sound of the amp is very good (for me) it has a clearly hearable noise even from 1-2m away from the speakers (not hum from rails).

I used BC560C for Tr1a and Tr1b; MPSA92 for Tr3 & Tr4 and 2N5551 for Tr2 as I couldn't find the ones specified. Tr9, 10 are 2SC5200.
I used quality components where it was needed, silverd mica for C8, 1% metal film resistors in the bridge, polypropylene for 680nF, Epcos 3uH and OPA627 (different sound but almost same noise as TL071).

The output dc is 26mV, I can't measure if it's oscillating or not but I tried to remove Tr9, Tr10 from the heatsink and without an input signal they remain cold/lukewarm. R30/31 get hot after a couple of minutes.

What do you suggest? Does changing transistors help?

Thank you,
Zsolt
 
#4
Partially resolved

Seems that a 2.2nF between bc of Tr10 resolved the problem and it's very quiet now, however if I touch the ground plane with a metallic object (screwdriver, anything) gets noisy again.

For sure it was oscillating and the capacitor between bc of Tr10 resolved partially the problem or at least in the audio spectrum.

Is it possible that it's oscillating because of the output transistors 2SC5200 wich are too fast (30MHz)? If so, can you recommend a TO-264 alternative, but MJL21194 wich I couldn't find here in Romania...

Thanks
 
#5
I think the problem is too much open loop (current) gain - Although you can fit a cap (QUADs C19) across tr10 the best option is to remove Tr3 ( a later QUAD change) keeping Tr4 as ZTX753 in my configuration.

This is shown on my schematic QUAD405-M12368-DCD-Mod-3 which can be got by clicking the link "schematic diagram" at the bottom of my original 405 mod page

Another problem is that Tr10 is biased on more than required for a current dumping configuration whereas Tr9 is almost "off" but at present I have not had time to add my findings to the website but keep an eye on the 405-2 page for updates.

Regards Keith
 
#8
Hi Roger,

I also removed R18 and as you wrote the base of Tr4 is from the collector of Tr1, just like in a Quad 405-2 iss. 7 (see the comment above the Revision 5, March 2007 schematics)

I haven't removed C7, R17 and I keep them in place until I can check for stability with an oscilloscope.

Ideed, in the QUAD405-M12368-DCD-Mod-3 schematics that Keith recommended it looks like R18 should remain and should be connected to Tr1 collector + Tr4 base, but I think it's a drawing mistake.

Have you made a pcb for the schematics (could you share it?) or modified an original one?

Regards,
Zsolt
 
#9
Yes it's a mistake or rather a style omission - I was trying to show Tr3 / R18 both in place and removed - I will grey out R18 on the next revision which should help.

It is difficult to publish stuff as I do without having a team around to check everything so consider yourselves part of the team and please continue to provide such feedback.

Meanwhile I shall try find time to finish the QUAD405-2 stuff and the CD stuff and the LS3/5A stuff ........

best Regards

Keith (Snook)
 
#10
vzs said:
Have you made a pcb for the schematics (could you share it?) or modified an original one?
Click to expand...


Hello Zsolt,

I would not mind sharing my layout! I have not got the time to finish any boards. The schematic is simular to Keith Snook's, just a few changes as I want a lower low frequency cutoff. There is room for larger value capacitors, even two in parallel if needed/wanted. The layout itself more or less follows the original Quad 405-2. I would be glad for all kinds of comments! I am only a laymen in electronics... Seems, that the file I wanted to attach is too large. You can find it here (I hope it works...): http://www.file-upload.net/download-423351/quad405_3.pdf.html


Roger
 
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