quad 44/405-2 another hum problem

[graeme uk]

I have a quad 44/405-2
It has been upgraded with modified boards and output transformers, and a dual mono power supply.
I think i have a problem with the 44 though.
It makes a dirty sounding hum, but only at high volumes with all sources unplugged, the sound is different/louder/quieter depending what source is selected.
Does anyone know this equipment well enouigh to say whats the likely cause?
Its not much of a problem because i never listen to it that loud anyway, but all the same, something aint right.

Im pretty sure its not the power amp as its only just been sorted and is dead silent without the pre linked up.

I hadnt used the pair much, or checked for hum at high volume before the power amp was modded.

Any ideas?

 

[Ketalar]

/.../
I think i have a problem with the 44 though.
It makes a dirty sounding hum, but only at high volumes with all sources unplugged, the sound is different/louder/quieter depending what source is selected.
/.../
I hadnt used the pair much, or checked for hum at high volume before the power amp was modded.

Any ideas?

Same problem here but only with tape (both) and phono inputs.
Curioulsly, it appears after around one minute, as if a capacitance slowly charges.
I've read somewhere that in early '44s chassis ground was not connected to mains ground. Dunno if it helps.

Regards,

 

[AndrewT]

early '44s chassis ground was not connected to mains ground

Quad would not do this.

 

[Ketalar]

Sure.
I should have written that there could have been different ground connection methods between early an late '44s...
Anyway, this peculiar hum problem exists.
The input cards are connected to a board where also sits the PSU elements (except the transformer), maybe one of them radiates ?

 

[Ketalar]

Here's what I found :
http://db.audioasylum.com/cgi/m.mpl?forum=amp&n=5050&highlight=quad+44+mains&r=&session=
Maybe it could help.
Best regards,

 

[AndrewT]

Hi Ketalar,
are they referring to Quad11 or 44 or 405?

 

[Ketalar]

Hi Ketalar,
are they referring to Quad11 or 44 or 405?

(My english may be poor)
Hello,
Thanks for the reply.

They refer to QUAD II fed by a 44 :
He writes the II has no ground connection, except via the signal lead.
In my opinion, it means that its power cord has only 2 wires, no mains ground wire.
In this case (!), if the 44 signal output ground is not connected to the mains ground, this leaves the amp floating and "hot". Idem for the inputs.

I will check my preamp, to see if my hum problem is solved by connecting (or not) signal gnd to mains gnd. However, as my 44+405 have 3-wire power cords, that would add a ground loop and worsen my hum problem !

 

[AndrewT]

However, as my 44+405 have 3-wire power cords, that would add a ground loop and worsen my hum problem

This quite normal and generally does not give rise to hum.
Many manufacturers can get this right and I have heard this combination and I cannot recall any hum.

 

[Ketalar]

I checked my '44 : the audio gnd and mains gnd are not connected together, as per the schematics, by the way.
Voltages on input cards pins are OK, but I dont'have a 'scope to investigate further.
As the hum problem arises from only 3 on the 5 inputs, I wonder if some 4066 could be defective...
Is it possible ?