Unbalanced to balanced input conversion? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th February 2009, 05:29 AM   #1
SQLGuy is offline SQLGuy  United States
diyAudio Member
 
Join Date: Jan 2009
Default Unbalanced to balanced input conversion?

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
Attached Images
File Type: jpg psaudio200c.jpg (98.4 KB, 1030 views)
  Reply With Quote
Old 20th February 2009, 03:36 PM   #2
diyAudio Member
 
Join Date: May 2008
not pheasable\possible the - side of the differential pair is needed for feedback,
  Reply With Quote
Old 20th February 2009, 03:57 PM   #3
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
Default Re: Unbalanced to balanced input conversion?

Quote:
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 .....
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 21st February 2009, 05:18 PM   #4
SQLGuy is offline SQLGuy  United States
diyAudio Member
 
Join Date: Jan 2009
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.
  Reply With Quote
Old 21st February 2009, 06:29 PM   #5
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Quote:
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
  Reply With Quote
Old 21st February 2009, 07:28 PM   #6
SQLGuy is offline SQLGuy  United States
diyAudio Member
 
Join Date: Jan 2009
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.
  Reply With Quote
Old 21st February 2009, 09:11 PM   #7
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
Quote:
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
  Reply With Quote
Old 22nd February 2009, 02:53 AM   #8
tedr is offline tedr  United States
diyAudio Member
 
Join Date: Jul 2006
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
  Reply With Quote
Old 22nd February 2009, 03:12 AM   #9
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
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.
__________________
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 22nd February 2009, 06:28 AM   #10
SQLGuy is offline SQLGuy  United States
diyAudio Member
 
Join Date: Jan 2009
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
unbalanced to balanced conversion neazoi Chip Amps 4 5th August 2008 01:45 PM
Line input transformer for unbalanced to balanced conversion okapi Chip Amps 23 12th January 2008 06:29 PM
using a transformer for balanced to unbalanced conversion h_a Analog Line Level 47 9th January 2007 11:16 AM
ESP 101 with balanced/unbalanced input Cloth Ears Solid State 2 17th June 2006 12:12 PM
unbalanced/balanced/unbalanced conversion using transformers vladimir Solid State 10 27th February 2003 07:54 PM


New To Site? Need Help?

All times are GMT. The time now is 09:23 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2