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vzs 28th April 2006 10:14 AM

Quad 405 hum problem
Hello all!

I recently finished to modify my home made Quad405 following Bernd Ludwig's tips and the input stage as specified at and everything worked perfectly up to a point.

I used to test my amplifier on the floor because the wall plug was easier to access from there :) , when I moved it on my table a strange low frequency hum appeared. I moved back to the floor and it went dead quiet again.

I use a 400VA toroidal transformer, made here in Romania. The input of the amplifier is in the air, so no signal at all, the DC offset on the output is around 0.2mV, so it is ok. The input OP is an OPA627 with stabilized source (7815, 7915 and 24V zenners before them).

I tried everything from changing the power cables to even ticker ones, to double the caps in the rectifier from 10.000uF to 20.000uF, but the problem remained when testing it on the table.

I'm out of ideeas and I don't want to place my amplifier on the floor :) so I'm looking forward to any ideeas.

Many thanks,

edl 28th April 2006 10:27 AM


I'm sure that you have got a ground loop - because of bad earthing.

vzs 28th April 2006 10:58 AM

Hello edl,

I tought that I might have a ground loop somewhere but the thing that I could not understand and drive me crazy is that it's working fine if the whole stuff (toroidal + PSU + 1 side Quad) is on the floor.

One thing I forgot to mention is that in both cases I have the earthing connectec to the PSU ground via a 10R.
However, when I tested it on the floor I tried to remove the earthing 10R and the hum was there too, like testing it on the table;
and by putting my hands near the input (not touching it) a modulated sound appeard in the speakers.

For this reason I suspect that is somehow related to the electromagnetic field of the toroidal. Also, if the toroidal is on the table it has a small 50Hz hum/noise comming from it, however this noise disappears on the floor (everything beeing connected in the same manner as on the table). I live in a flat wich has a metal structure, maybe that's the reason my toroid feels better there? Should I try to isolate the toroidal in some metal case?


dcd 13th May 2006 07:42 PM


I see from your comments that you applied my non inverting front end modification to your 405 - I hope you found it made a great improvement - did you also move C8 as I suggest ??

with regard to the hum problem - You may have noted that I wire every ground to the buss between the supply caps including the connection to the heatsink after removing the bolt - Soldered a wire on to the PCB where the bolt connects the PCB to heatsink - take this to the centre of the PSU caps or the 0V output terminal.

Hum can also be a problem if the input sockets are not connected to the chassis directly at the point they are mounted - also try to keep the connections from pre-amp to 405 together if they are separate cables.

I hope this helps

Best regards

Keith (dc-daylight ltd.)

vzs 17th May 2006 07:47 AM


Thank you, indeed the input sockets were not connected to the ground. I connected them and I also chnaged the PSU as you suggested.
After I connected the sockets to the ground the hum disappeared, however, a small ssssss noise remained wich was not there before. It's not a constant hum, but it's a faint noise audible in the bass and in the tweeters too.

I don't have an oscilloscope to see the output but I observed that the DC offset on my voltmeter is fluctuating now between +- 1mV.

Regarding to your question: the answer is yes :) I found your non inverting front end modification a great improvement. With my TL071 the noise went so down that I was realy undecided if I have to change them to the new OPA627, but of course I was curious and I did the modification. The noise I think remained the same, however the sound of the amplifier is much more pleasant. Also the DC offset went down from 39mV to around 0.5mV.

As a note: the operational is decoupled by 100nF right on the IC's pins and by 100uF after the stabilizers. I had to use stabilizers with the OPA627 because the original zenner + resitor started to warm up.
From the sound/oscillation point of view, is it important where do I solder the ground of the stabilizers regarding to the other components ground (like: R1, R3, C2 etc.) ?

Do you have any suggestion how to get rid of that faint noise? Could it be that my amplifier is sort of oscillating now?

Best regards,

AndrewT 17th May 2006 08:22 AM


the original zener + resitor started to warm up
running the zener warm is achieveable. Much more likely is that the Zener will run hot. This seems to do no harm to the following circuit.
Now that you have the supply working you may want to consider running a resistor Zener//cap AFTER the regulator. An experiment that may sound better.

That hum. Was it electrical? or mechanical coming from the transformer vibrating the table?

vzs 17th May 2006 09:16 AM

2 Attachment(s)


you may want to consider running a resistor Zener//cap AFTER the regulator
I had to run 24V Zenners before the 7815/7915 regulators because their max voltage is +35V/-35V.

Do you suggest me to put back the original 3.3k (or some smaller values) R7 and R8 after the regulators (red square in the attachement picture)?

Regarding to the hum, it was an electrical hum. A small mechanical hum could be heard because the transformer is not fixed but the one I'm talking about was clearly comming from the speakers.


AndrewT 17th May 2006 02:42 PM

do you really have a resistor in the Vrail feed to the opamp after the regs? This will kill effective regulator action.

I am suggesting that the the opamp MAY operate more cleanly if a zener stabilised voltage was fed to it rather than the dirty PSU voltage that comes out of these one chip regulators.

The easy way to achieve this is to add an extra regulating stage after the 7824 & 7924. The extra stage would be a dropping resistor (270r) and Zener (15V 500mW). This would provide about 30mA of peak current from the Zener and any additional current demand from the opamp would be met by the decoupling caps.

AndrewT 17th May 2006 02:46 PM

the ground symbol on the input side looks the same as the ground symbol on each of the regulator and decoupling returns.

Have you returned each of these grounds SEPARATELY to your central ground? Otherwise you may get contamination of your input signal from ground rail modulation. I wonder if the hiss you heard comes from here? or is it simply low level noise from the gain of your circuits?

vzs 17th May 2006 03:30 PM



the ground symbol on the input side looks the same as the ground symbol on each of the regulator and decoupling returns
No, I haven't returned them separately to my central ground and I'm susspecting that this is the cause of the faint hiss.
Maybe the regulators are ok as they are and I don't need any extra regulating stage but a separate ground return for the decoupling caps and the regulators.

This faint hiss is not the low level noise comming from the amplifier's gain. I changed the regulation stage because of the higher current need of the OPA627. Before of that the noise could be heard only if you sticked your ears to the speakers, especially after appling the non inverting design for the input.
Using the original regulation design for the OPA627 the sound was suppressed (maybe is not the right word), especially listening to piano (Keith Jarrett) the tail of the notes were clearly distorted and the resistance's R7 and R8 and zenners were getting hot after a couple of minutes.

That's why I tried the regulators for the input as in tvicol's design:

I have to try to separate the regulators and decoupling caps return from the rest of the input. Thanks.

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