Hum problem with pseude dual mono amp

[kin]

Hello everyone!
As the title suggests, I have built an amp with the following configuration and I have a less than usual hum problem. I have tried the 2 ways of connecting the input ground and the symptoms are quite similar: If i plug in only one channel(doesn't matter witch one of them) there is no hum or noise whatsoever, but when I plug both I get a very loud hum. I have also tried to tie together the 2 star-grounds and the hum is lower but still very loud.

In my drawings there is no safety ground because where I live we have two-prong AC mains. However, there is AC coupling between each amplifier ground and chassis though 220 nF film cap.

What are your suggestions regarding this problem? Many thanks in advance!

[kin]

...

[mjf]

hello.
have you tried to connect the two stargrounds to a single point on the chassis straitways,without that caps?perhaps this can help...........
greetings............

[Jan Dupont]

Hi kin,

1. Are your input RCAs isolated from the chassis ?
2. Does the PSU Ground connections (Gnd Left/Right) have connection to the Signal Gnd in on the PCB ?
3. How is your PSU Gnd's connected to chassis and PSU ?

If not allready done this way, try isolate the RCA from the chassis. If there is a Gnd Left/Right to Input Gnd connection on the PCB, try setup one again.

If there are no connections between Gnd's on the PCBs, connect the RCA Gnd's together, and connect this RCA Gnd to a common GND point between the big filter caps (as in setup two, but RCA
Gnd's should connect to a common Gnd beyween the PSU caps). This common Gnd point (now a star Ground should the be connected to chassis as chassis Gnd).

Always make sure that you only have ONE Gnd connection to each circuit, connector or chassis.

If you still got problems, try to post a photo here showing your wiring.

[jaycee]

Remember that pretty much every source component will join left and right signal grounds.

Try adding a 10 ohm resistor between signal and power ground

[kin]

Thanks everyone for the advice!

@mjf
I have tried connecting the two stargrounds together(with little improvement) but not to the chassis, I will give it a try.

@Jan Dupont

1. Yes
2. Yes, in fact the pcbs have a ground plane.
I don't think this is the problem though, because, as I said above, if I connect to the input RCA only one channel(that is the cable witch comes form the source, the internal wiring is not changed) there is no noise, problems occur only when i connect both channels. Also, with the source disconnected(that is both inputs floating), there is no noise.
So this makes me think that there is a difference between left and right ground and somehow hum appears into my signal.

3. No, because my mains is 2-prong.
Could this be the problem? I will try connecting starground to chassis and post the results. Speaking of this, I have seen in Self's book that he recommends connecting gnd to chassis in the vicinity of input RCA's, what are your thoughts on this?

I have also tried connecting a volume potentiometer, using both above setups. When in setup one, there is noise even at low levels. With setup two, at minimum there is no noise but it rises as I crank the volume up and at max there is the same noise as in the case when i had no volume pot. Maybe this info is useful.

[Jan Dupont]

kin

I think this is your Ground loop.
As you normally have common Ground in your source (preamp) you create a Ground loop through that.

[tiefbassuebertr]

to avoid unwanted ground loops by 2-ch amps in same cabinet, the royal way is always to use dual mono power supplies (i. e. two identical independend versions include two transformers) so as outsourced power supplies (i. e. additional enclosures).
Now the right place between left and right GND is the input GND. At same places I connect the metal enclosure for power amps (now there are optimized screen function).
In all the other cases I can't use the mentioned royal way und unfortunately I must choise an other GND connection approach. Residual hum effects don't disappear in all cases

-

[Michael Chua]

Originally posted by kin
I don't think this is the problem though, because, as I said above, if I connect to the input RCA only one channel(that is the cable witch comes form the source, the internal wiring is not changed) there is no noise, problems occur only when i connect both channels.

Hi kin

Jan is quite right.

Take one interconnect and disconnect the shield wire at one end.

This will break the ground loop. There should be no hum.

Do remember that the normal grounded interconnect must not be unplugged otherwise you will get a loud hum from the lifted interconnect.

Regards
Mike

[kin]

I am very grateful to you for your suggestions.
I have tried the other suggestions, that is connecting the two star-grounds together and then to the chassis. I as suspected, connecting it to the chassis made no difference. In fact, i tried connecting the chassis gnd at input, as per D self, also no difference. What is certain is that when I connect the 2 stargrounds together i get considerably lower hum, however not neglijable.

Regarding the ground loop formed at the source, i am aware of it, but that happens to every unbalanced amplifier as far as I can tell. I will try disconnecting one shield and report here the results(however I think this solution is somehow a compromise, as the loop formed by the input current of that channel will be considerably greater).


@ tiefbassuebertr

I know the absolute best solution would be two separate power supplies, and i think that the problem(or at least part of it) is the psu. Maybe converting it by using only two rectifier bridges and having only one star-ground would reduce the hum.