Loop breakers

[roender]

The VAS current is 12mA?

 

[felixx]

Cut the 100nF capacitor in parallel with 10ohm on input ground

Done.Same thing.

There is a regulated power supply for the LTP and VAS?

Yes there is.A really good one.I also tried VAS on the same voltage with power stage.Same thing.

The VAS current is 12mA?

Is 13,6mA.I think is ehough.

Did it start humming when connecting together the grounds at the RCAs or was it when connecting to the power supply? Did it do this with open inputs, shorted inputs, inputs connected to each other with the mono cable or was it connected to the crossover? Was it in more than one of these situations?

-it start humming exactly when I connect together RCA's.When the input's RCA is open,no problem.
-I tried to short the inputs(RCA-screen short) with the cable in.Same humm.I tried to short the inputs with the cable off----no humm.
When the cable is connected to the cross,the humm is prezent like the version with the single cable.
The rail noise from tranny is something else.The humm nature perception is different.

I have 98db speakers and all that humm is prezent to my ears only from 15-20cm.From 30-40cm disapears,but still want to do the right thing.

 

[AndrewT]

I know you said you did, but I still think you have not tried re-locating the audio star ground off the smoothing capacitor plate.

 

[felixx]

Andrew
I will try to put the star ground close to the RCA's or between and see the effect.
On previous attempt the star ground still was close to the big caps....probabily too close.

 

[AndrewT]

I will try to put the star ground close to the RCA's or between and see the effect.
On previous attempt the star ground still was close to the big caps....probabily too close.

Hi,
it might be better midway between the PSU and the power ground of the amp pcb.
Then bring a single wire from the RCAs to this new location and disconnect the PCB signal ground from the audio ground.
The signal ground on the PCB gets grounded via the screen to the RCAs then to the audio ground.
What does your DMM read when the hum is present? both mVdc and mVac.

 

[roender]

Do you have Douglas Self book "Audio Power Amplifier Design Handbook" ?
Take a look at page 166 figure 6.4 and at page 170 figure 6.9

 

[felixx]

Hi,
it might be better midway between the PSU and the power ground of the amp pcb.
Then bring a single wire from the RCAs to this new location and disconnect the PCB signal ground from the audio ground.
The signal ground on the PCB gets grounded via the screen to the RCAs then to the audio ground.
What does your DMM read when the hum is present? both mVdc and mVac.

The DMM said this:
-on a.c....0.000V...dead.
-on c.c.....4,5mV and that value is not stable.....3,3mV to 6,9mV....max.

I will try to cutt the 10 ohms resistor to izolate completely and to relocate the grounds via RCA's to star point.

 

[AndrewT]

Hi,
looks like your DC scale is 200.0mVdc, but your AC is less sensitive @ 2000mVac.
Shame, it won't show hum of 0.5mV which may be what you are hearing with 98db speakers.

If you can borrow a more sensitive 200.0mVac DMM or better, you can move cables around and turn toroids to see the effect in real time.

 

[ricobasso]

I'm only going to say this once more and then I'm going away to hit my head against a brick wall.

You know which loop is causing the buzz/hum because it it the one which includes the external link of the two input shields on the phono interconnect. Fact.

Even with 2 X 10 Ohms the loop current is enough to develop too much hum at the input. e.g. you have 0.5mV hum at the speakers. Your amplifier gain is 44 so that means you have 11uV hum at the input. Say the rsistance of the input cable shield is 0.1 Ohm. That means the loop current needed to develop this 11uV in series with the input will be only 110uA. If you are still not convinced, try increasing the 10 Ohm resistors to 22 Ohms. If the hum reduces you have confirmed that this is the loop causing the problem.

You cannot eliminate this loop. So, all you can do is reduce the area of that loop by changing the route of the input wiring.

 

[ricobasso]

You cannot eliminate this loop. So, all you can do is reduce the area of that loop by changing the route of the input wiring.

I take that back. As an alternative, you could add differential balanced inputs.

 

[ricobasso]

So. What happened Felixx? Did you fix it or did you give up?

 

[felixx]

ricobasso
I still can't contact you....also I tryed with a "help" from administrator but...
I want to ask you some other issues.

 

[ricobasso]

I can't see any problems in my profile.

I have always allowed "Bulletin Board Administrators and Moderators To Send You Email Notices" and enabled "Private Messaging"

I have now also set "Send you an email when you receive a Private Message" to YES.

If you still have problems you could try speakerguru"AT"hotmail"DOT"co"DOT"uk (change the usual anti-spam parts)

 

[AudioFreak]

ricobasso,

You need to set the option "Hide email address" to no. This will allow members to contact you.

 

[Eva]

My recommendation: Wire RCA input grounds to central PSU ground. Wire amplifier module input grounds also to central PSU ground and *not* to the input connectors. Wire power ground of each amplifier module independently to central PSU ground. Wire speaker grounds to central PSU ground too.

 

[ricobasso]

ricobasso,

You need to set the option "Hide email address" to no. This will allow members to contact you.

OK done that now