Unbalanced to balanced input conversion?

[SQLGuy]

Hi folks,

I'd like to convert this amplifier to balanced inputs (XLR), in the hopes of increasing its CMRR and making it more resistant to ground loop noise.

Would it be possible to bring a phase negative input around to the right side of the JFETs and mix it in without have to add another buffer stage?

I would appeciate any thoughts, and especially specific ideas as to the needed circuit changes.

By the way, the "input from DC servo" is another 10K resistor from a TL-081 based DC servo, so the after-the-op-amp gain of the DC servo is the same as that of the NFB loop.

Thanks,
Paul

[bosedtobosed]

not pheasable\possible the - side of the differential pair is needed for feedback,

[sakis]

Originally posted by SQLGuy
Hi folks,

I'd like to convert this amplifier to balanced inputs (XLR), in the hopes of increasing its CMRR and making it more resistant to ground loop noise.


Would it be possible to bring a phase negative input around to the right side of the JFETs and mix it in without have to add another buffer stage?

I would appeciate any thoughts, and especially specific ideas as to the needed circuit changes.

By the way, the "input from DC servo" is another 10K resistor from a TL-081 based DC servo, so the after-the-op-amp gain of the DC servo is the same as that of the NFB loop.

Thanks,
Paul

he he he you make it sound like ground loops are a balanced input issue .....

well its not ....actually is more ground loop techincues and more about grounding properly .... a balanced door will not change much if your installation is poor ....

PA expirience says so ....but this is just my opinion .....

[SQLGuy]

What do you suggest for grounding, then? Should I be breaking off ground pins on most of my equipment's power cords?

It's a bi-amped setup with:

Outlaw 990 Preamp
Behringer DCX2496 Crossover
SUMO Andromeda II Amp (Hi/Mid)
PS Audio 200c Amp (Low)
Paradigm Seismic 12 (Sub)
Bryston 2B-LP (Rears)

Every piece of equipment has a ground connection for its power cord.

The Bryston is unbalanced but has a ground defeat switch. With ground defeated I don't get ground loop noise from the Bryston.

The SUMO and the Paradigm have balanced inputs. I don't get ground loop noise from them.

The previous bass amp was an Adcom 555ii. It did not have a grounded power cord. I didn't get ground loop noise from it. The PS Audio ground is not designed to be defeated.

Neccessarily, with this much equipment, and this much power draw, I am not going to plug everyting into one outlet.

The noise is not super loud hum, but it's louder than I'd like it to be. Disconnected from the system, the noise from the PS Audio is about 300uV... connected to the system it's about 5mV. I would have thought that adding the 20dB or so of common mode rejection that balanced inputs would bring would help quite a bit here.

[fotios]

Originally posted by SQLGuy
What do you suggest for grounding, then? Should I be breaking off ground pins on most of my equipment's power cords?

It's a bi-amped setup with:

Outlaw 990 Preamp
Behringer DCX2496 Crossover
SUMO Andromeda II Amp (Hi/Mid)
PS Audio 200c Amp (Low)
Paradigm Seismic 12 (Sub)
Bryston 2B-LP (Rears)

Every piece of equipment has a ground connection for its power cord.

The Bryston is unbalanced but has a ground defeat switch. With ground defeated I don't get ground loop noise from the Bryston.

The SUMO and the Paradigm have balanced inputs. I don't get ground loop noise from them.

The previous bass amp was an Adcom 555ii. It did not have a grounded power cord. I didn't get ground loop noise from it. The PS Audio ground is not designed to be defeated.

Neccessarily, with this much equipment, and this much power draw, I am not going to plug everyting into one outlet.

The noise is not super loud hum, but it's louder than I'd like it to be. Disconnected from the system, the noise from the PS Audio is about 300uV... connected to the system it's about 5mV. I would have thought that adding the 20dB or so of common mode rejection that balanced inputs would bring would help quite a bit here.

Try a simple check to find how much of your units have earthed their power supply 0V node, with a ohmmeter. Place one of test leads on the earth contact of the mains cable plug. Place the other test lead on the shield of signal input sockets. Make a scratch on a paper: place a dot for each input shield point and for each mains earth point. According your measurement results, draw lines between dots which are shorted. Then you can see any existing gnd loop on the finished scratch. Remember that, avoidance of gnd loops between multiple connected units has the same value like the star point in power supplies gnd node.

Fotios

[SQLGuy]

Sorry, but I don't understand the point of this exercise.

I can tell you that the crossover has its chassis ground connected to its signal ground, and that the PS Audio amp also has its chassis ground connected to its signal ground.

As I mentioned before, plugging all of this equipment into a single outlet, which is what it sounds like you're suggesting, is not practical. Currently I have multiple paths to earth ground (through the separate power cords and through the patch cables), and I need a solution that will mitigate that in a pratical manner.

If I had balanced inputs to the PS Audio, I could float the signal shield at the amp and take the signal just from the differential between the XLR + and -. At that point I would only have one path to ground from the amp, instead of two.

[fotios]

Originally posted by SQLGuy
Sorry, but I don't understand the point of this exercise.

I can tell you that the crossover has its chassis ground connected to its signal ground, and that the PS Audio amp also has its chassis ground connected to its signal ground.

As I mentioned before, plugging all of this equipment into a single outlet, which is what it sounds like you're suggesting, is not practical. Currently I have multiple paths to earth ground (through the separate power cords and through the patch cables), and I need a solution that will mitigate that in a pratical manner.

If I had balanced inputs to the PS Audio, I could float the signal shield at the amp and take the signal just from the differential between the XLR + and -. At that point I would only have one path to ground from the amp, instead of two.

No, no, i am not suggesting a single outlet. I suggest only, the earth of outlets being like a star point. In your case it is not possible because x-over and PS amp. have earthed their 0V supply node. Make a simple external experiment. Place insulating tape (of course temporarily!) over the earth contact of the mains plug of x-over unit in a such way that its chassis is not earthed more. Check it with the buzzer. Be carefull that the x-over chassis is not in contact with any other unit chassis. Try your system again, and check if the noise dissapears. Let me know the result.

Fotios

[tedr]

probably a lot easier to add an external isolation transformer, you can buy an XLR adapter with transformer inside. Be sure that your connections do not join pin 1 and chassis.

Some good reading on the subject

RANE

http://www.rane.com/note151.html


Doug Self

http://www.dself.dsl.pipex.com/ampins/ampins.htm

Ted

[Conrad Hoffman]

What is the total power draw? I run three power amps, crossover, preamp, and all signal sources from a single power strip. Everything properly earthed and no hum problems. When I've needed a balanced input, it involved adding an opamp as the easiest way to do it. Probably won't solve this problem though. IMHO, something in the collection has poorly designed grounding.

[SQLGuy]

To get signal to the amp I was running some custom XLR to phone cables I'd made, with couplers to another pair of regular phono patches. This evening I rebuilt the custom cables with some longer shielded pair wire. I connected shield to XLR chassis, and left the shield unconnected at the phono end. (There is nothing special about the connection... I am just taking ground and signal + from the XLR pins to the inner wires of the shielded pair, and leaving signal - unused.)

The result was quite a bit more hum and noise than with the previous cables. Defeating the ground pin on the PS Audio fixed most of that, and rerouting the patch cables away from everything else helped a bit more... reducing the total noise to about 4mV.

I think I need to look into some better cables, among other things.